Individual sound perception
The characteristic measurement for the perception of sound by the sense of hearing is the acoustic pressure as well as the frequency. The physical unit for acoustic pressure is Pascal (Pa), the one for frequency is Hertz (HZ). This means that 1 Hz = 1 wave per minute.
The frequency determines the pitch, the pressure the volume.
The frequency range of 16 - 16000 Hz (typical range of audible of an human ear) is divided logarithmically into octaves and third-octaves.
Frequence spectrum at octave and third octave band
|Octave band||fm, octave||31,5||63||125||250||500||1000||2000||4000||8000||16000|
|Third octave band||fm, third octave||31,5||40||50||63||80||100||125||160||200||250|
Criteria curve A
The sound perception is individual from one to another on the frequency. For that reason there are defined four different weigthing curves based on the frequency weigthing of acoustic events. The most common curve for frequency-related weigthing of acoustic events is the internationally agreed weigthing curve A. Levels that are rated by that curve are described with the unit dB.
Calculation of A sound pressure level
Calculation of the A-sound pressure level out of the octave band range, e.g. gas engines, 300 kW, 1.500 * 1/min, 1m at exhaust pipe.
The table shows the differences between A-reated and un-rated levels of an gas engine.
|Octave band medium frequence||31,5||63||125||250||500||1000||2000||4000||8000||Sum|
|measured sound level [dB]||91||92||93||94||95||96||92||88||83||102,3|
|rated sound pressure level [db(A)]||51,6||65,8||76,9||85,4||91,8||96,0||93,2||89,0||81,9||99,5|