Nature of Sound, The (1948)

[Music] [Music] sound is all around us everywhere it beat at our eardrums until much of the time we're hardly conscious of it but on this particular Saturday morning Jimmy Foster suddenly became very conscious of it because a sound made him jump or was it really the sound that made him jump it seemed as though the post was vibrating well he soon find out whether there was a vibration along with the sound the scrap of wood showed him him that when the bell sounds it does [Music] vibrate Jimmy's ears were now wide open to the sounds around him his eyes were open too suddenly he saw the steam from a whistle way over there but didn't hear it until several seconds later Jimmy became curious about sound and decided to look up his cousin Phil Hansen Phil is a radio ham runs an amateur transmitter he should know if sound is vibration Phil how does it get from one place to another mostly through the air Jimmy you see the vibration of my voice box makes the air vibrate between us then when the air vibrates your eardrum you hear what I'm saying vibration of vibration of voice box you can prove that air Carri sound but but maybe maybe you've seen a demonstration of a bell under a glass jar with normal air inside the jar you could hear the Bell clearly but when the vacuum pump drew the air out of the jar there was nothing inside to carry the vibrations of the Bell nothing until the air was let back into the jar so you need air for sound to travel through you need something but it doesn't have to be air here I'll show you we'll use a microphone which is a sort of mechanical air now it picks up sound vibrations and changes them into electrical impulses these go through the amplifier to the speaker which changes them back into Sound Vibrations now the sound of my voice travels to the microphone through air doesn't it I guess so sure well sound actually travels better and faster through solid things iron or aluminum or glass here hold the microphone over here now what do you hear nothing and neither does the microphone or we' hear it from the speaker now I'll hold my watch up against the aquarium you hold the microphone against your end now I hear it that's because sound travels better through the glass than it does through air now since a watch is waterproof I guess that proves that sound also travels better through water that's right and speaking of water it can also show us how sound travels toss a rock in a pool and watch how the ripples start from the Splash and travel out and if there's something floating on the surface you'll see that it's the disturbance that moves and not the water as a whole sound travels the same way start this Bell vibrating then slow it down and we see that it pushes out and compresses the air then Springs back causing a lower pressure then pushes out again as this goes out and places of slightly higher air pressure or compressions travel outward separated by places of slightly lowered pressure or expansions the distance between one compression and the next is called the wavelength of a sound as is the distance from one expansion to the next of course the frequency is the number of waves per second when these vibrations reach your eardrum you hear the sound but remember it's the vibrations that travel not the air say those vibrations travel pretty fast don't they pH compared to some things yes but not compared to the speed of light light travels at the rate of about 186,000 m/ second sound travels through the air at about 1100 ft per second that's why I saw the twistle before I heard it and with your watchs you could measure the distance between you and the whistle because light travels so fast you see the steam from the whistle almost instantly but the sound traveling at 1100 ft a second now if it takes 5 Seconds to reach you you know that the sound is 5times 1100 ft or more than a mile away say I can have a lot of fun measuring distance by sound and there's another trick you can do with a watch you know what this is sure it's a light reflector it's also a sound reflector stand over there a minute Jimmy you say it's a sound reflector sure sound can be reflected almost like light now what do you hear why nothing I guess all right now listen when I hold it so the reflector directs the sound say that's wonderful it sounds as if it's right next to my ear yes sound reflects pretty much like light or a rubber ball or like a spring try this Jimmy hook one end over the reflector now pull it out straight that's it now the spring is just like air and silence no disturbances no compressions strike one end of it Jimmy why the compressions travel right along and bounce off the ends back and forth right and when sound bounces back that way it's an echo in motion picture studios and radio stations reflected sounds can cause trouble they often hang draperies to kill some of the reverberation a combination of hard and soft walls usually gives the best sound quality sound quality now what's there well quality it's one of the three characteristics of sound we've already mentioned the other two you know what they are well let's see quality is one uh you said something about frequency is that one that's almost it but let's call it pitch which depends mainly on frequency what about how loud it is right loudness pitch and quality I can show you all three show me a sound sure by turning on this oscilloscope it draws a picture of a sound uh be careful Jimmy it belongs to my radio club now watch this is a picture of my voice as I talk Jimmy can I see my voice hey there it is now about these characteristics of sound we'll start with loudness make a sound on the Sarina Jimmy now louder the louder I blow the tall where the lines get that's right try it again the height or amplitudes of the jumps changes according to the loudness or intensity of the sound and another thing watch how the sound falls off in intensity as I move away from the microphone so you see the loudness of any sound you hear varies with its intensity and its distance from you and this is shown by the amplitude of the jumps shallow waves for a soft note and high waves for a loud note now what do you suppose shows a pitch maybe maybe a number of waves huh almost right right Jimmy here try running a scale on this and [Applause] watch the number of waves does change with the pitch we call the number of waves each second frequency and the length of the Waves changes Too Short waves for a high note long waves for a low note now you can hear two sounds of the same loudness and pitch and still they sound different and that's where quality comes in right try singing the five vowels on the same note a e i o u now compare my note to yours me me the waves seem to have different shape yes the quality of the sound determines the shape of the wave we can have two sounds have the same loudness and pitch and still they're very different and that's that that's that but it's not all by any means there's a whole lot more to know about sound if you really want to go into the subject but remember those three characteristics and remember that all sound is vibration it won't travel through a vacuum but it does travel through Air at the speed of about 1,00 ft a second it travels even better through solids and through water it travels in waves of compressions and expansions with those points in mind Jimmy you can go on to a thorough study of the nature of sound that's for me the nature of [Music] sound