Sound
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.
Human Perception
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. [1]
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. [1]
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. [1]
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. [1]
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. [1]
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. [1]
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. [1]
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. [1]
Sound patterns
Beat
“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.”[1]
Reflection
“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.[1]
Refraction
“Refraction” of sound is when sound waves change direction because of a change in what they are traveling through.[1]
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.[1]
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.[1]
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.[1]
Interference
“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.[1]
Diffraction
“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 a 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.[1]
Absorption
“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. [1]
Doppler effect
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.[1]