Many forms of energy exist as waves. Waves have these features.
Crests--the high points on a wave
Trough--the low points on a wave
Wavelength--the distance between 2 neighboring crests or troughs
Amplitude--the height of a wave
Frequency--the number of crests passing through a place per second
The SI (metric) unit for frequency is the hertz (Hz). The unit called megahertz is one million hertz. If one million crests pass a given place in one second, the frequency is one megahertz. Hertz is sometimes described as cycles per second. Frequency is related to wavelength. The shorter the wavelength, the faster the frequency. The longer the wavelength, the slower the frequency.
Sound is a wave-form phenomenon which is produced by vibration. When you talk, your vocal cords vibrate creating waves which travel through the air. When you whistle, your lips vibrate. Musical instruments have reeds or strings that vibrate. When you are sick or yell a lot, your vocal cords may become swollen to the point of where they have difficulty vibrating. When this occurs, you have laryngitis and you lose your normal voice.
For sound waves to travel they need a conductor. All matter can conduct sound waves, but some matter conducts better than others. For example, air is a better conductor of sound waves than water.
A vacuum is a place that has no matter. Without matter, sound waves cannot occur. "In space no one can hear you scream." On the moon, verbal communication had to occur electronically.
Substances which trap sound waves are called insulators. Such things as carpet, furniture, acoustical tile and drapes trap sound waves. If you have ever talked in an empty house, you have noticed that it sounds loud because there is almost nothing to trap your sound waves. An echo occurs because sound waves bounce off of something and return to your ears. If sound waves reach your ears, travel on and bounce off of something and return to your ears, you hear the sound again.
Sound waves take time to travel. The speed of sound is the speed at which sound waves travel. Sound does not travel at the same speed in all kinds of matter. In general, sound travels faster in a solid than in a liquid, and faster in a liquid than in a gas. This is because the molecules are closer together in a solid than a liquid, and closer together in a liquid than a gas. However, because more tightly packed molecules result in more friction, sound waves travel further in a gas than in a liquid, and further in a liquid than in a solid. The speed of sound also depends on the temperature of the conductor. The warmer the conductor the faster sound waves can travel. Most guns shoot bullets at a speed greater than the speed of sound. So if someone was shot in the head and killed at the impact of the bullet, they would never hear the sound of the gun shot. In normal air, the speed of sound is 1,087 feet per second. This is equal to about 740 miles per hour.
When a moving object, such as an aircraft, reaches a speed where the molecules of the air can no longer be pushed aside, the air forms a barrier. If this barrier is broken, the compression of the air molecules produces heat which can cause the aircraft to burn up or disintegrate. This hot, compressed air acts like thunder and produces a shock wave known as a sonic boom. In 1947 the Bell X-1 became the first aircraft to successfully break the sound barrier. It did this at a high altitude where the air is thin. It had swept-back wings, and was constructed of heat resistant materials. A very small scale sonic boom occurs when you snap your fingers or clap your hands.
Not all sound waves can be perceived by people. We can only hear sound waves from 20-20,000 hertz. Vibrations slower or faster than this cannot be heard by people. However, other animals, such as dogs, can hear vibrations that are faster than this. A dog whistle, produces vibrations above 20,000 hertz.
Sound which can be heard has qualities. One such quality is pitch. Pitch refers to how high or low a sound is. The shorter the vibrating object, the higher its pitch. Shorten a guitar or violin string, and the pitch becomes higher. Strike a short string on a piano and it produces a high pitched sound. The thicker a vibrating object, the lower the pitch. The thick strings on a guitar or a piano produce low pitch sounds. Men have thicker vocal cords than women, and their voices are usually lower in pitch. The tighter the vibrating object, the higher the pitch. Tighten a guitar string and the pitch will become higher. Helium tightens human vocal cords and causes people to talk in a higher pitch.
Another quality of sound is loudness. The greater the amplitude of the sound waves, the louder the sound. Loudness is measured in units called decibels (db). A sound of zero decibels is the quietest sound which can be heard by the human ear. A sound of 120 decibels is the loudest sound which can be heard by the human ear without producing pain. Prolonged exposure to any sound above 90 decibels can cause hearing loss. Normal conversation is 65 db; a rock concert, 120 db; a jet airplane, 140 db; a whisper, 20 db.
The external, fleshy portion of the ear which is trumpet-like in
shape is called the pinna. Its purpose is to collect vibrations
and channel them into the auditory canal. There they strike the
tympanic membrane or ear drum which starts to vibrate. On the
other side of the ear drum are the ossicles. The ossicles are the
three smallest bones of the body called the hammer, anvil and the
stirrup. Their function is to amplify the vibrations of the ear
drum. The stirrup exerts piston like pressure against a structure
called the oval window of the cochlea. The cochlea is a long,
curled-up, fluid filled canal which is lined with millions of
microscopic hair cells which are connected to nerve fiber. If the
ripples caused by the piston-like pressure exerted by the stirrup
bends certain hair cells, the connecting nerve fibers are
activated. These nerve fibers feed into the auditory nerve which
takes the sound message to our brain.