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 Sound – Sound needs a medium to travel through. It can be solid, liquid, or gas. Sound cannot travel through a vacuum.
Sound level
Sounds from the cup travel along the string as tiny vibrations. They are shown here as a wavy line.
Sound
Any vibration through a material, either a gas, a liquid, or a solid.
Sound, unlike light, must have
a medium to travel through. That can be demonstrated by placing a bell in a flask and then pumping out the air. As the number of air molecules is reduced, the sound weakens until, when there is a vacuum in the flask, the bell cannot be heard ringing at all.
We cannot detect the same
range of sounds as other living things. For example, dogs can hear a dog whistle that is too high a frequency for human ears. Sonar (ultrasound), used to detect objects underwater, is also a sound wave out of the range of human hearing.
When a sound reaches our ears, it puts slight pressure on the diaphragm of the ear, and that in turn causes changes in the ear that send electrical signals to the brain.
Sound waves move through different materials at different speeds. Sound moves at different speeds through gases, liquids and solids. The speed of sound in dry air at 0°C is about 33m/s. The speed of sound in seawater is 1,490m/s. In steel the speed of sound is 5,000m/s.
Speaking into the air sends vibrations to the cup.
Sound is measured by its wavelength, its frequency and its intensity. The peak value of each wave is measured in watts per square centimetre.
(See also: Acoustics; Decibel; Distortion of sound; Diffraction of sound; Doppler effect of sound; Echo; Hertz (Hz); Loudspeaker; Musical instrument; Noise; Refraction of sound; Reverberation; Sound level; Soundproofing; Spectrum; Timbre; Tone.)
Sound level
You hear sound because a wave
of air pressure makes your eardrum move. This power can be measured in watts. Sound levels can also
be described in terms of decibels (dB). On the decibel scale the smallest sound that can be heard is 0dB. This sound has an intensity
of about 15 watts per square centimetre. Sound becomes painful (because the pressure on the ear is too great) at about 135dB.
The decibel scale does not rise evenly; instead, the pressure doubles for each 3dB increase. The difference between a 5W amplifier and a 10W amplifier is
3dB. Between the quietest sound that can be heard and pain, the pressure doubles about 45 times. The highest level the ear can tolerate is 1,000 billion times
the quietest. That is an amazing range of sound levels – called a dynamic range and far beyond anything a loudspeaker is capable of generating. In a totally quiet environment the ear is so sensitive that it can just pick up the sound of blood flowing in the ear. At
this level the eardrum vibrates a distance smaller than the diameter of a hydrogen molecule.
Music levels vary from about 50dB for quiet background music to 100dB for a full orchestra all playing together, to 120dB for a very loud rock band.
The human ear is most sensitive to sounds in the range of 20Hz
to 20,000Hz (20kHz), the normal range of speech.
An orchestra covers sound levels from 20dB in quiet passages to 100dB in the loudest parts. If you were to record this on disc or
tape, you would need a system that covers a range of 80dB. The old long-playing records (vinyl) reach 50 to 70dB; a CD can cover 96dB.
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