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Advanced Signal Processing Part 2 Lecture 3/25/20
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Dr. Jodi Baxter >>Okay, so what is the purpose of directional microphones or directional technology?
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So directional technology was developed in the 20s and 30s and what I remember of this -
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- unfortunately I did not write it down -
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-was that it was actually initially developed for places like stadiums,
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for football games, basketball games in order to have audibility
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for what was going on on the court or on the field,
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and not have what was happening in the crowd
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or the stadium interfering as much with that sound.
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The first hearing aid that was directional came out from Oticon in 1972 for the -
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- that hearing aid in the 80s,
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it was that you were purchasing a directional hearing aid.
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And so that was implemented in the way of having that single microphone with two ports,
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which we had talked about back in the hearing aid components lecture.
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So it was a very small number of hearing aids
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that were sold that were directional hearing
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aids and not really stayed that way and even declined throughout the 80s.
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So there wasn't a lot of availability or options in analog hearing aids,
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and it wasn't until the early 90s that the option became available to switch
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manually from an omnidirectional hearing aid or hearing aid mode program to a directional mode.
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And this was again still pretty limited in terms
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of what the size of that directional array was,
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and it was fixed and it was using,
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again, a manual toggle between the two modes.
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So Phonak actually was the first manufacturer to come out with the
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concept of a twin microphone or having two omnidirectional microphones and
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since then, we've been really able to run with that technology.
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So I would like to encourage you to take a couple of minutes to go back
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to the hearing aid components lecture and review
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the two types of directional microphone technology that we discussed,
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which was the single microphone that had two different ports,
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which I believe is not used at this point anymore.
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And the multi microphone system,
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meaning that there are multiple,
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typically two per hearing aid omnidirectional microphones.
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And then now, because we have this binaural communication,
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they're really calling it a microphone array or a dual
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microphone, so meaning the communication between those microphones
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provide the hearing aid with information on where
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it should be having maximal sensitivity to sound.
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So when we're talking about a polar plot or a polar sensitivity pattern,
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we're really looking at the three-dimensional -
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- dimensional space surrounding the
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microphone and where it is most sensitive to sound.
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Again, if you take a few minutes to go back to the hearing aid components lecture and review that,
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that's really kind of determined by those two
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design parameters that we initially talked about.
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So the external delay, meaning the port spacing between the two microphones,
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which is typically between 4 and 12 millimeters,
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and then that internal delay which,
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at this point, is typically done mathematically or electrically.
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So a polar plot is a plot of the hearing aid output
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as a function of the angle of the sound source in the horizontal plane.
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So typically the way polar plots are developed
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is by having a keymar or an acoustic mannequin in the center of a speaker array,
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a 360-degree array of speakers,
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and then plotting the output of the hearing aid,
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depending on where the signal of interest or sound source is,
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what speaker it's coming from.
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It's also often referred to as the directional sensitivity pattern.
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A vocabulary word you need to know is the null
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point which is the point of greatest attenuation or the least amount of sensitivity.
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So if we look at figure 1027 here,
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we're looking at - - this kind of center area is where the acoustic mannequin is,
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and then this is the speaker array,
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and you can see the dotted is the omnidirectional BTE.
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Remember we talked about, it's really not truly omnidirectional if it's
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mounted on someone's head because of the head shadow effect.
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So my guess by looking at this is that the hearing aid is a mounted on the right ear,
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because we have greatest sensitivity here and a little bit less on this side.
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And then we have the directional sensitivity pattern here,
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so greatest sensitivity in this area,
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and then a null point here and less sensitivity to this side of the head.
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And then what's important to also understand is that this varies by frequency as well.
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So remember that directional microphones operate by sampling at multiple locations,
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and comparing the sound sample at those locations.
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So remember the external delay is simply the spacing between the two microphones,
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or the microphone ports. And calculating that spacing or calculating the external delay by dividing the spacing itself.
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So the physical distance between the speed of sound.
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The internal delay, back in these original directional microphones,
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where there was one microphone,
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two ports- - a damper was put into the port of the rear microphone or the back microphone.
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So this was typically like a wire mesh acoustic damper.
