Behavior of Sound Waves - Sound | HowStuffWorks Behavior of Sound Waves Like light waves and other waves, sound waves are reflected, refracted, and diffracted, and exhibit interference. Reflection Sound is constantly being reflected off many different surfaces. Most of the time the reflected sound is not noticed, because two identical sounds that reach the human ear less than 1/15 of a second apart cannot be distinguished as separate sounds. When the reflected sound is heard separately, it is called an echo. Sound is reflected from a surface at the same angle at which it strikes the surface. This fact makes it possible to focus sound by means of curved reflecting surfaces in the same way that curved mirrors can be used to focus light. It also accounts for the effects of so-called whispering galleries, rooms in which a word whispered at one point can be heard distinctly at some other point fairly far away, though it cannot be heard anywhere else in the room. (Statuary Hall of the United States Capitol is an example.) Reflection is also used to focus sound in a megaphone and when calling through cupped hands. The reflection of sound can pose a serious problem in concert halls and auditoriums.

In a poorly designed hall, a speaker's first word may reverberate (echo repeatedly) for several seconds, so that the listeners may hear all the words of a sentence echoing at the same time. Music can be similarly distorted. Such problems can usually be corrected by covering reflecting surfaces with sound-absorbing materials such as draperies or acoustical tile. Clothing also absorbs sound; for this reason reverberation is greater in an empty hall than in one filled with people. All these sound-absorbing materials are porous; sound waves entering the tiny air-filled spaces bounce around in them until their energy is spent. They are, in effect, trapped. The reflection of sound is used by some animals, notably bats and toothed whales, for echolocation—locating, and in some cases identifying, objects through the sense of hearing rather than the sense of sight. Bats and toothed whales emit bursts of sound of frequencies far beyond the upper limits of human hearing, as high as 200,000 Hz in the case of whales. Sounds with short wavelengths are reflected even from very small objects. A bat can unerringly locate and catch even a mosquito in total darkness. Sonar is an artificial form of echolocation.

Refraction When a wave passes from one material to another at an angle, it usually changes speed, causing the wave front to bend. The refraction of sound can be demonstrated in a physics laboratory by using a lens-shaped balloon filled with carbon dioxide to bring sound waves to a focus. Diffraction When sound waves pass around an obstacle or through an opening in an obstacle, the edge of the obstacle or the opening acts as a secondary sound source, sending out waves of the same frequency and wavelength (but of lower intensity) as the original source. The spreading out of sound waves from the secondary source is called diffraction. Because of this phenomenon, sound can be heard around corners despite the fact that sound waves generally travel in a straight line. Interference Whenever waves interact, interference occurs. For sound waves the phenomenon is perhaps best understood by thinking in terms of the compressions and rarefactions of the two waves as they arrive at some point. When the waves are in phase so that their compressions and rarefactions coincide, they reinforce each other (constructive interference). When they are out of phase, so that the compressions of one coincide with the rarefactions of the other, they tend to weaken or even cancel each other (destructive interference).

The interaction between the two waves produces a resultant wave. In auditoriums, destructive interference between sound from the stage and sound reflected from other parts of the hall can create dead spots in which both volume and clarity of sound are poor. Such interference can be reduced by use of sound-absorbing materials on reflecting surfaces. On the other hand, interference can improve an auditorium's acoustical qualities. This is done by arranging the reflecting surfaces in such a way that the level of sound is actually increased in the area in which the audience sits. Interference between two waves of nearly but not quite equal frequencies produces a tone of alternately increasing and decreasing intensity, because the two waves continually fall in and out of phase. The pulsations heard are called beats. Piano tuners make use of this effect, adjusting the tone of a string against that of a standard tuning fork until beats can no longer be heard. Up Next Reflection, Refraction, and Diffraction Sound Waves and Music - Lesson 3 Behavior of Sound Waves Reflection, Refraction, and Diffraction Boundary Behavior Reflection, Refraction, and Diffraction Like any wave, a sound wave doesn't just stop when it reaches the end of the medium or when it encounters an obstacle in its path.

Rather, a sound wave will undergo certain behaviors when it encounters the end of the medium or an obstacle. Possible behaviors include reflection off the obstacle, diffraction around the obstacle, and transmission (accompanied by refraction) into the obstacle or new medium. In this part of Lesson 3, we will investigate behaviors that have already been discussed in a previous unit and apply them towards the reflection, diffraction, and refraction of sound waves. Reflection and Transmission of Sound When a wave reaches the boundary between one medium another medium, a portion of the wave undergoes reflection and a portion of the wave undergoes transmission across the boundary. As discussed in the previous part of Lesson 3 , the amount of reflection is dependent upon the dissimilarity of the two media. For this reason, acoustically minded builders of auditoriums and concert halls avoid the use of hard, smooth materials in the construction of their inside halls. A hard material such as concrete is as dissimilar as can be to the air through which the sound moves; subsequently, most of the sound wave is reflected by the walls and little is absorbed. Walls and ceilings of concert halls are made softer materials such as fiberglass and acoustic tiles.

These materials are more similar to air than concrete and thus have a greater ability to absorb sound. This gives the room more pleasing acoustic properties. Reflection of sound waves off of surfaces can lead to one of two phenomena - an echo or a reverberation. A reverberation often occurs in a small room with height, width, and length dimensions of approximately 17 meters or less. Why the magical 17 meters? The effect of a particular sound wave upon the brain endures for more than a tiny fraction of a second; the human brain keeps a sound in memory for up to 0.1 seconds. If a reflected sound wave reaches the ear within 0.1 seconds of the initial sound, then it seems to the person that the sound is prolonged. The reception of multiple reflections off of walls and ceilings within 0.1 seconds of each other causes reverberations - the prolonging of a sound.

