When selecting a microphone for a given application it is necessary to understand the different specifications and parameters so that the best microphone can be chosen.
There are many different microphone specifications, each one addressing a different element of its performance.
The different specifications or parameters have been addressed in different sections below.
Microphone sensitivity specifications
There are various different ways in which the microphone specification for sensitivity can be given, however they all relate to the electrical output to a given sound level in terms of pressure.
One of the most native methods of expressing the sensitivity is to provide the output, typically in millivolts, mV for a given pressure expressed in Pascals, or the former notation of Newton/m2. Even older specifications may use the notation of mV per µbar or mV per dyne/cm2.
Another notation is to express the output level as dB referenced to 1 Volt (note that 1 mV is -60dB referenced to a Volt). When referencing an voltage level to 1 Volts the dB should be given as dBV, i.e. referenced to 1 Volt. In this way 1 mV is -60dBV.
When specifying an output voltage for a given pressure input, the impedance of the load must also be mentioned. The reason for this is that the voltage output of the microphone varies with the load impedance. Often a value of impedance of 600Ω may be used but this is far from the overall standard.
This one microphone specification for sensitivity may read -60dBV/Pa into 600Ω
Microphone impedance specification
The actual impedance of the microphone itself is of great importance. If the microphone has a high source impedance then its output will fall considerably of a low impedance amplifier is used. Also the effect of the capacitance of the cable will have a significant effect.
The output impedance of the microphone is essentially the impedance that any load would see – it is the source impedance or resistance of the microphone itself. It is measured in Ohms.
The microphone source impedance forms a potential divider with the load impedance – typically the impedance of the preamplifier to which it is connected – and this has the effect of reducing the voltage output of the microphone as a result of the source and load impedances forming a potential divider.
Typically these days microphones have a low impedance, often around 200Ω, but in the days of valve or tube amplifiers, which had a high input impedance, many microphones were able to have a high source impedance.
Frequency response specification
As the name indicates the microphone specification for frequency response indicates how the microphone handles the different frequencies in the audio range.
Some low end microphones may give the upper and lower frequencies of the range but sadly this is unlikely to mean much as they tend not to give the amount by which the response has fallen.
Sometimes a response will be given as 50 Hz to 15kHz for -6dB points, indicating that at 50 Hz and 15 kHz the response has fallen by 6dB when compared to the mid range.,p>The more normal way is to provide a plot of the response curve. Response charts are normally averaged over several units to give a typical curve for the microphones. High end microphones may well come with their own response chart indicating the exact response for that particular item.
Pressure gradient units often show several bass curves for different sources distances – this arises because of the proximity effect.
Some microphones such as the capacitor or condenser microphone that includes its own amplifier can be overloaded when high levels sounds are present. In this case maximum sound levels are specified. Typical maximum levels may be 20 – 30 Pascals (120 – 124 dB).
Dynamic microphones cannot be overloaded in this way and therefore no figures are given for them.
Microphones can introduce distortion and this is important in many instances. The microphone specification is in terms of the total harmonic distortion in terms of the sound pressure level that produces a certain amount of distortion.
Normally the sound level to produce 0.5% total harmonic distortion, THD, is used, although some budget models may specify the sound level to produce 1% total harmonic distortion.
A typical microphone specification for distortion may be in the form is the sound pressure level needed to produce 0.5% total harmonic distortion is 20Pa (Pascals).
A number of different types of connector are used with microphones. Some microphones may have leads directly attached and these may be terminated in a jack plugs.
The most commonly adopted approach used for quality microphones is for the microphone itself to have an XLR plug as part of the microphone assembly. It is then possible to use a standard XLR lead to connect it to the amplifier, mixer or whatever system with which it is to be used.
A growing number of microphones now have a USB connection. The microphone contains its own preamplifier and analogue to digital converter enabling it to provide digital data directly to the computer or whatever interface is being used.
There are many different types of directional pattern that microphones can posses. Each type of directional pattern lends itself to a different type of use.
There are many different microphone specifications that need to be considered when looking for a microphone. Different microphones with different specifications will lend themselves to different applications.
Accordingly it is necessary to look carefully at the microphone specifications before selecting a particularly microphone. Coupled with looking at the reviews, it will be possible to select the right microphone with the right specification for the particular use in mind.