Measurement Calibration
Canonical Definition
Measurement calibration establishes a known correction or reference for part of a measurement chain, such as microphone frequency response, microphone sensitivity, interface response, input full scale, or absolute sound-pressure level. Each calibration applies only to the stage and conditions it covers.
Calibration Types
| Calibration | What it establishes | What it does not establish |
|---|---|---|
| Microphone frequency-response file | Correction versus frequency for the stated microphone, orientation, and file | Absolute SPL unless sensitivity data is included |
| Microphone sensitivity or SPL calibration | Absolute pressure-to-digital-level relationship for the unchanged input chain | Flat frequency response or complete standards compliance |
| Interface loopback | Electrical input/output response, delay, gain, and possible channel mismatch | Microphone, loudspeaker, amplifier-load, or acoustic-field correction |
| Input full-scale calibration | Voltage corresponding to digital full scale at a defined gain setting | Waveform cleanliness outside the tested frequency, level, and load |
| Relative baseline | Repeatable before-and-after comparison with the same chain | Traceable absolute units |
Microphone File And Orientation
Use the calibration file that matches the microphone serial number and intended incidence angle. A file intended for on-axis or 0-degree use should not be silently reused for a 90-degree orientation. The microphone body, grille, windscreen, holder, and nearby surfaces can also influence high-frequency response.
Some USB microphone files include a sensitivity value that allows absolute SPL. Others only provide frequency correction. Read the file and manufacturer instructions instead of assuming.
Absolute SPL Calibration
Absolute sound-pressure measurement requires a known sensitivity reference. That can come from a microphone calibration file containing sensitivity, an acoustic calibrator, or a reference sound level meter used under a documented procedure. Changing operating-system input volume, interface gain, preamp gain, or microphone sensitivity after calibration invalidates the absolute reference.
IEC 61672 defines complete sound-level-meter instrument performance, including frequency weighting, time weighting, environmental behaviour, tolerances, and measurement uncertainty. Software plus a microphone file should not be advertised as a class-compliant meter unless the entire configured instrument chain has been evaluated accordingly.
Interface Loopback
- Connect an output to an input using a safe level and appropriate cable or attenuation.
- Confirm neither output nor input clips.
- Measure response, delay, channel balance, and distortion over the required range.
- Create or apply the correction only to measurements that use the same interface path and settings.
- Save the raw loopback measurement and record sample rate, gain, and cable path.
A loopback may expose interface rolloff or clock and latency behaviour, but it cannot correct the acoustic device under test.
Calibration Boost And Noise Floor
A calibration curve that adds large gain where the sensor or interface is weak also raises the displayed noise floor in that region. Do not mistake correction-amplified noise for real acoustic output. Limit or question extreme extrapolation beyond the calibration file's valid range.
Calibration Record
- microphone model, serial number, file name, orientation, and date
- whether the file contains frequency correction, sensitivity, or both
- interface, driver, sample rate, input gain, operating-system level, and loopback file
- calibrator or reference meter model, frequency, level, adapter, and before/after check
- software version and whether calibration data was applied to response, SPL, and distortion views