[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:01.31,0:00:02.76,Default,,0000,0000,0000,,- [Voiceover] If two waves overlap
Dialogue: 0,0:00:02.76,0:00:04.54,Default,,0000,0000,0000,,in the same medium, we say that
Dialogue: 0,0:00:04.54,0:00:06.65,Default,,0000,0000,0000,,there's wave interference.
Dialogue: 0,0:00:06.65,0:00:09.80,Default,,0000,0000,0000,,So this box here could represent a speaker
Dialogue: 0,0:00:09.80,0:00:11.88,Default,,0000,0000,0000,,and this could be the\Nsound wave it generates
Dialogue: 0,0:00:11.88,0:00:14.39,Default,,0000,0000,0000,,or it could represent a\Nlaser and this would be
Dialogue: 0,0:00:14.39,0:00:16.26,Default,,0000,0000,0000,,the light wave it generates or it could be
Dialogue: 0,0:00:16.26,0:00:18.75,Default,,0000,0000,0000,,some sort of ripple\Ntank generator and this
Dialogue: 0,0:00:18.75,0:00:21.12,Default,,0000,0000,0000,,is the water wave it generates.
Dialogue: 0,0:00:21.12,0:00:23.31,Default,,0000,0000,0000,,Regardless, if you had a second source
Dialogue: 0,0:00:23.31,0:00:25.66,Default,,0000,0000,0000,,of a wave and these were to overlap,
Dialogue: 0,0:00:25.66,0:00:27.68,Default,,0000,0000,0000,,you'd cause wave interference and what
Dialogue: 0,0:00:27.68,0:00:29.48,Default,,0000,0000,0000,,that would look like would\Nbe something like this.
Dialogue: 0,0:00:29.48,0:00:32.34,Default,,0000,0000,0000,,So let's say these are speakers.
Dialogue: 0,0:00:32.34,0:00:34.14,Default,,0000,0000,0000,,I like thinking about\Nit in terms of speakers,
Dialogue: 0,0:00:34.14,0:00:35.92,Default,,0000,0000,0000,,I think it's easy to think about
Dialogue: 0,0:00:35.92,0:00:37.92,Default,,0000,0000,0000,,and I put this speaker right next
Dialogue: 0,0:00:37.92,0:00:40.87,Default,,0000,0000,0000,,to the first speaker, side-by-side.
Dialogue: 0,0:00:40.87,0:00:42.100,Default,,0000,0000,0000,,So they'd be creating sound waves
Dialogue: 0,0:00:43.20,0:00:45.67,Default,,0000,0000,0000,,in this region and I wouldn't really have
Dialogue: 0,0:00:45.67,0:00:47.82,Default,,0000,0000,0000,,two sound waves necessarily.
Dialogue: 0,0:00:47.82,0:00:49.20,Default,,0000,0000,0000,,You can think of it as just having
Dialogue: 0,0:00:49.20,0:00:52.14,Default,,0000,0000,0000,,one total sound wave and how would we
Dialogue: 0,0:00:52.14,0:00:55.14,Default,,0000,0000,0000,,find the size of that total sound wave?
Dialogue: 0,0:00:55.14,0:00:57.93,Default,,0000,0000,0000,,Well if I put an axis in here.
Dialogue: 0,0:00:57.93,0:01:01.18,Default,,0000,0000,0000,,The axis will make it\Neasier to think about this.
Dialogue: 0,0:01:01.18,0:01:04.42,Default,,0000,0000,0000,,I'm going to put an axis\Nthrough here like this
Dialogue: 0,0:01:04.42,0:01:06.66,Default,,0000,0000,0000,,What I can do is I can\Njust ask myself this,
Dialogue: 0,0:01:06.66,0:01:09.86,Default,,0000,0000,0000,,I'm going to say, what was the value
Dialogue: 0,0:01:09.86,0:01:11.78,Default,,0000,0000,0000,,of the first wave, so I'm going
Dialogue: 0,0:01:11.78,0:01:13.35,Default,,0000,0000,0000,,to take that value.
Dialogue: 0,0:01:13.35,0:01:15.01,Default,,0000,0000,0000,,What was the value of second wave
Dialogue: 0,0:01:15.01,0:01:16.37,Default,,0000,0000,0000,,and I'm just going to add them up.
Dialogue: 0,0:01:16.37,0:01:17.95,Default,,0000,0000,0000,,To get the value of the total wave,
Dialogue: 0,0:01:17.95,0:01:20.78,Default,,0000,0000,0000,,I'd take that value of the first wave
Dialogue: 0,0:01:20.78,0:01:22.48,Default,,0000,0000,0000,,plus the value of the second wave
Dialogue: 0,0:01:22.48,0:01:24.70,Default,,0000,0000,0000,,and well I'd just get double in this case.
Dialogue: 0,0:01:24.70,0:01:26.27,Default,,0000,0000,0000,,I can come over to here okay.
Dialogue: 0,0:01:26.27,0:01:29.16,Default,,0000,0000,0000,,The value here plus the value there,
Dialogue: 0,0:01:29.16,0:01:30.58,Default,,0000,0000,0000,,I get double that point.
Dialogue: 0,0:01:30.58,0:01:31.74,Default,,0000,0000,0000,,It's not going to be as high
Dialogue: 0,0:01:31.74,0:01:33.14,Default,,0000,0000,0000,,because they weren't as high..
Dialogue: 0,0:01:33.14,0:01:36.41,Default,,0000,0000,0000,,Then over here I got zero\Nand zero is just zero.
Dialogue: 0,0:01:36.41,0:01:37.62,Default,,0000,0000,0000,,and you start to see what's happening.
Dialogue: 0,0:01:37.62,0:01:40.52,Default,,0000,0000,0000,,I can come down here very low
Dialogue: 0,0:01:40.52,0:01:43.42,Default,,0000,0000,0000,,or very negative and I\Nget double that down here
Dialogue: 0,0:01:43.42,0:01:44.87,Default,,0000,0000,0000,,and if I were to trace this out,
Dialogue: 0,0:01:44.87,0:01:49.43,Default,,0000,0000,0000,,what I would get is one\Nbig total sound wave
Dialogue: 0,0:01:49.43,0:01:50.97,Default,,0000,0000,0000,,that would look like this.
