Noise and Sound
Noise has been given a variety of definitions, depending on the circumstances in which it occurs and the effects it produces. Noise is different to sound. Sound is a variation in pressure that is detected by the ear. Rapid variations in pressure can be heard and is referred to as sound. The number of pressure variations per second is referred to as the frequency of the sound and is measured in hertz (Hz).
The range of human hearing extends from around 20 to 20 000 Hz. The number of pressure variations per second determines the pitch of the sound heard. The greater the number of pressure variations (the frequency); the higher the pitch; the lower the frequency, the lower the pitch. Because the range of sound pressure covers such a wide spectrum, a logarithmic system of measurement has been developed so that the numbers used to describe sound are manageable. The basic unit of sound is the decibel (dB).
When measuring a sound made up of different frequencies it is often useful to “weight” each frequency band appropriately, so that the measurement correlates better with the sound which a person would actually hear. The sum of the sound measured in each of these weighted octave bands is known as the broadband level. This is the overall, total sound level weighted in order to correspond with human hearing ability. For example, a dog whistle may have a sound level of 100dB, but it will have an A weighted sound level of around 0dB(A) as it is inaudible to humans.
A-weighting is the main way of adjusting measured sound pressure levels to take into account human hearing and our uneven frequency response. The weighting is usually achieved by using and electronic filter which is built into sound level meters, but it can also be done manually. It is expressed as, for example 5dBA.
The threshold of human hearing is around 5dBA. The noise level in a typical bedroom at night is between 20 and 30 dBA, while noise in an average office is around 60dB. Music in nightclubs average 115 dBA, which is of concern as 130 dB is considered to be the threshold of pain without causing hearing damage (unless exposure is prolonged). A sound in excess of 150 dBA is thought to produce instantaneous hearing damage.
The distinction between noise and sound is that noise is sound which is undesired by the recipient, whereas sound can be pleasurable or unwelcome.