This article walks through some of the most important microphone specifications and explains how they are derived.
When you read microphone specifications, it is extremely important that you understand how to interpret them. In most cases, the specifications can be measured or calculated in many different ways although the standard IEC 60.268-4 is the common ground for this.
This article is designed to help evaluate specifications in a meaningful way.
What you cannot determine from specifications
While microphone specifications provide an indication of a microphone's electro-acoustic performance, they cannot give you the total appreciation of how it will sound. Specifications can detail objective information but cannot convey any subjective sonic experience. For example, a frequency response curve can show you how faithfully the microphone will reproduce the incoming pure sinusoidal frequencies, but not how detailed, well dissolved or transparent the result will be.
The decibel (dB) scale
The basis for most microphone specifications is the decibel scale. The dB scale is logarithmic and is used because of its equivalence to the way the human ear perceives changes in sound pressure. Furthermore, the changes in dB are smoother and more understandable than the very large numbers that might occur in (linear) pressure scales (Pascal, Newton or Bar). The dB scale states a given pressure in proportion to a reference pressure, mostly 20 micro Pa (µPa = 10-6
Pa). The reference pressure 20 µPa is equal to 0 dB. Please note that 0 dB does not mean, that there isn't any sound; it only states the lower limiting sound pressure level of the average human ear's ability to detect sounds.
The frequency response curve illustrates the microphone's ability to transform acoustic energy into electric signals, as well as whether it will do so faithfully or introduce coloration. Take care not to mistake frequency response for frequency range. The frequency range, will give you a rough indication of the frequency area the microphone can reproduce sound in, within a given tolerance. The frequency range is sometimes also referred to as "bandwidth". d:dicate™ 4006 Omnidirectional Microphone:
Frequency Range: On-axis: 20Hz - 20kHz ±2dB
Multiple frequency response curves
Manufacturers of professional equipment will always provide more than one frequency response curve, as it is essential to see how the microphone will respond to sound coming from different directions and in different acoustic sound fields.
On-axis response demonstrates the microphone's response to sound coming directly at it, on-axis, towards its diaphragm (0°). Be aware that on-axis response may be measured from different distances, which may influence the response on directional microphones because of the proximity effect. Hence, it should always be stated at which distance directional microphones (gradient microphones) are measured.
Diffuse field response
The diffuse field response curve illustrates how the microphone will respond in a highly reverberant sound field; an acoustic environment where the sound has no specific direction but where all directions are equally probable. The reflections from walls, floor, ceiling, etc. are as loud, or louder, than the direct sound, while the sound pressure level is the same everywhere. This is especially interesting when considering omnidirectional microphones, because they are able to register the full frequency range at lower frequencies. The diffuse field response will show a roll-off in the higher frequencies, partly due to the air's absorption of higher frequencies.
Off-axis response reveals the microphone's response to sound coming at it from different angles. This is particularly interesting when you want to discover how a directional (i.e. cardioid) microphone will eliminate sound coming from other angles than directly towards the diaphragm. Even though the off-axis responses are attenuated on directional microphones, it is of extreme importance that these curves also show a smooth frequency response, otherwise off-axis coloration (curtain effect) will be introduced.
A polar plot is used to show how certain frequencies are reproduced when they enter the microphone from different angles. The polar diagram can provide an indication of how smooth (or uneven) the off-axis coloration will be.
A reference point on the outer circle is defined, often by a 1 kHz sinusoidal tone aiming the microphone directly towards its diaphragm (0° = on top of the circle). Each shift between emphasized circles normally indicates a -5 dB step, unless otherwise indicated. In this way, you will be able to determine how much weaker the signal will be around the microphone for certain frequencies, commonly defined in standard octave bands within the relevant frequency range of the microphone (i.e. 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, 8 kHz, and 16 kHz).
The response curves should be smooth and symmetric to show an uncolored sound. Extreme peaks and valleys are unwanted and the response curves should not cross each other. From the polar plot, you can also see how omnidirectional microphones usually become more directional at higher frequencies (the bigger the microphone, the more directionality at high frequencies).
