INTERFERENCE: WAVE VERSUS WAVE When a tuning fork is struck, a listener hears a sustained tone-as long as fork and listener remain stationary. But if the fork is turned, or if the listener walks around it, the volume of the sound rises and falls. This variability can occur whenever two or more sets of sound waves meet. It is due to interference--eaused in this case by the interaction of waves from the two prongs of the fork. Sound waves are formed of alternating zones of high and low pressure called compressions and rarefactions. When waves from different sources mesh, compression to compression, rarefaction to rarefaction, reinforcement occurs, and the sound is louder. But when compressions of one wave series coincide with rarefactions of another, the sound is diminished. Interference is partially responsible for the “dead” spots in some auditoriums. Where reflections interfere with one another the clarity is affected. But interference decreases the volume and deadens the sound.
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Patterns of Interference
Two series of sound waves are shown here interfering with each other, creating areas of differing loudness. The two sounds, like those from the prongs of a tuning fork, are identical:each has the same frequency and is of the same intensity. But in competing for air space they overlap each other. When the waves coincide -where the red and blue lines are superimposed the sound is loudest. Where the waves alternate (dark areas), they tend to cancel each other: a “null” is formed and the sound’s loudness is diminished to the point of inaudibility.
DIFFRACTION: TURNING THE CORNER An executive seated in his office can hear the clacking of his secretary’s typewriter clearly, even though it is out of sight, separated from him by a partition. A pedestrian passing a busy schoolyard hears the shouts and laughter from within in spite of the thick concrete wall surrounding the playground. This “sound around a corner” effect is so common that neither man gives it a second thought. Yet the sounds they hear are getting to them .. by a complex phenomenon known as diffraction. Sound waves normally keep traveling in the direction they start out in. But through diffraction they can go around an obstacle by creating a new series of waves. These secondary waves radiate from the obstacle as though it were the source of the ‘ sound (opposite). Inside! diffraction usually works together with reflection, pushing sounds around corners and up stairs. But even alone diffraction is surprisingly potent-so much so, in fact, that it can squeeze nearly as much sound through a door open only an inch or two as through a wide-open doorway. Sending music around a corner, diffraction
lets a listener hear an unseen organ . The edges of the Gothic doorway act as secondary sound sources to create new series of waves that repeat the shape of the arch. Because the secondary waves cross each other’s paths, the sound is not so clear as that heard by the man standing in the path ofthe original sound waves.
ABSORPTION: SOAKING UP SOUND When sound hits a curtain, a rug or an acoustic tile, it is literally soaked up, like water into a sponge. This absorption of sound waves is due to a characteristic all absorptive materials share: like the sponge, they are extremely porous. When waves enter these materials, they bounce around aimlessly in myriad air pockets until they have lost much of their energy. Actually, the energy of motion has been transformed into heat. Under normal conditions the rise in temperature is so minute that it is detectable only by instruments. But sometimes the conversion of sound into heat can have serious effects. In one experiment mice were exposed to sound of extremely high intensity. After only 10 minutes or less, the sound energy trapped by the animals’ highly absorptive fur had produced so much heat that the hair was badly burned. To prevent most of the noise generated in one room from getting into an adjacent room , the sound-resistant wall in this diagram both reflects and absorbs sound waves. Some waves bounce away and back into the room; the remainder of the sound penetrates the wall. Which because of its absorptive qualities dissipates most of the sound waves’ energy. Only a small amount goes right through the wall. Moving from one medium to another, the waves refract as they enter and leave the wall. though this bending has no part in lessening the sound. How an absorptive material dissipates the energy of the sound of a man’s voice is depicted in this sequence of drawings. The sound reaches a section of special acoustic tile. Further magnification shows how the sponginess acts as a trap. Within it, sound loses intensity as it rebounds repeatedly. By the time it finally rescapes, it is reduced .to a mere whisper of its former self. How an absorptive material