converting mike output voltage to SPL

[MCPete]

My understanding is that the output voltage of most microphones is directly proportional to sound pressure. First assume that this is true for the microphone that I want to use to measure relative SPL by reading the output voltage of the mike. Is it true that

SPL difference = 20*log(e2/e1)

where e1 and e2 are the voltage outputs of the mike where I make some change and I want to know the resultant change of SPL?

In effect I'm asking how a sound level meter converts the voltage output of its mike to a dB reading without going into the required conversion circuitry or how to assign an absolute dB reading for a given output voltage.

I've constructed an electret condenser mike with a preamp using the NE5534 opamp with ACL = 50 to do some special type of SPL reading nearfield to a driver that at the moment I don't want to reveal (top secret! for now). We all have our delusions, or maybe not.

Thanks to all for any knowledgeable input about this.

-Pete

[Svante]

Originally posted by MCPete
My understanding is that the output voltage of most microphones is directly proportional to sound pressure. First assume that this is true for the microphone that I want to use to measure relative SPL by reading the output voltage of the mike. Is it true that

SPL difference = 20*log(e2/e1)

where e1 and e2 are the voltage outputs of the mike where I make some change and I want to know the resultant change of SPL?

Yes, for a linear microphone, voltage is directly proportional to the sound pressure. However, many electret microphones are not linear when it comes to high levels. This is a bit tricky, because it is har to notice. There is no sharp clipping, but a graph of voltage level vs sound level is not always a straight line.

Anyway for the linear microphone, the formula is correct.

Originally posted by MCPete

In effect I'm asking how a sound level meter converts the voltage output of its mike to a dB reading without going into the required conversion circuitry or how to assign an absolute dB reading for a given output voltage.

Hmm... It has to have some circuits that does the conversion of the voltage to dB. Furthermore it must be calibrated.

Originally posted by MCPete


I've constructed an electret condenser mike with a preamp using the NE5534 opamp with ACL = 50 to do some special type of SPL reading nearfield to a driver that at the moment I don't want to reveal (top secret! for now). We all have our delusions, or maybe not.

Thanks to all for any knowledgeable input about this.

-Pete

Ok, nearfield measurements can result in high levels, high enough to provoke the non-linear behaviour I mentioned above.

My recommendation would be to compare your design with a "known good" level meter. It is quickly done by holding both meters near a loudspeaker cone and varyling the sound level.

[bjorno]

Hi,

Is this the kind of circuits you’re looking for? See 1(1) or:

http://www.linear.com/pc/productDeta...54,C1086,P7526

http://www.linear.com/pc/downloadDocument.do?id=24931

http://www.ortodoxism.ro/datasheets/...412AD636_b.pdf

b

1(1)

[MCPete]

Thank you, Svante, for confirming my formula for calculating SPL difference and indicating how to go about checking linearity of my mike. Actually at this point I am happy with measuring SPL change only and don't want to tackle the more difficult pursuit of building the circuits supplying an absolute dB reading.

Also thanks to bjorno for the references to circuits and ICs for constructing a sound level meter. Given the available free time that would be interesting to pursue.

Regards,
Pete