Sound is a disturbance caused by variations in pressure that travels as longitudinal waves through a medium such as air or water, disturbing molecules along the way. The study of sound is known as acoustics. Humans detect sound waves through their ears. When a twig breaks, it pushes the air molecules. Sound waves get weaker as they expand, while they are moving away from the source of the sound. A longer wavelength has a lower frequency since the waves come less often than a high frequency. Rarefaction is the low pressure part of the sound wave while compression is the “pushing” part of the wave. A changing distance to the object generating the sound affects the frequency and pitch; this is the Doppler effect.
Sympathetic vibrations of an object are caused by sound and are at the object’s resonance frequency. A violin string’s harmonics, or various vibrations, produce its timbre or specific sound; the harmonics include the fundamental frequency and the overtones. Some sounds interfere with other sounds, resulting in different sounds. High frequencies are absorbed first by a rough surface such as the walls of an anechoic chamber. A hard, smooth surface reflects sound. Reverberations are sound reflections arriving almost with the direct sound, like in a room. Echoes are sound reflections from farther away. They arrive more after the direct sound does, like in a canyon. Some sound diffracts around things.
A bat’s brain navigates in total darkness using the echos of the bat’s chirps, which are too high in pitch for humans to hear. The higher the sound wave frequency, the higher the note. However, changes in frequency do not match precisely changes in pitch, so musical instruments have to be tuned by ear. Sound wave strength or intensity also does not match what it sounds like. Therefore, while intensity is measured in microbars, the loudness it causes is measured in decibels. Some kinds of jet engine are quieter than others and Santa Barbara, California was the first U.S. city to outlaw sonic booms.
Sound is caused by the vibration of an object, and its effect is a physiological occurrence in the brain, which also may occur to some extent when reading words or music. The vibration causes waves in a medium which are detected by the ear. The effect of written and physical sound is affected by the organisms ability to hear it. 
Sound Wave Propagation
Scientist Robert Boyle had a bell in a glass jar which contained a vacuum, and he could not hear the bell until there was air in the jar. The vibrating object pushed air molecules. Compression is when the molecules bump together. Refraction is when they bounce apart, causing the wave to move away from the vibrating object. Marin Mersenne timed echoes to determine that sound travels at 1,038 feet per second. William Derham, watching and listening for a cannon boom, determined that sound energy traveled at 1,135 feet per second. Others determined that sound moves faster when the air is warmer. 
The higher the compression (or sound wave), the louder the sound. The distance between the waves—known as frequency— determines the pitch or note. The higher the frequency of the waves, the higher the pitch is. 
When sound waves bump into each other, the resulting waves are different than the original waves. For example, when two equivalent waves are in phase, they occur at the same time and add to each other causing the sound to be louder, but when the equivalent waves are out of phase or in between each other they cancel each other out and there is silence. 
A vibrating tuning fork produces one tone (pure tone). A pure tone is graphed as a sine wave, shaped like an "s". Pythagoras pointed out that the longer the string of the same tightness, the lower the note. Fourier claimed the string produces several higher tones (overtones, harmonics) in harmony with the main note of the string, because of vibration waves going along the length of the string. 
When one of two tuning forks of the same size is struck, the other begins to vibrate also. This is an example of resonance or sympathetic vibration. The body of a violin affecting the sound of the violin is also. 
Sound intensity (loudness) is measured is decibels after Bell, the inventer of the sound-only telephone. A sound ten times stronger is ten decibels louder, 100 times as intense is 20 decibels louder, and 1,000 times more intence is only 30 dedibels louder, because of the ear’s protective mechinism. 
Reverberation is sound echoing and reechoing. Wallace Sabine measured reverberation in a lecture hall using a stopwatch. He then measured the effect of sound absorbing materials such as seat cushions. He found they reduced the duration of the reverberation. 
While the physiologist would say that sound has: pitch, loudness and quality, the physicist would say it has: frequency, intensity and overtones. A “sound source” is a vibration such as from a bell, drum, tuning fork, clarinet reed or piano wire. Sound is transmitted through a material medium – solid, liquid or gas – by molecules pushing those in front of them. Each molecule moves just a little as a wave moves through the material like on the surface of water only three dimensional. Where there is no solid, liquid or gas, sound is not transmitted by molecules pushing other molecules.
