Physical Science Lesson 3 Notes

Page 1

Sound is a form of energy that’s produced when waves are created by certain kinds of vibrations. A vibration is defined as the periodic motion of particles. Sound can also refer to the sensations produced when these waves reach the eardrum. To produce sound, a vibration of some kind must occur. A point in a body vibrates when the point moves back and forth along a straight line, in the same manner, at regular intervals of time. A sound lasts as long as the vibration continues. As soon as the vibration stops, the sound ceases. Every characteristic of a sound depends on some physical property of the vibration or on some characteristic of the wave motion that produces it.

When a small stone is dropped into a pond of still water, circular ripples, or waves, spread out over the surface of the water. These waves consist of a series of crests separated by troughs. The crest is the highest point of a wave, and the trough is the lowest point of a wave. When the spring is pulled, each coil is stretched. When the spring is released, each coil tends to return to its original position. The coils tend to move past their normal (resting) positions because of inertia. As a result, adjacent coils are compressed and the wave progresses along the coil. Reflections from the point of suspension then travel in the opposite direction, and the disturbances in the coil continue until friction damps out the vibrations.

Each molecule of water remains at the same horizontal distance from a certain reference point and simply moves up and down along a vertical line. However, the visible displacement of any portion of the string is in a vertical direction, because each point on the string moves up and down alternately.

The direction that a body’s molecules move determines the wave class. If the molecules of a body move at right angles to the direction the wave travels in, the wave is called a transverse wave. If the molecules of a body move parallel to the direction the wave travels in, the wave is called a longitudinal wave. A wave produced on the surface of a body of water or along a string is a transverse wave. Stretching and releasing a coiled spring produces a longitudinal wave. Sound waves in the air are also longitudinal waves.

To be transmitted, a wave motion needs a substance, or a medium, through which to travel. As a transverse wave travels through a body, each molecule in the body moves back and forth along a straight line at right angles to the direction in which the wave travels. During a complete cycle, a molecule moves from its normal position to its highest position, then moves back through its normal position to its lowest position, and finally returns to its normal position. This series of movements is repeated in each cycle of the wave. As the cycle progresses, these same molecules will occupy different positions. Molecules that are in phase occupy similar positions in a vibrating body, move in the same direction, and undergo similar changes in position in the same amount of time.

When a molecule is subjected to a longitudinal wave, the molecule moves parallel to the direction in which the wave travels. A molecule subjected to the action of a longitudinal wave first travels in the same direction as the wave, from its normal position to its most distant point on one side of its normal position. Then, the molecule changes direction and moves back through its normal position to a corresponding point on the opposite side of its normal position. Finally, the molecule returns to its normal position, thus completing one cycle. Most of the molecules, however, are in different phases of their vibrations. Thus, at a certain instant, the molecules experience compression; that is, they’re crowded closely together in some portions of the body. At the same instant, the molecules in other portions of the body are in regions of rarefaction; that is, they’re spread farther apart.

The shape of the curve that represents the wave motion is called the sine curve. A wave’s characteristics can be described by numerical quantities. The maximum distance a molecule has been moved from its normal position by the wave or vibration is its amplitude.

For a transverse wave, the amplitude is measured at right angles to the direction of travel of the wave, and is the distance from the horizontal reference line to a molecule at a crest or trough of the wave. For a longitudinal wave, the amplitude is measured parallel to the direction of travel of the wave It represents the maximum distance the molecule has moved from its normal position. Small open circles represent the normal positions of the molecules, and the solid circles represent the positions of the molecules at some instant after the waves have been produced.

The length of a wave, called the wavelength, is the distance from one molecule in a body to the nearest molecule that occupies a similar position and moves in the same direction. For either a transverse wave or a longitudinal wave, the wavelength is measured parallel to the direction of travel of the wave. The wavelength of a longitudinal wave may be represented as the distance between the two nearest molecules that didn’t vibrate, or that are at maximum compression or as the distance between the two nearest molecules that are at maximum rarefaction.

The period of a wave is the time required for the wave to travel a distance equal to its wavelength, or one complete cycle. The period is measured in seconds. The number of waves that pass a particular point in the body in a unit of time is called the frequency. When a body is vibrating, the frequency is the number of vibrations completed in a second.

The frequency of a series of waves can be calculated by dividing 1 by the period. This relationship can be expressed by the following formula:

f = 1⁄T

In this formula, f stands for the frequency, which is the number of cycles (complete repetitions per second), and is expressed in hertz. (The symbol for hertz is Hz.) T stands for the time period in seconds.

