100 mm

ELECTRO-TECHNOLOGY FOR INDUSTRYTechnologyFrequency range and sound pressure levelThe range of human hearing is from 20 to 20,000 Hz.Deeper sounds (infrasound) and higher sounds (ultrasound)cannot be heard. The human ear is most sensitive to soundbetween 500 Hz and 3 kHz. With regard to volume, a soundpressure of 2/10,000 = 0.0002 μbar is just barely audible.This limit value is called ‘hearing threshold pressure’.A sound pressure of 200 μbar and above causes pain.This is known as the pain threshold.In order to make the hearing range more manageable interms of numbers, the ratio of the actual measured soundpressure to the hearing threshold pressure is converted toa logarithm. This logarithmic relationship is known as thesound pressure level and is expressed in decibels (dB).The equation is:Lp= 20 x logmeasured sound pressure in μbar dBhearing threshold pressure in μbar100 000 000 μPa10 000 000 μPa1 000 000 μPa100 000 μPa10 000 μPa1 000 μPa100 μPa140 dB (A)130 dB (A)120 dB (A)110 dB (A)100 dB (A)90 dB (A)80 dB (A)70 dB (A)60 dB (A)50 dB (A)40 dB (A)30 dB (A)20 dB (A)10 dB (A)20 μPa0 dB (A)Basic principles of acoustic audibilityThe loudness of a sounder is expressed in dB (A) and measuredat a distance of 1 metre (USA 10 feet). The smallestincrease in the sound level that the human ear can detect is3 dB. An increase of 6 dB is equivalent to a doubling of thesound pressure. An increase of around 10 dB is perceivedas being twice as loud.Reduction in the sound pressure level inrelation to the distance between the sounderand the listener’s ear0Lower frequencies (at the same sound level) are perceivedto be quieter. This is all the more pronounced at lower soundlevels.Alarm signals can be better heard when the differencebetween the frequency of the ambient noise and that ofthe sounder is greater. Interfering factors are, for example,damping, fog, obstructions, wind speed and direction, rainand air humidity.Sound pressure reduction (dB)-10-20-30-35-40-50ExampleA doubling of the distance to the source of the sound isequivalent to a reduction in the sound level of around 6 dB,e.g. there is a sound pressure level reduction of 35 dB at adistance of 58 m.1 2 4 8 16 32 64 128 25658Distance (m)27