| |
Sensitivity
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.
Omni-directional
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.
Uni-directional ( 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. |
|
|
|
[Figure1]
|
Bi-directional
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.
|