Since sound waves travel at about 340 m/s at room temperature, it will take approximately 0.1 s for a sound to travel the length of a 17 meter room and back, thus causing a reverberation ( recall from Lesson 2 , t = d/v = (34 m)/(340 m/s) = 0.1 s). This is why reverberations are common in rooms with dimensions of approximately 17 meters or less. Perhaps you have observed reverberations when talking in an empty room, when honking the horn while driving through a highway tunnel or underpass, or when singing in the shower. In auditoriums and concert halls, reverberations occasionally occur and lead to the displeasing garbling of a sound. But reflection of sound waves in auditoriums and concert halls do not always lead to displeasing results, especially if the reflections are designed right. Smooth walls have a tendency to direct sound waves in a specific direction. Subsequently the use of smooth walls in an auditorium will cause spectators to receive a large amount of sound from one location along the wall; there would be only one possible path by which sound waves could travel from the speakers to the listener.

The auditorium would not seem to be as lively and full of sound. Rough walls tend to diffuse sound, reflecting it in a variety of directions. This allows a spectator to perceive sounds from every part of the room, making it seem lively and full. For this reason, auditorium and concert hall designers prefer construction materials that are rough rather than smooth. Reflection of sound waves also leads to echoes. Echoes are different than reverberations. Echoes occur when a reflected sound wave reaches the ear more than 0.1 seconds after the original sound wave was heard. If the elapsed time between the arrivals of the two sound waves is more than 0.1 seconds, then the sensation of the first sound will have died out. In this case, the arrival of the second sound wave will be perceived as a second sound rather than the prolonging of the first sound. There will be an echo instead of a reverberation. Reflection of sound waves off of surfaces is also affected by the shape of the surface. As mentioned of water waves in Unit 10 , flat or plane surfaces reflect sound waves in such a way that the angle at which the wave approaches the surface equals the angle at which the wave leaves the surface.

This principle will be extended to the reflective behavior of light waves off of plane surfaces in great detail in Unit 13 of The Physics Classroom . Reflection of sound waves off of curved surfaces leads to a more interesting phenomenon. Curved surfaces with a parabolic shape have the habit of focusing sound waves to a point. Sound waves reflecting off of parabolic surfaces concentrate all their energy to a single point in space; at that point, the sound is amplified. Perhaps you have seen a museum exhibit that utilizes a parabolic-shaped disk to collect a large amount of sound and focus it at a focal point. If you place your ear at the focal point, you can hear even the faintest whisper of a friend standing across the room. Parabolic-shaped satellite disks use this same principle of reflection to gather large amounts of electromagnetic waves and focus it at a point (where the receptor is located). Scientists have recently discovered some evidence that seems to reveal that a bull moose utilizes his antlers as a satellite disk to gather and focus sound. Finally, scientists have long believed that owls are equipped with spherical facial disks that can be maneuvered in order to gather and reflect sound towards their ears.

The reflective behavior of light waves off curved surfaces will be studies in great detail in Unit 13 of The Physics Classroom Tutorial . Diffraction of Sound Waves Diffraction involves a change in direction of waves as they pass through an opening or around a barrier in their path. The diffraction of water waves was discussed in Unit 10 of The Physics Classroom Tutorial. In that unit, we saw that water waves have the ability to travel around corners, around obstacles and through openings. The amount of diffraction (the sharpness of the bending) increases with increasing wavelength and decreases with decreasing wavelength. In fact, when the wavelength of the wave is smaller than the obstacle or opening, no noticeable diffraction occurs. Diffraction of sound waves is commonly observed; we notice sound diffracting around corners or through door openings, allowing us to hear others who are speaking to us from adjacent rooms. Many forest-dwelling birds take advantage of the diffractive ability of long-wavelength sound waves. Owls for instance are able to communicate across long distances due to the fact that their long-wavelength hoots are able to diffract around forest trees and carry farther than the short-wavelength tweets of songbirds.

Low-pitched (long wavelength) sounds always carry further than high-pitched (short wavelength) sounds. Scientists have recently learned that elephants emit infrasonic waves of very low frequency to communicate over long distances to each other. Elephants typically migrate in large herds that may sometimes become separated from each other by distances of several miles. Researchers who have observed elephant migrations from the air and have been both impressed and puzzled by the ability of elephants at the beginning and the end of these herds to make extremely synchronized movements. The matriarch at the front of the herd might make a turn to the right, which is immediately followed by elephants at the end of the herd making the same turn to the right. These synchronized movements occur despite the fact that the elephants' vision of each other is blocked by dense vegetation. Only recently have they learned that the synchronized movements are preceded by infrasonic communication. While low wavelength sound waves are unable to diffract around the dense vegetation, the high wavelength sounds produced by the elephants have sufficient diffractive ability to communicate long distances. Bats use high frequency (low wavelength) ultrasonic waves in order to enhance their ability to hunt. The typical prey of a bat is the moth - an object not much larger than a couple of centimeters.

Bats use ultrasonic echolocation methods to detect the presence of bats in the air. But why ultrasound? The answer lies in the physics of diffraction. As the wavelength of a wave becomes smaller than the obstacle that it encounters, the wave is no longer able to diffract around the obstacle, instead the wave reflects off the obstacle. Bats use ultrasonic waves with wavelengths smaller than the dimensions of their prey. These sound waves will encounter the prey, and instead of diffracting around the prey, will reflect off the prey and allow the bat to hunt by means of echolocation. The wavelength of a 50 000 Hz sound wave in air (speed of approximately 340 m/s) can be calculated as follows wavelength = speed/frequency wavelength = (340 m/s)/(50 000 Hz) wavelength = 0.0068 m The wavelength of the 50 000 Hz sound wave (typical for a bat) is approximately 0.7 centimeters, smaller than the dimensions of a typical moth. Refraction of Sound Waves Refraction of waves involves a change in the direction of waves as they pass from one medium to another.

