Learning About Sound (1974)

[Music] there was once a time that movies were silent except for the music that was played with [Music] them then sound came [Music] this film takes a close look at the sounds we've always taken for granted [Music] vibrations are the source of all sounds it's not hard to believe the drum vibrates but what is the source of this [Music] sound the bow causes the string to vibrate and this generates the sound [Music] vibrations in the gong have enough energy to keep this pingpong ball bouncing we'll use the light from a laser to see the vibrations of a tuning fork a mirror is attached to the end of the fork as it moves the reflected beam moves the rotating mirrors provide a horizontal motion the fork is electrically [Music] driven we can see vibrations and we can feel vibrations but how do they reach our ears as sound one small drop of water yet its energy disturbs the entire surface as it moves out in waves energy from a vibrating object moves through the air in waves when these vibrations in the air reach our ears we interpret them as sound what would happen to the sound of the Bell if we removed the air around it we're filling the jar with smoke when the smoke is gone most of the air will be gone a pump is used to remove the air the oscilloscope displays the sound of the Bell without a medium such as air there is no sound unlike water waves sound waves travel in all directions if you had a point source of sound its energy would radiate like this let's look at a model of a small section of the radiating wave we'll use this suspended spring to illustrate the action of waves in one plane waves can be created by both pushing and pulling on the spring while the energy of the wave moves from one end of the spring to the other any given coil simply vibrates back and forth let's analyze these sounds on an oscilloscope 4 and A2 vibrations n vibrations and 18 vibrations high notes have more vibrations per second than low notes the number of vibrations per second is called the frequency without changing the frequency notice the difference the loudness of a note is related to the strength of its vibrations we call this amplitude frequency and amplitude are two qualities of sound but how fast does sound travel the sound of this axe moves through the air in all directions from a distance the the sound arrives late how fast does sound travel we'll use this abandoned track for our demonstrations first we measure off 300 M these flags Mark the two positions we'll use a starting gun as our source of sound up close the smoke and the sound go together now 300 M away under these conditions sound travels about 300 M per [Music] second does sound travel through solid objects like the steel rail if so how fast let's find out we can hear the hammer through the rail with a stethoscope it seems to reach us almost at the moment of impact in fact sound travels 10 times faster through steel which is a solid than it does through air do you think sound travels through water how would you measure its speed [Music] a person with normal hearing can hear approximately 20 to 20,000 vibrations per second yeah okay I understand you could you use better speech what's that that was better speeech it's a box some people cannot hear sound as we do for them even learning to speak is difficult since they cannot hear the sounds they make this child is using a device to see the sounds of speech and then practice duplicating them good oh good oh good good that was real good Patrick which one do you want to do good good good good good boy that's right even people with good hearing cannot hear certain sounds this dog whistle for example is too high pitched for human ears high-pitched sounds we cannot hear are called ultrasonic each of these metal bars gives off a high pitched sound when struck the oscilloscope can display even the ultrasonic tones while all we hear is the tap of a mallet [Music] [Music] one of the greatest Pleasures we derive from sound is music musical instruments generate sound in different ways but in every instrument the sound comes from some form of VI [Music] vibration here mallets strike the strings setting up mechanical vibrations we can also generate Sounds by vibrating electrons in a circuit this synthesizer can make a wide variety of Sounds here the key these are merely electronic switches the shape of the wave and therefore the quality of the sound can be easily [Music] altered these are all sounds of the same frequency and amplitude yet each sounds different [Music] the quality of most musical instruments is a complex mixture of Vibrations by analyzing these patterns the engineer finds Clues to the electronic combinations necessary to copy a sound [Music] [Music] [Music] [Music] here is the real cello notice the complexity within each [Music] wave now the copy the oscilloscope allows us to see clearly the difference between the two [Music] sounds sound can be music to our ears or noise our understanding of the physics of sound may help us diminish noise as a pollutant sometimes we can quiet a source of Noise by simply changing the material it is made of the metal garbage can makes more noise because it vibrates more than the plastic one here is another potential noise maker a good Muffler however can reduce the noise but how do you put a muffler on a Jet Plane Engineers have discovered that certain frequencies of sound are more annoying to the human ear than others jet engines are now being designed to minimize these annoying frequencies this particular aircraft is a lot quieter than other planes the same size there are many positive steps we can take to quiet the environment but it will probably never be as quiet as it was in the good old days and even then we were trying to use our knowledge of sound to make some things louder [Music] [Music] [Music] [Music]