there's a bug in the water. that's the top view, you're looking down. you're like a bird,

you're looking right down in the water. there's a bug sitting there. and this is a lake that's perfectly smooth. but now what the bug starts to do, is the bug starts to giggle up and down. and that bug giggles up and down, the bug is a vibration, yeah? and that's vibrating on a nice surface, yeah? we're gonna make what? begin with the w. ripples. [laughter] all right, all right. we're gonna make waves, gang. we're gonna make waves, okay? and the waves kind of look like this. where i draw the line that represents like the crest of a wave, like it might just-- sort of be like this. can you see what i'm saying? okay? so this represents crest the waves. and the waves go out in all directions. and the fact that they're circles is kind of evidence that they go out in all direction at the same speed. is that true? like the one faster over here, then the waves started to be like that, yeah? - right. - okay? so, the circles are evidence of the same speed, okay? now, what would be the frequency of the waves compared to the frequency of the vibrating bug? in other words, if you put your hand here,

the wave would hit your hand, yeah? certain frequency? how would that-- --compare to the-- --of the bug? - same, same. - same, same. isn't that right? the frequency of a wave is the same as the frequency of the vibrating source. makes sense, doesn't it? let's suppose now that the bug swims in this direction. what the bug is gonna do is the bug is gonna swim in the direction of the waves that it's producing and it continues to vibrate a little bit, yeah? wouldn't the wave pattern look something like this? can you kind of be seeing this? in fact, what direction will the bug be swimming, gang? i mean, you could tell it's going that way, right? well, the bug might be at rest and the water is going this way. i mean, motion is relative anyway, yeah? okay. now, if i put my hand over here, would i get a higher speed wave, a lower speed wave, or the same speed wave?

check your neighbor. and make sure you're listening to the question that's being asked. how many people say, "hey, the speed of the wave is gonna be the same." yay, that's right. the speed of the wave doesn't change. what does change, gang? how frequently that wave encounters you as different, yeah? so i can get a different frequency. when i put my hand here, higher frequency or lower? - higher. - do you see it? over here. that change in frequency due to motion of the source or the receiver is what we call the--named after a person. effects. [laughter] very good. it is called, it's a doppler effect. that's right. the doppler effect is the change of frequency when something moves toward you or away from you, that sort of thing. the change of frequency due to motion. doppler effect. let's suppose the bug swims faster and faster.

and other bug generates waves that's sort of like this. i got a question for you, gang. how fast is that bug swimming compared to wave speed? same, same. what do you say? - same, same. - same, same. isn't that right? the bug is going as fast as the waves it produces. now, you say to the bug, "hey, mr. bug go a little faster, honey." and the bug--that bug has got a big problem, because what's in front of the bug? waves. all the waves the bug has generated in the forward direction all piled up. that's constructive interference, when we talked about interference last time? constructive interference. and so it's all piled up in front there, huh? and so the bug finds it very, very difficult to go faster. come on, bug go-- bug say "i can't go faster. there's a wave barrier here." do you see what the wave barrier is, gang? but that bug can go faster. that bug-- --right down the other side and man, smooth sailing. so the bug can break through that barrier. and when the bug does that--

you see we get something like this? now the bug is swimming out in front of the waves its produces. and here's the bug right here. the bug says, "hey, honey, smooth sailing." and you know what? you see where these waves overlap here, here, here, here, here? they don't overlap here, here, here. but notice right along the edges where they all overlap, we get a v. you've watch bugs swimming through the water. you've watch ducks swimming through the water. you've watch birds swim. you've watch boats go. and you always notice a v-shape behind, yeah? and you've always says, "yeah, i wonder what they make in the v wave?" and now you know the answer. that v wave is a super position of a whole lot of circles that all ganged together on the edge and give you constructive interference out here and there you go. isn't that nice? this we call a bow wave, usually made by the bow of a boat. but you know what, gang? these ideas apply to sound, because in the case of an aircraft going faster than the speed of sound, it's gonna leave