The speed of sound waves in air is much slower than the speed of light. One way it was timed was by looking at the flash of a gun and timing how long it took for the sound to be heard. It was calculated to be 1087 ft/sec at 32 F. The speed of sound increases as the temperature of the air increases: 1.1 ft/sec per 1 F. How far lightning is away can be calculated if the temperature is known, and physics books have such homework problems in them. Sound travels slowest in gasses and fastest in solids. Thus, sound speed is a property of the material the waves are in and the medium’s temperature.
As a bullet flies, air keeps moving out of the way causing sound waves. Two waves forming a V shape are caused by the projectile (described as if two dimensional). The V is wider when the projectile is moving slower. If the speed of the sound is known as well as the angle of the waves to the projectile (from a high speed movie camera) the speed of the projectile can be calculated using trigonometry.
On a larger scale, when a jet flies faster than sound moves through the air, it pushes the air molecules faster than sound waves would. The jet itself acts like sound waves. It is like more than one sound wave combined, resulting in a loud thundering sonic boom from the intense V wave of the jet passing by. The speed of a supersonic object divided by the speed of sound for that temperature is the Mach number. For example, Mach 1 is a speed. It is equal to about 770 mph.
Music is different from noise in that is has a tune. Music contains distinct pitches. A high voice can sing high pitches and a low voice can sing low pitches. Savart’s wheel demonstrates that a regular frequency of vibration causes a definite pitch which could be notated, while irregularity of frequency has no definite pitch. Like, if you held the end of a playing card a little against the spokes of a bicycle wheel, a higher pitch would result when the wheel is moving faster.
“Beat” is when two slightly different frequencies of sound waves combine to cause a sound with rising and falling loudness. This is because when waves are in the same place they join and become a bigger wave and when they are between each other they cause a flat “surface.”
“Reflection” is when sound waves bounce off a flat surface. “Plane reflection” is when the sound bounces off a flat surface at an angle equivalent to the angle it came at the surface, like a ball bouncing off the side of a pool table. “Parabolic reflection” is when the sound bounces off a curved surface and disperses like the beam of light from a flashlight. “Elliptical reflection” is when the sound bounces off a curved surface and focuses on a point at some distance from the origin of the sound. For example, if one whispers on one side of the room, the sound is focused by the elliptical surface and the whisper can be heard at a point on the other side of the room.
“Refraction” of sound is when sound waves change direction because of a change in what they are traveling through.
For example, when the air is 32 degrees F, the sound travels at only 740 mph. At 68 degrees F, it travels at 767 mph: 27 mph faster. Because of this, when sound goes from a cool area into a warm area, the sound waves change direction. The day the cannons were fired in London because Queen Victoria died the sound refracted so much it reflected off the sky. The cannons were heard 90 miles from London although they were not heard only 40 miles from London.
When the sound wave enters the warm layer of air at an angle, one end of the wave speeds up first. This somehow causes the angle (of the wave in relation to the line between cool and warmer air) to become less. “Simple refraction” is when the wave direction change is less than to critical angle. “Critical angle” is when the wave ends up traveling parallel to the line between cool and warmer air.
A larger refraction causes a reflection. When the warm air is a lot warmer than the cooler air, the sound bounces off of the warm air heads back toward the ground – still moving away from the source of the sound. When a man yells on a winter morning on one side of a lake, the sound waves might bounce off the sky and be heard on the other side of the lake, but not in the afternoon when the ground is warmer.
“Interference is when sound waves bump into each other, decreasing the quality of the sound. Sound waves are areas of high followed by low pressure or closeness of molecules. The high areas are compressions and the low are rarefactions. When the compression of one wave fills in the rarefaction of another, the volume is lowered. When two compressions join, the sound is unnaturally louder. The sound waves of a tuning fork interfere with each other to some extent. If the tuning fork is turned while sounding, the volume rises and falls from interference. "Dead" spots in auditoriums are caused by sound waves canceling each other, causing the volume of those spots to be less.