When working with fractions, it’s often easier to perform calculations by rewriting the equation as:

f = 1 ÷ 1⁄6

rather than

f = 1⁄1⁄6

When completing a division problem with fractions, remember to keep the first number as is, change the division sign to a multiplication sign, and take the reciprocal of the second number.

f = 1 × 6⁄1

Here the first number (1) is kept as is, the sign is changed, and the fraction is changed from 1⁄6 to 6⁄1. The whole number 1 can also be written as 1⁄1.

f = 6⁄1

f = 6 Hz

This means that six full cycles of the wave are completed in one second.

The velocity of a wave is the total distance traveled by the wave in a unit of time. A wave travels a distance of one wavelength in the time equal to the period of the wave. Consequently, the distance traveled in a unit of time can be found by dividing the wavelength by the period.

V=I/T

In this formula, v stands for velocity in meters per second and represents the distance traveled by the wave in a specified period of time. The variable I stands for wavelength in meters, and T stands for the time period in seconds.

if a particular wave has a period of 1⁄6 second, the frequency of the wave can be calculated as follows:

f = 1⁄T

f = 1⁄1⁄6

f = 1 ÷ 1⁄6

f = 1 × 6⁄1

f = 1⁄1 × 6⁄1

f = 6⁄1

f = 6 Hz

if a wave has a period of 0.005 seconds, its frequency can be calculated as shown.

f=1Tf=1T

f=10.005f=10.005

f = 200 Hz

if a wave has a wavelength of 6 meters and a period of 0.005 seconds, the velocity of the wave can be calculated as follows

V=I/T

V=6/0.005

V = 1,200 m/s

Sounds are the result of vibrations traveling through a medium. The medium determines the sound’s velocity, and the number of vibrations determines the sound’s amplitude.

The crest is the highest point of a wave; the trough is the lowest point of a wave.

In a transverse wave, the molecules of a body move at right angles to the direction the wave travels. In a longitudinal wave, the molecules of a body move parallel to the direction the wave travels.

Amplitude is the maximum distance a molecule has been moved from its normal position by the wave or vibration.

Wavelength is the measurement between the two nearest molecules of a wave that are in phase with each other.

The period of a wave is the time it takes for the wave to travel a distance equal to its wavelength, one cycle.

Frequency is the number of vibrations per second.

Velocity is the distance traveled by the wave in a unit of time.

Page 2

A sound always results when a body vibrates in a medium that transmits the energy given off by the body. the vibrating body must produce waves with a suitable amplitude and frequency. the vibrating body must produce waves with a suitable amplitude and frequency for a person to hear the sound. The waves must be transmitted through a suitable medium and the person’s ears must be sufficiently sensitive to respond to the sound.

Vibration is the primary source of sound. Resonant (echoing) cavities have some influence on the quality of the sounds heard.

As a sound wave travels through a medium, the wave uses up energy. As a result, the amplitude of the wave becomes smaller and smaller. The greater the force applied to a body to cause vibration, the greater the amplitude of the resulting sound wave at the beginning.

For a person to hear a sound, the frequency of the sound wave must fall within certain limits. The lowest frequency that can be detected by a person with normal hearing ability is about 20 Hz, while the upper-frequency limit is usually between 15,000 Hz and 20,000 Hz. The ability to hear the higher frequencies decreases with age. Infrasonic vibrations have such a low frequency that the waves produced by them can’t be heard by a human being. if the amplitude is great enough, the vibrations may be felt. A vibration that has a frequency greater than the upper limit for normal hearing is called an ultrasonic vibration. In industry, ultrasonic vibrations are used to detect flaws in the interior of large steel castings and to remove grease or other foreign material from the surfaces of delicate and complicated machine parts.

Sound waves can travel through solid, liquid, or gas. But sound doesn’t travel through a vacuum. (A vacuum is a completely empty space that doesn’t contain any matter—not even air.) In general, a solid conducts sound better than a liquid, and a liquid conducts sound better than a gas. A solid body is elastic if, after the body is stretched or compressed by a relatively large force, its molecules quickly return to the original positions when the force is removed. A solid body is inelastic if its molecules don’t return to their original positions in the same circumstances.

Some building materials are designed so they don’t conduct sound efficiently. Such materials, called sound insulators, are often used in walls and floors to prevent sound transmission between rooms.

