98 Bedrock SMxx operating manual © Bedrock Audio BV, 2019
Annex B. Manufacturer’s instructions for
periodic testing according to IEC-61672-3.
B.1. Insertion of electrical signals for models with BAMT1 microphones
Periodic testing of the Bedrock SMxx in its capacity as a Sound Level Meter is to take place
in accordance with the procedures as described in IEC-61672-3:2014. This standard re-
quires the manufacturer to specify a means of inserting electrical signals.
For models featuring a BAMT1 microphone (BAMT1-MP201 or BAMT1-MP309), the
procedure to enable insertion of electrical signals is to fit an adapter on the BAMT1 pre-
amplifier (replacing the microphone capsule) with a capacitance that matches the MP201
and MP309 capsule (16 pF + 2 pF). These are available from multiple vendors (contact
Bedrock Audio for more information).
B.2. Insertion of electrical signals for models with BAMT2 microphones
For models featuring a BAMT2 microphone, a different procedure must be adopted, since
the BAMT2 does not feature a detachable capsule. The BAMT2 is an electret microphone
and preamplifier assembly; as a whole, this assembly is colloquially called “the micro-
phone.” The BAMT2 is removable from the device, allowing insertion of electrical signals
into the system via the XLR connector. This enables test labs to carry out tests for which
insertion of electrical signals is needed, in accordance with IEC-61672-3:2014.
For electrical insertion of signals into the XLR input, a test interface for the XLR input is
available through Embedded Acoustics (article number 20179842). This interface matches
the BAMT2 in signal level and impedance, and allows input of electrical test signals via a
BNC connector.
B.3. Microphone response measurements
Microphone response measurements can be done using either of two methods:
• Using the the B&K 4226 Multi-frequency calibrator (set to pressure field calibration
mode), taking free-field to pressure field corrections given in Annex A into account.
Add these factors to the response measured with the mulittone calibrator to obtain the
free-field response.
• Using an eletrostatic actuator (GRAS RA0015, GRAS 14AA or equivalent) taking ac-
tuator to free-field corrections given in Annex A into account. Add these factors to the
response measured with the actuator to obtain the free-field response.
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