Live Sound International | Backstage Class - How to Read Microphone Specs

When reading microphone specifications, it is extremely important to understand how to interpret them. In most cases, the specifications can be measured or calculated in many different ways.

While microphone specifications provide an indication of a microphone¹s electro-acoustic performance, they will not give a total appreciation of how it will sound. Specifications can detail objective information but cannot convey the subjective sonic experience. For example, a frequency response curve can show how faithfully the microphone will reproduce the incoming pure sinusoidal frequencies, but not how detailed, well dissolved or transparent the result will be.

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 pressure scales (Pascal, Newton or Bar). The dB scale states a given pressure in proportion to a reference pressure, mostly 20µPa. The reference pressure 20µPa is chosen equal to 0dB. Please note that 0dB does not mean, that there isn¹t any sound; it only states the lower limiting sound pressure level of the average human ear¹s ability to detect sounds.

The frequency response curve illustrates the microphone¹s ability to transform acoustic energy into electric signals, and whether it will do so faithfully or will introduce coloration. Take care not to mistake frequency response for frequency range. The microphone¹s frequency range, will only give a rough indication of which frequency area the microphone will be able to reproduce sound within a given tolerance. The frequency range is sometimes also referred to as bandwidth. (See Figure 1).

Figure 1: Frequency Response of a DPA Type 4006 Omnidirectional Microphone. Frequency Range: On-axis: 20Hz - 20kHz ±2dB.

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.

Figure 2: DPA Type 4011, Cardioid Microphone.

The on-axis response demonstrates the microphone¹s 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.

The diffuse field response curve will illustrate how the microphone will respond in a highly reverberant sound field. This will be an acoustic environment where the sound has no specific direction, but where all directions are equally probable. The reflections from walls, floor and ceiling are as loud or louder than the direct sound, and 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 in the lower frequencies. The diffuse field response will show a roll-off in the higher frequencies, partly due to the air¹s absorption of higher frequencies.

Off-axis Responses The off-axis responses will reveal the microphone¹s response to sound coming 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 extremely important that these curves also show a straight frequency response, as it will otherwise introduce off-axis coloration (curtain effect).(See Figure 2).

A polar diagram 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. (See Figure 3).

A reference point on the outer circle is often defined by a 1kHz sinusoidal tone aimed directly towards the microphone¹s diaphragm (0° = on top of the circle). Each shift between the circles normally indicates a -5 dB step, unless otherwise indicated. In this way, it is easy to determine how much weaker the signal will be around the microphone for certain frequencies, commonly 5kHz, 10kHz, 15kHz and 20kHz.

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 diagram, it is also easy to see how omnidirectional microphones usually become more directional at higher frequencies.

The equivalent noise-level (also known as the microphone¹s self-noise) indicates the sound pressure level that will create the same voltage as the self-noise from the microphone will produce. A low noise-level is especially desirable when working with low sound pressure levels, so the sound will not ³drown² in noise from the microphone itself. The self-noise also dictates the lower limitation in the microphone¹s dynamic range.

1. The dB(A) scale will weight the SPL according to the ear¹s 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 to 30 dB.

Sensitivity expresses the microphone¹s 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 1kHz (measuring microphones at 250Hz). As an alternative, the sensitivity can be submitted according to the American tradition, which states the sensitivity in dB, relatively to 1V/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 2dB.

Figure 3: DPA Type 4006 Omnidirectional Microphone.

Sensitivity:
Nominally 10mV/Pa; -40dB re. 1V/Pa
unloaded (at 250Hz) Max difference 1dB

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 SPL¹s easily supersede the RMS value by more than 20dB.

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.

A commonly used level of THD is 0.5% (1% is also often seen), which is the point where the distortion can be measured, but not heard. Ensure that the THD specification is measured for the complete microphone (capsule + preamplifier), as many manufacturers only specify THD measured on the preamplifier, which distorts much less than the capsule. The distortion of a circular diaphragm will double with a 6dB increase of the input level, so other levels of THD can be calculated by using this factor.

Total harmonic distortion:
142dB SPL peak (<0.5% THD)
148dB SPL peak (<1% THD)

Microphone specifications do not tell the whole story about a microphone¹s quality, and are 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.

This text was provided by DPA Microphones through their Microphone University program. To check out this and other topics, visit www.dpamicrophones.com and click on Microphone University.