“Diffraction” is when sound makes a sharp turn. The wave expands as it moves away from the source of the sound. When the wave goes through an open door, for example, it is expanding enough it to bump the molecules immediately to its left and right. This causes another wave a fraction as loud to expand from the edge of the door. An ant standing close to the left or right of the door would hear the sound. This sound would not be as loud as it would if standing in front of the door and hearing the primary waves. The secondary waves, starting at the edge of the door, do interfere somewhat with the primary waves.
“Absorption” is when sound waves lose energy and thus loudness by echoing around within a porous material. Examples of porous material are curtains, rugs, and acoustic tiles. Each time the wave bumps into the solid part in the porous material, some of its energy of motion goes into the solid and is stopped by the mass of the solid. The solid heats up a little from this. A mad scientist once exposed some mice to extremely high energy sound. After ten minutes or less, the intense sound waves had burned their fur. 
The “Doppler effect” is when the sound is of higher pitch or lower because the sound source is moving. When a race car is coming toward the listener, its speed decreases the distance between one wave and the next, or else causes all the waves to arrive faster like a fly in the car. As the car moves toward the audience, the car’s sound to the audience is of higher tone than to the driver within the car. When the race car is moving away, each wave arrives less often than when the car was coming toward the listeners. When the car is moving away, its sound is lower notes than when the car was coming toward the grand stand.
Subcategories of Sound
Types of sound include speech, music, noise; dissonant, harmonic, masking (such as low tones making high more difficult to hear); underwater sound, and so forth. Also, the word sound has denotations such as bay, measure of depth, sensible, and apparent meaning.
A frequency (note) can be painfully loud. If that tone is turned down too much, it becomes too soft to hear. Some frequencies are too low in pitch to hear; some are so far above the highest note of a piano they are too high too hear. Frequencies too high to hear by a human with good hearing are ultrasonic. Some animals can hear tones twice as high as humans can hear.
The molecules in a sound wave’s rarefaction are in the same phase. Molecules in its compression are in another phase of the wave. A sound wave’s undisturbed shape is like that of a bubble. The sound source is in the center of the expanding spherical compression of molecules.
Loudness depends on frequency, intensity and distance. Scientists Weber and Fechner found that dividing the intensity of a note by the softest intensity it can be heard at and then finding the logarithm of the result approximates how loud the note will seem. When the tone seems 1 bel louder, the intensity is ten times stronger. Humans can tell the difference between tenths of a bell, so loudness is measured in decibels. Using a sound meter, how much street sound a closed window keeps out of a room can be calculated with algebra.
As a sound wave gets a bigger diameter by moving away from the sound source, its intensity gets less. Sound intensity getting less because of distance is called attenuation. An attenuated sound has become weaker because of distance traveled and an amplified sound has been made stronger. The increase in loudness is called the gain. There are algebraic problems about whether a speaker is loud enough for a listener at a given distance and so forth. Each problem would take a long section to explain well.
Quality of a note refers to differences in the sound produced by different sources. A tone from a single frequency is a pure tone. However, a violin’s string vibrates in more than one way at the same time. The tone from the entire length moving back and forth is the fundamental tone. The tones from part of the length moving one way while another part is moving another way are called overtones, also called harmonics. They happen at the same time as the fundamental, and add to and subtract from the fundamental’s wave. The note played is the fundamental. Its quality is affected by the length’s harmonics. Two good singers have a different sound quality, partly because their different vocal chords produce different overtones.
- ↑ 1.0 1.1 1.2 Science Service, Science Program. 1968. Sound and Hearing. USA: Nelson Doubleday, Inc. and Odhams Books Ltd.
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 Stevens, S., Warshofsky, F., & others. 1965. Sound and Hearing. Chapter one. Life Science Library, Time-Life Books, New York, NY: Time, Inc.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 Harris, N. Hemmerling, E. 1980. Introductory Applied Physics, Fourth Edition. Chapter 19. USA: McGraw-Hill, Inc.
- ↑ Phillips, R., and others. Sound. 1971. Funk & Wagnalls New Encyclopedia, Volume 24.
- ↑ Gove, P. & others. 1961. Sound. Webster’s Third New International Dictionary of the English Language. Springfield, Massachusetts: G & C Merriam Company