Refraction, or bending of the path of the waves, is accompanied by a change in speed and wavelength of the waves. So if the media (or its properties) are changed, the speed of the wave is changed. Thus, waves passing from one medium to another will undergo refraction. Refraction of sound waves is most evident in situations in which the sound wave passes through a medium with gradually varying properties. For example, sound waves are known to refract when traveling over water. Even though the sound wave is not exactly changing media, it is traveling through a medium with varying properties; thus, the wave will encounter refraction and change its direction. Since water has a moderating effect upon the temperature of air, the air directly above the water tends to be cooler than the air far above the water. Sound waves travel slower in cooler air than they do in warmer air. For this reason, the portion of the wavefront directly above the water is slowed down, while the portion of the wavefronts far above the water speeds ahead. Subsequently, the direction of the wave changes, refracting downwards towards the water. This is depicted in the diagram at the right.

Refraction of other waves such as light waves will be discussed in more detail in a later unit of The Physics Classroom Tutorial . Jump To Next Lesson: Science Explorations: Journey Into Space: Sound and Ultrasonics | Scholastic.com Sound and Ultrasonics William C. Vergara The everyday world is filled with all kinds of sounds -- the murmur of voices, the blaring horn of an automobile, the distant putt-putt of a lawn mower, the buzzing of a mosquito. Each of these common sounds is different from the others. And it is easy to tell all of them from hundreds of other sounds. There are still other sounds, however, that are too high-pitched to be heard by the human ear. These sounds are called ultrasonic sounds, or ultrasound. Some animals, such as dolphins and bats, make and hear these sounds. Scientists have built special instruments to produce and detect ultrasonic sound. In this article you will read how sounds are produced and why they are different from one another. You will also read how ultrasound has been put to use. How Sounds Are Produced All sounds are produced by a certain kind of motion. Suppose you stretch a rubber band and then pluck it.

The band moves back and forth so rapidly that its motion appears blurred. The sound you hear comes from its motion. Such very fast, back-and-forth motions are called vibrations. Vibrations are the source of all sounds. Pluck a guitar string. You will hear a musical sound. If you place your finger lightly on the string, you can feel the vibrations. They produce the sound. When you stop the vibrations, you stop the sound. Sound Travels Through a Medium Sound travels from a vibrating object to your ear by means of a medium, or sound carrier. The medium may be a solid, a liquid, or a gas. Air, a mixture of several gases, is the most common carrier of sound as far as people are concerned. If there is no medium to carry the sound to our ears, we cannot hear the sound. For example, the moon has no air. Astronauts on the moon cannot speak directly to each other but must communicate by radio. The Speed of Sound A person seated beyond the outfield of a ballpark sees a batter swing at the ball. An instant later the person hears the crack of a bat as it hits the ball.

You usually hear a clap of thunder a short time after you see the bolt of lightning that caused the sound. Such common experiences demonstrate that the speed of sound is slower than the speed of light. Careful measurements show that the speed of sound depends on the medium. For example, sound travels through the air at a speed of about 1,100 feet (335 meters) a second. Sound travels about 4 times as fast in water as it does in air. Sound travels even faster in solids. In steel and in aluminum, the speed of sound is about 15 times faster than it is in air. Changes in temperature can change the medium enough so that sounds travel through it at a different speed. For example, sound travels slightly faster in warm air than in cold air. However, at a given temperature all sounds in the same medium have the same speed. Sound Waves Sounds travel from a vibrating object to your ear by means of compression waves in the air. To understand what a compression wave is, you can try an experiment with a coil spring toy made up of many flat metal coils. (One such toy of this type is called a Slinky.) The spring stretches open easily and then returns to its original shape.

This flexibility makes it useful for the experiment. First, find a yardstick or other stick about 3 feet (1 meter) long. Slide it through the center of the spring. Ask a helper to hold one end of the stick firmly. With your right hand hold your end of the stick and one end of the spring at the same time. With your left hand hold the free end of the spring. Now you can stretch the spring back and forth along the stick. If you slide your left hand back and forth along the stick in a regular motion, a wave will move down the outstretched spring from one end to the other. The individual turns of the spring move only slightly out of position, yet the wave motion travels all along the coils. To understand how this happens, think about the movement of the spring in slow motion. When you first move the end of the spring toward yourself, the coils nearest your hand are compressed. When you let go, the coils immediately spring apart, or expand, which compresses the next section of coils along the spring. One such compression and expansion is called a cycle. Scientists call a wave made of many compressions and expansions a compression wave.

Many compression waves pass along the spring as your hand continues to jiggle the end of the spring. Molecules of air behave in much the same flexible way. When pushed by a vibrating object, air molecules bunch up, or compress, and then expand. The effect is passed along to neighboring molecules as compression waves of sound. The invisible compression waves travel through the air, gradually becoming weaker and weaker until the sound waves die out altogether. Characteristics of Sound Waves A vibrating object sends out sound waves through the air in all directions. The waves travel outward in ever widening "shells," one inside the other. If you could see sound waves, they would look somewhat like round balloons of different sizes, one inside the other. The balloons would represent the compressions of air molecules, and the space between the balloons would represent the expansions. The vibrating body, the source of the sound, would be in the center of the smallest balloon. Wavelength and Frequency The distance between one compression and the next is called the wavelength of a sound wave. The human ear can hear sounds whose wavelengths in air are from a little less than 3/4 inch (2 centimeters) to about 69 feet (21 meters) long.