Dialogue: 0,0:01:50.97,0:01:52.66,Default,,0000,0000,0000,,So these have been amplified.
Dialogue: 0,0:01:52.66,0:01:54.02,Default,,0000,0000,0000,,So that's one possibility.
Dialogue: 0,0:01:54.02,0:01:58.54,Default,,0000,0000,0000,,When two waves overlap,\Nyou can get this case
Dialogue: 0,0:01:58.54,0:02:00.42,Default,,0000,0000,0000,,where the peaks match the peaks
Dialogue: 0,0:02:00.42,0:02:02.80,Default,,0000,0000,0000,,and the valleys match the valleys
Dialogue: 0,0:02:02.80,0:02:05.13,Default,,0000,0000,0000,,and you get constructive interference.
Dialogue: 0,0:02:05.13,0:02:08.32,Default,,0000,0000,0000,,So notice how each valley matches
Dialogue: 0,0:02:08.32,0:02:10.14,Default,,0000,0000,0000,,the valley, each peak matches the peak
Dialogue: 0,0:02:10.14,0:02:14.52,Default,,0000,0000,0000,,and this is called,\NConstructive Interference
Dialogue: 0,0:02:14.52,0:02:17.54,Default,,0000,0000,0000,,because these constructively combine
Dialogue: 0,0:02:17.54,0:02:19.77,Default,,0000,0000,0000,,to form one bigger wave.
Dialogue: 0,0:02:19.77,0:02:22.60,Default,,0000,0000,0000,,So this is Constructive Interference.
Dialogue: 0,0:02:22.60,0:02:24.15,Default,,0000,0000,0000,,So what would you hear?
Dialogue: 0,0:02:24.15,0:02:27.23,Default,,0000,0000,0000,,If your ear was over here somewhere
Dialogue: 0,0:02:27.23,0:02:30.35,Default,,0000,0000,0000,,waiting to hear this sound.
Dialogue: 0,0:02:30.57,0:02:33.24,Default,,0000,0000,0000,,What you'd actually hear is a loud note.
Dialogue: 0,0:02:33.24,0:02:34.92,Default,,0000,0000,0000,,This would be much louder than it was.
Dialogue: 0,0:02:34.92,0:02:36.89,Default,,0000,0000,0000,,It would be twice as loud in fact.
Dialogue: 0,0:02:36.89,0:02:37.74,Default,,0000,0000,0000,,Which makes sense.
Dialogue: 0,0:02:37.74,0:02:39.26,Default,,0000,0000,0000,,You've got a second speaker in here.
Dialogue: 0,0:02:39.26,0:02:40.74,Default,,0000,0000,0000,,It's twice as loud.
Dialogue: 0,0:02:40.74,0:02:42.17,Default,,0000,0000,0000,,That makes sense.
Dialogue: 0,0:02:42.17,0:02:44.44,Default,,0000,0000,0000,,What's a little bit harder to understand
Dialogue: 0,0:02:44.44,0:02:45.98,Default,,0000,0000,0000,,is you can also have something called,
Dialogue: 0,0:02:45.98,0:02:47.48,Default,,0000,0000,0000,,Destructive Interference.
Dialogue: 0,0:02:47.48,0:02:49.02,Default,,0000,0000,0000,,What would that look like?
Dialogue: 0,0:02:49.02,0:02:52.11,Default,,0000,0000,0000,,Well, imagine you had two speakers
Dialogue: 0,0:02:52.11,0:02:54.74,Default,,0000,0000,0000,,but they looked like this so that the peak
Dialogue: 0,0:02:54.74,0:02:57.14,Default,,0000,0000,0000,,of the first one lined\Nup not with the peak
Dialogue: 0,0:02:57.14,0:02:58.97,Default,,0000,0000,0000,,of the second one but with the valley
Dialogue: 0,0:02:58.97,0:03:01.25,Default,,0000,0000,0000,,of the second one and the valley of
Dialogue: 0,0:03:01.25,0:03:03.36,Default,,0000,0000,0000,,the first one lined up with the peak
Dialogue: 0,0:03:03.36,0:03:04.76,Default,,0000,0000,0000,,of the second one.
Dialogue: 0,0:03:04.76,0:03:06.99,Default,,0000,0000,0000,,These are out of phase we say.
Dialogue: 0,0:03:06.99,0:03:09.43,Default,,0000,0000,0000,,Before, when they looked like this.
Dialogue: 0,0:03:09.43,0:03:12.11,Default,,0000,0000,0000,,These waves we say are in phase,
Dialogue: 0,0:03:12.11,0:03:13.29,Default,,0000,0000,0000,,because they look identical.
Dialogue: 0,0:03:13.29,0:03:14.91,Default,,0000,0000,0000,,The peaks match up with the peaks.
Dialogue: 0,0:03:14.91,0:03:16.54,Default,,0000,0000,0000,,The valleys with the valleys.
Dialogue: 0,0:03:16.54,0:03:18.08,Default,,0000,0000,0000,,These are out of phase.
Dialogue: 0,0:03:18.08,0:03:20.08,Default,,0000,0000,0000,,How far out of phase are they?
Dialogue: 0,0:03:20.08,0:03:24.60,Default,,0000,0000,0000,,We say that these are\N180 degrees out of phase.
Dialogue: 0,0:03:24.61,0:03:29.14,Default,,0000,0000,0000,,So these are 180 degrees out of phase.
Dialogue: 0,0:03:29.14,0:03:31.20,Default,,0000,0000,0000,,The phase refers to what point on
Dialogue: 0,0:03:31.20,0:03:34.64,Default,,0000,0000,0000,,the wave cycle is the wave at
Dialogue: 0,0:03:34.64,0:03:37.95,Default,,0000,0000,0000,,and these two are starting\Ncompletely separately
Dialogue: 0,0:03:37.95,0:03:39.23,Default,,0000,0000,0000,,which is 180 degrees.