a conical-shaped wave behind. and that's called a shock wave. so anything that moves faster than the speed of sound is gonna have the sound all ganged up and form that high-pressure cone. this is really high-pressure air, just like with water that would be high wave, yeah? and now high-pressure air, when you encounter that down below-- let's suppose this is an aircraft now, not a bug. the person up here says, "hey, wow, look at the aircraft, man, silence." "no sound at all. "hey, they finally made it. they finally made it. no, boom." okay? and this thing here, finally-- wash right over and when that hits the listener, the listener can't tell the difference between that and like dynamite going off. i used to prospect in colorado. we'll be way out in the mountains and sometimes we'd hear-- --hey, is that someone prospecting, someone nearby, someone making some dynamite holes, and we didn't know. we did--there it is, military aircraft. military aircraft, but you couldn't tell.

we couldn't tell the difference between dynamite going off and the sonic boom, because when this wave makes -- with the ear, the high- pressure air-- --just like dynamite, same thing. and you get that loud, loud sound. sonic boom. produced by what? the incidence of a shock wave-- but you know what? there's a misconception going around that sonic boom is produced by an aircraft, just when it punches through the sound barrier. as if going at a speed of sound. you know when the aircraft go to speed of sound. man, that high-pressure air is all piled up in front of the wing, and it used to be way back in the 1940s, they thought an aircraft could never go faster than the speed of sound. 'cause you're going as fast as the speed of sound, man, you've got that high-- like a brick wall in front of your wings of your aircraft. you can't get through that high-pressure air. forget it. and low and behold, it turns out they could get through it, okay? they could get through. and they entered the whole age of faster than sound aircraft. and when an aircraft is going faster than sound--

nice and steady, steady, steady no opposition, it keep going, going, going. and it drags behind that v behind it. if something goes as fast as the speed of sound, we call it mach 1. if it goes twice as speed of sound, we call it, mach 2. guess what its called if it goes three times as speed of the sound? check your neighbor. mach, ernst mach, german physicist of long ago, named after him, mach 1, mach 2. let's talk about cars. mach 1, right? mach 1 means when you have a car go 750 miles an hour or about 340 meters per second. that's fast. sound travels fast. not compared to light-- about a million as fast as light, but nevertheless sound. about 750 miles an hour. so if you're in an aircraft going 600 miles an hour, you don't produce a shockwave. that's because the waves don't pile up in front of you. i mean, you're traveling along like this, okay, and your wave is sort of like this.

see, the waves are still going faster than you're going. it's only as you go as fast as the sound. of course then you get like this, okay? and you know what, those waves don't pile up anywhere but right here. so when an aircraft is going to the speed of sound and a little bit faster, down here they don't get a sonic boom. you only get the sonic boom when it's going appreciably fast enough so you that you can generate this shockwave like that. so the misconception is, people think that airplanes produce sonic booms going as fast as sound. not true. or just going faster than the sound, not true. the sonic boom you hear might have been produced by an airplane that might have cracked the sound barrier an hour ago. and it drags it continuously behind. so that's a misconception. that is only when the aircraft picks up that extra speed that the sonic boom is made. no true. all the things with sonic booms you can answer, all the questions that might come up can be answered by considering the analogous more is easily visualized case of a motor boat in the water.

you're with your friends, and you're sitting at the water's edge, and you're having your picnic. and we're talking about a river now, okay? and up the river comes a speed boat. and when that speed boat races by, what happens to your lunch that's right at the river's edge? gets sloshed, right? and when you get sloshed, there someone say, "oh, the boat might just went faster than the speed of the water waves." is that true or false? that might happen way, way, way down range. that boat has been dragging that v behind it continuously. then people are out in the fields or out of the water, whatever, and an aircraft goes by and you get that sonic boom. that sonic boom probably is been dragging behind that aircraft. the shock wave has been dragging behind the aircraft, but ever since it exceeded the speed of sound. and so like if aircraft go from san francisco to new york faster than the speed of sound, guess what there's a trail underneath there. wreckage, honey, wreckage, because that shock wave is gonna drag right across the ground

