Only
a small percentage of all acoustical measurements are performed in a well defined and/or well controlled environment of an e.g. acoustical laboratory – on the contrary most acoustical measurements
are done under not really controlled conditions. Here are some hints on how to use our microphone.
Sound
Fields:
Free
field: There
are no reflecting objects, only the microphone disturbs the sound field.
Diffuse
field: There
are many reflecting surfaces or sound sources so that the sound waves arrive from all directions.
Pressure
field: This
is found in small confined spaces like sound calibrators.
Depending
on the nature of the sound field an appropriate microphone, which is optimized for the sound field could be selected and used. Unfortunately there are many practical situations where the sound field is
not really of a well defined type. This application note should give you an idea on how to measure with a free field response microphone.
The
free field microphone is the most common in use, chosen on tradition but we should know about the sound field.
The following picture shows both the free field and the diffuse field response of a free field microphone.
The
diffuse field response is not easy to measure, because it is not easy to generate a truly diffuse sound field over a wide frequency range but there is a known procedure to estimate the diffuse frequency
behavior of a free field microphone.
From literature we know, that a microphone’s random (diffuse) incidence response can be approximated by measuring the 90 deg incidence response relative to a single sound source.
While it is
an approximation only iSEMcon has measured the 90deg response of many EMX-7150
microphones and used the averaged data to evaluate a 19th order polynomial. This
is now used to derive the “diffuse field” response from the microphones free
field response data.
Typical freefield measurement:
Speaker measurement. The microphone should target to the sound source (speaker).
Typical diffusefield measurements:
Concert SPL monitoring (normally at FOH), Room Acoustics measurement (RT60): the microphone should not target to the sound source. Let it target to the ceiling. This is the most practical way
Diffusefield Sensitivity and Level Correction
A diffuse sound field is characterized by the sound arriving at the capsule from all directions. The CPX12/12 is a freefield equalized measurement microphone with a linear frequency response referring
to a 0° sound incidence. It‘s diffusefield sensitivity level correction is calculated by measuring and averaging the directional characteristics (according to IEC 61183). The corrections factors
are as follows:
|
Frequency
(kHz) |
<1 |
1 |
1.25 |
1.6 |
2 |
2.5 |
3.15 |
4 |
5 |
6.3 |
8 |
10 |
12.5 |
16 |
20 |
|
Magnitude
correction |
0 |
0 |
0.1 |
0.18 |
0.22 |
0.33 |
0.62 |
0.85 |
1.37 |
1.83 |
2.66 |
3.51 |
4.54 |
6.15 |
8 |