Sound Waves (1960s)

[Applause] in this Echo chamber at the building the search station at Watford the sound of a gunshot takes about 20 seconds to die away the surfaces are hard and opposite walls are not quite parallel to each other how does the sound travel from one wall to another does it travel as a wave and if so how can we tell let's start with a familiar object a loud speaker producing a simple sound is it moving some things move so fast that we can't see yes it's moving all right one way we can see this is to use a stroboscope and make it move in slow motion one like this another way is to take a highspeed cine shot and then to slow it down you might like to think why both methods give us the same effect of slow motion we took a high-speed film with this loud speaker vibrating at a frequency of about 50 Cycles a second here is a view from the back of the loud speaker it moves like a piston and here is a closeup of the front edge of the speaker this loud speaker is also giving a note of about 200 Cycles a second now a fairly low note with a simple control I can raise the [Music] [Applause] frequency until now it's about a th Cycles a second let's go back to a very low frequency I spoke about the loudspeaker vibrating but we prefer to use the word oscillating the cone of the loudspeaker is now oscillating at about 26 cycles per second what is the air doing air is the medium surrounding this loud speaker through which the sound must travel or be transmitted does the air move in a steady stream as it does in a wind or is there some sort of pulse or wave which sends the sound along each part of the medium moves back and forth but the medium itself doesn't travel along with the wave you can see that clearly in the slinking first at normal speed and now in slow motion second each part of the medium moves back and forth just a little later than the part before and the wave machine shows this clearly third energy Travels With The Wave let's see if any of these apply to sound waves air is the medium around the loudspeaker but we cannot see it however a candle burning in the air can help us when I switch the loud speaker on candle is obviously affected but here there are many drafts and so we shielded the flame and again used the cic camera as a stroboscope to slow the motion down we placed the candle and speaker in front of a cine camera running at a slightly different frequency the flame and the speaker cone shown by the white Mark are clearly moving from side to side together here is the Flame by itself in fact moving back and forth about 26 times a second the flame from above appears rather like a DOT on our wave machine except that there each dot went up and down this is rather odd the loud speaker Moves In and Out and the candle in the surrounding medium air carrying the loudspeaker sound seemed to go with it it's difficult to see how this idea we have of a wave with the medium moving at right angles to the direction of the wave can explain our loud speaker experiment perhaps our toes can help imagine these two Tes to represent air par Les I haven't joined them together with springs because air particles move about much more freely this is the outline of the loud speaker and when it oscillates the candle flame and so presumably the air moves in this direction and it somehow squeezed together this is rather like the air on a bicycle pump which is squeezed air is springy and resists being pressed perhaps sound can be transmitted and explained in this way here's a model of how a sound wave might travel assume that the loud speaker is still vibrating we've arranged the toes in a line ahead to be rather like the air particles in front of the loud speaker and as the wave travels down to the end and back you can see each to moves back and forth very much like the candle flame the air particles are not connected together but they can push each other with these Springs so now we have two kinds of wave motion in the first kind the parts of the medium move back and forth at right angles to the direction of the wave this is called a transverse wave water waves and those on our machine move like this in the second kind of which sound waves are an example the parts of the medium move back and forth in the same direction as the wave and this is called a longitudinal wave here is a method you might like to try for measuring the speed of sound RS which are longitudinal the source of sound is metal hitting metal here's one boy with a stopwatch and there are four others as a [Laughter] check rulers are put down starting from the source of sand the rulers help the boys to get a regular length of pace and the distance from the source to the wall is then paced out [Applause] notice that the boy producing the sound keeps the original sound and the echo in Step meanwhile the timekeepers measure the time taken by a certain number of outward and inward journeys of the sound knowing the distance of the s's journey and the time taken we can then calculate its speed if you don't already know the value of this speed I'd like you to find it out or measure it for yourselves and now what about the question of energy you will remember how earlier the movement of the loud speaker affected a candle flame some energy was transferred in this experiment a speaker is placed at one end of a long box and a candle flame at the other when the speaker is turned on enough energy is produced to blow the candle out so energy travels with a soundwave and we can concentrate this energy as we can with light energy by using a lens of some sort here is one this is a balloon filled with a gas which is not air with a microphone here and the L speaker there you should be able to hear the sound but first we'll have to switch off the microphone above and just use this microphone listen while I move the balloon up and down did you notice that the sound is concentrated or focused by the balloon well here's a rather difficult question for you which you can take down at the end of the program was the gas in the balloon more or less dense than air it's difficult to do these experiments in a normal room or even in a large studio like this because there are all sorts of disturbances especially from Echoes so we took our cameras to another chamber at the building research station which has quite opposite properties to the one we saw at the beginning of the program the walls ceiling and floor of this room are covered with foam polyurethene wedges 4 ft long there are 22,000 of them when the door is closed nearly all the sound in the room is absorbed and there is an eerie silence this is a particularly good place to carry out our sound experiments because of the lack of echo it is called an anico chamber and this one is the largest in Europe its primary use is in the study of sound transmission through various materials used in the construction of buildings the same gun we used before fired in this chamber is very unimpressive because of the wedges on the floor a working floor of wire mesh is stretched across the chamber 2 ft above the floor wedges now for the experiment you will all have noticed in your own sitting rooms that sound is not as we say very directional in fact you can sit anywhere and still hear the program coming from your radio set the sound seems to spill out in all directions for example sound from this loud speaker is heard not only here but here and around here we investigated this property in the anico chamber first by moving a microphone in front of a large loud speaker notice that we pick up most sound straight ahead and less either side of the center line on the other hand if we narrow down the width of the Sound Source it is quite a straight ahead as we should expect but the sound doesn't seem to fall off as much on either side perhaps this is not so expected so narrowing down the slit makes the sound quieter which is not surprising but it also makes it spread out more in all directions and perhaps this is more surprising does this fit in with our idea of sound as a wave motion if you haven't already done so I'd like you to compare the effect of water waves going through wide slits and then on this Ripple tank we narrow the slit down to make a narrow one we refer to this spreading out of waves as defraction and the fact that sound waves spread out around corners is strong evidence for the fact that there are wave motions let's experiment a little more some of you may have tried using two sorters in a ripple tank to produce this sort of pattern would we get a similar effect using two loud speakers we tried in the anur chamber the right hand speaker is on all the time and the microphone is in front of and between the speakers notice what happens when we switch on the left hand speaker the volume increases that's expected now move the microphone to one side and switch on the second speaker again [Music] now the volume decreases that seems more [Music] surprising if we now look at the speakers from behind and pass a microphone across in front of them this happens in this experiment with two loudspeakers we were only considering what happens in one line what do you think happens here and here well here's an experiment you can try for yourselves to find out set up two loudspeakers in the school playing field and connect them to the same oscillator switch on the two speakers both at the same frequency of about 1,000 cycles per second a group of listeners standing in front of the speakers moves back and forth until the sound seems to be at a maximum any one person will hear the volume of the note rise and fall as he moves at first we seem to have just a disorganized crowd but eventually a pattern like this forms an interference pattern you may think the pattern you just saw and the Ripple tank pattern are very different but each is part of this pattern which would be obtained if for example the loud speakers could send out their sounds in all directions and now I hope you've got a pencil and paper ready first what is the speed of sound second was the gas in the balloon more or less dense than air and here's a third question for you do high notes and low notes spread out equally when they pass through slits you can try this with them going around corners as well you might like to try that one out practically using say a drum and a whistle like this e [Music]