and hit everyone underneath there. so, you got a chicken coop there that's not built too well, right? all of a sudden-- and it is even worse, 'cause it turns out that the high-pressure region is followed by a low-pressure region. ever be waterskiing? then you go to the edges, hoop, you come up like that? what's on the other side. hoo, down. --on this side, called the bow wave, you know? i think not the bow wave, the tail wave, see? 'cause you get overlappings of high pressure and you get overlappings of low pressure. in the case of sound, you get overlappings of-- what do you call it, compressions. where all the compressions overlap, high pressure, where all the rarefactions overlap, low pressure is what you get. all of a sudden boom, high pressure followed by low pressure. this are like boom, boom and-- so it's even more devastating than sonic boom because of rapid high pressure followed by rapid low pressure. questions? what effect does it have when an aircraft

is flying higher in the atmosphere? if an aircraft flies higher, what effect would that have on people down below? well, let me ask you this question. if you're sitting with your friends and a boat comes by and it douses you, what effect would it have if the boat where further off shore and drove by? less effect or more effect or same, same? how many say everything's the same. same, same. [laughter] no. it's gonna be less, see. you'd be further away. so further away, you would have less damage. but then in the case of aircraft, how high can an aircraft go? i mean, aircraft needs air to fly. it's got to hit air. next time you're driving in a car, put your hand out the window. you put your hand like this. you see why aircraft fly. that air hit in the bottom. now, take your hand and go like that. down. is that what you do when you're in the water? you go like this. down, up, huh? 'cause that water bounces out that way, push you this way, see?

water bounces off this way, push you that way. the aircraft do the same-- - thing. - --thing. but you got to have the air to ride on. how about the people in the aircraft? people in the aircraft say, "what are those people down there complaining about? i don't get any problem." is that true? people in the aircraft hear the sonic boom too? they are making it. they ought to hear it, yeah? well, person riding a boat, okay, douses all the kids at the edge of the river. he said, "i don't know what everyone's complaining about. it seems nice and smooth to me." it turns out even a bullet, gang, a bullet, most bullets will travel faster than the speed of sound. so when a bullet goes over your head, what do you hear? - a crack. - a crack. and that crack you hear is what, gang? that's a sonic boom. that's a sonic boom, okay? now, what would you hear first, the crack due to the sonic boom or the crack due to the gun going off? sonic boom. check your neighbor.

okay, gang, what is the answer? what's the answer? how many say, "oh, they're both be the same." how many say, "no, no, no. "you'll hear the-- the bullet fired first "then the bullet moves, so you must hear the firing first and then you'll hear the crack." yeah, yeah, yeah. how many say, "no, no, no. "you're gonna hear the sonic boom first "and later on, you'll hear things in reverse. "and later on then you'll hear the crack of the bullet. woo, as strange as it may seem." yeah, yeah, yeah. how many say, "i don't be knowing the--" [laughter] that's my man. least gonna say it's-- it seems to me it will be the same because you hear the crack of the bullet and then you hear the crack of the bullet that's why you couldn't say anyway. so anyway you were-- oh, hear the crack of the bullet then you hear the crack of the gun. i'm sorry. i'm sorry, lee. i kinda get that su, su. screwed up. how about it, gang? it turns out you will hear the crack of the sonic boom. you hear the sonic boom crack

and then you'll hear the explosion of the gun. ain't that neat? 'cause it'll get you first. hey, how about the circus whip? a circus whip were a person snap and you get that snap sound. guess what that snap sound is? sb. i mean, msb. mini, a mini sonic boom. that's right, when you get that crack, honey, that circus whip is going faster than 750 miles an hour. the tip and it's making its own little sonic boom. it's the same as a towel, is it? with a towel? long towel maybe. yeah. the long towel snap, yeah, yeah. neat 'o, neat 'o stuff. speed of sound about 340 meters per second, but that's an air. and guess what's a poor conductor of sound? begin with "a". - air. - air. the best we got, honey, okay? it turns out like in water. in water, sound travels about four times faster. and in steel, about 15 times faster.

