# Q. Can I calculate the preamp gain difference between two mics?

Given the sensitivity of two mics, you can calculate precisely the preamp gain difference required to raise their signals to the same level.

I have two different mics, made by the same legacy German manufacturer, that I’m ‘shooting out’ to compare them as vocal mics, and generally to make sure they’re working properly. They’re amplified by two separate but identical preamps and I’m noticing that, to get the same level coming into the DAW from each, one preamp needs almost exactly 10dB more gain. Is there a way to calculate, from the published sensitivity specs, whether this discrepancy in gain is correct — or is it not so simple? Here are the published specs for the two mics:

Mic 1

• Max SPL: 127dB
• S/N Ratio: 82dB
• Sensitivity: 28mV/Pa

Mic 2

• Max SPL: 125dB
• S/N Ratio: 74dB
• Sensitivity: 8mV/Pa

SOS Forum post

Hugh Robjohns, SOS Technical Editor replies: The only relevant specs for matching output levels are the sensitivity numbers: the higher the sensitivity, the larger the output level for a given source.

When determining sensitivity, the acoustic reference is normally 1 Pascal (Pa) of air pressure, which equates to a sound pressure level of 94dB SPL. With that sound pressure acting on the mic’s diaphragm, the generated output voltage is measured in milliVolts, hence the sensitivity being given as mV/Pa.

The higher the sensitivity, the larger the output level for a given source.

Your mics are specified as 28mV/Pa and 8mV/Pa and to calculate the difference in output level, you use the standard formula: dB = 20 x Log (v1/v2). The figures from your mics give us: 28/8 = 3.5. And since Log 3.5 = 0.544, we get: 20 x 0.544 = 10.881dB.

Assuming the published specs are accurate, then, to perfectly match the output level of the two mics the quieter one must be amplified by 10.9dB more than the louder one. Your estimate of 10dB is therefore very close!