Dialogue: 0,0:03:39.23,0:03:40.70,Default,,0000,0000,0000,,You might think that means 360
Dialogue: 0,0:03:40.70,0:03:41.95,Default,,0000,0000,0000,,but think about it.
Dialogue: 0,0:03:41.95,0:03:43.97,Default,,0000,0000,0000,,If you turn around 360 degrees,
Dialogue: 0,0:03:43.97,0:03:45.43,Default,,0000,0000,0000,,you're actually back where you started.
Dialogue: 0,0:03:45.43,0:03:47.93,Default,,0000,0000,0000,,If we tried to make these 360 degrees
Dialogue: 0,0:03:47.93,0:03:50.31,Default,,0000,0000,0000,,out of phase, they'd look identical again
Dialogue: 0,0:03:50.31,0:03:52.52,Default,,0000,0000,0000,,because I've moved on\Nso far through a cycle
Dialogue: 0,0:03:52.52,0:03:53.85,Default,,0000,0000,0000,,that's it's back to where it started
Dialogue: 0,0:03:53.85,0:03:54.68,Default,,0000,0000,0000,,in the first place.
Dialogue: 0,0:03:54.68,0:03:57.09,Default,,0000,0000,0000,,So I want to move it 180\Ndegrees out of phase.
Dialogue: 0,0:03:57.09,0:03:59.54,Default,,0000,0000,0000,,That's exactly the opposite
Dialogue: 0,0:03:59.54,0:04:01.72,Default,,0000,0000,0000,,So that you get peak lining up with valley
Dialogue: 0,0:04:01.72,0:04:04.45,Default,,0000,0000,0000,,or if you like radions, this is called
Dialogue: 0,0:04:04.45,0:04:06.84,Default,,0000,0000,0000,,pi out of phase because pi and 180
Dialogue: 0,0:04:06.84,0:04:09.01,Default,,0000,0000,0000,,are the same angle.
Dialogue: 0,0:04:09.01,0:04:10.24,Default,,0000,0000,0000,,Alright so what happens here
Dialogue: 0,0:04:10.24,0:04:12.93,Default,,0000,0000,0000,,if I take these two speakers?
Dialogue: 0,0:04:12.93,0:04:14.60,Default,,0000,0000,0000,,I'm going to take this second speaker
Dialogue: 0,0:04:14.60,0:04:18.46,Default,,0000,0000,0000,,and I line it up right\Nnext to the first speaker.
Dialogue: 0,0:04:18.46,0:04:20.88,Default,,0000,0000,0000,,I get something that looks more like this.
Dialogue: 0,0:04:20.88,0:04:21.90,Default,,0000,0000,0000,,Look at how weird this looks.
Dialogue: 0,0:04:21.90,0:04:23.07,Default,,0000,0000,0000,,These are completely out of phase
Dialogue: 0,0:04:23.07,0:04:24.24,Default,,0000,0000,0000,,and what's going to happen is
Dialogue: 0,0:04:24.24,0:04:27.82,Default,,0000,0000,0000,,if I add my little axis to\Nhelp me think about this.
Dialogue: 0,0:04:27.82,0:04:31.05,Default,,0000,0000,0000,,I'm going to add an axis\Nstraight through here.
Dialogue: 0,0:04:31.05,0:04:32.46,Default,,0000,0000,0000,,Now I play the same game.
Dialogue: 0,0:04:32.46,0:04:34.85,Default,,0000,0000,0000,,What total wave do I end up with?
Dialogue: 0,0:04:34.85,0:04:37.05,Default,,0000,0000,0000,,Well, I take this value.
Dialogue: 0,0:04:37.05,0:04:38.64,Default,,0000,0000,0000,,I'm going to add up the\Nvalues just the same.
Dialogue: 0,0:04:38.64,0:04:40.24,Default,,0000,0000,0000,,I take the value of the first wave
Dialogue: 0,0:04:40.24,0:04:42.02,Default,,0000,0000,0000,,plus the value of the second wave.
Dialogue: 0,0:04:42.02,0:04:43.48,Default,,0000,0000,0000,,I add those up, one's a positive
Dialogue: 0,0:04:43.48,0:04:45.78,Default,,0000,0000,0000,,and one's a negative I get zero
Dialogue: 0,0:04:45.78,0:04:49.02,Default,,0000,0000,0000,,and then over here zero plus zero is zero
Dialogue: 0,0:04:49.02,0:04:51.69,Default,,0000,0000,0000,,and then the valley of the first wave
Dialogue: 0,0:04:51.69,0:04:53.81,Default,,0000,0000,0000,,is lining up with the\Npeak of the second wave
Dialogue: 0,0:04:53.81,0:04:56.42,Default,,0000,0000,0000,,and if I add these two points up,
Dialogue: 0,0:04:56.42,0:04:58.13,Default,,0000,0000,0000,,I get zero again and you probably see
Dialogue: 0,0:04:58.13,0:04:59.35,Default,,0000,0000,0000,,what's going to happen.
Dialogue: 0,0:04:59.35,0:05:01.31,Default,,0000,0000,0000,,I'm just going to get a flat line.
Dialogue: 0,0:05:01.31,0:05:04.06,Default,,0000,0000,0000,,I'm going to get a flat line and I'm
Dialogue: 0,0:05:04.06,0:05:06.05,Default,,0000,0000,0000,,going to get no wave at all.
Dialogue: 0,0:05:06.05,0:05:09.55,Default,,0000,0000,0000,,These two waves cancel and so we call this
Dialogue: 0,0:05:09.55,0:05:11.44,Default,,0000,0000,0000,,not Constructive Interference
Dialogue: 0,0:05:11.44,0:05:14.75,Default,,0000,0000,0000,,but Destructive Interference\Nbecause these have
Dialogue: 0,0:05:14.75,0:05:18.64,Default,,0000,0000,0000,,destructively combined\Nto form no wave at all
Dialogue: 0,0:05:18.64,0:05:19.86,Default,,0000,0000,0000,,and this is a little strange.