An object that vibrates 500 times per second causes a sound wave that vibrates at the rate of 500 cycles per second. This rate is called the frequency of the sound wave. Instead of cycles per second, the term hertz (abbreviated Hz) is often used. This unit was named for Heinrich Hertz, a German scientist. For example, a bell or a piano string that vibrates 250 times per second produces a sound wave with a frequency of 250 hertz. Depending on the rate of vibration, sound waves are described as being of low, medium, or high frequency. Thus, a 22-hertz wave is considered a low-frequency wave, while a wave of 7,000 hertz is a high-frequency wave. There is a relationship between the wavelength and the frequency. Short wavelengths have a high frequency. Long wavelengths have a low frequency. There is also a connection between the frequency and the pitch. Pitch A whistling or screeching sound has a high pitch. A deep or rumbling sound, such as you get from the low notes on a piano, has a low pitch. The pitch of a sound depends on its frequency.

A high frequency (many cycles per second) produces a high pitch. A lower frequency (fewer cycles per second) produces a lower pitch. You can show that the pitch of a sound depends on the frequency by plucking a stretched rubber band. If you pluck a loosely stretched rubber band, the frequency of the sound waves produced may be so low that you can see the vibrations of the rubber band. The sound you hear has a low pitch. Stretch the rubber band tightly and pluck it again. Its vibrations are so fast that you see only a blur, and the sound has a higher pitch. Think of how an electric fan sounds when you turn on the switch. At first the fan turns slowly, and the air particles around it vibrate slowly. A low-frequency sound wave is produced, and you hear a low-pitched hum. As the fan whirls faster, the air particles vibrate faster. A higher-frequency sound wave results, and you hear a higher-pitched hum. Amplitude, Loudness, and Intensity A sound wave is a disturbance of the particles in the medium, or sound carrier. As each particle is disturbed, it moves back and forth a tiny distance.

The distance each particle travels from its undisturbed position is called the amplitude of the sound wave. In general, the greater the amplitude, the louder the sound. Pluck a stretched rubber band gently. It vibrates gently and strikes the air particles gently. Each air particle is made to move only a short distance -- that is, the sound wave has a small amplitude. You hear a soft, gentle sound. If you pluck the rubber band with a greater force, each air particle moves a greater distance. The sound wave has a greater amplitude, and you hear a louder sound. Loudness depends on each person's sense of hearing. Loudness may not be the same for two people sitting next to each other and listening to the same sound. One person may have a keener sense of hearing than the other. Therefore scientists cannot measure the loudness of a sound because it may be different for each person who hears it. Scientists can, however, measure the intensity of a sound. Intensity refers to the amount of energy in the sound, no matter who hears it. Think back to how you made a rubber band produce sounds of different loudness. When you plucked the rubber band gently, it produced a soft sound.

When the rubber band was plucked with greater force, you put more energy into the sound. That produced a sound wave of greater amplitude. The sound had a greater intensity and was louder to your ears. It would sound louder to most people with normal hearing. Measuring Intensity of Sound Scientists can measure the intensity of a sound by measuring the amplitude of the sound wave. This intensity is called the sound level. It is measured in units called decibels. One decibel is the smallest change in intensity that can be detected by most people. Here are the decibel units of some sample sounds: Faintest whisper most people can hear: 0 decibels. Whisper loud enough to be heard 5 feet (1.5 meters) away: 20 decibels. Ordinary conversation at a distance of 3 feet (1 meter): 65 decibels. Traffic on a busy street: 80 decibels. Noisiest spot at Niagara Falls: 95 decibels. A loud, amplified rock band: 100 decibels. A sound level of 120 or more decibels is painful to the ears of most people.

If you stood near an 11-inch (280-millimeter) heavy military gun that was being fired, the sound would be very painful to your ears. The sound level is about 230 decibels. For this reason, soldiers usually use ear protectors when they are near a big gun or cannon that is about to be fired. Both the intensity and the loudness of a sound decrease rapidly as you move away from the sound source. Sound waves have energy. As the waves move away from their source, the energy spreads out over an ever increasing space. At a great distance from the source of the sound, the energy is so spread out that any one spot receives only a tiny bit of energy. If you are standing a great distance from the source of the sound, only a very small amount of the energy reaches your ears, and you hear only a tiny sound. Behavior of Sound Sound waves can behave in ways that produce a number of different special effects. Some of these effects may cause the sound to be repeated or to be louder or blurred. If you have ever heard an echo, you have heard one of these effects. Echoes Suppose you shout across a canyon.

Part of the sound wave you have produced with your voice strikes the opposite wall of the canyon and bounces back. The sound is reflected from the canyon wall. You hear the sound of your voice returning. This returned sound is what is known as an echo. In order for you to detect a definite echo, the reflecting surface must be at least 55 feet (16.75 meters) away from you. The reason for this has to do with the speed of sound and the way we hear. When you shout, at first you hear your own voice. This sensation of sound lasts in your mind for about 1/10 second. Thus, if you are to hear a separate echo, the sound must take at least 1/10 second to cross the canyon and return. This is the minimum time that must pass before the sensation of the original shout has died out in your mind. In this minimum time of 1/10 second, sound travels about 110 feet (33.5 meters). If the canyon wall is 55 feet away, the sound will make a round trip of about 110 feet.

You will be able to tell the difference between the shout and its echo, but you cannot do this if the wall is any closer. If the wall is more than 55 feet away, you may hear a clear, distinct echo of your shout as the sound wave bounces back. Reverberation The reflection of sound sometimes causes an effect called reverberation. In some large auditoriums and gymnasiums, for example, sounds are reflected many times from the floor, walls, and ceiling. When the sound waves bounce around between these surfaces, the original sound is repeated many times. But no distinct, separate echoes are heard. Each echo has mixed with the others that are reflected from various parts of the room. This overall effect is called reverberation. Reverberation is measured by making a sharp sound, like the crack of a pistol, and measuring the time it takes for all the echoes and re-echoes to die out. This time is called the time of reverberation. If the time of reverberation in an auditorium is too long, speech sounds run together and it is hard to understand the speaker. If it is too short, the room sounds "dead" and not enough sound reaches the listeners.