In the transmission of sound waves through air, a vibrating body causes a disturbance in the air surrounding the body. The molecules of the air move back and forth much as the segments of a spring move up and down, producing longitudinal waves.

Waves can be seen on such instruments as a seismograph, which records a vibration of the earth as a wavy line, and an oscilloscope, which gives a visual impression of alternating current in the form of a sine curve. For practical purposes, it’s more convenient to represent a sound wave graphically as a transverse wave rather than a longitudinal wave, because it’s easier to indicate the amplitude and the wavelength for a transverse wave.

Sound waves travel outward from a vibrating body in all directions. You may think of these waves as a series of expanding spheres, all having a common center at the vibrating body. The regions where the circles or arcs are spaced closer together indicate compressions, and the regions where these curved lines are spaced farther apart indicate rarefactions. The positions of the compressions and rarefactions of a longitudinal wave are true for only an instant of time. When these compressions and rarefactions pass by the ear, the ear will sense them as sound.

When sound waves are transmitted through a medium from a vibrating body, any curved surface passing through all the points of maximum compression at a uniform distance from the body is known as a wave front. A wave front has the shape of a surface of a sphere. For a particularly strong sound, such as that which might result from an explosion, the passage of the initial front might be felt as well as heard. While the wave front is different from the usual sound wave, it travels in the same way.

The intensity of a sound wave is the amount of energy passing through a unit area of the wave front in a unit of time. The amount of energy in a sound wave produced by a vibrating body depends primarily on the amplitude of the wave. As a sound wave travels through the medium surrounding a vibrating body, the total amount of energy passing through a wave front in a unit of time decreases slowly, but the area covered by the wave front increases rapidly as the wave advances. Therefore, the amount of energy passing through a unit area of the wave front in a unit of time must decrease rapidly. the intensity of a sound wave decreases rapidly as the distance from the vibrating body increases. Expressed mathematically, the intensity is inversely proportional to the square of the distance. Another factor that affects the intensity of a sound wave is the density of the medium. The intensity of a sound wave having a certain amplitude is greater if the medium has a high density than if the medium has a low density. Since the density of the air decreases as the distance above the earth’s surface increases, sound waves at high altitudes have a lower intensity than similar waves at the earth’s surface.

the human ear has three main compartments The outer ear collects the sound waves and leads them to the middle ear. The middle ear contains the eardrum, an elastic membrane that vibrates whenever sound waves strike it. When the sound waves reach the middle ear, the compressions and rarefactions of the waves cause the eardrum to vibrate with the same frequency as the source of the sound waves. These vibrations are passed along to the inner ear, which is filled with a fluid. As the vibrations spread out in this fluid, they’re transmitted to the delicate fibers of the auditory nerve that extend into the fluid. The impulses sent to the brain through the auditory nerve are interpreted by the brain as the sensation of sound. When you hear a sound, you can usually determine the general direction the sound is coming from; if the sound comes from your right side, the sound waves reach your right ear first. The difference between the phases of the waves reaching your two ears is detected by the ears and interpreted by the brain. If the sound comes from directly above, directly below, directly ahead, or directly behind, the sound waves reach both ears in phase. This fact can also be interpreted by the brain.

When a sound wave strikes a smooth, hard surface, the wave is reflected (turned back) by the surface. Sound waves can be reflected by the walls and ceiling of a room or by the steep side of a rocky cliff. incident wave (the wave that strikes the reflecting wall) and the normal line (the line perpendicular to the reflecting wall) is called the angle of incidence.

the frequency has nothing to do with the loudness of a sound

the change in intensity can be calculated as 1/R^2 where R is the distance from the source

waves always reflect off of a surface at the same angle that they hit the surface. the original angle is called the angle of incidence. the angle at which the wave bounces back is called the angle of reflection.

The velocity of a sound wave is extremely small compared to the velocity of light waves. In physics, elasticity is the tendency of a material to keep its shape when a force is applied to it and to return to its original shape when that force is removed. A dense material like steel maintains its shape better than a softer material like rubber; it has a higher elasticity than rubber. Sound travels faster through materials with higher elasticity.

As you previously learned, at 0°C (32°F), the velocity of a sound wave traveling through the air is approximately 331 meters per second. The velocity of a sound wave in air increases by approximately 0.6 meters per second for each degree Celsius increase in temperature.

The following formula can be used to determine velocity at a certain temperature:

v = 331 m/s + (0.6 m/s/C) × T

Here v stands for velocity and T stands for the temperature of the air. Notice that 331 m/s and 0.6 m/s/C are constants and will always be placed in the same spot in the formula.

