Let’s look at an example for the Shure SM58 and I’ll explain how it works out…
SM58: -54.5 dBV/Pa (1.85 mV)
This means it produces 1.85 millivolts when a 1 kHz frequency is broadcast at 94 dB SPL.
Now, let’s look into the dBV value. This is where things get a bit complicated. Think “less is more.”
It produces 54.5 decibels relative to 1 volt when a 1 kHz frequency is broadcast at 94 dB SPL. We know that decibel is a ratio measurement.
But here’s the kicker, it’s not always about sound pressure. dBV is a measurement of voltage.
Therefore, the ratio calculation is comparing 1.85 mV and a reference level of 1 volt. This means that when the SM58 is hit with our 94 dB SPL 1 kHz frequency, the microphone produces a signal 54.5 decibels BELOW 1 volt. This is why it’s a negative number, and is also why -50 dBV is more sensitive than -60 dBV.
SM58: -54.5 dBV/Pa (1.85 mV)
SM86: -50 dBV/Pa (3.15 mV)
SM57: -56 dBV/Pa (1.6 mV)
SM81: -45 dBV/Pa (5.6 mV)
SM93: -43 dBV/Pa (7.0 mV)
Notice how the SM86 is more sensitive than the SM58. Also, notice how the SM93 lav mic is the most sensitive. Considering the distance used in lav applications, it makes perfect sense.
Knowing about sensitivity can help you in buying the right microphone as well as knowing when to use a microphone. It can also explain why your microphones perform the way they do.
Take for instance the SM58 compared to the SM86. The SM86 is a condenser microphone, a type of microphone more sensitive based on the nature of the inner workings of the condenser microphones. The SM57 is a dynamic microphone with a 1.6 mV sensitivity. Compare that with the SM81 condenser microphone with a 5.6 mV sensitivity. Which would you want hanging over your cymbals and which would you want sitting up close on your snare drum?
Using a new microphone that’s hotter than the last, you’d need to use significantly less gain. Often times in the case of such sensitive condensers, you must go as far as engaging the channel Pad to decrease the amount of signal coming into the channel.
A simple rule-of-thumb is that condensers are more sensitive than dynamics and ribbons are more sensitive than condensers. Place a ribbon microphone in front of the TV while watching “Sleepless in Seattle” and it will cry every time. (I had to have one sensitivity joke, right?)
The Take Away
Microphones have many properties as can be seen in my one-page guide to vocal microphones. Microphone sensitivity is yet one more aspect that gives microphones their unique properties so you can use them in the right way and for the right applications.
Ready to learn and laugh? Chris Huff writes about the world of church audio at Behind The Mixer. He covers everything from audio fundamentals to dealing with musicians. He can even tell you the signs the sound guy is having a mental breakdown.