A certain amount of reverberation is necessary in auditoriums, especially for the enjoyment of music. Acoustics Acoustics is the science that is concerned with the behavior of sound. It also means the way sounds echo or reverberate in an auditorium or concert hall. People say, "The acoustics of this concert hall are very good," or "This auditorium has poor acoustics." Engineers and architects must take into account the problems of echoes and reverberation when they plan auditoriums, concert halls, and other public places. They must design the room so that there are no echoes. But they do want some reverberation, which is necessary for good acoustics. The amount of reverberation can be controlled by placing special materials on the floor, walls, and ceiling of a room. Porous materials, such as wool, felt, and cork, are good sound absorbers. When sound waves enter the pores of these materials, the vibrations of the particles of the air are stopped. Sound-reflecting materials can be positioned in a room, especially auditoriums and concert halls, to help create desirable reverberations.

Sound-absorbing tiles, called acoustical tiles, line the ceilings of many restaurants, reducing the clatter of dishes and the noise made by the voices of many people. Soft draperies, carpets, and wall hangings help to reduce reverberation in auditoriums and concert halls. The presence of people in an auditorium also affects the acoustics. The clothing and bodies of people absorb some of the sound waves. In an empty auditorium, listen to the way your voice sounds. It sounds different than when most of the seats are filled. For special experiments, scientists often use a room called an anechoic chamber. Anechoic means without echoes. The walls, ceiling, and floor of such a room are constructed of sound-absorbing materials built into compartments of various sizes. Different size compartments trap sounds of different wavelengths. The result is a room in which almost no sound is reflected. Such a chamber reveals the true sound quality of instruments and voices. The anechoic chamber is useful for making very accurate measurements of sound and for testing microphones and other devices. Natural Frequency When you strike an object and make it vibrate, the force of your blow determines the intensity of the sound wave.

But the frequency of the sound wave does not depend on the force of your blow. The frequency depends on the size, shape, and material of the object. Every vibrating object has its own frequency of vibration. This is called its natural frequency. For example, the metal of a bell vibrates at its own natural frequency no matter how hard the clapper inside the bell strikes the metal. A tuning fork -- a two-pronged "fork" used by musicians and singers to get a pure tone -- is constructed to vibrate at a certain frequency. If it is constructed to vibrate at 440 cycles per second, this is its natural frequency. Sympathetic Vibrations Suppose two neighboring objects have the same natural frequency. One of the objects is made to vibrate. Its sound waves reach the second object and strike against it. Because the frequency of the sound waves is the same as the natural frequency of the second object, the waves start the second object vibrating. Both produce the same note. The second object is in sympathetic vibration with the first. If you have two tuning forks of the same frequency, you can hear sympathetic vibrations. Hold each tuning fork at its stem and make one vibrate by striking it against the edge of a table.

After the tuning fork has sounded for a few seconds, stop its vibration by moving your hand up to the prongs. You can then hear the same sound coming from the second fork, because it is in sympathetic vibration with the first. Sympathetic vibrations can cause an avalanche. In a mountainous region, a large mass of rocks and snow may be piled loosely on a steep slope. The sound waves from a human voice or from some other source may cause some rock and snow particles to vibrate sympathetically. This small motion is sometimes enough to start the large mass of loosely piled material tumbling down the mountainside. Resonance An effect called resonance occurs when a number of small repeated pushes cause a large vibration. Resonance increases the amplitude, and thus the intensity, of a sound. For an example of how resonance occurs, think of a child sitting on a swing. You give the swing a push, and it starts moving in an arc. If you continue giving a small push each time the swing is at its highest point, the swing soon moves through a wide arc. Although each push is small, you have timed the pushes so that their effect adds up, producing the large back-and-forth motion.

The swing has a natural frequency at which it moves back and forth. Your pushes, if timed to this natural frequency, add up to make the wider arc. If you push at a faster or a slower rate, the swing comes almost to a stop. Resonance takes place only when you push at the natural frequency of the swing. The sounds made by wind instruments in music are reinforced (strengthened) by resonance. The resonance comes from a column of air inside the instrument. Thus, the sound of a trumpet or a bugle is reinforced by resonance of the air inside the instrument. Forced Vibration Sometimes an object can be made to vibrate at a frequency other than its natural frequency. Its back-and-forth motion is then called forced vibration. A common example of forced vibration is in the eardrum. When sound waves strike the eardrum, they cause it to vibrate at the frequency of the received sound wave. You can use a tuning fork to produce forced vibrations. Strike the tuning fork to make the prongs vibrate, and listen to the sound for a moment. The sound is quite soft. Now place the stem of the fork against the top of a table. The sound at once becomes much louder.

The vibrations of the fork force the tabletop to vibrate at the same frequency as the tuning fork. The table's larger size sets more air in motion than the tuning fork alone does, and you hear a louder sound. Many musical instruments are designed so that forced vibrations amplify their sounds. In a violin, for example, vibrating strings force the wood to vibrate -- thereby increasing the sound made by the strings. The forced vibration of the sounding board of a piano amplifies the sound made when a note is struck. Interference and Beats If two musicians play a tone of the same frequency, the two sounds combine. The result is a louder sound. But if two tones of only slightly different frequencies are played together, a series of loud and soft sounds is heard. This effect -- of loud and soft sounds occurring one after the other -- is called beats. Beats are caused by the interference of two sound waves. When the compressions and expansions of two different sound waves reach your ear at the same time, they reinforce each other. The sound becomes louder. But when the compressions of one wave reach your ear at the same time as the expansions of the other, they interfere with each other. For a moment, almost no sound is heard.

