Measurement microphone calibration
Microphones and preamplifiers we calibrate
- Brüel & Kjær (HBK) — 4189, 4190, 4191, 4193 and other 1/2" and 1/4" types
- GRAS Sound & Vibration — 40AE, 40AF, 40PH and related series
- PCB Piezotronics and Larson Davis — 377 and 426 series
- Norsonic — Nor1225, Nor1220 and matching preamplifiers
- Rion — UC-59, UC-53A and NH preamplifiers
- Microtech Gefell, ACO Pacific and BSWA
We are not an authorised service centre for any of these makers and have no commercial relationship with them. Calibration is performed against the standard, not against a manufacturer's procedure, so the make matters far less than the class of the instrument. If yours is not listed, ask — the list is what we see most, not a limit.
Turnaround
- Standard lead time
- 10 working days from receipt
- Express service
- Available on request, subject to laboratory schedule — quoted at booking
- Loan instrument during calibration
- On request, subject to availability
- Send with the instrument
- Accessories that affect the reading — preamplifier, extension cable, windscreen — and the date of the last calibration
The microphone is the first link in the traceability chain and the component that drifts. We calibrate working standard and measurement microphones by comparison, and determine frequency response by electrostatic actuator.
- Pressure sensitivity at 250 Hz and 1 kHz — accredited, CMC 0.15 dB
- Frequency response by electrostatic actuator, 100 Hz to 20 kHz — accredited
- Preamplifier gain and noise floor — traceable, outside the accredited scope
- Complete microphone-and-preamplifier chains — traceable, outside the accredited scope
Pressure or free-field — get this right before you order
A measurement microphone does not have one sensitivity, it has a sensitivity for a defined sound field. Pressure sensitivity describes its response when the sound pressure is uniform over the diaphragm. Free-field sensitivity describes its response to a wave arriving from a stated direction in an undisturbed field, where the microphone itself disturbs the field it is measuring. The two differ increasingly with frequency, and the difference is not small at the top of the range.
The consequence is practical. A microphone calibrated as a pressure microphone and then used in a free field, or the reverse, produces a systematic error that no amount of care in the rest of the measurement will remove. Tell us which one your application needs. If you do not know, tell us what the microphone is used for and we will tell you which applies.
Why the microphone is the part that drifts
Everything after the microphone in a modern instrument is digital and stable. The microphone is not: it is a mechanical device with a tensioned diaphragm a few micrometres thick, separated from a backplate by a gap of similar order, and it lives in the real world. Humidity, temperature cycling, airborne contamination and physical shock all act on it. In a tropical climate the first of those never stops.
This is also why a microphone should be calibrated as part of the chain it is used in, together with its preamplifier, rather than in isolation. The combination is what measures.
Should I send the preamplifier with the microphone?
Yes, send the combination you actually measure with. Note that microphone calibration to IEC 61094 is inside our accredited scope while preamplifier calibration is not — the certificate states which is which, so there is no ambiguity for an auditor.
Do I need to remove the protective grid?
No — send it as you use it, grid fitted. The grid affects the high-frequency response, so a calibration performed without it does not describe the instrument you actually measure with.