84 Bedrock SMxx operating manual © Bedrock Audio BV, 2019
85
Annex A. Correction factors for SPL
measurements (IEC-61672)
All of the microphone types distributed with the SMxx are designed as free-field micro-
phones. The BAMT1 modules feature detachable 1/2” capsules, whereas the BAMT2 has
a fixed 1/4” microphone. In this annex, data for each of the microphones are provided that
may be needed in any of the following cases:
• In case you are an end user who wishes to manually correct measurements for maxi-
mum accuracy. End users of the Bedrock SMxx who aim to achieve the highest possi-
ble accuracy may correct free field measurements with the (typical) free-field response
of the applied microphone. Similarly, they may wish to correct for the frequency-de-
pendent effect of the windscreen or for reflections against the instrument body.
• In case you are a test lab and need to perform periodic tests according to IEC-61672-
3. This standard defines the procedures for periodic testing of acoustic measuring in-
struments, in particular when used for measuring sound pressure levels. IEC-61672-3
requires the free field response to fall within certain tolerance limits, after application
of correction factors provided by the manufacturer. Those correction factors are given
in this Annex for each type of microphone offered with Bedrock instruments.
The following microphone data are provided for all microphone types that are available
with the Bedrock SMxx:
• Typical free field response. Since our microphones are free-field microphones, the
free field response should ideally be zero for all frequencies, but in practice small de-
viations occur, particular at higher frequencies.
• Typical actuator response. Calibration labs that use electrostatic actuators to verify
the microphone response need the free-field to actuator corrections specified in this
Annex.
• Typical pressure field response. Test labs that use a the multi-frequency calibrator for
periodic testing (such as the B&K4226) need to apply the free-field to pressure field
corrections specified in this Annex.
• Effects of reflections from the instrument body. Reflected sound from the instrument
body have a small but measurable impact on the measured sound field. The overall re-
sponse, including these effects, needs to fall within the tolerance limits of IEC-61672-3
during periodic testing.
• Effects of wind screens. The 32mm wind screens supplied with the BAMT1 and
BAMT2 microphones have a measurable impact on the microphone response. If these
windscreens are included in the scope of periodic testing, the overall response (includ-
ing wind screens) needs to fall within the tolerance limits.
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