Reference input voltage for microphjones during development

[drmachismo]

Hey All -

Wondering what everyone uses as their input signal when building gear that takes a microphone input. I looked at the spec sheet for an SM57 and found its sensitivity rating of -56 dBV/Pa. If I convert this to Vrms and use 60 dB SPL as a "quiet" level and 130 dB SPL as a "loud" level it would follow that the output for these two levels is:

-56 dBV/Pa = .001585 Vrms/Pa

...and since 1 Pa = 94 dB SPL

SM57 output in Vrms @ 94 dB SPL = .001585 Vrms

So...

SM57 output in Vrms @ 60 dB SPL = 60/94 * .001585 = 1 mVrms

and

SM57 output in Vrms @ 130 dB SPL = 130/94*.001585 = 2.2mVrms

Seems logical, but the SM57 spec also states that the sensitivity rating is for open circuit at 1 KHz. An input transformer would load the microphone and I would expect the voltage seen at the input of the mic transformer to be lower.

What are you all using as an input signal to simulate a mic input when tuning gain staging?

 

[Brian Roth]

Something is wacky with those numbers. 130 dB SPL should be much more that 2.2 mV. I recall yelling into a 57 connected to an old Simpson analog VOM and seeing some meter deflection on the most sensitive AC scale (2 VAC FS???) 2 mV wouldn't have been visible.

I know a LDC mic can produce nearly line level with a hot acoustic source.

Too late for me to crunch the math....lol.

Bri

 

[gyraf]

dB is a log scale, you can't multiply and divide those figs without srsly mucking up

you add and subtract in stead: If your reference level (for 1.5mV) is 94dB then

60dB is 34dB less than this - and those -34dB means "50 times smaller", i.e. 1.5mV/50 = 30uV ( https://sengpielaudio.com/calculator-db-volt.htm )

130dB is 36 dB more than the 94 - and this +36dB means "63 times larger", i.e. 1.5mV*63 = 95mV

/Jakob E.