Acoustic testing or quality control is a non-destructive comparative testing method based on the analysis of the natural vibration behaviour of a test object or the emitted noises of moving systems via the measurement of body or airborne sound. In common parlance, the frequency range audible to humans from approx. 16-20 Hz to max. 16-20 kHz is referred to as acoustic, the frequency range above this from approx. 20 kHz as ultrasonic. Both frequency ranges occur during acoustic testing.
The structure-borne sound is recorded either with contacting sound sensors or contactless vibrometers and the airborne sound is recorded with microphones. The determined vibration behaviour as well as the noise development are then compared and evaluated with the known behaviour of defect-free workpieces or assemblies as a reference.
The noise analysis for mechanical function testing is always carried out during operation and can detect gear faults, bearing damage, imbalances, etc. on assemblies. Sound analysis for material testing is carried out by stimulating the test specimen, e.g. by impact or sound waves, and is used to detect material defects and properties such as cracks, structural changes or to test hardening and tempering. Acoustic process control enables the monitoring of machining, assembly and joining processes, for example.

A test application is set up by teaching the sound signature of a series of good parts as a reference. During the test, the sound signature is automatically recorded and compared with the reference pattern as a setpoint. Deviations from this pattern that exceed the defined tolerance limits are detected as defects and result in an evaluation as a bad part.
The required sound transducers are selected specifically for the application. Contacting acceleration sensors or non-contacting laser and ultrasonic vibrometers serve as structure-borne sound transducers. Airborne sound is recorded with microphones, but the measurement can be affected by background noise.
Noise analysis for mechanical function testing is carried out during operation. For this purpose, e.g. in the case of electric motors, the drive is used or, e.g. in the case of gearboxes, an external drive is coupled. Mechanical assembly and machining errors such as gear errors, concentricity errors, grinding and bearing defects are detected as acoustic anomalies and the causes of the defects are evaluated.
For material testing, the test specimens are stimulated in a defined way, e.g. by impact or sound waves. The component-specific resonance frequencies depend on the size, the material and the external and internal structure of the component. If one of these properties changes, e.g. due to cracks or changes in the material and microstructure, the typical resonance frequencies change. By comparing the sound pattern with the nominal values, these faults can be detected.
In acoustic process monitoring, the quality of a process is monitored via compliance with or deviation from the characteristic sound pattern.