You can hear beats by using two tuning forks that have the same frequency. Change the frequency of one fork slightly by putting a thin rubber band around one of its prongs. Then strike the forks. You will hear the repeated loudness and softness as regular pulses. These are beats. It is annoying to hear more than four or five beats per second, and beats can spoil music. Musicians in an orchestra tune their instruments in unison so that all will produce the same pitch, or frequency. This eliminates the possibility of beats on identical pitches, and the instruments are said to be "in tune." However, this applies only to notes of the same name, such as all "A" notes. In modern, or "tempered," tuning, all other intervals are very slightly out of tune and actually generate beats. This system is used to equalize the various intervals between the half-steps of the musical scale. These beats, described as "shimmers," give the characteristic richness to groups of notes played together as chords. Doppler Effect Have you ever noticed the change in pitch of a train whistle or automobile horn as the vehicle approaches and then passes you? As the sound source comes toward you, the pitch sounds higher.

When the source is nearest you, you hear the sound at its normal pitch. Then as the source of sound moves away from you the pitch drops. This change in pitch is called the Doppler effect, after Christian Doppler, a 19th-century Austrian scientist. An automobile horn, like a train whistle, always vibrates at the same frequency. The apparent change in frequency in the sound wave is caused by the motion of the automobile. When the vehicle approaches you, it is moving in the same direction as the sound waves coming from the horn. The sound waves are pushed closer together. Your ear receives more vibrations each second than the horn actually sends out. This gives the effect of a higher frequency, and you hear a higher-pitched sound. After the car passes you, the opposite effect takes place. The horn still sends out the same sound waves but is moving away from the waves. The sound waves are spaced farther apart, and your ear receives fewer vibrations a second than the horn sends out. This gives the effect of a lower frequency, and you hear a lower-pitched sound. The Doppler effect occurs whenever distance is changing between the source of sound and the listener.

For example, you will hear a change in pitch if you are driving past a parked car whose horn is blowing. The amount of change depends on how fast the sound source or the listener is moving. If the speed of the sound source or the listener is great, the change in pitch is great. If the speed is slow, the change in pitch is less. Musical Sounds What determines whether a vibrating object will produce a noise, a pleasant sound, or a musical tone? When compressions and expansions of a sound wave follow each other in an even, regular order, the sound is pleasant. People usually call it music. When the compressions and expansions follow each other in an uneven, irregular pattern, the sound we hear is a noise. Hammering on a steel tank produces a loud noise. The metal of the tank vibrates unevenly. It sends out compressions and expansions that do not follow each other in a regular pattern. A note played on the piano is a musical tone because the strings that produce the tone send out a regular pattern of compressions and expansions. A skilled pianist plays enjoyable music by producing a series of regular vibrations that blend well together. A child banging on the piano keyboard produces unpleasant noises.

An exception to this rule is the sound of percussion instruments. Cymbals and most drums produce random combinations of frequencies, usually considered noises. However, when the bursts of noise are skillfully played in organized combinations, noise is converted into music. Fundamentals and Overtones A tuning fork is a specially constructed device that vibrates at only one particular frequency. However, most vibrating objects vibrate at several frequencies at the same time. To understand how an object can vibrate at several frequencies at once, here's an example. A string is stretched between two posts. When plucked in the middle, the string vibrates as a whole . When plucked halfway between the middle and either end, the string again vibrates as a whole. But each half of the string vibrates separately as well. Depending on where it is plucked, the string can be made to vibrate as a whole and also in three or more separate parts. The parts vibrate at a different frequency from the vibration of the whole string. The tone produced by the vibration of the string as a whole is called the fundamental tone of the string. The tones produced by the vibrations of the parts are called overtones or harmonics.

Musical instruments are designed to produce both a fundamental tone and overtones. In stringed instruments the strings vibrate as a whole and in sections, producing fundamentals and overtones. When a wind or woodwind instrument is played, the column of air in the instrument vibrates as a whole and in sections, producing both fundamentals and overtones. The metal or skin of percussion instruments produces overtones as well as fundamentals. The different combinations of fundamentals and overtones give the sound of each musical instrument its special quality. Two instruments playing the same note sound different because each instrument produces different overtones. You might say that each has its own "recipe" of fundamentals and overtones. Like different musical instruments, each human voice has its own sound-wave recipe. The vocal cords in each person's throat vibrate with frequencies and overtones that depend on the size, thickness, length, and tension of the cords. The amount and force of the air passing between the cords as a person speaks also helps determine the kind of sound waves produced. This makes it possible for you to identify many different voices, just as you can tell the difference between the sound of a piano and a flute.

Ultrasonics The human ear generally cannot hear sound waves whose frequency is higher than about 20,000 hertz. Such high-frequency waves are called ultrasound or ultrasonic waves. ("Ultra" means "beyond," and "sonic" means "sound.") Ultrasonic waves may have frequencies as high as millions of hertz. They can never be heard by a person. Ultrasonic waves can be produced in several ways. One method requires a thin slice of a quartz crystal. (Quartz is a common mineral. It has the unusual property of responding to a changing, or alternating, electric current by vibrating in step with the current.) High-frequency alternating current is fed into the quartz. The crystal vibrates in step with the current and produces an ultrasonic wave. Another method makes use of an electromagnet with a special core usually made of the metal nickel. When a high-frequency alternating current is passed through the electromagnet, the nickel core expands and contracts slightly. This sets up ultrasonic vibrations in the surrounding medium. Uses of Ultrasound Ultrasonic waves have a very short wavelength. (Short wavelengths have a high frequency.) Because of this, many waves -- and their energy -- are packed into a small space.

