While an ordinary microphone picks up sound vibrations through the air, this microphone picks up vibrations conducted by a solid mass. Air-borne sounds attenuate more quickly than solid-borne sounds. This microphone picks up sounds before they radiate into the air, which means that it can pick up minute sounds that would otherwise be inaudible in air.
The microphone uses a moving-coil-type electroacoustic transducer. Because it is not necessary to have a converting circuit that changes the output impedance of the transducer from high to low, the microphone noise is less than in the electrostatic type or piezoelectric type of microphone. The noise of the moving-coil microphone is determined only by the thermal noise of the resistance of the coil.
Since this microphone does not have the diaphragm that an ordinary microphone has to detect air vibrations, its sensitivity is low with respect to air-borne sound. In fact, because it is insensitive to air-borne sound and sensitive to solid-borne sound, it can clearly detect the sound of a small moving object.
The microphone unit consists of a magnetic circuit, coil, contact plate and contact pin. The contact pin is attached to the coil via the contact plate and is the mechanism that catches the vibration. The position of the contact pin is adjustable along its axis in order that it stays in contact with the solid plate on which the insect is crawling. The vibrations of the plate are conveyed to the coil through the contact pin. An electric current is generated in the coil as a result of the coil moving in the magnetic field.
When the contact pin is shaken with constant acceleration, output response decreases in nearly inverse proportion to the frequency of shaking. The sensitivity is -58 dB (re: 1 V) at 1 kHz and 0.1g (g=9.8 m/s2 : acceleration due to gravity).
The 'Insect Microphone' has been used to pick up a snail's heartbeat and footsteps of ants and antlions.
Recently, a portable type of insect microphone was developed. This model can be used in an outdoor environment and can be put in direct contact with objects or attached to a microphone stand.
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