To put it simply, an audio interface usually does three things – it uses its internal mic preamps to boost the signals to line level, it takes care of the A/D – D/A conversion and it creates the bridge to send and receive digital audio to and respectively from a computer. So let’s compare the reference output level of the SM58 to the standard recording level – the difference is huge as the standard +4 dBu recording level is 663.8 times higher than the reference output of the SM58! To get that very low signal to a line level appropriate for recording, a microphone preamp is needed. (millivolts) The standard professional recording level is +4 dBu which converted to volts is 1.228 V. Most microphones output a maximum of around 30-40 mV. For example, one of the most popular all-purpose microphone, the Shure SM58, has a sensitivity of 1.85 mV 94 dB SPL. What that means is that for a certain acoustical pressure of 1 Pa or 94 dB SPL your microphone will output “x” mV. ( “x” being an arbitrary value, Pa or Pascal being a unit of measure for pressure). It is usually written as “x” mV 94 dB SPL / 1 Pa. If you have ever researched the specifications of one of your microphones you probably came across the sensitivity of your device.

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