Acoustic calibration helps users understand the performance of instruments used to measure sound. It establishes a relationship between instrument readings and reference values under specified conditions, with measurement uncertainty taken into account.
A sound level meter may display a stable reading, but that alone does not show that the measurement is reliable. The microphone, electronics, settings and measurement conditions all influence the result. Calibration provides evidence about instrument performance at the points tested.
What is acoustic calibration?
Acoustic calibration applies measurement standards to sound measurement equipment. Depending on the instrument and method, the work may establish microphone sensitivity, sound pressure level response or the output of an acoustic calibrator.
Calibration and adjustment are separate activities. Calibration establishes how readings relate to reference values. Adjustment changes the instrument so that it gives specified indications. An instrument may need calibration again after adjustment. These distinctions follow the terminology in ILAC-G24 / OIML D 10.
Which instruments need acoustic calibration?
The appropriate work depends on what the instrument measures and how it will be used.
| Instrument | Purpose | Examples of characteristics to assess |
|---|---|---|
| Sound level meter | Measures sound levels | Level indication and response across selected levels and frequencies |
| Acoustic calibrator | Provides a reference sound signal | Generated sound pressure level and frequency |
| Measurement microphone | Converts sound pressure into an electrical signal | Sensitivity and frequency response |
| Personal noise dosimeter | Records sound exposure over time | Level response and exposure measurement functions |
| Octave-band analyser | Separates sound into frequency bands | Filter response and band-level measurements |
These examples describe possible areas of assessment, not a universal test list. Audiometers and underwater hydrophones also involve acoustic measurements, but require specialist methods suited to their applications.
Field checks and laboratory calibration
A field check usually involves fitting a compatible acoustic calibrator onto the microphone and observing the instrument's response to a known signal. It provides a useful check of the measurement chain at the applied level and frequency.
For example, the Brüel & Kjær Type 4231 product data specifies nominal outputs of 94 dB and 114 dB at 1 kHz. These values describe that model; users must follow the instructions and applicable corrections for their own calibrator and microphone combination.
A satisfactory field check at one frequency does not demonstrate performance across all frequencies, levels or instrument functions. Laboratory work can assess a broader set of characteristics using the agreed method and test scope.
IEC 61672-3:2013 describes periodic tests for applicable sound level meters. Its scope covers a limited set of key tests under the conditions of testing. It should not be described as proving every aspect of instrument performance.
A practical measurement workflow
- Identify the equipment. Record the meter, microphone, preamplifier and calibrator models and serial numbers. Check that the combination is suitable for the task.
- Inspect the condition. Look for damage, contamination, loose connections and battery problems. Follow the manufacturer's handling instructions.
- Perform and record the initial check. Use the specified settings, adaptor and procedure. Keep the observed reading and any correction or adjustment in the record.
- Complete the measurement. Record the settings, position and relevant environmental conditions so another person can understand how the result was obtained.
- Check again afterwards. Where required by the measurement procedure, compare the final check with the initial result. Investigate a change that exceeds the applicable acceptance criterion.
Agree acceptance criteria before measuring. Do not use an arbitrary difference as a universal pass limit. A failed final check calls for a review of the affected measurements, rather than simply adjusting the meter and keeping the earlier results without assessment.
What should you review on the certificate?
A certificate is most useful when its results match the instrument's intended use. Review the following information when accepting equipment back into service:
- The identity of the instrument and components covered.
- The method, settings, test points and relevant conditions.
- The measured results, errors or corrections, with their units.
- The reported measurement uncertainty and traceability information.
- Any adjustment, repair, limitation or excluded function.
- The acceptance criteria and decision rule, if a conformity statement is provided.
Ask for results before adjustment when they are needed to assess earlier measurements. Results obtained only after adjustment may not show how the instrument performed when it arrived at the laboratory.
For more on reviewing results, see our guide to reading calibration certificates.
How often should acoustic instruments be calibrated?
There is no single interval suitable for every acoustic instrument. Consider the manufacturer's guidance, applicable requirements, frequency of use, operating conditions, previous results and the consequences of an incorrect measurement.
Review the interval using evidence from calibration history and intermediate checks. Damage, repair or an unexplained change in response may justify earlier evaluation. ILAC-G24 / OIML D 10 provides guidance for selecting and reviewing recalibration intervals.
Choosing a calibration laboratory
Send the laboratory your instrument details and explain the measurement task. Ask which ranges, frequencies and functions its proposed service covers, and what uncertainty it can report.
If accredited calibration is required, check that the relevant work is included in the laboratory's accreditation scope. Also confirm which components and accessories should accompany the meter. A clear request helps prevent a certificate that covers fewer functions than you need.
Common questions
Does an acoustic calibrator also need calibration?
Yes. Its reference output needs an appropriate calibration programme. Using it to check another instrument does not establish the accuracy of its own output.
Does calibration automatically make a meter accurate?
Calibration provides information about performance. Adjustment or repair may be needed, and the user still needs to assess whether the results support the intended task.
Is a passing field check enough?
It provides evidence at the check conditions. It does not replace broader testing where the measurement method or equipment control programme requires it.
Keep the certificate, field-check records and measurement settings together. This makes it easier to investigate changes and explain the basis of a reported sound measurement.
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