Microphone specs can look confusing, but ultimately it is possible to understand the numbers once you know what the values are telling you. This guide will help you make sense of them and help you to select the right microphone for the job.

The Decibel

The dB scale is used because it can efficiently describe the very high range of values found in audio. It is always used to show one value compared to another. 

“94dBSPL” is the value relative to a reference of 0dBSPL. “A” is added when any measurement has been “weighted”, or filter, to mimic how we hear.  “dB” used on its own can be used to show the amount of change from one value to another.  For more on dBs check out our complete guide to Decibels.

Frequency Response Plots

These show how a microphone can be expected to respond to different frequencies. A mic with a flat response may sound neutral, whereas a mic with a “bump” or a dip at a certain frequency may be more suitable for a specific task or be described as having “character”. 

The chart above shows the response of Austrian Audio’s OC18 microphone. This mic has a 4dB lift at 5 and 10 kHz, which could help improve intelligibility with certain voices, for example.

Frequency response charts show frequency in front of the mic but will tell us less about how the mic will respond to sounds arriving from the sides or rear. Highly directional mics will have a varying off-axis frequency response that changes as the mic is moved.

A flat frequency response will often be described as a neutral sound whereas some mics have a boost around 4kHz which are often described as a presence bump. It will help you decide if you want a neutral sounding mic or one that has ‘character’.

Polar Response Diagrams

These show the level drop in a mic’s pickup at any point relative to the front. It’s helpful to think of the diagram as an aerial view “map” imagined from above but remember this map actually describes a sphere around the mic.

The point directly in front of the mic, sometimes called ‘on-axis’ is shown as zero degrees. The rings around the mic’s pickup pattern show the level drop in dB relative to sound in front of the mic.

The plot above uses colour to show how the mic’s pickup pattern varies according to frequency. For example, the pale blue plot shows that at 8 kHz the mic becomes less directional.

The polar diagrams are useful if you are using multiple microphones in the same space as it shows how the mic will pick up, or not, sounds coming in off-axis (not from the front). These are also useful if you need a microphone to reject unwanted sounds coming from places other than the direction you are pointing the microphone at.

THD <1% and Max SPL

SPL stands for Sound Pressure Level and is used when talking about acoustic sounds in air, as opposed to electrical signals inside the microphone. THD stands for Total Harmonic Distortion. The THD figure is quoted in dBSPL (peak and average) that states the upper limit of sound pressure input before distortion occurs. Below this figure, Total Harmonic Distortion (THD) is less than 1%. 

Under high SPLs, distortions can be both mechanical and electrical. Mechanical diaphragm excursions are limited by the dimensions of the capsule, causing non-linearities (distortion) into the preamp circuit. Electrical distortion occurs when there is insufficient preamp headroom available. While attenuator or ‘pad’ switches can help avoid electrical overloads, they cannot reduce mechanical ones.

The THD figure should be a combination of mechanical and electrical distortions. Max SPL on its own is less useful as it does not distinguish between the <1% figure and the point that mic sustains mechanical damage. 

Effectively this is telling you how loud the sound the microphone can handle. If you are putting microphones near loud sounds then this figure is important.

Sensitivity

This figure describes how high the mic’s electrical output will be with a sound source of a fixed known sound pressure level. Mics with higher sensitivity are suitable for quiet sources, whereas those with low sensitivity are suitable for loud ones.

For the acoustical input, sometimes 94dBSPL is used instead of 1 Pascal; the two are the same thing. It is simple to convert from mV to dBV using an online resource, or with a calculator:

For example, to convert from 13mV to dBV enter as follows:

The result of -37 is the mic’s output in dB relative to 1 volt. 

This calculation can also be used to work out how much more or less gain one mic will need compared to another on the same source. A dynamic mic with a sensitivity of 1mV/Pa will need ten times (20dB) more gain than a condenser mic with a sensitivity of 10mV/Pa.

Signal to Noise Ratio (SNR)

“Noise” is any unwanted artifact in the wanted signal. Most of us will have heard electrical noise in various gear in the form of hiss. 

All audio gear generates noise, including microphones. SNR describes the difference between the mic’s wanted signal and its own noise, sometimes called ‘self-noise’. The output voltage during complete silence is subtracted from the sensitivity figure to give the difference in dB. The higher the SNR figure is, the better.

This is important at the extremes. If you are recording a very quiet sound source, then you need a microphone with a very low self-noise.

Self Noise, a.k.a. Equivalent Input Noise (EIN)

Self-noise figures are related to SNR. Often called Equivalent Input Noise, the figure describes an acoustical noise level that would be equivalent to the mic’s own electrical self-noise. The SNR is subtracted from 94dBSPL to give the EIN figure in dBA

Condenser mics have more self noise than dynamics and ribbons, and the latter’s self noise is so low it is often not included in the specs. That said, a condenser’s output is far bigger than its noise compared to a dynamic or ribbon’s output and its own (thermal) noise. This means condenser mics’ SNR performance still makes them favourable for quiet sources. 

Output Impedance, a.k.a. Nominal Impedance

A mic’s components collectively have a total output impedance, as seen by the mic preamp. Impedance is frequency-specific and is usually averaged out across the frequency spectrum; this is “Nominal Impedance”.

If the mic output impedance is too high and destination’s input impedance too low, the mic can be “loaded” by the mic pre, losing several dB of mic sensitivity and changing the frequency response. Too high an output impedance can also result in higher thermal self noise in dynamic or ribbon mics.

If the mic output impedance is too low, distortion can occur at high SPLs with condenser mics. A lower impedance circuit will potentially draw more current from the phantom power supply than is available, with the result being clipping as the mic’s circuitry is starved of current.

Load Impedance, a.k.a. Terminating Impedance

This is the minimum input impedance value for the destination that the mic can work into without any problems. Generally speaking, the input impedance should be at least five times, some say 10 times, greater than the output impedance. 

Putting It All Together - What To Look For

What do good specs look like when it comes to maximising technical quality?

Preferable sensitivities would be anything upwards of 1mV/Pa for a dynamic mic, 5mV/Pa for a small-diaphragm condenser, and 10mV/Pa for a large-diaphragm condenser. Most mics from the established brands will usually be able to exceed these figures.

A very quiet studio noise floor would be around 15dBSPL(A), so ideally we would want a mic’s noise to be under that. An SNR value of 80dB or higher would be favourable as it will deliver an EIN of 14dBA, just under the ambient noise floor. The lower the EIN, the better.

An output/input impedance ratio of between 1:5 and 1:10 is considered normal, and is achieved with almost any modern mic and pre combination. For example if your preamp’s input impedance is quoted as 2kΩ, using a mic with a 200Ω nominal impedance will fulfil this. Another common mic preamp input impedance is 1.2kΩ which is 1:6 and so ‘in the zone’.

What Specs Cannot Tell Us

Preconceptions based on the specs shouldn’t lead us into choosing a mic that doesn’t “agree” sonically with the source. You might be surprised how well your trusty dynamic sounds on a certain voice or your large-diaphragm condenser mic does on a bass cab, for example.

Given a choice of mics, the ultimate decider must be which one sounds best for the source, as determined simply by listening.