Other factors that affect the velocity of a sound wave in air are the amount of moisture in the air and the velocity of the wind. The density of water vapor is less than that of dry air. Therefore, the higher the humidity (that is, the more water vapor there is in the air), the lower the density of the air. For this reason, sound travels through the air more rapidly in damp weather than in clear weather. The presence of wind also affects the velocity of a sound wave traveling through the air. As the wind moves the air, sound travels more rapidly in the direction that the wind is blowing and more slowly in the opposite direction. A strong wind also has a noticeable effect on the resistance offered to the passage of a sound wave through the air and on the amplitude of the wave. If a sound wave is traveling in the direction of the wind, the sound carries farther and seems louder. When a wave is traveling against the wind, the range of the sound is reduced and the loudness of the sound is diminished.

The velocity of a sound wave traveling through a liquid medium is much greater than the velocity of a similar wave traveling through a gas. the velocity of a sound wave is 1,439 meters per second if it’s traveling through fresh water, which has a density of 1,000 kilograms per cubic meter (kg/meter3). The effect of the temperature of a liquid on the velocity of a sound wave can be neglected.

When a sound wave travels through a solid body consisting of an elastic material, the velocity of the wave is much higher. the velocity of a sound wave traveling through steel (which is almost perfectly elastic) is about 5,060 meters per second. The velocity of a sound wave traveling through an inelastic solid, such as lead, is approximately 1,402 meters per second.

if both the source of the sound waves and the person hearing the sound remain stationary, the frequency of the sound waves reaching the person also stays constant. The frequency isn’t affected by the nature of the medium or by the distance between the source of the waves and the person hearing the sound. The frequency of the sound waves reaching a person’s ear may be changed noticeably, however, by either increasing or decreasing (at a fairly rapid rate) the distance between the source and the person. For example, imagine that you’re in a moving car. Another car passes you, moving in the opposite direction, and blows its horn. The sound of the horn changes as the other car speeds by yours because the frequency of the sound waves reaching your ears changed abruptly when the two cars moved apart. This apparent change in the frequency of sound waves, caused by a rapid change in distance between their source and a person hearing the sound, is called the Doppler effect. The frequency of the sound waves reaching a person’s ear is modified whenever there’s relative motion between the source of the sound and the observer. While standing at a railway or subway station as a train passes, have you ever noticed that the pitch of the train’s horn changes suddenly? The apparent change in the pitch is explained by the Doppler effect.

If two sources simultaneously produce equal sounds, you hear a sound of double intensity, because the waves reinforce each other.

The resultant wave travels in the same direction as each component wave and is in phase with the component waves. It has the same frequency and the same wavelength as the component waves, but the amplitude of the resultant wave is twice as great as the amplitude of either component wave. The two component waves support, or reinforce, one another. Reinforcement occurs whenever a molecule is moved in the same direction under the action of each component force acting alone.

Now, assume that the two waves are out of phase with each other by one-half of a wavelength—the crests of one wave correspond to the troughs of the other wave. In this case, the component waves move each molecule through the same distance, but in opposite directions. The resultant motion of each molecule is zero, and the two component waves interfere with one another. Interference occurs whenever a molecule is moved in opposite directions under the separate action of each component wave. In this example, the resultant wave (which isn’t really a wave) can be represented by a straight line.

If two sources produce equal sounds that are out of phase you wouldn't hear any sound. The two waves interfere with each other. Of course, waves may travel through the medium along a common path at the same time, have the same frequency and the same amplitude, but be out of phase by a distance that isn’t exactly one-half of the wavelength. In such a case, the resultant wave won’t have the shape of a sine curve; it will be irregularly shaped.

two simultaneous waves that have different frequencies. the resultant wave that has two beats. A beat occurs at each instant when the reinforcement of the two component waves is at a maximum.

The greatest amount of reinforcement occurs at each point where the crests or troughs of the two component waves are in phase. At other points, there’s maximum interference between the component waves, and the molecules remain in their normal positions.

When two sound waves having slightly different frequencies pass through a medium along a common path at the same time, pulsations are heard in the sound produced by the combined waves. These pulsations, or beats, are caused by the alternate reinforcement and interference of the component waves.