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And the purpose was to introduce a time delay
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by slowing down the sound as it arrived to the rear microphone.
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So if you think about it, sound coming from behind the hearing aid
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user would arrive to this microphone first and then travel to the front microphone.
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By introducing the damper here and flowing down that sound,
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the goal is that the sound that arrives to the
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rear port and goes to the underside or bottom of the diaphragm,
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would arrive at the same time as the sound arriving to the front microphone,
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which is sent to the top or the upper side of the diaphragm.
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And if they are arriving at the same time,
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then there's essentially no net pressure on the diaphragm and that sound is canceling.
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It's canceled out. So the more exactly matched the phase is,
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the greater the attenuation is.
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Now that manipulation or that relationship between the internal delay
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and the external delay is primarily done mathematically or electronically.
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And that is what determines what a polar plot looks like.
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So again, omnidirectional means that the microphone is equally sensitive to
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sounds from all direction, all directions.
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Again when we're looking at one that's a perfect circle like this,
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it's suspended in free space,
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so it's not taking into account the hearing aid user wearing the
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hearing aid. So again, our head shadow effect.
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This is good when there - - it's a
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quiet environment, because it gives the patient access to all sounds in the environment.
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And then this is an example of a directionality pattern,
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a cardioid pattern is what it's called because it looks like a little heart.
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When you're looking at this type of polar plot,
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the hearing aid microphone is sensitive to sounds in front of the hearing aid user,
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so that's gonna be the zero degree here,
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and there's a large null point or a point where it's not as sensitive to
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sounds behind the hearing aid user.
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The other thing that's important to
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understand and we'll see here in a minute is that these polar plots are
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also frequency-dependent. So this is a series of other versions of cardioid -
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- or polar plot - - you can ignore this one.
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I actually pulled this from something that was not hearing aid
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related just so that - - I thought it was the best image I could find.
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So we also have a - - let's see,
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super cardioid here, so again more sensitive to sounds behind the user than a plain cardioid,
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but narrow sensitivity to sounds in front compared to a regular cardioid.
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Same thing with a hyper-cardioid,
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so even more narrower or more directional in the front,
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as you can see - - although it's not as obvious.
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But again, greater sensitivity to sounds in the rear and these kind of null points
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on the sides. Figure 8, or a bi-directional is equally as sensitive to the front and back,
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but not as sensitive to sounds to the side.
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Yeah, so just options of what sensitivity and null points are.
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Okay, so directivity index is -
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- (break in audio) - - - What makes this a little bit less straightforward
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and a little bit more confusing is at this point,
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directionality in all digital systems are adaptive;
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they switch automatically and they're multiband.
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So each frequency is often doing something different.
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It's not as global as it used to be.
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So, but some more terminology that's useful to understand.
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Fixed directionality is when the sensitivity pattern does not adapt or does not change.
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Fixed directionality does not actually have to be what I have written
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on this slide here which is one null point behind the patient or a cardioid.
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Fixed just really means that it's not adaptive and at this point,
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we do have the ability to set up fixed directionality in most hearing aids as a
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separate program, so times that I may choose to do this is if,
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for example, a patient is still continuing to struggle,
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and it doesn't seem like the adaptive
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algorithms of the hearing aid are as sensitive as the person needs it to be
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or they're describing something of it kind of coming in and out of directionality,
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I could put a fixed kind of tight directional channel so a beam former,
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which we'll talk about here in a minute,
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directly in front of them, so that they
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can manually override those decision rules that are in the hearing aid
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programming and the patient can use their phone app or their program button
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on the hearing aid to go into that directional program.
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That tends to be a little bit more effective
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in terms of providing a better signal-to-noise ratio,
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but there are some challenges too;
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for example, the patient has to know how to use it,
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so how to actually change into it,
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but they also have to remember when to change into it.
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They have to physically be able to do that;
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again either the push button on the hearing aid,
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or through their app. And they also have to remember to get out of it which,
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anecdotally, is often a challenge of,
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okay, now I'm leaving this restaurant and I chose to
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put my hearing aid into the directional program and now I'm leaving and I
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completely forgot to move it out of that.
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Which then, say those two individuals get in the car,
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they're really going to struggle if they're in a fixed directional program and the array is
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focused on sounds directly in front of them.