you know, you kind of make do that when you have-- you know, you got your two ears. do you ever wonder how was it that someone can make a sound over here and you know it's there or here and you know it's there? how do you know? how do you know where the sound is coming from? how many people have never wondered about things like this before? [laughter] okay. well, we're wondering about it now, yeah? huh? it's kinda neat. you know what happens when the sound is made over here, guess which ear it gets to first? right. this one and then this one. so what i have inside here, is that compute-- computer computer, okay? and what i do is that-- there. okay. so-- there, okay? you do that with stereo. yeah, you really do it. you adjust, so that you-- you know, where you get all fouled up is when you do it under the water, under the water. do you ever played tag with your friends clicking rocks together? how many people have never done that with they're friends, click rocks together under the water? okay. you know what you gonna do this weekend out of the bay, huh? and you click the rocks together and see if you can find out

where the rocks are clicking from. it turns out its very difficult. you know why? because even if it's over here, it gets to this ear and this ear almost the same, four times the speed of sound, gang. you are not equipped for that judgment, a dolphin is. dolphin got two ears. dolphin-- okay. kind of hard with no neck, but never was it, yeah? [laughter] yeah. yet, you get the idea. you know about ultrasound, yeah? ultrasound, high, high-frequency sound. high-frequency sound, very, very small wavelengths. and those small wavelengths can discern a thing. you know about how they send the sound waves into the stomach of a person having a baby, something like that. you don't want to send x-rays, do you? i mean, unless you don't like the kid already. [laughter] ultrasound waves, really, really neat. this ultrasound go right through, reflect and give the information, echoes, huh? the echoes of high-frequency sounds. and guess what maneuvers the same way in the sky at night time. begin with a b, end with at. - bats. - bats. right on. bats do the same thing.

how do the bats catch little insects on the fly? they send out beep, beep, beep, beep. the beep hits the insect, bounces off, be-beep. beep. got. ain't that nice? ain't that nice? picks it up by sonar, sonar. that means radar echo. lee. why when lower frequency sounds tell you where the insects-- a lower frequency not so good because a lower frequency is a long wavelength. remember we talked it before about seeing things small with long wavelengths like atoms? you just can't do it. detail is not picked up. so what the bat does, it makes a chirp that we can't even hear. the chirps we hear are probably the leftover part. in a high frequency, have a small wavelength and can detect small, small things. it turns out dolphins too. dolphins perceive most of their environment with the sense of sound. there's a way down there. it gets dark gang-- gets murky, doesn't it?

and how does a dolphin get around? in fact, the dolphins kinda got us beat in a little way because a dolphin can communicate with others with the same sense that they receive information. we receive most of our information by sight, huh? now, how do you give that back to other people in terms of sight? we can't. so we've invented a language. we have letters of the alphabet. we have grammar. we have words. so someone can tell you, boy, did i have a neat time today. the physics that i saw was blah, blah, blah, blah, blah, blah, blah, yeah? and we have a language. and we've always wondered, how come that dolphins don't seem to have maybe like-- they don't an alphabet language, yeah? what's they're alphabet? how many letters, you know? what are they're word processors like? how do the dolphins do it? and there's some neat ideas on that. and the way the dolphins do it goes something like this-- it's speculated. what the dolphin does-- the dolphin swimming over a nice great big terrain and big valley down the all-- nice stuff down there. and dolphin go-- --come back in. so that-- coming in as the image that the dolphin has, yeah?

