The pick-up efficiency of a microphone may vary depending on the direction from which the sound comes. This characteristic can be an unavoidable consequence of the design of the microphone or a conscious design to obtain properties for specific purposes. Some microphones also have variable characteristics. This is usually illustrated graphically in diagrams showing the characteristics. In this case the microphone is located in the centre with its front directed upwards in the diagram.
“Omnidirectional” microphones are not aimed in any direction but pick up the sound all around them with the same sensitivity. This type of microphone is used to advantage when recording group calls and when shooting films. A directional microphone is called a Cardioid and is especially sensitive to sound from a certain direction, which is generally at the front. Sound from the side and rear will therefore not be heard as well. The advantage of these microphones is that you are better able to screen out unwanted ambient sounds. Supercardioids are more powerfully directional but have slightly greater pick-up capability from the rear.
“Figure of eight” (bidirectional) is a description of how the microphone is designed in terms of its directional characteristics. A figure of eight picks up most sound from the front and rear of the microphone, which makes it most sensitive to sounds directly from the front or rear.
A practical example could be to connect an external microphone to the hearing aid’s electrical input – the microphone can then be either omnidirectional or directional. If it is positioned centrally on a table, it should be omnidirectional and there is then little gain in the signal-to-noise ratio. If a directional microphone is passed round and used near the mouth of the different speakers the gain becomes significant.
Microphone sensitivity is dependent on the signal it can provide at a given sound pressure, which is expressed in volts per dB sound pressure. (V/dB SPL). The sensitivity varies between the types of microphone and also between models. When measuring it is important to have full control over the sensitivity of the measurement microphone.
The microphone will have some self-generated noise, the less the better. The noise originates partly from the material noise, i.e. the movement of the atoms in the microphone, coil and diaphragm themselves. As far as condenser and electret microphones are concerned, most of the noise comes from their built-in amplifiers.
The microphone cannot reproduce sound pressures that are very high. If the sound pressure is too high, the microphone can also be damaged. It is customary to indicate the maximum SPL of the microphone, which then indicates the maximum number of decibels (dB) or sound pressure to which the microphone can be exposed.
There are many types of microphones, but nowadays microphones are mainly divided into two categories: dynamic microphones and condenser microphones. Both these types generate signals from changes in the air pressure. When the moving plate (diaphragm) vibrates in time with the sound wave, the distance between the plates and hence the capacitance is changed. The changes in capacitance can then be converted to an electrical signal. Since the diaphragm can be made extremely thin and light, its flexibility is extremely good and the signal is therefore of extremely good quality. An electret microphone is in principle a condenser microphone in which the external voltage supply is replaced by materials with a permanent electric charge, known as electrets.
In a dynamic microphone the sound waves are converted to electric signals when a diaphragm with an electric coil moves in a fixed magnetic field. The microphone is therefore robust and withstands a high sound pressure, but due to the higher weight in the moving system, the diaphragm and coil are less well able to handle rapid changes and high frequencies than a condenser microphone. A dynamic microphone is robust and is mainly used in PA systems and in communications equipment.
There is a wide range of microphones that we have not covered here. Write a post if you want to add a comment. If you want to discuss a specific technology and to obtain advice on how to proceed with a project, please contact our application engineers and sales staff.