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If they're driving, they're not going to hear the person to the right of them in the car at all.
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So it's important that if you're gonna set up a fixed program,
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you've got to choose the patient wisely,
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and you really have to counsel on when to use it and when not to use it.
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Another challenge again of that is,
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say, two people are at a restaurant and then the server comes up and starts talking,
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the person in that fixed program -
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- again, of course, depending on their hearing loss and other features in the hearing aid,
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they may not have any awareness that the server has come up and started talking,
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because they have a null point in that spot,
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you know, to the side and really only has good sensitivity
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or ability to pick up sounds from directly in front of them.
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Other scenarios where I've set up fixed directional programs,
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if you think of someone in a wheelchair or someone who's driving children,
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maybe, you know, parent bus driver -
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- I actually have set up a fixed directional program for a bus driver.
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Think about it. Where that person needs to hear is actually
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not in front of them but sounds behind them.
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So we often have the ability to make a fixed directivity pattern,
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you know, in front, behind to the side,
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depending on what the need of your patient is,
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but that - you need to have, you know,
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lengthy conversations and counseling sessions
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on what their need is and how to use it effectively;
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otherwise they could potentially get into trouble.
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In a fixed sensitivity pattern,
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that time delay is constant. In an adaptive pattern,
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that directivity pattern typically is changing
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to maximize the audibility from a target direction or more often,
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the most dominant sound source.
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So, in general, these are usually more effective than the fixed arrays,
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because it's rare that an individual only wants to hear from
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one direction. So in this, that time delay is constantly varied,
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which then changes the polar plot constantly to provide the most attenuation for,
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you know, depending on what the sound level is to the side,
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from behind. Again, there are typically,
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just like what we talked about in the last lecture,
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digital - -the digital noise reduction lecture -
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-there are usually rules or criterion in
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place for the hearing aid to decide,
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okay, at this point, so for example,
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when sound exceeds 55 or 65 DB SPL,
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which I would say is typically really conservative or more often,
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it's like a 70 DB SPL and the SNR is of a certain range in this
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frequency band - - the hearing aid will go into a directionality mode.
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But when that criterion has not been met,
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the hearing aid is functioning omnidirectionally.
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Again, this is multi-channel,
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so this is happening at multiple frequency channels at any given time.
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So in- - within any frequency band,
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the hearing aid is looking for a noise source,
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detecting this overall DB SPL,
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detecting the signal-to-noise ratio,
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and then applying those different time delays in each channel.
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And so the goal is to reduce multiple noise sources that are spatially separated,
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if they're occurring in different frequency regions.
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Okay, so beamforming directionality.
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This really came to be when we had the
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development of that, again, intraoral or intra ear wireless communication
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because that essentially allowed us to then have four microphones because,
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again, we have two on each hearing aid that communicate with one another.
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So this allowed us to move towards even more narrow directionality.
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When we're talking about beamforming,
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though, it's exactly that. So you can think of it as a beam and I really like this graphic here,
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where the purple is what is within that beam or that very narrow directionality,
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that is getting good emphasis or good sensitivity and everything outside of
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that beam is attenuated. So a traditional dual mic system
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has about a sixty degree angle,
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whereas beamforming technology can be as
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tight as about 45 degrees. The limitations,
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I've kind of already talked about so,
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for example, if you're using
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this beam in a restaurant setting and then the server comes up to the side,
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they're not going to have probably even awareness -
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- again, of course, depending on degree of hearing loss in the speaker and a number of other things,
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but much less awareness of sound coming from other directions.
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So I have some people, for example,
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who may use this in church, but again,
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they're not gonna be able to communicate at that point with the
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person sitting next to them.
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So counseling is really, really important
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when you are choosing to use this type of directionality.
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And you can see here, I'm pretty sure this is Signia.
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I do like this app, although I don't use that manufacturer
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as often, but this is a really nice way for the patient to visually understand and
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control what they - - where their directional beams are pointing to and then you can see down here at the bottom,
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they also have the ability to control how wide or narrow that beam is.
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In general, the smaller that angle is,
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the more favorable the signal-to-noise ratio is.
