Basic Algorithm/formula to convert mic db output to db SPL

I have a mic that outputs its level in decibels (the range this mic outputs seems to be
between around -55db to 0db) and would like to find an algorithm that converts this value
into decibel SPL.

I have done some research and have used this formula that seems to do the basic job, but the
values it outputs dont seem to be very accurate.

Here is the formula im using now dbspl(db) = 20*log10( (10^(db/10)/.000002))

where db is the decibel output value of the mic.

Thanks for any help! Start a New Thread

I have a mic that outputs its level in decibels (the range this mic outputs seems to be
>between around -55db to 0db) and would like to find an algorithm that converts this value
>into decibel SPL.
>
>I have done some research and have used this formula that seems to do the basic job, but the
>values it outputs dont seem to be very accurate.
>
>Here is the formula im using now dbspl(db) = 20*log10( (10^(db/10)/.000002))
>
>where db is the decibel output value of the mic.
>
>Thanks for any help!
>

Hi!

First you need to know what is the reference value that the mic uses for the levels. It sounds (as your max is 0dB) that it might be the maximum value of the dynamic range of the mic, i.e. Lmic *log10(vmic/vmax). Where vmic is the voltage from the mic and vmax is the upper limit of the range. Sound pressure levels are defined as SPL *log10(peff/pref), where pref is 20 myPa, and peff is the rms value of the measured pressure. Thus you need to calibrate your mic to find out what the relation between peff and Lmic is. Assuming peff=vmic: SPL *log10(10^(Lmic/20)*vmax/pref). With the formula you are using now you assume that your microphone (or rather analyzer) outputs pressure levels with 1 Pa as the reference value.

J Start a New Thread

A mic output level is in volts, not just dB which is only relative. To convert the mic's output voltage to dB SPL, you have to know the mic's sensitivity, for example 8 mV/Pa. A quality mic manufacturer would provide this data.

First divide the mic's output voltage by its sensitivity to give a level in sound pressure in units of Pascals (N/m^2). Then compute the SPL in dB where the reference level of 0 dB SPL = 20 uPa. The mic has to be followed by a preamp to give plenty of voltage gain (Av) so that you can measure the voltage easily.

So the whole formula is:

dB SPL = 20 * log10 { [ (preamp output voltage / Av) / mic's sensitivity] / 20 uPa ].

Hope this helps.

Gene Goff
Ashly Audio, Inc. Start a New Thread