Some notes on ultrasound microphones
| We are often asked for charactreristics of ultrasound microphones. Due to physical effects, the frequency response of an ultrasound microphone can not be as flat as a audio microphone which may have very flat frequency responses (<+-3dB) up to 15 kHz. |
In general, at higher frequencies the frequency response becomes more and more rough. This effect can be reduced by damping the capsule (and reducing its size), which unfortunately will degrade the sensitivity dramatically. Therefore, we have to live with that compromise between flatness and sensitivity. However, most common bat species emit sounds in a frequency range (10 ... 100 kHz) which is well covered by electret microphones with small damping. The freqeuncy ranges specified for ultrasound microphones for bat sound recording are commonly based on the maximum frequency which can be detected. In other words, the amplitude at the maximum frequency (or somewhere between) may be much lower than at mid-range frequencies. This means, that these specified ranges are not 3 dB bandwidths!
If a flat frequency response (+-2dB) is important for your application, it is recommended to use a 1/4" or 1/8" metal-diaphragm measurement microphone. These (expensive) microphones have a flat and fully documented frequency response, are rugged and resistant against humidity. Unfortunately, the sensitivity of these microphones is relatively poor and most of them require a polarization of voltage of 200V (requiring extra powering modules). Manufacturers of such microphones are Brüel & Kjær, G.R.A.S and Larson-Davis.
Another effect in microphones (both audio-range and ultrasound types) is, that the frequency response also depends on the direction from which the sound is received (directionality increases with higher frequencies). Additionally, high frequencies will be more attenuated than lower ones. This effect will become very significant in large distances between sound source and microphone.
Therefore, care should be taken to interpret amplitude differences of signals at different frequencies.
Last modified on 25 November 2002, Raimund Specht