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So evidence has shown that when we,
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in the lab setting, the angle goes from 60 degrees,
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which is a traditional directional mic system to about 40 degrees -
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- 45 degrees - - that can make an improvement as great as 1 DB in the signal-to-noise ratio.
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So there are a number of manufacturers
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that are implementing this beamforming technology,
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although not all of them. What you're looking at here on these polar plots -
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- these were Signia, although not the latest platform now,
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and we're looking at, you know,
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them bragging about their beamforming technology
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in their hearing aids versus other manufacturers,
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but mostly what - - I just liked the ability to look at how those polar plots may vary,
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again, for different manufacturers,
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different technology. Some manufacturers use split band or split channel directionality.
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So what this is, is the hearing aid is directional in the
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high frequencies, but omnidirectional and the low frequencies.
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This cutoff point of where it becomes directional
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versus omnidirectional may be fixed by the manufacturer;
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it may be something that's able to be adjusted by the clinician or audiologist.
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Or it may vary depending on the patient's hearing loss.
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Typically that cutoff point is around two thousand Hz or so.
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The purpose of this is that direction -
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- directional systems typically have a low frequency roll-off naturally.
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That is then compensated for by applying an extra
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boost of amplification in the low frequencies.
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This is called equalization.
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But when that low frequency boost is provided,
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it then affects the effectiveness of the directionality of the system.
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At the same time, if you don't do it,
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the patient may complain that the hearing aid sounds too tinny or too unnatural.
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The advantages of having this concept of directional
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in the higher frequencies and omnidirectional in the low frequencies
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is that it does more closely mimic the directivity of the human ear.
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So we take advantage of the directionality,
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but also we preserve the environmental awareness and low frequency cues that happen,
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such as the interaural timing difference.
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If the equalization concept -
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- so again, that idea that the low frequencies are boosted
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in order to compensate for that low frequency
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roll-off that happens naturally with directional systems,
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then the individual's own voice,
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wind noise, other low frequency sounds,
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may be overamplified. But this is gonna sound more natural to the hearing aid user.
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The disadvantage of having this concept of split
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band or split channel directionality is that
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there will be no signal-to-noise ratio improvement in the
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low frequencies, which is where noise often occurs.
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So sometimes that may not be great for hearing and noise.
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The other concept to keep in mind is that all
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open fit hearing aids really are split directionality.
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And the reason for that is, remember in the low frequencies,
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if you have something that's truly open,
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like open dome, we're really not providing
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gain in those frequencies at all and the sound is able to enter the ear canal
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naturally, the way it normally does.
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A hearing aid isn't going to be acting on
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any sound that is entering the ear canal naturally.
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So if it's not being processed by the hearing aid,
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then it's not getting any directionality.
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There have definitely been times working
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with patients where I may move to more occluding or closed canal,
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by way of dome or earmold sooner than I thought I would based on their hearing loss.
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So they may have, you know, decent low frequency hearing,
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but I may choose to go with something a little bit more occluding because I really need
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that directionality, noise reduction,
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any of that to take place in the low frequencies.
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This is not common but it's something that I definitely think about
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and have to balance as I'm making decisions.
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Again, this is typically not the way I'll go right off the bat,
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because most of the time, if you include the ear more than it should be,
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the patient is not going to be satisfied with the sound quality or the
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naturalness of their own voice,
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particularly, because of the occlusion effect.
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But if I'm continuing to have a patient struggle or complain about hearing and noise or,
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you know, things that make it sound like the noise reduction
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aren't doing their job, and the patient is in a completely open dome,
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that may be something that I consider changing.
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So again, we're looking at polar plots here.
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But what we're looking at now is the polar plot by frequency.
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So you can see there's different colors to demarcate the different frequencies,
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so 500, 1000, 2000, 4000. And this graph is really meant to show you
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both in a completely open ear and also with a
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hearing aid that is intentionally doing split band directionality,
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the low frequencies, so 500 and 1000 hz are essentially omnidirectional versus two and four,
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which are getting some level of directionality.
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So again, these, you know, anything that falls outside of this red
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or this gold region for the two and four K,
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the hearing aid is really not picking up.
