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. The concept of acoustic field is explained in our October 2018 issue of Waves (Issue 10, p.18, “Measurement microphones explained”) and in the Brüel & Kjær microphone handbook (available for download at no cost on www.bksv.com).
- Measurements outdoors or in anechoic chambers will require a free-field microphone
- If you plan to make measurements inside, for example, within a car cabin, you will need a diffuse-field microphone (a ½-inch free-field would underestimate the sound pressure about 6 dB at 20 kHz)
- Measurements performed in small, closed couplers, for example, artificial ears used to measure headphone performance, or close to hard, reflective surfaces, require a pressure-field microphone
Are you looking for a microphone for more specialized applications? We have several models to choose from:
- Very high- and ultra-low frequency measurements require microphones specially designed for one extreme or the other, not both. Types 4964 and 4193 are prefect for ultra-low-frequency measurements, but for the very highfrequency end, a microphone like Type 4138 is needed (up to 140 kHz)
- Extremely high sound pressure levels require a robust microphonelike Type 4941
- Inflow measurements may require a flush-mounted or small-footprint microphone like Type 4948/4949
- 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
In most cases, once you have defined the application, you can refine and choose between a:
- 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.