Microphones for bat detectors
Most bat calls are very high pitched - they are too high for us to hear. These are called "ultrasonic" sounds. Bat detectors require a special microphone that will work in the ultrasonic range. There are three main types that can be used; piezo sensors, capacitance microphones, including electrets, and the new "MEMS" (micro-electro-mechanical sensor) microphones.
Piezo sensors are often used in ultrasonic range measurement. They use a special crystal which produces a voltage when it is stressed by an incoming sound wave. They are robust - "tough as old boots" and produce a high output. This made them very popular for early bat detector designs. They are made to be particularly sensitive at a pre-set frequency - their "resonant frequency" which depends on the size of the crystal used. Common types have resonant frequencies of 40, 45 or 50kHz.
Many bats use echolocation calls in this range; but the narrow signal range means others will not be detected.
Most microphones used for normal sound recording fall into this category. They comprise a pair of plates with a small gap between. A small charge is applied to the plates, and when they vibrate the voltage between them changes, creating an electrical signal. Capacitance microphones are capable of very high quality reproduction, but their requirement of a rather high bias voltage makes them less suitable for portable applications. Also they are generally fragile, large and not very sensitive.
The requirement for robust compact capacitance microphones has led to the development of first ECM (Electret), then MEMS microphones.
Electret condenser microphones
Unlike older capacitance microphones the electret type does not need a large bias voltage as it uses a special diaphragm which has a charge "fixed" within the material. Because the signal generated is small electret microphones have a built-in amplifier, so they need a supply voltage generally around 1.5 - 5V. This is not usually a problem as the portable equipment they are used with will usually have a suitable supply.
Electret microphones are inexpensive and robust. While their frequency response is characterized for the audio range (20Hz - 20kHz) many can continue to work well into the lower end of the ultrasonic spectrum (say 100kHz).
The sensing element uses a silicon membrane which is micromachined to produce audio sensors. This creates a very small cavity and a thin light membrane. The plates of this capacitor are very close together so a low bias voltage is sufficient - in the region of 5V. The microphone package includes an ASIC (application specific integrated circuit) which measures the capacitance and produces an output signal. MEMS microphones typically have the same sensitivity as electrets, but a much better noise performance. They can be made very small, and are often used as microphones in smartphones and tablets. Again the performance is characterised in the audio range, but many types have a good performance in the ultrasound range.