Dialogue: 0,0:05:19.86,0:05:22.100,Default,,0000,0000,0000,,How can two waves form no wave?
Dialogue: 0,0:05:22.100,0:05:25.89,Default,,0000,0000,0000,,Well, this is how you do it.
Dialogue: 0,0:05:25.89,0:05:27.89,Default,,0000,0000,0000,,And what would our ear here if we had
Dialogue: 0,0:05:27.89,0:05:30.26,Default,,0000,0000,0000,,our ear over in this area again,
Dialogue: 0,0:05:30.26,0:05:31.43,Default,,0000,0000,0000,,and we were listening.
Dialogue: 0,0:05:31.43,0:05:33.55,Default,,0000,0000,0000,,If I just had one\Nspeaker, I'd hear a noise.
Dialogue: 0,0:05:33.55,0:05:35.42,Default,,0000,0000,0000,,If I just had the second speaker,
Dialogue: 0,0:05:35.42,0:05:36.75,Default,,0000,0000,0000,,I'd hear a noise.
Dialogue: 0,0:05:36.75,0:05:38.50,Default,,0000,0000,0000,,If I have both the first and second
Dialogue: 0,0:05:38.50,0:05:41.79,Default,,0000,0000,0000,,speaker together, I don't hear anything.
Dialogue: 0,0:05:41.79,0:05:44.89,Default,,0000,0000,0000,,It's silent, which is hard\Nto believe but this works.
Dialogue: 0,0:05:44.89,0:05:46.48,Default,,0000,0000,0000,,In fact, this is how noise canceling
Dialogue: 0,0:05:46.48,0:05:48.93,Default,,0000,0000,0000,,headphones work if you take a signal
Dialogue: 0,0:05:48.93,0:05:51.07,Default,,0000,0000,0000,,from the outside and you send in
Dialogue: 0,0:05:51.07,0:05:54.84,Default,,0000,0000,0000,,the exact same signal but flipped.
Dialogue: 0,0:05:54.84,0:05:57.96,Default,,0000,0000,0000,,Pi out of phase or 180\Ndegrees out of phase.
Dialogue: 0,0:05:57.96,0:06:00.98,Default,,0000,0000,0000,,It cancels it and so you can fight noise
Dialogue: 0,0:06:00.98,0:06:04.20,Default,,0000,0000,0000,,with more noise but exactly out of phase
Dialogue: 0,0:06:04.20,0:06:06.24,Default,,0000,0000,0000,,and you get silence in here, or at least
Dialogue: 0,0:06:06.24,0:06:08.11,Default,,0000,0000,0000,,you can get close to it.
Dialogue: 0,0:06:08.11,0:06:10.14,Default,,0000,0000,0000,,Now you might be wondering how do we
Dialogue: 0,0:06:10.14,0:06:13.10,Default,,0000,0000,0000,,get a speaker to go 180\Ndegrees out of phase?
Dialogue: 0,0:06:13.10,0:06:14.20,Default,,0000,0000,0000,,Well it's not too hard.
Dialogue: 0,0:06:14.20,0:06:15.85,Default,,0000,0000,0000,,If you look at the back of these speakers.
Dialogue: 0,0:06:15.85,0:06:17.69,Default,,0000,0000,0000,,Let me make a clean view.
Dialogue: 0,0:06:17.69,0:06:19.12,Default,,0000,0000,0000,,If you look at the back of these speakers,
Dialogue: 0,0:06:19.12,0:06:20.95,Default,,0000,0000,0000,,there will be a positive terminal
Dialogue: 0,0:06:20.95,0:06:23.23,Default,,0000,0000,0000,,and a negative terminal or at lease inside
Dialogue: 0,0:06:23.23,0:06:24.99,Default,,0000,0000,0000,,there will be and if you can swap
Dialogue: 0,0:06:24.99,0:06:27.76,Default,,0000,0000,0000,,the positive terminal\Nfor the negative terminal
Dialogue: 0,0:06:27.76,0:06:30.34,Default,,0000,0000,0000,,and the negative terminal\Nfor the positive terminal,
Dialogue: 0,0:06:30.34,0:06:33.96,Default,,0000,0000,0000,,then when one speaker's\Ntrying to push air forward,
Dialogue: 0,0:06:33.96,0:06:36.38,Default,,0000,0000,0000,,there's a diaphragm on this speaker
Dialogue: 0,0:06:36.38,0:06:37.91,Default,,0000,0000,0000,,moving forward and backwards.
Dialogue: 0,0:06:37.91,0:06:39.56,Default,,0000,0000,0000,,When one speaker is\Ntrying to push air forward
Dialogue: 0,0:06:39.56,0:06:41.32,Default,,0000,0000,0000,,the other speaker will be trying
Dialogue: 0,0:06:41.32,0:06:43.22,Default,,0000,0000,0000,,to pull air backwards and the net result
Dialogue: 0,0:06:43.22,0:06:45.31,Default,,0000,0000,0000,,is that the air just doesn't move
Dialogue: 0,0:06:45.31,0:06:46.76,Default,,0000,0000,0000,,because it's got equal and opposite
Dialogue: 0,0:06:46.76,0:06:49.56,Default,,0000,0000,0000,,forces on it and since\Nthe air just sits there,
Dialogue: 0,0:06:49.56,0:06:51.31,Default,,0000,0000,0000,,you've got no sound wave because
Dialogue: 0,0:06:51.31,0:06:53.14,Default,,0000,0000,0000,,air has to oscillate\Nto create a sound wave
Dialogue: 0,0:06:53.14,0:06:54.98,Default,,0000,0000,0000,,and you get Disruptive Interference.
Dialogue: 0,0:06:54.98,0:06:56.63,Default,,0000,0000,0000,,So that's how you can create a speaker
Dialogue: 0,0:06:56.63,0:06:58.55,Default,,0000,0000,0000,,pi out of phase.
