Dynamic microphones use the principle of electromagnetic induction. The dynamic microphones from HOLMCO work on the moving-coil principle. They contain a diaphragm that is fixed to a moving coil. The coil is positioned in a static magnetic field generated by a permanent magnet. As the sound waves hit the microphone, they set up vibrations in the diaphragm, which are transferred to the coil. The movement of the coil in the magnetic field induces a signal voltage that is proportional to the incident sound.
Dynamic microphones do not need a power supply. They can tolerate very high sound pressures and are also extremely durable and reliable, so are ideal for use in harsh environmental conditions.
The electret microphone is a special type of condenser microphone. A moving diaphragm and a fixed back plate form a parallel-plate capacitor, which is charged with a DC voltage. The sound waves hitting the microphone make the diaphragm vibrate. As a result, the capacitance varies, which causes voltage variations across the resistor that are proportional to the incident sound signal.
In the electret microphone, an electric charge is permanently “frozen” in the diaphragm or in the back plate. This removes the need for a DC voltage for the transducer, so an electret microphone only needs a power supply for its output amplifier.
The electret microphone has the merits of good signal quality and low cost. Electret microphones are extremely small, making them easy to fit even into compact housings. They are relatively insensitive to electromagnetic interference, which is particularly important for digital radio communications.
The directionality or polar pattern of a microphone describes how its sensitivity varies with the angle of incidence of the sound. It is basically determined by the mechanical design of the microphone.
A microphone that has an omnidirectional pattern is equally sensitive to sound waves from any direction. It will pick up all ambient sounds. It is used, for example, when several people are speaking from different positions, or if a microphone is fixed and the person speaking has the freedom to move anywhere in the room.
The omnidirectional microphone is relatively insensitive to pop and wind noise. In addition, it does not exhibit the proximity effect common to directional microphones.
A microphone with a cardioid pattern picks up sound primarily from the front and from the side. The microphone largely screens out sound coming from behind.
This directional pattern is the ideal choice when the interfering noise source lies behind the microphone.
The cardioid is the directional pattern that provides the best suppression of unwanted noise coming from behind.
The directional response of the supercardioid lies between the cardioid and the hypercardioid. It attenuates sound incident from the side more strongly than the cardioid, and attenuates sound coming from behind more strongly than the hypercardioid. The null angle, at which the microphone has minimum sensitivity, lies at ±125°.
If one thinks of the space around the microphone as divided into two hemispheres whose dividing line is the microphone diaphragm, then for the supercardioid, the sound pick up from the front hemisphere (±90°) is maximized compared with the rear hemisphere. Looked at the other way round, the supercardioid is the directional pattern that provides maximum suppression of sound from the rear hemisphere compared with the front hemisphere.
Like the cardioid, the hypercardioid favours sound coming from the front. The speech pick-up angle, however, is narrower than for the cardioid and supercardioid, with greater attenuation of sounds from sources positioned to the side. In exchange, sound incident from behind is not suppressed as strongly by a hypercardioid as by the cardioid. The null angle lies at ±110°. An omnidirectional microphone with the same sensitivity at 0° picks up four times as much sound as a hypercardioid. In other words, compared to the omnidirectional microphone, the hypercardioid picks up only a quarter of the sound coming from all sides (for the cardioid and figure-of-eight it is a third in each case). The hypercardioid is the directional pattern that picks up the least ambient sound.
Figure of eight
A microphone with the figure-of-eight pattern detects sound from two opposite directions. Sound coming from the side is suppressed. The figure of eight is the directional pattern that provides the best suppression of unwanted noise from the side.
The frequency response describes how the ratio between noise level at the microphone and voltage produced at its output varies as a function of the frequency.
The frequency response of a microphone must satisfy different criteria depending on the application. HOLMCO microphones are used for speech transmission. For this application, there is no need to transmit the entire audible frequency range, as is the case with music transmission, for instance. Instead, only the voice frequency range lying between 300 Hz and 3.4 kHz needs to be transmitted. In fact, especially where there is loud ambient noise, it is often better to restrict transmission to the voice frequency range. Background noise whose frequency lies outside this range is then not picked up by the microphone in the first place.
While Hi-Fi designers seek a linear (flat) frequency response, this is not always desirable for speech transmission. For instance, especially if background noise is superimposed on the speech signal, intelligibility is improved by selective amplification of the higher voice frequencies.
The sensitivity of a microphone is the ratio of the AC output voltage to the sound pressure. There are a raft of different standards around the world that apply to sensitivity. HOLMCO uses the free-field open-circuit sensitivity for the dynamic microphone specification, and the free-field nominal sensitivity for electret microphones. Both these values are measured under acoustic free-field conditions at a frequency of 1 kHz, a sound pressure of 1 Pa and a distance of 1 m.
Electret microphones tend to deliver a higher output voltage than dynamic microphones. Usually, however, the microphone is connected to a radio set, a mixing desk or some other equipment containing an amplifier, which can compensate for the different levels of input signal.
The sensitivity of dynamic microphones also depends on the winding resistance of the coil. The larger the resistance, the higher the sensitivity of the microphone.
The proximity effect only occurs with directional microphones i.e. not with microphones with an omnidirectional characteristic. When the distance between noise source and microphone is very small (less than 10 cm), the low frequencies are overemphasised. The microphone sounds muffled and unnatural. This effect becomes more accentuated as the distance shortens and the frequency lowers. The proximity effect also depends on the directivity. The more pronounced the directional pattern, the stronger the proximity effect. This phenomenon does not occur at all with omnidirectional microphones, but is most severe for a figure-of-eight pattern. A possible countermeasure is to reduce the sensitivity in the lower frequency range. The proximity effect restores an approximately linear frequency response in this range. These microphones should then only be used for close-range speech, however, because otherwise the low frequencies would be lost at larger distances.
Pop and wind noise
Plosive sounds such as “p” or “t” produce pop noise when speaking close to the microphone. Wind noise occurs during outdoor use if wind hits the microphone diaphragm directly. The stronger the directivity of a microphone, the more prone it is to pop and wind noise. An omnidirectional microphone is barely susceptible, whereas hypercardioid or figure-of-eight microphones are very sensitive to pop and wind effects. Pop and wind noise can be muffled effectively by a foam wind shield (pop shield).