Frequency Response Calibrations Of Unlisted Microphones - BRUEL & KJAER 4226 User Manual

9) Using the
Frequency Step
when conditions are stable, note the level detected by the microphone or measuring
instrument. A stable condition is signified at each step when the LED for the corresponding
frequency lights up continuously (not flashing).
At all frequencies, the sound level detected by the microphone or measuring instrument should
be, within specified tolerances, equal to the nominal level, that is, 94, 104 or 114 dB. Table 3.1
shows the specified sound level tolerances (in decibels) applicable to both linear and A-weighted
responses relative to the nominal level according to IEC and ANSI standards for types 0, 1 and 2
sound level meters. Table 3.1 is a guide because it also allows for the influence of the instrument
case on the sound field. You should also be aware of whatever tolerances apply to the nominal
sound level produced in Type 4226's coupler (see the Specifications).
Table 3.1
Tolerances (in dB)
relative to the nominal
level at each selected
frequency for a sound
level meter with linear
and A-weighted
responses. Influence of
instrument case on
sound field included
3.5

Frequency Response Calibrations of Unlisted Microphones

For microphones not listed under step 4 in section 3.4, and ¼" microphones that must use
Adapter DP-0781, we advise you to set the
Traditionally, the pressure response of a microphone is determined normally without a
protection grid on the microphone. With Type 4226, however, all tests are performed with the
protection grid in place. The presence of the protection grid together with the impedance of the
microphone's diaphragm will influence the sound field such that the sound level sensed at the
diaphragm does not always match the actual sound level produced in the coupler. Therefore,
discrepancies will occur between the actual and sensed (or measured) sound levels. These
discrepancies are normally negligible at frequencies below 4 kHz, but can be several decibels at
higher frequencies.
Because of this, the pressure and free-field response of an unlisted microphone cannot be
determined directly, but indirectly by comparison. This is done by first performing a pressure
response test of the microphone measured with Type 4226 and comparing this with the
traditionally obtained pressure response of the microphone (usually provided on the
microphone's calibration chart). This will indicate what corrections need to be applied to the
Type 4226 results for each of its selected frequencies. These corrections will be valid for all other
microphones of the same type, that is, the same geometry, but not of other types. This is because
switch, step through the frequency range of interest. At each step,
Nominal
Frequency (Hz)
31.5
63
125 to 1000
2000
4000
8000
12500
16000
Type 0 Type 1
±1 ±1.5
±1 ±1.5
±0.7 ±1
±0.7 ±1
±0.7 ±1
+1, –2 +1.5, –3
+2, –3 +3, –6
+2, –3 +3, –∞
switch to Pressure.
Sound Field
CHAPTER 3
17
Operation
Type 2
±3
±2
±1.5
±2
±3
±5
+5, –∞
+5, –∞