Sound waves are characterized by compression and expansion of the medium as sound energy moves through it. This represents the pressure component of sound. At the same time, there is also back and forth motion of the particles making up the medium. Particle motion is described by displacement, velocity, and acceleration.
Geophones are devices that generate electric signals proportional to the particle velocity in a sound wave. They are most commonly used to measure sound waves propagating through the earth, called seismic waves. Geophones can also be used to measure particle velocity in underwater sound waves. The name geophone is derived from the Greek words “geo” meaning earth and “phone” meaning sound.
The active element of a geophone consists of a mass hanging on a spring.
In seismic applications, the case of the geophone is rigidly attached to the earth and moves with it as a seismic wave goes by. The mass mounted on a spring, however, tends to remain motionless and therefore moves relative to the case. The motion of the mass is measured using a permanent magnet attached to the case and a coil of wire wrapped around the mass. When a conducting wire moves through a magnetic field, a voltage proportional to the velocity of the wire through the field is generated. The voltage is recorded. The output of a geophone is therefore proportional to the particle velocity in the wave. A geophone is designed to be sensitive only to the component of velocity parallel to the axis of its case. Three geophones are therefore required to measure all three components of particle velocity (for example, the vertical and two horizontal components).
In order to use geophones to measure the particle velocity in underwater sound waves, they must be mounted in a package that moves with the water as the sound wave goes by. Geophones mounted in glass floats that have the same density as seawater and move with it have been used to measure the acoustic particle velocity associated with sound waves at low frequencies (1–20 Hz).