The science of noise and noise terminology

• The science of noise
• Noise terminology

The science of noise

Noise is usually defined as unwanted sound so the question really is what is Sound? Sound may be defined as any pressure variation (in air, water or other medium) that the human ear can detect. The most familiar instrument for measuring pressure variations in air is the barometer. However, the pressure variations which occur with changing weather conditions are much too slow for the human ear to detect - and hence do not meet out definition of sound. But, if variations in atmospheric pressure occur more rapidly - at least 20 times a second - they can be heard and hence are called sound. (A barometer cannot respond quickly enough and therefore cannot be used to measure sound).

The number of pressure variations per second is called the frequency of the sound, and it measured in Hertz (Hz). The frequency of a sound produces its distinctive tone. Thus, the rumble of distant thunder has a low frequency, while a whistle has a high frequency. The normal range of hearing for a healthy young person extends from approximately 20 Hz up to 20 000 Hz (or 20 kHz) while the range from the lowest to highest note of a piano is 27,5 Hz to 4186 Hz.

These pressure variations travel through any elastic medium (such as air) from the source of the sound to the listener’s ears. You probably already have some idea of the speed of sound for the familiar rule for determining how far away a thunder storm is: count 3 seconds per kilometre from the time you see the lightning until you hear the thunder. This time interval corresponds to a speed of sound in air of 1238 km/hour. For acoustic and sound measurement purposes, this speed is expressed as 344 meters per second at room temperature.

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Noise terminology

Frequency The rate of repetition of a sound wave. The subjective equivalent in music is pitch. The unit of frequency is the Hertz (Hz), one hertz is equivalent to one wave per second. A thousand hertz is often denoted kHz, e.g. 2 kHz = 2000 Hz. Human hearing ranges approximately from 20 Hz to 20 kHz. For design purposes, the octave bands between 63 Hz to 8 kHz are generally used. The most commonly used frequency bands are octave bands, in which the mid frequency of each band is twice that of the band below it. For more detailed analysis, each octave band may be split into three one-third octave bands or in some cases, narrow frequency bands.

The Decibel dB A sound's intensity, or loudness or volume is measured in bels, named after Alexander Graham Bell. A bel is a very large measurement so the loudness scale is shown in decibels, (dB, one-tenth of a bell) The ratio of sound pressures which we can hear is a ratio of 10⁶ (one million : one). For convenience, therefore, a logarithmic measurement scale is used. The resulting parameter is called the ‘sound pressure level’ (L_p) and the associated measurement unit is the decibel (dB). As the decibel is a logarithmic ratio, the laws of logarithmic addition and subtraction apply.

Equivalent Continuous Sound Level Another index for assessment for overall noise exposure is the equivalent continuous sound level, L_eq. This is a notional steady level which would, over a given period of time, deliver the same sound energy as the actual time-varying sound over the same period. Hence fluctuating levels can be described in terms of a single figure level.

Statistical Noise Levels For levels of noise that vary widely with time, for example road traffic noise, it is necessary to employ an index which allows for this variations. The L_eq, the equivalent continuous sound level, has been adopted in this country for the assessment of road traffic noise. The L₉₀, the level exceeded for ninety per cent of the time, has been adopted to represent the background noise level. A-weighted statistical noise levels are denoted L_A10, dBL_A90 and such like. The reference time period (T) is normally included, for example. dBL_A10,5min or dBl_A90,sh.

See also: What is a statutory nuisance.