Acoustic Sensors

Abstract

The fundamentals of acoustics are given in Sect. 3.10. Here we will discuss the acoustic sensors for various frequency ranges. The audible range sensors are generally called the microphones, however, the name is often used even for the ultrasonic and infrasonic waves. In essence, a microphone is a pressure transducer adapted for transduction of sound waves over a broad spectral range, which generally excludes very low frequencies below few Hz. The microphones differ by their sensitivity, directional characteristics, frequency bandwidth, dynamic range, sizes, etc. Also, their designs are quite different depending on the media from which sound waves are sensed. For example, for perception of air waves or vibrations in solids, the sensor is called a microphone, while for operation in liquids, it is called a hydrophone (even if liquid is not water – from the Greek name of the mythological water serpent Hydra). The main difference between a pressure sensor and an acoustic sensor is that latter does not need to measure constant or very slow changing pressures. Its operating frequency range usually starts at several hertz (or as low as tens of millihertz for some applications), while the upper operating frequency limit is quite high – up to several megahertz for the ultrasonic applications and even gigahertz in a surface acoustic wave (SAW) device.