The choice of sensor decides the success of an acoustic inspection – more than any software. Measuring microphone level in a 95 dB hall fails. Mounting an accelerometer on a vibration-decoupled comfort test sees nothing. This article helps make the right choice.
Three sensor families, three physical principles
Microphone (airborne sound)
Measures pressure fluctuations in air. Classical 1/2" free-field microphones cover 20 Hz – 20 kHz at ±1 dB. Pro: contactless, represents customer perception. Con: extremely sensitive to ambient noise – mostly unusable in loud production halls.
Accelerometer (structure-borne sound)
Piezo element converts mechanical acceleration into voltage. Range typically 0.5 Hz – 10 kHz, top units up to 50 kHz. Pro: very robust against hall noise. Con: contact problem – contact pressure and mount position influence the result.
Laser vibrometer
Measures velocity via Doppler effect – contactless, with µm/s resolution. Range up to > 1 MHz. Pro: ideal for light or hot parts, no mass loading. Con: expensive (10–80 k€), alignment-sensitive, requires optical access.
Comparison matrix
| Criterion | Microphone | Accelerometer | Laser vibrometer |
|---|---|---|---|
| Frequency range | 20 Hz – 20 kHz | 0.5 Hz – 50 kHz | 0.1 Hz – > 1 MHz |
| Noise immunity | ★☆☆☆☆ (very low) | ★★★★★ (very robust) | ★★★☆☆ (medium) |
| Mounting | contactless | glued / screwed / magnetic | contactless, optical |
| Part reaction | none | mass loading possible | none |
| Inline capability | only in noise enclosure | yes, robust | yes, with robot tracking |
| Cost / sensor | 200–2,000 € | 150–1,500 € | 10,000–80,000 € |
| Customer perception | ★★★★★ | ★★☆☆☆ | ★★☆☆☆ |
Decision guide
Comfort & sound quality (customer ear): microphone is mandatory – only in a quiet environment (hemi-anechoic room).
Inline EOL in noisy halls: accelerometer on the part or test rig. Hall noise is not "heard", inspection robust against neighbouring lines.
Resonance analysis on small or hot parts: laser vibrometer to avoid mass loading.
High-frequency signals (> 20 kHz): acoustic emission or laser. Standard microphones drop out.
Hybrid setups in practice
In many RTE projects we deliberately combine two sensor principles: an accelerometer secures the robust OK/NOK decision, a microphone validates against the originally subjective assessment. This combines inline robustness with customer relevance without requiring an acoustic chamber on the line.
Conclusion
The "best sensor" does not exist – only the right one. For every inspection task we recommend a short feasibility study with two or three sensor types in parallel. If you are unsure what works on your line, request a free initial analysis.
