Distortion, THD <1%
Specifies the max SPL (RMS and peak) below which the total harmonic distortion is less than 1%.
A significant component of the microphone is the diaphragm. If the transducer is a condenser type, the diaphragm has a position in front of a backplate. The space between the two is in the range of 20-50 µm. When placing the microphone in a high-SPL situation, it is evident that there is a limit to diaphragm excursion, at least when pushed in the direction of the backplate. Similarly, the diaphragm material itself has a limit to how "stretchable" it is in either direction. These limitations cause amplitude non-linearity, also called distortion.
Besides the diaphragm and the backplate, a condenser microphone needs an electronic interstage that converts the high impedance of the transducer into a relatively low impedance to feed longer cable-runs. The electronics design may be a source of non-symmetrical behavior that can also be a source of distortion. (However, CORE by DPA technology is a successful attempt to improve on this).
Although manufacturers are continuously trying to improve the microphones, there always exist limits to the microphone systems that eventually may cause distortion.
One form of distortion is clipping. When the waveform changes shape from pure sine to some degree of flattop curve (time-domain), harmonics occur in the spectrum (frequency domain). It is the amount of these unintended frequency components, the Harmonic Distortion, being expressed as a percentage of the input signal.
At DPA, we specify the SPL up to THD <1%. This value is worth knowing as it forms the basis for calculating the dynamic range of a microphone. Dynamic range is the difference between the RMS-level at which a THD of 1% occur, and the noise floor (self-noise of the microphone, RMS, A-weighted). In addition, the related peak-level is measured and stated in the specifications.
DPA measures the THD at one frequency. The chosen frequency depends on the microphone type (omnidirectional or directional).
Why is THD only measured at one single frequency? Due to practicalities. It is difficult to produce a sound source that can deliver an SPL of (for example) 160 dB with zero distortion, especially if this source should cover the whole frequency range.
At DPA, omnidirectional mics are measured by applying a B&K 4221 High-Pressure Microphone Calibrator. The directional mics are measured using a unique acoustic tube designed by DPA.
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. Typically, the preamplifier distorts much less than the capsule; thus, defining a more extensive dynamic range than is actually available.
At low levels, the distortion should always be below 1%. Increasing the SPL increases distortion. Therefore, what is specified is the max SPL (RMS and peak) at which the THD does not exceed 1%.
Ref: IEC 60268-4 Sound System Equipment - Part 4: Microphones
clause 14.2: Total harmonic distortion
Microphone max SPL, THD 10%
Microphone’s response to extreme sound pressure.
This parameter is also called “Overload SPL.” In many recording situations, it is practical to know the maximum Sound Pressure Level (SPL) a microphone can handle and what output voltage to expect in that situation. 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.
For general specifications, the SPL at which a THD of 0.5% or 1% occur is useful, because this is the point at which you start to detect audible distortion.
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.
However, DPA specifies the microphones' maximum peak-SPL. The definition of the max SPL is when the output reaches a THD of 10%. The measurement is carried out at one single frequency, including both capsule and preamp.
Presenting this specification indicates that the microphone delivers increased signal also after the 1% THD is passed. In addition, this specification provides a useful max-value for the input section of wireless systems.
(Note: In some brands’ specifications, the max SPL indicates the maximum sound pressure level at which the microphone does not break up! This measure is of no practical use unless you are in the spacecraft business.)
Ref: IEC 60268-4 Sound System Equipment - Part 4: Microphones
clause 15.2: Overload sound pressure
Rated impedance
Output impedance, as stated by the manufacturer.
The output impedance of a professional microphone should be low when compared to the input impedance of the preamp, usually ten times lower.
The output impedance of condenser microphones is basically determined by resistors. Thus, the impedance is constant with frequency (contrary to dynamic microphones, where the coil/magnet/suspension has an influence that may result in a less constant impedance with frequency. The nonlinear impedance in some cases may affect the frequency response of the microphone. The reason for calling this specification “rated impedance” is that the manufacturer is allowed to call it what he finds best to describe the overall impedance value.
The impedances of a DPA microphones are constant with frequency.
Ref: IEC 60268-4 Sound System Equipment - Part 4: Microphones
clause 10.2: Rated impedance
Minimum load impedance
The minimum input impedance of the external preamp.
A complete condenser microphone has a capsule and an internal preamplifier. When connecting to the outer world, an external preamplifier is applied. The microphone should be able to deliver a decent voltage into the input of this external preamplifier. However, if the load is too heavy (too low input impedance), there is a risk for a reduction of the microphone’s output signal.
Thus, it is practical to know the minimum load impedance allowed without any loss of signal.
(Somebody may – for emergency reasons only of course – do a passive split from one microphone into two inputs. If that is the case, the load impedance is less than the lower of the two input impedances!)
Ref: IEC 60268-4 Sound System Equipment - Part 4: Microphones
clause 10.3: Rated minimum permitted load impedance
Cable drive capability
Long cables may degrade a signal. The loss will usually set in at higher frequencies first (the cable may act as a low-pass filter). To avoid this situation, DPA states the maximum cable run without any significant loss.
A typical value for DPA microphones is 100 m (328 ft).
This information is not required by any standard.
Output balance principle
Microphone signals are weak compared to line-level signals, perhaps in the range of 100 times smaller. However, we connect the microphones using long cables. So, to minimize the noise induced into the microphone cables, it is crucial to use balanced lines.
In most microphone types (or rather in most microphone’s output amplifiers), DPA uses a principle called “Active Drive.” The Active Drive has balanced impedance (the same impedance to ground from both pin 2 and 3. By that, the effect of induced electrical noise is massively reduced. (See CMRR).
Whereas the impedance is balanced, this is not the case with the signal. The signal runs only on pin 2. Pin 3 is silent. The advantage of this is a simple and clean circuit that delivers sufficiently high output.
Ref: IEC 60268-4 Sound System Equipment - Part 4: Microphones
clause 16.1: Balance of the microphone output
CMRR
CMRR stands for Common Mode Rejection Ratio (also interpreted as Common Mode Range Rejection). This measure indicates the efficiency of the impedance balancing. It is a measure of the microphone’s ability to suppress electrical noise, which may be collected predominantly by the wires connecting the microphone to the preamp.
The CMRR is measured in the frequency range of 50 Hz to 20 kHz.
Specs not found
There are many more specs than the above mentioned. At DPA, we test the microphones on many other parameters: wind, pop, humidity, EMC, to mention a few. They are not listed at this point. However, in the future, you may find more lines in the spec sheet (even though most think that there is already more than enough information.) DPA wishes to deliver as much useful information to our users as possible.
What you cannot determine from specifications
While microphone specifications indicate 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.
Conclusion
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 correct microphone.