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I've built an adjustable sized speaker and I want to get frequency response curves for it.

My setup to test it is a sine wave generator connected to a power amplifier connected to the speaker which plays into a microphone. The voltage going into the speaker is pretty much constant aside from at low frequencies where it drops off (due to the amplifier).

I have measurements of the voltage going into the speaker as well as the measured voltage in the microphone. I also have the microphones sensitivity.

-39dB re 1V/Pa @ 1kHz (12mV/Pa @ 94dB SPL) ±2dB @ 1kHz

I looked it up but I can't figure out how to convert this to an SPL scale and also how to include the differing input voltage. How do I convert this to SPL and do I need to take any other measurements to do so? I also have the amplifiers specs.

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    You will also need the frequency response of the microphone. – audionuma Apr 10 '18 at 15:06
  • Also the distance between the speaker and the mic, and you really want to make those sort of measurements in free space (take the speaker outside, point it upwards and place the mic a known distance above it being the poor mans approach). An alternative is to use a gated measurement but that takes some more faffing about with a suitable analyser. – Dan Mills Apr 11 '18 at 20:41
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If you want the freq response curve (transfer function) of your speaker, or room, or any piece of gear you can use this software or other similar programs:

https://www.rodetest.com/

Just hook up an audio interface, a line out to a power amp, and a mic and you can send sweeps thru just about anything. Much quicker I think.

If you're really fancy you can use a measurement mic and an acoustic calibrator (basically a tone generator for mics) to calibrate your preamp levels going in. Usually you will generate about a 94dBSPL signal at the microphone and adjust your preamp level so the program recognizes that the incoming signal level in the cable corresponds to 94dBSPL in the air. This is to account for the different sensitivities of various mics.

But if you want to do it the hard way you can use your mic's sensitivity rating (look at the specifications or data sheet) to calculate dBSPL from your voltage measurements. Probably more accurate to just calibrate and measure SPL directly though.

Hope this helps.

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