Sound Pressure Level Measurement
Canonical Definition
Sound pressure level is a logarithmic measure of acoustic pressure relative to 20 micropascals. A reported SPL result must identify microphone or meter calibration, placement, frequency weighting, time detector or averaging interval, signal, and test context.
Weighting Answers Different Questions
| Weighting | General purpose | Car-audio use |
|---|---|---|
| A weighting | Emphasises frequencies important to hearing-risk and many environmental or occupational assessments | Exposure and dose context; not a full-band system-output curve |
| C weighting | Broader response with less low-frequency attenuation than A | Some high-level and calibration tasks where bass contribution matters |
| Z weighting | Nominally flat over the instrument's stated range | Broadband engineering or system-output checks when the full spectrum is wanted |
The correct choice follows the question. Do not declare A weighting useless because it reduces bass, and do not use Z weighting for a hearing-exposure claim simply because it produces the larger number.
Detector And Averaging
| Reading | Meaning |
|---|---|
| Fast | Exponential time weighting with a shorter response, useful for quicker level changes |
| Slow | Longer exponential time weighting that produces a steadier display |
| Peak | Maximum instantaneous or sample-based peak according to the instrument |
| Leq | Equivalent continuous sound level over the stated measurement interval |
| SEL | Sound exposure level representing total acoustic energy normalised to a reference duration |
Fast, slow, peak, Leq, and SEL are not interchangeable. Record the detector and time interval with the number.
Calibration And Instrument Class
Absolute SPL requires a sensitivity reference or acoustic calibration. IEC 61672 specifies complete time-weighting, integrating, and sound-exposure meter performance and defines class 1 and class 2 categories. A software meter and microphone calibration file do not automatically prove full class compliance for the assembled system.
Measurement Contexts
- Tuning level: repeatable listening-position level for sweeps, channel matching, and safe workflow.
- Maximum output: defined signal, placement, weighting, detector, duration, vehicle state, and stop condition.
- Competition: organisation-specific sensor, placement, bandwidth, signal, and vehicle rules.
- Product testing: standard or lab procedure with controlled environment and distortion or compression criteria.
- Hearing exposure: A-weighted level integrated with duration and repeated exposure.
Hearing Exposure
NIOSH uses 85 dBA over eight hours as an occupational reference and a 3 dB exchange rate: each 3 dB increase halves the corresponding duration. This is a long-term occupational risk framework, not a guarantee that every exposure below it is harmless or every brief exposure above it causes immediate injury. High-level car-audio tests should be remote and brief whenever possible.
| Average level | NIOSH reference duration |
|---|---|
| 85 dBA | 8 hours |
| 88 dBA | 4 hours |
| 91 dBA | 2 hours |
| 94 dBA | 1 hour |
| 97 dBA | 30 minutes |
| 100 dBA | 15 minutes |
Maximum-Output Test Record
- meter or microphone, calibration, input range, and overload indication
- exact sensor placement and orientation
- vehicle state, doors, windows, seat positions, engine, voltage, and temperature
- signal, frequency, bandwidth, duration, weighting, detector, and peak-hold behaviour
- distortion, compression, protection, excursion, voltage, thermal, and hearing-safety stop criteria