When a rock hits water, waves spread in circles. In exactly the same way a sound source creates waves or pressure variations in the air.
These pressure variations are perceived by the ear as sound.
Sounds vary widely in characterstics. Simple sounds such as pure tones contain one frequency component only, while complex sounds such as speech or noise consist of many frequency components. Most of the everyday sounds we hear are complex sounds.
The frequency of a sound is the number of cycles of a sound wave in one second. The unit of measurement is hertz (Hz).
The frequency of a sound increases as the number of cycles per second increase. Vibrations between 20 and 20,000 cycles per second are interpreted as sound by a healthy individual. A high-pitched sound could be a piccolo flute or a bird singing. Low-pitched sounds could be thunder heard from far away or tones from a bass guitar.
The term dB (deciBel) and the dB scale is used all over the world for the measurement of sound levels. The deciBel scale is a logarithmic scale where a doubling of sound pressure corresponds to a 6 dB increase in level.
It is very important to realize that the term 'dB' has different meanings and is not a fixed value like the volt or the meter etc. The value of a dB depends on the context in which it is used.
Very often, sound pressure is expressed either in dB SPL, where the weakest sound detected by a normal hearing person is 1000 Hz or in dB HL which corresponds to the normal hearing threshold of a specific sound, (e.g. from 125 Hz to 8000 Hz), 0 dB SPL and 0 dB HL are therefore not the same.
Examples of different sound intensities as expressed in dB(HL):
- 180 dB: Rocket at take-off
- 140 dB: Jet engine at take-off
- 120 dB: Rock band
- 110 dB: Loud thunder
- 90 dB: City traffic
- 80 dB: Loud radio
- 60 dB: Ordinary conversation
- 30 dB: Very soft whisper
- 0 dB: Softest sound a person can hear