Equivalent noise level
The equivalent noise level (also known as the microphone's self-noise) indicates the sound pressure level that will create the same voltage the microphone actually generates. A low noise level is especially desirable when working with low sound pressure levels so the sound will not "drown" in noise from the microphone itself. The self-noise also dictates the lower limitation in the microphone's dynamic range.
There are two typical standards:
1. The A-weighted RMS-measure is related to the ear's sensitivity, filtering out low frequency noise. A good result (very low noise) on this scale is usually below 15 dB(A).
2. The CCIR 468-1 scale uses a different weighting and a quasi-peak detection, so in this scale, a good result is below 25-30 dB. d:dicate™ 4041-SP Omnidirectional Solid State Microphone:
Equivalent noise level A-weighted: Max. 7 dB(A) re. 20 µPa
Equivalent noise level CCIR 468-1: Max. 19 dB
There is link between the size of the microphone’s membrane and its ability to be quiet ( i.e. exhibit low noise figures). Typically, a larger diaphragm will lead to lower self-noise. This is why the d:screet™ 4060 Miniature Microphone, which has very low self-noise when compared to similar-sized microphones, still measures an equivalent noise level of 23 dB(A) re. 20 µPa.
Sensitivity expresses the microphone's ability to convert acoustic pressure to electric voltage. The sensitivity states what voltage a microphone will produce at a certain sound pressure level. A microphone with high sensitivity will give a high voltage output and will therefore not need as much amplification (gain) as a model with lower sensitivity. In applications with low sound pressure levels, a microphone with a high sensitivity is required in order to keep the amplification noise low.
According to the IEC 268-4 norm, the sensitivity is measured in mV per Pascal at 1 kHz (measuring microphones at 250 Hz). As an alternative, the sensitivity can be submitted according to the American tradition, which states the sensitivity in dB, relatively to 1 V/Pa, which will give a negative value. A serious microphone manufacturer will also state tolerances in sensitivity, according to production differences - such tolerances would normally be in the region of 2 dB. d:dicate™ 3506 Stereo Kit with 4006 Ominidirectional Microphones:
Sensitivity, nominal, ±2 dB: 10 mV/Pa; -40 dB re. 1 V/Pa unloaded (at 250Hz) Max difference 1 dB
SPL handling capability
In many recording situations, it is essential to know the maximum Sound Pressure Level (SPL) a microphone can handle. Please note that in most music recording, maximum peak SPL easily supersedes the RMS value by more than 20 dB. The RMS value indicates a kind of average SPL, not the true peak level.
It is important to know the SPL at which a certain Total Harmonic Distortion (THD) occurs.
A commonly used level of THD is 1% (0.5% is also seen), which is the point at which you start to detect audible distortion.
At DPA, we specify the SPL for 1% THD, measured at 1 kHz. This figure is important to know as it forms the basis for calculating the dynamic range of a microphone. Dynamic range is expressed as the difference between the level at which a THD of 1% occur (at 1 kHz), and the noise floor (self-noise of the microphone, RMS A-weighted).
When comparing different brands of microphones, ensure that the THD measured includes the complete microphone (capsule + preamplifier), as many manufacturers only specify THD measured on the preamplifier. The preamplifier distorts much less than the capsule; thus, specifying a wider dynamic range than is available.
In general, the distortion of a circular diaphragm will double with a 6 dB increase of the input level, so you can calculate other levels of THD by using this factor.
DPA also specifies the microphones' maximum SPL peak. This is defined by a THD of 10% at 1 kHz - still including both capsule and preamp. Presenting this specification is to indicate that the microphone delivers increased signal also after the 1 % THD is reached.
In some brands' specifications, the max SPL indicates the maximum sound pressure level at in which the microphone does not break up! This measure is of no practical use unless you are in the spacecraft business.
To summarize, it is important to know the SPL at which a certain Total Harmonic Distortion (THD) occurs (i.e., 1% THD).
Microphone specifications do not tell the whole story about a microphone's quality. There is no substitute for the sonic experience. Although microphone specifications may not be fully comparable between manufacturers, when properly evaluated they do provide useful objectivity and will help in the search for the optimal microphone.