Glossary of Microphone Terminology
By John Vellone
Sensitivity is an electrical output produced in a microphone in response to an input with a specified sound level.Sensitivity is expressed in decibels (dB), the standard unit for indicating the ratio of power output to input, which is defined by public standards in individual countries. It is commonly accepted that 0 dB is the ratio of an open terminal output of 1V obtained to a sound pressure of 0.1Pa define it as 0dB=1V/1Pa or 0dBm= 1mW/0.1Pa. The input sound pressure is ten times larger at 0dB=1V/1Pa than at 0dB=1V/0.1Pa; that is, the sensitivity expressed is greater by 20dB at 0dB=1V/1pa than at 0dB=1V/0.1Pa. While the standard 1mW/0.1Pa is based on power. Thus, when the impedance is 600, for example, the sensitivity is expressed in terms of voltage as 1mW=0.774V/0.1Pa.
Note : 0.1Pa=1μbar
◆ Frequency Response
Frequency Response is the range of frequencies a microphone can pick up such as 80 to 12,000Hz.This is a key factor to be checked when you compare a certain microphone with others. The frequency range should be determined to be optimum for individual applications; hence a wider frequency range is not always desirable.
For example, it is as narrow as 300 to 5,000Hz for communications and paging microphones where intelligibility is essential.
In some microphones of this type, the peak frequency lies between 2,500 and 3,500Hz. For deep bass and delicate highs, the frequency range must between 50 and 15,000Hz.
◆ Nominal Impedance
“Nominal Impedance” is the internal impedance seen by the output terminal of a microphone, indicating a value at 1,000Hz.
◆ Directional Characteristics
This represents the characteristics of the microphone to pick up sound from a particular direction.
This property of the microphones implies an even and equal sensitivity for all sounds irrespective of their directions. Such microphones are suitable for picking up the exact acoustic ambience around the place of the use.
● Unidirectional ( Cardioid)
Such microphone have maximum sensitivity for sound coming from the front. Since sound from other directions is not picked up, a clean response is obtained. Howling and undesirable effects of multi-microphone use are thus effectively eliminated.
* Aim the null of the polar pattern at the offending noise source. The null is the angle off-axis where the mic is least sensitive. Different polar patterns have nulls at different angles. Shown below and in Figure 1. are the null angles for various polar patterns.:
|Cardioid: 180 degrees.
Super-Cardioid: 125 degrees.
Hyper-Cardioid: 110 degrees.
Bi-directional: 90 degrees.
Fig. 1. Polar Patterns
These microphones have high sensitivity for the sound both from the front and the rear but do not catch the sound coming from the sides at all. A representative model of such microphones is the ribbon type microphone.
◆ Prevention of Howling
To prevent howling, precautions are necessary with respect to the microphone, the amplifier, the speaker and other acoustic systems.
a) Among acoustic systems it is necessary to minimize the peak of a specific frequency and to make the frequencies as flat as possible.Curtains and sound absorbers are used in the room to minimize dispersion and standing-waves of the sound.
b) The distance between the speaker and the microphone must be as long as possible.
c) The microphone and the speaker used must have certain directivity.
d) The degree of sound reproduction is such that the voice talked close to the microphone can be heard faintly from the speaker.
◆ Balanced and Unbalanced Types.
Depending on the type of the output cable, there are two types of microphones: the balanced type in which the cable has two cores and the unbalanced type where the cable has a single core. The balanced type has the following salient features.
a) External ham or noise has no effect on the performance.
b) A long cable can be used.