and with that image, that dolphin has then the view of that world, yeah? now, the dolphin goes to some friend and goes-- --and puts that image directly in the mind of the friend. no language needed, gang. maybe the dolphin embellishes a little bit, lies a little like-- you know? [laughter] so we do that and we're cutting music, right? i mean, a little mistake, we cut in here and there or embellish this frequency and cut that one down. maybe the dolphin does that too. maybe the dolphin hears-- --and goes--okay? [laughter] and a person say no way, honey. okay? how do they say no way, honey? i don't know, a little blips. kinda neat that the dolphin at least can communicate its environment with the same sense that it receives it, yeah? we can't do that. so who is better off? us. we got a thing-- you know what-- that separates us from the dolphins? fingers. maybe not mind, fingers. we got fingers. we can do things. ain't that nice? how can a dolphin type, you know? [laughter]

would you expect sound to travel faster in warm air or cold air or make no difference from what you guys know about the theory of matter? warm air means the molecules-- cold air means the molecules-- yeah? and which kind of air do you think you can get one molecule tap in the next, the next, the next? by the way, you know how sound travels, don't you? when i strike the tuning fork, vibrates back and forth. doesn't it hit a molecule, another molecule hit another, another, another and doesn't it cascade through? picture the whole room with ping-pong balls right now. i take a big paddle and i-- wouldn't those balls hit one another? now, one ball here fly across the room. when i speak to you and go, "hello." it's not like the molecule come right across hit your ears. "oh, it sound like-- that doesn't happen.

does it? what happens when i say hello? i just-- all the molecules start moving. like the ping-pong balls go-- can you see them all moving in your minds eye? how many say, "no, i can't picture that. "you'd have to show me more. i just can't picture things like--" show a hands. okay. now, that's what happen. so anyway, what do you think? warm air or cold air communicate the sound faster? check your neighbor. how many of you saying i think the sound would travel a little bit faster in the warm air? how many say, "nope, that make sense, "but you wouldn't have asked the question if it made sense. "you asked the question 'cause it was the opposite. i think sound probably travels faster in the cold air." how many say, "no, i think the speed of sound "has nothing to do with air molecules and moving it.

"it's just 340 meters per second if you don't believe it, here it is right here"? [laughter] well, it turns out, gang, if the air molecules are already jiggling faster, then they'll tap the neighbor all the quicker. and so the faster moving air molecules will send sound a little bit faster than slow moving air molecules. so sound--the speed of sound depends upon the temperature of the air. okay? and your book gives the value as to how much it increases per degree or something like that. i can't remember what kind of thing, - but you got in there, yeah? - right. question. if it moves more quickly in air where the molecules are moving more, when we talk about it moving in a solid or a liquid, which obviously, the molecules are not moving as much, how does that compare them too? yes, yes, yes. that's a very good point, dave. how the sound travels has to do with something i haven't even talked about. a lot of people think it has to do with the density, that if something is very, very dense, sound will travel very, very quickly through it. like a piece of putty is very dense.

or a piece of lead is very dense. would you expect sound to travel very well in a lead pipe or a steel pipe? steel. if i bounce the lead pipe and bounce the steel pipe off the floor, which would-- well, if i drop the two, which would bounce off the floor, the lead pipe or the steel pipe? steel pipe. the steel pipe. it turns out that the speed of sound has to do with the elasticity of the substance. when you deform something, how quickly will it snap back? and steel is very, very, elastic. so sound travels very nicely through steel. but it's not as dense as lead. and even the density of the air falls out. it doesn't have to do with the density of the air or the density of the medium. has to do with the elasticity of the medium, and then the second order effect is this, what we're talking right now, the temperature of the medium. rest assure sound will travel a little faster in a warm day than a cold day.

and let me give you some evidence of that. it's something that occurred to me when i was a kid. i couldn't figure it out. and later on, when i got into physics--boom-- the answer to my question came. when i was a kid, i used to be a boy scout, and we used to go camping up at camp nihan, saugus, massachusetts. and we'll camp in one side of the lake. this is the lake. there will be another scout troop over here. and we'll have our tent set up here. and we'd tent out here, you know? and here's our troop here. here's their troop over here. they've got their tents. okay? in the daytime, you could yell over to this side, "hey. hey, you guys. how are you doing?" and they'll say, "speak a little louder, i can't hear you." "how you doing?" "fine. how you doing?" "oh, okay." "how's it going?" "oh"--you know these heavy conversations you have, yeah? but it turns out, we could just barely hear each other. but at nighttime, at nighttime--