Scientists have found ways of putting this concentrated energy to use in scientific research, in industry, and in medicine. Ordinary sound waves move outward in all directions. But the very short wavelengths of ultrasonic waves make it possible to beam ultrasound in a straight line. Scientists have used straight ultrasonic beams in systems called sonar to locate submarines, schools of fish, and other solid objects underwater. In sonar systems, sound reflections, or echoes, are used to detect the solid objects. A pulse, or burst, of ultrasonic waves is sent out from an instrument on a ship. The ultrasonic waves travel underwater in a straight line until they strike a solid object. The waves are then reflected and received at an instrument on board the ship. To measure the depth of the ocean a type of sonar system called a fathometer is used. An operator aboard a ship beams an ultrasonic wave down into the water. The time it takes for the echo of the sound wave to return to the instrument on the ship gives the operator the depth of the water. Ultrasound is used in industry to detect flaws hidden deep inside metals or other solid materials. An ultrasonic wave is beamed into the part to be tested.

The way in which the beam is echoed back into a receiving device tells whether there is a defect in the material and where the defect is. An ultrasonic wave passed through a liquid makes the liquid vibrate very fast. The vibration can shake up paint to mix it thoroughly, homogenize milk by breaking up the fat particles, and clean tools, machine parts, dishes, and other solid objects. Ultrasound is used experimentally to break down the cell walls of bacteria. In medicine it is used to help detect brain injuries, to trace blood flow through various parts of the body, and for certain kinds of surgery. In dentistry, a dental drill controlled by ultrasonic vibrations can neatly penetrate or cut into tooth enamel, creating very little friction or heat in the gum tissue. Certain ultrasound frequencies can be used to penetrate muscles and create heat, thereby stimulating circulation. An interesting application of ultrasound is in ultrasonic holography, a method of producing three-dimensional pictures with ultrasound. This is possible because ultrasound waves can easily travel through soft body tissues. When they reach denser layers of bone and cartilage, the waves are reflected back. Computers store and compare the images to generate three-dimensional pictures.

With this technique it is even possible to safely detect the form of an unborn baby inside its mother's womb. Ultrasonics and Animals Some animals can hear ultrasonic sounds. For example, dogs can hear the high-pitched sound of a dog whistle that most people cannot hear. There are some animals that have built-in sonar systems. Dolphins have a sort of sonar system that they use to locate fish and obstacles in the water. Bats, too, have a sonar system. They find their way about in darkness by uttering ultrasonic squeaks that are echoed back to them from solid objects like walls. Because animal sonar is so efficient, scientists are studying these animals to learn their "ultrasonic secrets." AES E-Library » Loudspeakers and Rooms for Sound Reproduction—A Scientific Review AES E-Library Loudspeakers and Rooms for Sound Reproduction—A Scientific Review The physical measures by which acousticians evaluate the performance of rooms have evolved in large performance spaces—concert halls. They rely on assumptions that become progressively less valid as spaces get smaller and more acoustically absorptive.

In listening rooms the loudspeakers and the rooms interact differently below and above a transition region around 300 Hz, similar to the Schroeder frequency in large rooms. Above this transition we need to understand our reactions to reflected sounds; below it the modal behavior of the space is the dominant factor. A review of the scientific literature reveals that natural reflections in small rooms are at levels where they are perceptible, and their subjectively judged effects range from neutral to positive. At low frequencies the long-standing problem of room resonances can be alleviated substantially through the use of multiple subwoofers, thereby providing similarly good bass to several listeners in a room. A provocative observation has to do with human adaptation to the complexities of reflective rooms, and the extent to which it allows us to localize sounds correctly in direction and distance, and to hear much of the true timbral nature of sound sources. In the case of loudspeakers, an analysis of comprehensive anechoic data is found to be sufficient to provide a good prediction of sound quality, above the low-bass frequencies, as subjectively judged in a normal room. Although the interactions of loudspeakers and listeners in small rooms are becoming clearer, there are still gaps in our understanding.

A number of these are identified and are good opportunities for future research. Author: Toole, Floyd E. Farina et. al. 6 Ambiophonic Principles for the Recording and Reproduction of Surround Sound for Music - Part 6 Angelo Farina, Ralph Glasgal, Enrico Armelloni, Anders Torger 4. VIRTUAL AMBISONICS SURROUND In most cases, the stereo recording provided on commercial CDs is obtained with good spatial information related to the position of the sound source and their direct soundfield, but very little "ambience" information is encoded in the source material. In fact, in traditional stereo reproduction, it is quite annoying to hear, superimposed on the direct sound coming from front, the discrete reflections and reverb which should arrive from the sides, above and behind the listener in a good concert hall. So sound engineers tend to place their microphones very close to the sound source, and to shield them from "annoying" reflections and reverb coming from the back of the room. A realistic replication of a music listening experience requires that the whole three-dimensional sound space be reconstructed in the reproduction space.

This space can be obtained with the Ambisonics technique, although with very limited definition or localization of the direct sound source location. The basis for the Ambisonics method is the description of the spatial properties of the sound field by means of the B-format signal: it is a 4-channel signal, obtained from the sound pressure captured by an omnidirectional microphone (called W) and by the three first-order spherical harmonics of the sound pressure field, corresponding to the response of three figure-of-eight microphones aligned with the axes of a 3D Cartesian reference system (called X, Y and Z). It must be clear here that the inventors of the Ambisonics technique [ 2 , 18 ] did not know anything about modern energy analysis of sound fields [ 19 , 20 , 3 ], although their very original theories were in a certain sense anticipatory of these subsequent modern developments. In practice, the spherical harmonics of the sound pressure field were easily confused with the Cartesian components of the particle velocity vector, as they are coincident, following the Eulerï¿½s equation, in the case of plane, progressive waves.

