Once the frequency range and sound level range of interest is defined, the next question to ask is, “Where will I use my microphone?”
For most uses, the type of sound field is the main parameter to consider.
Microphone sound field
- Free-field microphone: Measurements outdoors or in an anechoic chamber.
- Diffuse-field microphone: Measurements inside - i.e., within a car cabin, you will need an (a ½-inch free-field would underestimate the sound pressure about 6 dB at 20 kHz)
- Pressure-field microphone: Measurements performed in small, closed couplers. As an example, to measure headphone performance, or close to hard, reflective surfaces.
For specialized measurement applications, Brüel & Kjær provide several models to choose from:
- Very high- and ultra-low frequency measurements require microphones specially designed for one extreme or the other - but never both. Microphone type 4964 and type 4193 are perfect for ultra-low-frequency measurements. However, for the very high-frequency end you will need a microphone like Type 4138 is needed (up to 140 kHz)
- Extremely high-pressure sound levels require a robust microphone such as microphone type 4941. Inflow measurements may require a flush-mounted or small-footprint microphone like microphone type 4948 or type 4949
- Particularly small spaces, awkward placements, or harsh environments can also require specialized microphones. Type 4182 is a good start – its design provides a very high acoustic impedance probe tip (to minimize the effect on the measurement) and it can be used to measure in temperatures up to 700°C
Prepolarized or externally polarized?
In most cases, once you have defined the application, you can refine and choose between:
- Prepolarized microphone (less expensive and simpler cables, easier conditioning, greater reliability in extreme humidity)
- Externally polarized microphone (often with a higher dynamic range and able to withstand higher temperatures).
Finally, remember that choosing a microphone with a larger membrane will generally lower the inherent noise and increase the sensitivity, while microphones with a smaller membrane will be able to measure higher sound pressure levels, have an extended frequency range and be less sensitive to the angle of incidence of the incoming sound wave.