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What is noise and how is it measured?

What is noise and how is it measured?

In science and engineering, noise is an undesirable signal that obscures a wanted signal. So we could just say that noise is unwanted sound.

It’s not that simple however. Some noise is good – just ask Beethoven. So what is this invisible yet powerful force of nature that is experienced by a myriad of species every day to detect danger, communicate and navigate?

Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid or gas. Sound waves have a number of properties including frequency, wavelength, amplitude, speed, intensity and direction. The subjective or perceived magnitude of a sound is called its loudness and will depend on frequency and sound pressure.

The frequency, also referred to as pitch, is measured in hertz (Hz). Most of us hear best between 500Hz and 4000Hz which is referred to as the speech frequency range although this varies with age, sociological influences and physical factors.

The internationally recognized unit of measurement for sound pressure or wavelength, sometimes referred to as volume, is the decibel (dB). Because the human range of hearing is so vast a logarithmic scale is used for decibels, ranging from 0 dB (hearing threshold) to 120-140 dB (pain threshold).

This logarithmic scale is an important concept because 20 decibels is actually 10 times greater than 10 decibels, and 30 decibels is 100 greater than 10, which explains why achieving an apparently incremental real-world sound reduction between rooms of, say, ‘just’ 5dB can be challenging.

 

dB

Situation

 

0

Near silence

20

Whispered conversation

45

Typical occupied office

50

Typical occupied classroom

65

Gymnasium/restaurant

70

Factory/railway carriage

90

Traffic/machinery

100

Nightclub

110

Rock concert

120

Airplane at take-off

140

Gunshot