In a generic sound field, though, pressure and velocity exhibit significant phase and gain mismatch, and these quantities should not be confused at present. This is not a problem for "synthetic" B-format signals, obtained by panning a single mono track with proper gains, computed simply by using the values of the cosines of the angles between the intended direction of the sound with the three Cartesian axes; it is though a great problem for signals captured from a "true" sound intensity probe, which captures the real physical quantities (pressure and particle velocity components). The widely employed Soundfield microphone has a strange, intermediate behavior: it is close to a true sound intensity probe when the wavefront has little curvature, but deviates from it in the cases of strong curvature. However, it does not achieve the theoretical behavior of a pure cosineweighted pressure microphone. In a generic, reactive sound field, deriving accurate three-dimensional information from measurements done with a Soundfield microphone is not an easy task: although not corresponding to the definition of a B-format signal, it is simpler to process true pressurevelocity recordings obtained with a sound intensity probe, because the relationship between these physical quantities is mathematically known.

But the signals coming out from a Soundfield mike are not so easily interpreted, because the published theory describing its behavior [ 18 ] is valid only with plane, progressive waves. The process described here for creating a threedimensional soundfield surrounding the listener is called "Virtual Ambisonics" because it is not based on native B-format recordings, but on B-format signals reconstructed by convolution of the original stereo recordings with B-format impulse responses. As the B-format signal needs to be "decoded" for feeding a three dimensional loudspeaker array, and being that this decoding process is implementable as another convolution with a set of proper decoding filters [ 21 ], it is possible to connect the two convolutions into a single one: a set of threedimensional IRs can be derived, which can be used directly as filters, applied by convolution to the original stereo recording, and then use them to drive the loudspeakers in the reproduction array.

This combination can be seen as the synthesis of the impulse response obtainable by a virtual microphone, characterized by strong directivity, and pointing in the direction, relative to the listener, of the specific loudspeaker being considered, when the sound field is produced, in the original concert hall, by a sound source located on the stage. As this surround methodology need not be as accurate as the binaural one, just two positions of the sound source can be considered on the stage, as shown in fig. 2 , corresponding to a generic "L" and "R" positions. Thus, for each loudspeaker in the reproduction space, two "3D Impulse Responses" are defined, named sL,3D and sR,3D respectively: the speaker feed is obtained as the sum of the results of the convolution of the two original signals with these filters. 4.1 Measurement of 3D Impulse Responses in theatres and concert halls M.Gerzon [22] first proposed to start a systematic collection of 3D impulse responses measured in ancient theatres and concert halls, for assessing their acoustical behavior and preserving it for the posterity.

His proposal found sympathetic response only very recently, with the publication of the "Charta of Ferrara" [23] and the birth of an international group of researchers who agreed on the experimental methodology for collecting these measurements [24] . Only a small number of theatres have yielded a complete three-dimensional impulse response characterization up till now. Among them, we employed for the present work the IRs measured in three Italian theatres: - Gran Teatro La Scala in Milan - Teatro Comunale in Ferrara - Teatro Verdi in Trieste The following table reports the main technical data regarding the measurement technique employed in each of these three rooms: Room 44100 Hz The measured three-dimensional impulse responses of these three theatres can be downloaded from: HTTP://pcangelo.eng.unipr.it/public/AES19. Figs. 18 , 19 and 20 show, for each theatre, a schematic plan of the room with the positions of the sound sources and of the microphone. Fig. 18 Plan of La Scala in Milan Fig. 19 Plan of Teatro Comunale in Ferrara Fig. 20 Plan of T.

Verdi in Trieste It must be noted that La Scala was in opera configuration, but the other two were in concert configuration, with a reflective orchestra shell mounted on the stage. Some details are required regarding the three different kinds of microphones employed in these three rooms. In La Scala, a "virtual" 7-omnis microphonic array was employed [ 25 ], obtained by moving a single omnidirectional pressure microphone (B&K type 4166) into 7 close positions, and measuring a separate impulse response at each of them. Fig. 21 7-omnis microphonic array The geometry of the array is shown in fig. 21 .

From these 7 IRs, it is easy to extract 4 processed IRs, the first being simply the pressure response in the central microphone, and the other three being the particle velocity components along the three axes computed by means of the classic Eulerï¿½s relationship, with the finite differences approximation commonly employed in sound intensity analyzers: The same approach is employed for deriving pressure and velocity components from the 6 IRs measured at the Teatro Comunale in Ferrara by means of the three-dimensional sound intensity probe B&K type WA0447, which is shown in fig. 22 . Fig. 22 B&K type WA0447 sound intensity probe Fig. 23 The Soundfield MK-V microphone In this case there is no central pressure microphone, so the pressure signal has to be derived simply as the arithmetic mean of the 6 signals measured around the central virtual position. It must be noted, however, that this fact introduces some minor artifacts, as the 6 signals summed together are not perfectly coherent, and this introduces some high-frequency amplitude fluctuation, and a certain degree of smearing in the time domain. In the third case, a standard Soundfield microphone was employed, as shown in fig. 23 .

This unit is equipped with a special electronic processor, which extracts the 4 signals labeled W, X, Y and Z. As discussed earlier, in very reactive sound fields (close to the sound source, in a small, highly reverberant room) none of these three microphonic probes produces exactly the theoretical B-format signal (spherical harmonics of 0th and 1st order of the pressure field). But in the halls studied here, the sound source was very far away and the room was quite dry compared to its huge size (Italian theatres are known for their low reverberation times compared to north-European concert halls of the same size); this is demonstrated by fig. 24 , which shows the measured reverberation times in the three theatres. Fig. 24 Reverberation time of the three theatres Consequently, it can be assumed that in these three cases the measured pressure-velocity 4-channelIRs are a reasonable approximation of the theoretical Bformat signals, and thus they can be processed with classic Ambisonics-like math for extracting the responses of virtual microphones, with proper directivity patterns, and pointing in any desired direction.

Everything You Always Wanted to Know About Concert Hall Acoustics Everything You Always Wanted to Know About Concert Hall Acoustics (but you couldn't afford the consulting fee)

Question: The behavior of sound in rooms and concert halls is a separate science. what is its name?
Answer: Acoustics