Dialogue: 0,0:06:58.55,0:07:00.14,Default,,0000,0000,0000,,You might be wondering, I don't want
Dialogue: 0,0:07:00.14,0:07:01.78,Default,,0000,0000,0000,,to mess with the wires on the back
Dialogue: 0,0:07:01.78,0:07:03.41,Default,,0000,0000,0000,,of my speaker in fact, you shouldn't
Dialogue: 0,0:07:03.41,0:07:05.81,Default,,0000,0000,0000,,so you don't get shocked but if I've got
Dialogue: 0,0:07:05.81,0:07:08.19,Default,,0000,0000,0000,,two speakers in phase\Nlike this, I'm stuck,
Dialogue: 0,0:07:08.19,0:07:11.23,Default,,0000,0000,0000,,I can't get Destructive Interference
Dialogue: 0,0:07:11.23,0:07:12.66,Default,,0000,0000,0000,,but yeah you can.
Dialogue: 0,0:07:12.66,0:07:14.13,Default,,0000,0000,0000,,Even if you don't mess with the wires,
Dialogue: 0,0:07:14.13,0:07:15.92,Default,,0000,0000,0000,,and don't, don't try this at home,
Dialogue: 0,0:07:15.92,0:07:18.02,Default,,0000,0000,0000,,you can still take this speaker,
Dialogue: 0,0:07:18.02,0:07:20.86,Default,,0000,0000,0000,,remember before when these where
Dialogue: 0,0:07:20.86,0:07:22.95,Default,,0000,0000,0000,,in phase we'd just line them up like that,
Dialogue: 0,0:07:22.95,0:07:24.85,Default,,0000,0000,0000,,Constructive Interference but I don't
Dialogue: 0,0:07:24.85,0:07:26.82,Default,,0000,0000,0000,,have to put them side-by-side.
Dialogue: 0,0:07:26.82,0:07:29.26,Default,,0000,0000,0000,,I can start one speaker\Na little bit forward
Dialogue: 0,0:07:29.26,0:07:30.29,Default,,0000,0000,0000,,and looks what happens.
Dialogue: 0,0:07:30.29,0:07:33.06,Default,,0000,0000,0000,,We start to get waves\Nthat are out of phase.
Dialogue: 0,0:07:33.06,0:07:36.20,Default,,0000,0000,0000,,So my question is how far forward
Dialogue: 0,0:07:36.20,0:07:38.11,Default,,0000,0000,0000,,should I move this speaker to get
Dialogue: 0,0:07:38.11,0:07:40.44,Default,,0000,0000,0000,,Destructive Interference\Nand we can just watch.
Dialogue: 0,0:07:40.44,0:07:43.90,Default,,0000,0000,0000,,So I'm just going to try\Nthis and when we get to
Dialogue: 0,0:07:43.90,0:07:48.14,Default,,0000,0000,0000,,this point there, now we're out of phase.
Dialogue: 0,0:07:48.14,0:07:49.94,Default,,0000,0000,0000,,Now I have Destructive Interference
Dialogue: 0,0:07:49.94,0:07:53.81,Default,,0000,0000,0000,,and so how far did I\Nmove my speaker forward?
Dialogue: 0,0:07:53.81,0:07:56.88,Default,,0000,0000,0000,,If we look at it, here was the front
Dialogue: 0,0:07:56.88,0:07:59.24,Default,,0000,0000,0000,,of the speaker originally, right there.
Dialogue: 0,0:07:59.24,0:08:01.19,Default,,0000,0000,0000,,Here's the front of the speaker now.
Dialogue: 0,0:08:01.19,0:08:02.100,Default,,0000,0000,0000,,If you look at this wave, how much
Dialogue: 0,0:08:02.100,0:08:06.10,Default,,0000,0000,0000,,of a wavelength have I moved forward.
Dialogue: 0,0:08:06.10,0:08:07.59,Default,,0000,0000,0000,,The amount of wavelength that you had
Dialogue: 0,0:08:07.59,0:08:10.100,Default,,0000,0000,0000,,to move forward was 1/2 of a wavelength.
Dialogue: 0,0:08:10.100,0:08:12.58,Default,,0000,0000,0000,,So if you take two speakers that are
Dialogue: 0,0:08:12.58,0:08:14.98,Default,,0000,0000,0000,,in phase and you move one
Dialogue: 0,0:08:14.98,0:08:17.68,Default,,0000,0000,0000,,1/2 a wavelength forward you get
Dialogue: 0,0:08:17.68,0:08:19.14,Default,,0000,0000,0000,,Destructive Interference again.
Dialogue: 0,0:08:19.14,0:08:21.70,Default,,0000,0000,0000,,Again, if my ear's over here, I'm not
Dialogue: 0,0:08:21.70,0:08:23.37,Default,,0000,0000,0000,,going to hear anything.
Dialogue: 0,0:08:23.37,0:08:24.98,Default,,0000,0000,0000,,Even though these two waves started off
Dialogue: 0,0:08:24.98,0:08:26.95,Default,,0000,0000,0000,,in phase, move one 1/2\Na wavelength forward,
Dialogue: 0,0:08:26.95,0:08:29.43,Default,,0000,0000,0000,,they line up so that it's Destructive,
Dialogue: 0,0:08:29.43,0:08:33.19,Default,,0000,0000,0000,,I get no noise but if I take this away.
Dialogue: 0,0:08:33.19,0:08:34.65,Default,,0000,0000,0000,,We go back to the beginning here.
Dialogue: 0,0:08:34.65,0:08:36.84,Default,,0000,0000,0000,,Take my speaker, we start over.
Dialogue: 0,0:08:36.84,0:08:39.53,Default,,0000,0000,0000,,If you move it forward a whole wavelength,
Dialogue: 0,0:08:39.53,0:08:41.79,Default,,0000,0000,0000,,so I take this here, keep moving it,
Dialogue: 0,0:08:41.79,0:08:43.76,Default,,0000,0000,0000,,keep moving it and then\NDestructive Interference
Dialogue: 0,0:08:43.76,0:08:48.10,Default,,0000,0000,0000,,Whoa, here we go, Constructive\NInterference again.
