Microphone Sensitivity: One Hot Microphone

Microphone sensitivity becomes a topic of discussion as soon as it becomes a problem.  I’d hazard a guess that most people don’t think about microphone sensitivity until the moment when a microphone picks up a sound that explodes out of the house speakers, to which someone says, “that microphone’s really hot!”

The definition of microphone sensitivity

Microphone sensitivity is the measure of the microphone’s ability for converting acoustic pressure into an electric voltage.  The higher the sensitivity, the less amplification required to bring the sound to a useable level on the mixer channel.  The less the sensitivity, the greater the amplification required.  Talking about amplification is in the area of the mixer’s pre-amplifier.  This pre-amp boosts the signal coming into the mixer. Depending on your mixer, it will be called by different names such as head amp or pre amp.

Microphones which appear significantly more sensitive than others, in the signal chain, are thought of as “hot” or as a “hotter” microphone.

Digging into the details

Sensitivity is measured against a specific baseline in a specific way.  That is to say, it’s measured against a specific sound level, on-axis to the microphone, in a free sound field.  Even more specifically, the microphone sensitivity is based on a 1 kHz frequency at 94 dB SPL.  Specifically speaking, that is.

The measurement of acoustic pressure converted into electric energy is listed in two ways.  It can be listed as milli-volts (mV) produced per Pascal unit.  It can also be declared as dBV per Pascal unit.  Don’t worry too much about the new “Pascal unit” terminology.  A Pascal unit is the unit for pressure and in this case, it’s a constant of 94 dB SPL.

Shure SM58 sensitivity example

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 milli-volts 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 microphone is hit with our 94 dB SPL 1 kHz frequency, the microphone produces a signal 54.5 decibels BELOW one volt.  This is why it’s a negative number.  This is also why -50 dBV is more sensitive than -60 dBV.

Shure microphone sensitivity examples

Vocal Mic’s

• SM58: -54.5 dBV/Pa (1.85 mV)
• SM86: -50 dBV/Pa (3.15 mV)

Instrument

• SM57: -56 dBV/Pa (1.6  mV)
• SM81: -45 dBV/Pa (5.6  mV)

Lav

• SM93: -43 dBV/Pa (7.0 mV)

Notice how the SM86 is more sensitive than the SM58.  Also, notice how the lav. mic is the most sensitive.  Considering the distance used in lav applications, it makes perfect sense.

Practical application

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 general rule-of-thumb has been that condensers are more sensitive than dynamics and ribbons are more sensitive than condensers.  However, I don’t think that’s really the case, especially with ribbons.  Much of it depends on the use of the microphone.  For example, a broadcasters ribbon microphone doesn’t have the same sensitivity needs as an instrument ribbon microphone.   Microphone sensitivity is an important microphone property which must be considered when picking the right microphone for a specific need. Don’t assume one microphone is more sensitive than another just because of the microphone type.

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 property that goes towards giving the microphone its unique properties so you can use it in the right way and for the right applications.