How to Understand Microphone Specifications

When you purchase a microphone, you may be confused by all the microphone specifications listed on the package or the manual. The microphone specifications have a great effect on the sound of recordings. Today we are going to introduce same basic microphone specifications to help you choose the right microphone.

The decibel (dB) scale

Decibel scale is a logarithmic scale used to describe the intensity of sound. Decibel scale references units of sound pressure that begins at '0' dB as the threshold of human hearing. The chart below shows how that scale works. This is the commonly used scale when people talk about sound "loudness" in the real world.

Frequency Response

The frequency response is the output level or sensitivity of a microphone over its operating range from lowest to highest frequencies. Generally two types exist:

 Flat frequency response:

All audible frequencies (20Hz-20kHz) have the same output level. This is most suitable for applications where the sound source has to be reproduced without changing or “coloring” the original sound such as recording.

Tailored frequency response

 A tailored response is usually designed to enhance a sound source in particular application. For instance, a microphone may have a peak in the 2-8kHz range to increase intelligibility for live vocals.is the output level

On-axis response

The on-axis response demonstrates the microphone's response to sound coming directly on-axis towards its diaphragm (0°). Be aware that the on-axis response may be measured from different distances, which may influence the response on directional microphones because of the proximity effect. 

Off-axis response

Off-axis response refers to an audio source that is not directly in front of a microphone. This will result in off-axis coloration; a distortion or change in the frequency response of the reproduced audio signal. Often this coloration is put to good use. For example, many engineers intentionally set up mics on guitar amps so that they are slightly off access to control the amount of high frequencies captured.

Diffuse field response

Microphones are divided into 3 types according to their response in the sound field: free field, pressure, and diffuse field (or called random incidence). The diffuse field microphone is designed to respond uniformly to signals arriving simultaneously from all angles.

Signal-to-Noise Ratio (S/N)

In analog and digital communications, signal-to-noise ratio, often written S/N or SNR, is a measure of signal strength relative to background noise. The ratio is usually measured in decibels (dB) using a signal-to-noise ratio formula.

Equivalent noise level

Another term for a microphone’s self-noise. Basically, this indicates the sound pressure level that will create the same voltage as the self-noise from the microphone will produce.

There are two typical standards:

1. The dB(A) scale will weight the SPL according to the ear's sensitivity, especially filtering out low frequency noise. Good results (very low noise) in this scale are usually below 15 dB(A).
2. The CCIR 468-1 scale uses a different weighting, so in this scale, good results are below 25 - 30 dB.

Sensitivity

The sensitivity of a microphone is the electrical response at its output to a given standard acoustic input. This is expressed as the ratio of the input pressure to the electrical output (voltage or digital word). The standard reference input signal for microphone sensitivity measurements is a 1 kHz sine wave at 94 dB sound pressure level (SPL), or 1 pascal (Pa, a measurement of pressure). A microphone with a higher sensitivity value has a higher level output for a fixed acoustic input than a microphone with a lower sensitivity value. Microphone sensitivity in decibels (dB) is typically a negative number; therefore, a higher sensitivity is a smaller absolute value.

SPL handling capability

In many recording situations it is essential to know the maximum Sound Pressure Level (SPL) the microphone can handle. Please note that in most music recording maximum peak SPL's easily superseded the RMS value by more than 20 dB. The RMS value indicates an average SPL and will not show the true SPL peaks.

It is important to know:

1. The SPL where a certain Total Harmonic Distortion (THD) occurs.
2. The SPL where the signal from the microphone will clip, that is the waveforms will become squares. This is the term: Max. SPL and it refers to peak values in SPL.

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