Dialogue: 0,0:08:48.10,0:08:49.51,Default,,0000,0000,0000,,That's a whole wavelength.
Dialogue: 0,0:08:49.51,0:08:52.81,Default,,0000,0000,0000,,So if you move it forward\Na whole wavelength.
Dialogue: 0,0:08:53.05,0:08:56.62,Default,,0000,0000,0000,,Look, there's one whole\Nwavelength forward.
Dialogue: 0,0:08:56.62,0:08:58.31,Default,,0000,0000,0000,,So the front of the speaker was here
Dialogue: 0,0:08:58.31,0:08:59.96,Default,,0000,0000,0000,,now the front of the speaker's here.
Dialogue: 0,0:08:59.96,0:09:02.71,Default,,0000,0000,0000,,This is an entire wavelength.
Dialogue: 0,0:09:02.71,0:09:04.71,Default,,0000,0000,0000,,I get Constructive Interference.
Dialogue: 0,0:09:04.71,0:09:06.67,Default,,0000,0000,0000,,Now I'm going to hear a loud sound again.
Dialogue: 0,0:09:06.67,0:09:08.39,Default,,0000,0000,0000,,I'm going to hear twice the noise
Dialogue: 0,0:09:08.39,0:09:10.86,Default,,0000,0000,0000,,that there would be if\NI just had one speaker.
Dialogue: 0,0:09:10.86,0:09:12.68,Default,,0000,0000,0000,,So the moral of this story is that
Dialogue: 0,0:09:12.68,0:09:14.72,Default,,0000,0000,0000,,even if you have speakers\Nthat are in phase,
Dialogue: 0,0:09:14.72,0:09:17.40,Default,,0000,0000,0000,,you can get Destructive Interference
Dialogue: 0,0:09:17.40,0:09:19.60,Default,,0000,0000,0000,,depending on the difference in the length
Dialogue: 0,0:09:19.60,0:09:21.44,Default,,0000,0000,0000,,that these two waves travel.
Dialogue: 0,0:09:21.44,0:09:24.74,Default,,0000,0000,0000,,In other words wave two\Nis traveling this far
Dialogue: 0,0:09:24.74,0:09:25.60,Default,,0000,0000,0000,,to get to my ear.
Dialogue: 0,0:09:25.60,0:09:28.04,Default,,0000,0000,0000,,I'm going to call that x2 and wave one
Dialogue: 0,0:09:28.04,0:09:30.52,Default,,0000,0000,0000,,is traveling this far to get to my ear.
Dialogue: 0,0:09:30.52,0:09:31.82,Default,,0000,0000,0000,,I'm going to call that x1.
Dialogue: 0,0:09:31.82,0:09:33.79,Default,,0000,0000,0000,,If I took the difference\Nbetween these two,
Dialogue: 0,0:09:33.79,0:09:36.53,Default,,0000,0000,0000,,I'd be finding the path length difference.
Dialogue: 0,0:09:36.53,0:09:38.60,Default,,0000,0000,0000,,The difference in path lengths that
Dialogue: 0,0:09:38.60,0:09:40.27,Default,,0000,0000,0000,,these waves are traveling and that
Dialogue: 0,0:09:40.27,0:09:41.58,Default,,0000,0000,0000,,would be this amount.
Dialogue: 0,0:09:41.58,0:09:42.92,Default,,0000,0000,0000,,This is the difference right here.
Dialogue: 0,0:09:42.92,0:09:44.72,Default,,0000,0000,0000,,I'm going to call it delta x because
Dialogue: 0,0:09:44.72,0:09:46.72,Default,,0000,0000,0000,,it's the magnitude of the difference
Dialogue: 0,0:09:46.72,0:09:49.50,Default,,0000,0000,0000,,between these two lengths and we saw
Dialogue: 0,0:09:49.50,0:09:52.59,Default,,0000,0000,0000,,that if this equals\Nlambda it was constructive
Dialogue: 0,0:09:52.59,0:09:55.07,Default,,0000,0000,0000,,and if it equaled a 1/2 a lambda
Dialogue: 0,0:09:55.07,0:09:57.87,Default,,0000,0000,0000,,it was destructive but those\Naren't the only values.
Dialogue: 0,0:09:57.87,0:09:59.93,Default,,0000,0000,0000,,We can write down an\Nimportant result here.
Dialogue: 0,0:09:59.93,0:10:03.90,Default,,0000,0000,0000,,If delta x, the path length difference was
Dialogue: 0,0:10:03.90,0:10:07.18,Default,,0000,0000,0000,,lambda or it turns out 2 lambda will work
Dialogue: 0,0:10:07.18,0:10:10.11,Default,,0000,0000,0000,,or 3 lambda, imagine\Nmoving the second speaker
Dialogue: 0,0:10:10.11,0:10:13.27,Default,,0000,0000,0000,,one more whole wavelength.
Dialogue: 0,0:10:13.41,0:10:15.62,Default,,0000,0000,0000,,Well, you'd be perfectly\Nback in phase again
Dialogue: 0,0:10:15.62,0:10:17.55,Default,,0000,0000,0000,,because you'd align back up perfectly
Dialogue: 0,0:10:17.55,0:10:21.39,Default,,0000,0000,0000,,or 3 whole wavelengths\Nagain, perfectly in phase.
Dialogue: 0,0:10:21.39,0:10:25.08,Default,,0000,0000,0000,,Any integer wavelength including zero
Dialogue: 0,0:10:25.08,0:10:26.51,Default,,0000,0000,0000,,because zero is just the case where
Dialogue: 0,0:10:26.51,0:10:29.40,Default,,0000,0000,0000,,the speaker was right next to speaker one.
Dialogue: 0,0:10:29.40,0:10:30.76,Default,,0000,0000,0000,,Where these two speakers were lined up
Dialogue: 0,0:10:30.76,0:10:32.56,Default,,0000,0000,0000,,right next to each other,
Dialogue: 0,0:10:32.56,0:10:35.11,Default,,0000,0000,0000,,you'll get Constructive Interference.