During any period of 1⁄4 second, there will be two instants at which the crests or troughs of the component waves are exactly in phase and maximum reinforcement occurs. If there are two beats in 1⁄4 second, there will be 8 beats in 1 second. This frequency is equal to the difference between the frequencies of the component waves (48 Hz – 40 Hz = 8). In general, the number of beats produced per second by two component waves equals the difference between the frequencies of the waves in hertz.

Music teachers use a device called a tuning fork, which is simply a steel fork with two prongs. When either prong of a tuning fork is struck, both prongs vibrate with a common frequency, which is always the same regardless of the force of the blow. Because the prongs of a tuning fork are relatively small, very little air is set in motion and only a weak sound is produced. if the handle of a vibrating tuning fork is pressed against a large surface (such as the top of a wooden table) the entire surface will vibrate with the same frequency. The vibrations of this surface are called forced vibrations. As a result of these forced vibrations, a much greater volume of air is set in motion and a much louder sound is produced. Forced vibrations are produced whenever a vibrating body is placed in contact with another body.

If two tuning forks that vibrate with the same frequency are placed near each other, and one of the forks is struck a blow that causes it to vibrate, the other fork will soon start to vibrate without being touched. The vibrations that are set up in the second fork are called sympathetic vibrations, and the two forks are said to be in resonance. Resonance occurs when a body’s molecular wavelength sends vibrations to another body that result in the production of another wave. Resonance can also be produced with two bodies that aren’t alike.

A musical sound causes a pleasant sensation when the wave producing it strikes a person’s eardrum. The wave producing a musical sound usually has a perfectly uniform frequency, and any changes in the wave’s amplitude are gradual. A pure tone is produced by applying the force exactly at the center of the length of the string. The part of a musical instrument that vibrates in resonance with a sound wave produced by a stretched string is called the sounding board. The sounding board of a piano is a large, flat piece of wood. In a violin or guitar, the sounding board is the hollow box to which the strings are connected. Forced vibrations of the sounding board increase the loudness. the reed’s frequency of vibration doesn’t determine the frequency of vibration of the air column in the tube. The frequency of vibration of the air column in any wind instrument is determined entirely by the effective length of the tube. While a kettle drum can be tuned to some extent, a membrane can’t usually be adjusted to produce a tone with a certain definite pitch. A bell can be constructed to produce a sound with a certain definite pitch. The nature of the sound produced when a cymbal is struck usually depends on the manner in which the blow is applied.

The pitch of a vibrating string can be affected by the string’s length, tension, and mass. These three rules apply to stretched strings on instruments:

The longer a stretched string is, the slower it will vibrate, and the lower its frequency will be. The lowest or deepest notes are therefore produced by the longest strings in a piano or harp.

The greater the tension in a stretched string, the faster it will vibrate, and the higher its frequency will be.

The heavier or thicker a string is, the slower it will vibrate and the lower its frequency will be. Thus, the strings that produce the lowest notes in a piano or harp are thicker than the other strings (in addition to being longer).

The frequency of a vibrating string is inversely proportional to its length. The fundamental frequency is the lowest note that can be produced by the stretched string. A sound produced by a portion of a stretched string vibrating with a frequency higher than the fundamental frequency for the string is called an overtone. Multiplying a string’s fundamental frequency by a whole number (such as 2, 3, or 4) gives the frequency of an overtone. An overtone that has a frequency that’s a whole-number multiple of the fundamental frequency of the string is called a harmonic. An overtone with a frequency equal to two times the fundamental frequency is called the second harmonic. An overtone with a frequency three times the fundamental frequency is called the third harmonic, and an overtone with a frequency equal to four times the fundamental frequency is called the fourth harmonic.

Sounds can be defined according to three characteristics: pitch, loudness, and quality. Pitch is the effect of the frequency of sound on a person’s ear, or the frequency received by the ear. The actual frequency, or pitch, is usually not of interest in the comparison of two sounds; it’s the ratio of two pitches that’s important. This ratio is the relative difference in pitch between these two sounds. The relative difference in pitch is important in musical tunes because it indicates how much higher or lower one sound is as compared to another sound. The loudness of a sound depends on the energy content of the sound and, consequently, on the energy of the source. Loudness is a complex factor; when it’s correlated with the response of the human ear, it depends on frequency and on the level of the sound. the human ear responds to a much smaller difference in energy for faint sounds than for more intense sounds and also perceives sounds of different frequencies as having different intensities, even though they have the same energy. The range of loudness that the ear can respond to is very large. At a frequency of about 3,000 Hz, where the ear is most sensitive, the energy of the faintest audible sound differs from the energy of sounds loud enough to be painful by a factor of 10^13.

