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Sound theory

Sound

Sound is something we hear as a result of vibration of air, and is created bycompressed and expanded air pressure fluctuations. The pressure variations

propagates as sinus formed sound waves. Each unique frequency has its own

characteristic sound wave, repeatedly cycled over a period.

Sound can be measured by either by use of a sound pressure level meter or sound

intensity level meter. The sound pressure is measured in Newton/m2 or Pascal, and

sound intensity is measured in Watt/m2.

Amplitude

The maximum distance between top and bottom curve of the sound wave is explained

as amplitude, and characterize the sound loudness. Long amplitude means high sound

level (noisy), and short amplitude low sound level (quiet).

Change of amplitude do not change the sound wave length.

Frequency

The frequeny is measured as the number of cycles the period is repeated per second.

The measurement unit is HERTZ (cycles per second), abbriviated Hz.

Depending on size of sound waves, the repeatedly cycle over a period is different. Low

frequency sound is characterized by long sound waves, and contain much energy, while

high frequency sound is characterized by short waves,and contain low energy.

Humans hearable range is from 20 to 20 000Hz.

Human ear frequency response

Human ear has an enormous range of sensivity to sound pressure, and can discern

sound of 0dB, whilst threshold of pain is 120dB. The ear is not equally sensitive in all

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frequencies. Acceptance to sound pressures in lower frequencies are greater than the

same sound pressures in higher frequencies,and the difference is more significant at

lower amplitudes. The ear is most sensitive in 4000Hz, and less sensitive from 125Hz

and below. The ear's response to the frequencies are used in the sound technique by

introducing " sound filters" to simulate ear's frequency response. Four (4) sound filters

have been introduced, filter "A", "B","C" and "D". Originally it was intended that A filter

should be used for sound pressure levels up to 55dB, B filter from 55-85dB, and C filters

at higher amplitudes. Today we are most common with use of A and C filters (known as

dBA and dBC). D filters are most used in aircraft noise measurements, as the filter has

a special frequency response at 4000Hz and corresponds to dominanting frequency for

jet engines.

Period

The period is measured in seconds, and is defined as the time it takes to repeat on

cycle.

Wavelength

The wavelenght is different from low frequency sound to high frequency sound, and

characterize the distance it takes to repeat on cycle. (1 period).

Example:

Temperature: 20 oC : 63 Hz: Wavelength= 5,4 meter

Temperature: 20 oC : 20 000 Hz: Wavelength= 17 centimeter

Desibel

Measuring unit for sound is Desibel, orignated by Bel. (1 Bel = 10 desibel). The actual

measuring value is compared to a fixed reference value. By use of the logaritmic scale

log 10, the relations are done as follows:

Example:

Measured sound intensity level (SIL): 10-8 Watt/m2.

Desibel, dB: 10 log 10-8/ 10 log10-12= 10 log 104= 40 desibel

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Reference values:

Sound power level (SWL) : reference value 10-12 Watt

Sound pressure level (SPL) : reference value 2 x10-5 Pascal

Sound intensity level (SIL) : reference value 10-12 Watt/m2

Noise

Noise is undesirable sound, and affect people in different ways, from noise which are

casually disturbing to noise which are dangerous to the health. The interpretion of noise

are indivually judged, but acoustic engineers need a more objective way to characterize

the level of noise by use of sound level meters.

Determination of Sound Levels (explained by Analogue between Heating and Sound

Radiation)

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Physical characteristics of sound

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