Dialogue: 0,0:10:35.11,0:10:38.15,Default,,0000,0000,0000,,The waves line up perfectly,\Nit's going to be constructive
Dialogue: 0,0:10:38.15,0:10:42.100,Default,,0000,0000,0000,,and we saw if, delta x\Nequals a 1/2 wavelength
Dialogue: 0,0:10:42.100,0:10:46.04,Default,,0000,0000,0000,,it was destructive but\Nthat's not the only case.
Dialogue: 0,0:10:46.04,0:10:48.91,Default,,0000,0000,0000,,Any odd 1/2 integer here.
Dialogue: 0,0:10:48.91,0:10:50.57,Default,,0000,0000,0000,,So I can't do 2 over 2 because that
Dialogue: 0,0:10:50.57,0:10:52.15,Default,,0000,0000,0000,,would be lambda again.
Dialogue: 0,0:10:52.15,0:10:57.11,Default,,0000,0000,0000,,I could do 3 lambda over\N2 or 5 lambda over 2
Dialogue: 0,0:10:57.11,0:10:59.52,Default,,0000,0000,0000,,or 7 lambda over 2.
Dialogue: 0,0:10:59.52,0:11:02.45,Default,,0000,0000,0000,,Any of these will give me\NDestructive Interference
Dialogue: 0,0:11:02.45,0:11:04.78,Default,,0000,0000,0000,,because they'll cause these peaks
Dialogue: 0,0:11:04.78,0:11:06.48,Default,,0000,0000,0000,,to match up with valleys.
Dialogue: 0,0:11:06.48,0:11:07.91,Default,,0000,0000,0000,,The whole thing would flat line.
Dialogue: 0,0:11:07.91,0:11:09.37,Default,,0000,0000,0000,,I'd get no sound.
Dialogue: 0,0:11:09.37,0:11:11.06,Default,,0000,0000,0000,,This is an important result.
Dialogue: 0,0:11:11.06,0:11:12.42,Default,,0000,0000,0000,,If you've got two speakers that are
Dialogue: 0,0:11:12.42,0:11:14.52,Default,,0000,0000,0000,,starting off in phase.
Dialogue: 0,0:11:14.52,0:11:16.93,Default,,0000,0000,0000,,In other words they both\Nstart off the same way
Dialogue: 0,0:11:16.93,0:11:19.37,Default,,0000,0000,0000,,and by that I mean one speaker sends out
Dialogue: 0,0:11:19.37,0:11:23.94,Default,,0000,0000,0000,,it's wave going up, the other
Dialogue: 0,0:11:23.94,0:11:25.29,Default,,0000,0000,0000,,sends out it's wave going up.
Dialogue: 0,0:11:25.29,0:11:26.76,Default,,0000,0000,0000,,There both at the same cycle.
Dialogue: 0,0:11:26.76,0:11:30.04,Default,,0000,0000,0000,,If the only difference is\Nthe path length difference,
Dialogue: 0,0:11:30.04,0:11:31.55,Default,,0000,0000,0000,,this is an important result that let's you
Dialogue: 0,0:11:31.55,0:11:33.30,Default,,0000,0000,0000,,determine whether there's constructive
Dialogue: 0,0:11:33.30,0:11:36.84,Default,,0000,0000,0000,,or destructive interference\Nbut you might ask,
Dialogue: 0,0:11:36.84,0:11:38.72,Default,,0000,0000,0000,,hold on, what if...
Dialogue: 0,0:11:38.72,0:11:40.61,Default,,0000,0000,0000,,See this was assuming there was no phase
Dialogue: 0,0:11:40.61,0:11:42.39,Default,,0000,0000,0000,,difference to start off with.
Dialogue: 0,0:11:42.39,0:11:44.47,Default,,0000,0000,0000,,What if you did the old switcheroo
Dialogue: 0,0:11:44.47,0:11:46.11,Default,,0000,0000,0000,,on the back of one of these speakers
Dialogue: 0,0:11:46.11,0:11:47.64,Default,,0000,0000,0000,,and you swapped the positive end
Dialogue: 0,0:11:47.64,0:11:50.02,Default,,0000,0000,0000,,for the negative end so\Ninstead of coming out
Dialogue: 0,0:11:50.02,0:11:54.59,Default,,0000,0000,0000,,upward, the second one\Nwas coming out downward.
Dialogue: 0,0:11:54.59,0:11:56.00,Default,,0000,0000,0000,,Then what would happen?
Dialogue: 0,0:11:56.00,0:11:57.76,Default,,0000,0000,0000,,Well you might be able to guess.
Dialogue: 0,0:11:57.76,0:12:00.82,Default,,0000,0000,0000,,Now, this results just going to flip-flop.
Dialogue: 0,0:12:00.82,0:12:04.93,Default,,0000,0000,0000,,In other words, if I\Nlook at this case here
Dialogue: 0,0:12:05.30,0:12:06.79,Default,,0000,0000,0000,,Look at, now we start off with speakers
Dialogue: 0,0:12:06.79,0:12:10.08,Default,,0000,0000,0000,,that are out of phase to begin with.
Dialogue: 0,0:12:10.08,0:12:12.100,Default,,0000,0000,0000,,This time, if I start off with zero
Dialogue: 0,0:12:12.100,0:12:15.10,Default,,0000,0000,0000,,path length difference, I get destructive
Dialogue: 0,0:12:15.10,0:12:16.70,Default,,0000,0000,0000,,destructive instead of constructive.
Dialogue: 0,0:12:16.70,0:12:19.55,Default,,0000,0000,0000,,If I move this a whole wavelength forward,
Dialogue: 0,0:12:19.55,0:12:20.88,Default,,0000,0000,0000,,there's a whole wavelength,
Dialogue: 0,0:12:20.88,0:12:23.15,Default,,0000,0000,0000,,I get destructive again.