The quality of a sound may be described as pure, mellow, average, harsh, or discordant. A sound has a pure quality if it’s produced by a single wave with constant frequency and there are no overtones. A mellow sound is produced when the only overtones are small, even-numbered harmonics, such as the second, fourth, and sixth harmonics. An average sound is produced when the combination of waves includes all the small-numbered harmonics, such as the second, third, fourth, fifth, and sixth harmonics. A sound is harsh if the combination of waves producing it includes relatively high, odd-numbered harmonics, such as the eleventh, thirteenth, and fifteenth harmonics. A discordant sound is produced by overtones that have high frequencies and that aren’t harmonics.

Musical sound produced by a wave with a certain definite frequency is called a note. The diatonic scale, or natural scale, in music, is a series of eight notes that are related so as to permit the formation of the greatest number of harmonious ratios of frequencies. The notes of a scale are identified by the seven capital letters A, B, C, D, E, F, and G. The first and eighth note of any particular scale have the same letter, and the scale is said to be in the key of this letter. One note that has a suitable frequency is selected as the foundation for the scale. The foundation note, or keynote, may have any desired frequency. The entire interval from the eighth note to the first note, or keynote, is called an octave. The scale starting with the keynote having a frequency of 264 Hz is called the middle scale of the key of C.

Modern pianos are tuned on an even-tempered scale, meaning that each key is tuned an equal and precise frequency above or below the surrounding keys. The eight notes in a scale that correspond roughly to the frequencies for the key of C on the diatonic scale are produced by striking the white keys, and the five notes that correspond to the additional frequencies on the even-tempered scale are produced by striking the black keys.

If the temperature of the air is 15°C (59°F), the corresponding velocity of the sound wave would be faster than at 0°C (32°F). Use the formula noted to determine the velocity.

v = 331 m/s + (0.6 m/s/C) × T

v = 331 m/s + (0.6 m/s/C) × 15°C

v = 331 m/s + 0.6 × 15

v = 331 + 9

v = 340 m/s

The occurrence of beats may be explained by referring back to parts A and B of the previous figure. Part A shows 10 cycles of a wave that has a frequency of 40 Hz. Part B shows 12 cycles of a wave that has a frequency of 48 Hz. The time required for each number of cycles to occur can be calculated as follows:

10 cycles/40 Hz=14 second

12 cycles/48 Hz=14 second

For example, if two waves have frequencies of 65.4 Hz and 87.3 Hz, the ratio of these frequencies is

87.3/65.4

Sound travels poorly in gases, better in liquids, and best in elastic solids. Sound can’t travel through a vacuum.

Infrasonic vibrations can’t be detected by the human ear; ultrasonic vibrations are higher than can be detected by normal hearing.

A wave front is any point of a wave that passes through all points of maximum compression.

The energy contained in a wave front decreases rapidly as the front moves away from the vibrating body.

Sound waves are collected in the outer ear, leading them to the middle ear and vibrating the eardrum; vibrations are transferred to the inner ear’s fluid, activating the auditory fibers and finally sending signals to the brain.

A sound is reflected when it strikes a smooth, hard surface.

The angle between the incident wave and the normal line (perpendicular to the reflecting wall) is the angle of incidence.

The angle of incidence equals the angle of reflection.

Sound in air travels at 332 meters per second at 0°C.

A sound reverberates when the waves are reflected at different times and intensities.

Sounds are absorbed by surfaces if the surfaces contain enough air pockets to trap sound waves.

Sounds may be refracted (bent) based on atmospheric conditions and wind interference.

Sounds are diffracted, or split into elementary waves, when the sound waves strike objects and are reflected, causing a new series of waves.

Sound waves can be combined or reinforced if they share the same frequency or a multiple of that frequency and are in phase with each other.

Sound waves negate or interfere with each other if they have frequencies such that the troughs of one wave correspond with the crests of the other wave.

Two bodies may experience resonance when the sound waves of the first body cause sympathetic vibrations in the second body.

The Doppler effect is the apparent change in frequency when the source of sound waves changes rapidly.

An overtone is a sound produced by a portion of a stretched string vibrating with a frequency higher than the fundamental frequency for the string.

A whole-number multiple of the fundamental frequency is called a harmonic.

A sound is characterized by its pitch (frequency), loudness (amplitude), and quality (overtones and harmonics). Sound quality can be further categorized as either discordant or harmonious.