[whistles] you could hear these guys whispering in their tents. you can hear them so clearly. i can remember that now why the-- "wow, at nighttime, i can hear those guys. "how come i can hear them so good at nighttime? in daytime, i can't." and i didn't know the answer to that. and it took me a long time to find the answer to that. has anyone ever experienced something like that? it's not all the time true, but under special conditions, sometimes you can hear a lot clearer over long distance than other times. let me tell you what happened, okay? it turns out that over here-- let's suppose over here, someone says, "hello. how are you?" and we get sound waves coming off like this. and the sound keeps going. now, it turns out, the direction of sound will always be at right angles to the waves. so, for example, like this, it's like that. and up here, it's right angle, right angle, right angle.

see that? so when all the air has the same temperature, then the sound will travel at equal speeds in all directions and form all these circles. yeah? but how about in the case where, maybe at nighttime, it's relatively cool above the water, and relatively warm up above? that's so-called temperature inversion. when that happens-- how about this wave? will this wave get to here or will it be further along? in the warm air, it would get to here. in the cold air, it would get to here. and up here, maybe, up to here. so you know what's gonna happen to the wave, gang? the wave is gonna be bent more like that. when it gets over here like this. now, my rays, being perpendicular, are pitched.

ooh, look at that. see that? and maybe up here, and down here. and it turns out, if you have a bigger ear, you can hear better. when your friends are far away, does it really help to put your hand like that? does it? yes, it does. okay? ask a rabbit. what's a rabbit doing all the time, right? yeah. the bigger the ears, the better you can hear. does it help to go like this? "hello, you guys." yeah, it does too. you're--like a megaphone, yeah? for the ear too. okay. so what you're doing is you're getting more energy in the air at nighttime than you are in the day 'cause the sound is being bent. the waves are being bent. we don't say waves bend because one part is going faster than another. we have a name for that. what's it called, gang? - refraction. - refraction. we say the wave is refracted. one part is going faster than the other because of a speed change. what underlies the phenomenon of refraction?

some people say, "what? you gotta have a speed change there somewhere." what do you say? begin with y and with p. try it. yup. yup. and that's true. so refraction is a change in the direction of a wave by virtue of a speed change. refraction. sound refracts like this. does that have something to do with the refraction of light? it turns out that anything that we call a wave will exhibit the properties of waves. and one of those properties is refraction. and here, i'm talking about sound refracts. guess what we're gonna talk about later, gang? - light-- - light refracts, okay. we talked about sound interference. later on, we'll talk about light interference. we talk about light interference, we'll find out that's why you get the pretty colors on gasoline spilling a wet street. you know? and the sound interference-- by the way, my cousin is deaf in one ear. he can't hear in one ear.

and you know why he can't hear in one ear? 'cause he spent many years with a jackhammer. that's loud sound. you know how tomorrow's jackhammers are gonna be, gang? tomorrow's jackhammers-- you know the sound they make, yeah? i see people out here in campus last semester doing those things with no earplugs. "yeah, i don't need any earplugs, man. i don't need any"-- you do need earplugs, honey. my cousin's deaf. but the sound, the sound, the sound--shaking. tomorrow's, gang, you know what tomorrow's jackhammers will do? they're developing them right now. tomorrow's jackhammers broadcast the sound to your ears. but exactly out of phase. so when a high pressure hits your ear, you get broadcast a low pressure. and guess what one does to the other? - cancel. - cancels it out. so the guy with a jackhammer tomorrow, you're going down the street and you hear this noise, and you see these two guys like this just chatting