Dialogue: 0,0:12:23.15,0:12:25.82,Default,,0000,0000,0000,,Two wavelengths forward,\Ndestructive again.
Dialogue: 0,0:12:25.82,0:12:27.17,Default,,0000,0000,0000,,Three wavelengths forward would be
Dialogue: 0,0:12:27.17,0:12:28.76,Default,,0000,0000,0000,,destructive again and so\Nthe integer wavelength
Dialogue: 0,0:12:28.76,0:12:31.61,Default,,0000,0000,0000,,this time are going to\Ngive me destructive.
Dialogue: 0,0:12:31.61,0:12:33.05,Default,,0000,0000,0000,,What about the half integers?
Dialogue: 0,0:12:33.05,0:12:35.03,Default,,0000,0000,0000,,Let's see, I'll go forward\Na 1/2 a wavelength.
Dialogue: 0,0:12:35.03,0:12:38.02,Default,,0000,0000,0000,,Look at this, perfectly in phase.
Dialogue: 0,0:12:38.02,0:12:39.44,Default,,0000,0000,0000,,It's going to be constructive.
Dialogue: 0,0:12:39.44,0:12:42.87,Default,,0000,0000,0000,,How about if I go 3 1/2 of a wavelength.
Dialogue: 0,0:12:42.87,0:12:45.65,Default,,0000,0000,0000,,Again, perfectly in phase, constructive.
Dialogue: 0,0:12:45.65,0:12:49.65,Default,,0000,0000,0000,,So, in this case it turns\Nout if you start off
Dialogue: 0,0:12:49.65,0:12:52.19,Default,,0000,0000,0000,,with speakers that were\Nalready phase shifted.
Dialogue: 0,0:12:52.19,0:12:57.19,Default,,0000,0000,0000,,If one speaker is pi\Nshifted from the other
Dialogue: 0,0:12:57.35,0:12:58.82,Default,,0000,0000,0000,,then we got another result here.
Dialogue: 0,0:12:58.82,0:12:59.89,Default,,0000,0000,0000,,We've got that...
Dialogue: 0,0:12:59.89,0:13:01.14,Default,,0000,0000,0000,,Well actually I'll just go back to my
Dialogue: 0,0:13:01.14,0:13:02.98,Default,,0000,0000,0000,,previous result, it's easier.
Dialogue: 0,0:13:02.98,0:13:06.93,Default,,0000,0000,0000,,We can just add a little addendum here if
Dialogue: 0,0:13:07.14,0:13:12.14,Default,,0000,0000,0000,,if one speaker is pi phase shifted,
Dialogue: 0,0:13:15.98,0:13:20.14,Default,,0000,0000,0000,,from the other speaker and remember
Dialogue: 0,0:13:20.14,0:13:22.98,Default,,0000,0000,0000,,these don't have to be speakers.
Dialogue: 0,0:13:22.98,0:13:25.32,Default,,0000,0000,0000,,They could be any wave source.
Dialogue: 0,0:13:25.32,0:13:27.27,Default,,0000,0000,0000,,If one speaker's a pi phase shift
Dialogue: 0,0:13:27.27,0:13:32.27,Default,,0000,0000,0000,,from the other speaker then\Nyou just flip-flop this.
Dialogue: 0,0:13:32.30,0:13:34.22,Default,,0000,0000,0000,,Then you just take this rule and now
Dialogue: 0,0:13:34.22,0:13:39.07,Default,,0000,0000,0000,,these give you constructive right here.
Dialogue: 0,0:13:39.07,0:13:41.30,Default,,0000,0000,0000,,These would give you constructive
Dialogue: 0,0:13:41.30,0:13:45.05,Default,,0000,0000,0000,,and these up here would\Ngive you destructive
Dialogue: 0,0:13:45.05,0:13:46.42,Default,,0000,0000,0000,,and so the whole thing just gives you
Dialogue: 0,0:13:46.42,0:13:48.50,Default,,0000,0000,0000,,the opposite result.
Dialogue: 0,0:13:48.50,0:13:51.21,Default,,0000,0000,0000,,Now the whole integer\Nwavelengths give you destructive.
Dialogue: 0,0:13:51.21,0:13:53.58,Default,,0000,0000,0000,,The 1/2 integer wavelengths\Ngive you constructive
Dialogue: 0,0:13:53.58,0:13:56.65,Default,,0000,0000,0000,,and I have to impress upon you the idea
Dialogue: 0,0:13:56.65,0:13:59.15,Default,,0000,0000,0000,,that this does not just\Napply for speakers.
Dialogue: 0,0:13:59.15,0:14:01.71,Default,,0000,0000,0000,,This applies for light and some sort of
Dialogue: 0,0:14:01.71,0:14:03.97,Default,,0000,0000,0000,,double slit experiment or light in
Dialogue: 0,0:14:03.97,0:14:06.45,Default,,0000,0000,0000,,a thin film experiment or sound
Dialogue: 0,0:14:06.45,0:14:08.66,Default,,0000,0000,0000,,with speakers or water waves.
Dialogue: 0,0:14:08.66,0:14:11.23,Default,,0000,0000,0000,,Any time that's the case, this rule holds
Dialogue: 0,0:14:11.23,0:14:13.11,Default,,0000,0000,0000,,in fact, this is the fundamental rule
Dialogue: 0,0:14:13.11,0:14:16.55,Default,,0000,0000,0000,,for almost all wave interference aspects.
Dialogue: 0,0:14:16.55,0:14:18.31,Default,,0000,0000,0000,,Is that the path length difference
Dialogue: 0,0:14:18.31,0:14:21.17,Default,,0000,0000,0000,,along with whether\Nthere's a pi phase shift,
Dialogue: 0,0:14:21.17,0:14:23.59,Default,,0000,0000,0000,,a relative pi phase shift between the two
Dialogue: 0,0:14:23.59,0:14:25.11,Default,,0000,0000,0000,,will determine whether you get
Dialogue: 0,0:14:25.11,0:14:28.52,Default,,0000,0000,0000,,constructive or destructive interference.