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It's not sensitive to it, so you can see the difference in sensitivity patterns
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between the microphone between those different frequencies.
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One manufacturer, G.N. Resound,
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uses asymmetric directivity and to be completely honest with you,
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the first time I heard about this type of directionality,
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and even now, as I read about it,
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it's hard for me to understand that it's effective.
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But the evidence shows that it is.
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What this is is, one hearing aid is always in
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omnidirectional and one hearing aid is always in directional.
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The theory behind this or the goal,
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is to maximize auditory awareness from any direction.
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I say that in quotes because we still have
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to remember that we have the concept of the head shadow coming into play here.
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But also having directionality at all times as well.
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They reported that this was developed initially to overcome a patient's inability to use the manual
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directional microphone, and the limitations that do exist of automatic
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environmental classification.
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The last I heard, this was still being used
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in Resound hearing aids, but hopefully the group that has been assigned to look
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at Resound can tell us more about it.
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Again, there is research to show that
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it's successful, so two different articles showed just as much directional
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benefit - - and then two articles that showed less directional benefit.
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So 1-1/2 to 2 dB in adults and children with mild to moderate hearing loss.
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Okay so let's talk a little bit about reverberation.
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Reverberation is when a sound source is reflected
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off room surfaces and so in this scenario we
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have a direct sound pathway,
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and multiple reflected sound pathways.
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As the distance from the speaker,
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or the sound source increases,
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the direct sound level will decrease,
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but the reverberant sound remains constant depending on the
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location of the reflective services.
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So, for example, if you are near a wall or a
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hard surface that may be a reflective surface,
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the reflective sound or the reverberant sound may even be higher than the direct sound.
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Critical distance is an important concept to understand.
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Critical distance is the point that the direct sound is equal to the reverberant sound.
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So basically once you're outside of that critical distance,
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the direct sound level is drastically reduced compared to the reverberant sound.
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And once we're outside of that critical distance point,
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the directional microphone and the digital noise
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reduction effectiveness are also reduced.
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So this is why counseling is very important,
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even with these great features
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that we have of directional microphones and digital noise reduction,
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the patient still has to be close to the speaker.
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This will improve the signal-to-noise
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ratio, this reduces the consequential effects of reverberation.
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This reduces the effects of distance,
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so think about your inverse square law.
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The further you move away from the sound,
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the greater the reduction of sound,
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and this also does allow the directional microphones to be more effective.
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Finally, it gives the patient access to visual cues from the speaker.
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Typically, critical distance is around six feet.
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Some of the manufacturers say that directionality really is most effective within 10 feet.
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So again, this is important for you to remember as the clinician.
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A perfect example of this is church.
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You know, even if the patient is in the very front row,
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they are rarely within 10 feet of the person talking.
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Now, again, this is a different type of environment because they do have speakers
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too - - but you can set up the most beautiful fixed directional program,
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but if the speaker is still 30 feet away,
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it's not going to be as effective.
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So this is why we need to have a constant conversation about,
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again, being close to the speaker for a variety of different reasons.
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Reverberation is a pretty significant challenge:
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the biggest challenge for directional microphones.
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If we look at our waveform graphs or figures on the
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right side of the slide here,
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what you can see happens is reverberation
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essentially fills in the modulation depth of the speech signal.
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So this is the same signal, 4.8 and 415,
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but this signal is sampled in a reverberant room.
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So you can see - - we have these really big peaks and valleys and
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modulation depths here that are essentially wiped away or gone in Figure 415.
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This isn't noise; this is just a reverberation.
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Average reverberation time is 4 seconds.
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The longer the time is, the more that modulation is filled in.
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So basically what's happening here is earlier parts of speech that are then
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reverberating are serving to mask part -
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- later occurring parts of speech.
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Another challenge with reverberation in terms of directional microphones is that
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essentially it's reaching the microphone evenly from all directions.
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So then those concepts of making a polar plot and
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putting in a null point really works minimally well when the noise is coming
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from every direction. So again,
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an adaptive algorithm tries to choose -
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- choose the pattern that is average from overall directions to attenuate the most noise.
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But that's going to be challenging when the noise is really coming from everywhere.
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So clinical application of directional microphones.
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At this point they're primarily implemented automatically and adaptively,
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but there are scenarios, as I
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described earlier in the lecture that they can be implemented as a manual program.
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You just have to make sure the patient knows how and when to
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successfully use the program.
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The mic port location orientation have to be
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correct and in the horizontal plane in order for them to be effective.
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The range of benefit of directional microphones is really highly dependent on the environment.
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So hopefully we've covered this in depth throughout this slide show,
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but factors, of course, are going to include the number,
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location and type of competing noise sources,
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reverberation time in the environment,
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the distance between the hearing aid user and sound of interest,
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and the fact that the hearing aids are operating on the principle that the speech is the signal
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of interest. And typically operating on the principle that the loudest speech is
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the clinical - - the signal of interest,
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which may not always be the case.
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Another thing to keep in mind is the effective venting on directionality,
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which we talked about briefly a couple of slides ago.
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So the hearing aids style doesn't -
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-has not been found in the research to have a significant impact on directionality.
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But venting or openness of the canal does.
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Again, because sound - - sound is leaking out of the ear canal,
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but also because sound is entering the ear canal unprocessed by the hearing aid.
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So if you look at figure 1116 here,
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we're looking at changes in low frequency DI,
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or directivity index. In AIDI which is the articulation index directivity
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index for a no-vent versus a 1-millimeter event,
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which is quite small, so that's gonna be a little bit larger than a pressure vent.
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A 2-millimeter vent, which is,
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then, a pretty large vent and then a completely open canal.
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But, you know, venting is valuable and omnidirectional is valuable,
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in that it does provide us with sound awareness from multiple directions.
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So what does the evidence tell us on directionality
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and speech recognition and noise?
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This is wide-ranging. It's been reported that up to 11 DB improvement
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or 70 percent improvement in word recognition in noise in a lab setting,
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when using directionality. A 2017 article by Todd Ricketts at Vanderbilt
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reported that directional microphones provide
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an advantage or improvement in signal-to-noise ratio about 42 percent of the time in school.
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This is important because for a number of years,
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it was said that we shouldn't be using directionality
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or noise reduction in pediatric fittings because we were
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potentially not able to take advantage of the concept of incidental learning.
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At this point it seems like enough evidence has
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disproven that directional microphone noise reduction would be valuable for kids in schools,
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but we will talk more about that in Hearing Aids II when we have a pediatric-specific lecture.
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Adaptive directionality and beamforming directionality
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have been shown to add an additional 1 to 2 DB
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of benefit over that concept of traditional fixed directionality.
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Listening effort - - uh, so I'd love to see the research coming out about listening effort.
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Both directional and beamforming technology have been shown to
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reduce listening effort, both behaviorally and subjectively compared
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to omnidirectional implementation.
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So, again, this is where we have to think
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about how we use this for patients.
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This study Chord et al. , found that 30% of
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users did not switch between settings and users often did not know when to switch.
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Anecdotally I will tell you that for the most part that seems to be true.
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I really try to avoid setting up specific programs for patients because
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even the most savvy patients seem to stop using them after a period of time,
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or still don't quite understand how to use them,
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when to use them, when to get out of them.
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I do do them, sometimes that you just absolutely have to,
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but I would be conservative with doing that and making sure you're counseling and also
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checking back in routinely because I troubleshooted a lot of problems
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because of excessive program use and lack of understanding of programs.
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One study with Terese Walden suggested that some
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of the switching algorithms may be too conservative,
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so it's - - the study suggested that 33% of the time,
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hearing users were in environments where a directional benefit may be -
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- the directional microphone may be beneficial;
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however, they only switch 5 to 17 percent of the time.
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I will tell you, also anecdotally,
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this has been my experience clinically;
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in the manufacturers that do have really precise data logging systems,
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I am shocked by how rarely they're actually going
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into their speech in noise program or programs
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where directionality and noise reduction should be -
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- are used, so that information can be valuable,
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especially when you have the ability to change how quickly the hearing aid is doing this.
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So again, looking at directional advantage,
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compared to omni, it's a wide range,
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and replicated by a number of studies.
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There does not seem to be a significant effect
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based on hearing loss in terms of omnidirectional versus directional.
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