Acoustic noise testing is a method for quality testing of components and assemblies (e.g. electric motors, pumps, air conditioning units) that emit noise during operation. Typical “noise and vibration patterns” are defined that stand for a “good” component. These are used to compare or evaluate parts of the same type whose quality is not yet known. If deviations or anomalies are found in the “noise picture”, this is a strong indication of faults in the component (e.g. assembly errors, defects in individual parts, insufficient greasing).

The acoustic noise test can be carried out objectively and reproducibly with the help of suitable test software. The method can therefore already be recommended for the evaluation of prototypes in the development phase and also later in production for monitoring series parts.

Acoustic material testing is a method for quality testing of components (such as cast parts, sintered metal parts, plastic parts, ceramic parts) based on acoustic resonance analysis. The components are externally stimulated to vibrate and show a typical “sound” with resonance frequencies that are unique for them.

The method uses the effect that various defects such as cracks or material changes have an influence on the resonance frequencies and change the acoustic pattern. If one now compares the resonant frequencies of components of the same type and detects anomalies, this is a strong indication of possible failures in the component. The method can be carried out objectively and reproducibly with the help of suitable testing software. It is well suited for series production and enables short cycle times due to the rapid testing and evaluation.

Microphones can always be used when the frequency range relevant to the test is covered and interference from ambient noise can either be avoided by technical measures or the ambient noise does not affect the relevant area. Microphones record airborne sound signals contactless. The detection is done spatially.

A typical application for microphones is acoustic material testing or sound testing. The typical frequency range of approx. 2 kHz up to a maximum of 50 kHz is covered and cost-effective solutions are also available for automated applications. Ambient noise can either be adequately contained by technical measures or the affected frequency range is masked out.

Acceleration sensors can be used if contacting during the measurement is generally possible and the frequency range relevant to the test can be covered.

Acceleration sensors record structure-borne noise at one point and are therefore independent of ambient noise. Several vibration directions can be recorded (triaxial). A sufficiently good contact with the component to be measured must be established for signal recording.

Acceleration sensors are used, for example, for cost-effective noise testing or vibration testing. Depending on the type, these are usually designed for a frequency range of up to 12 kHz. Depending on the type of contact, the frequency range is significantly reduced.

Laser vibrometers can be used if optical contact can be established with the measuring point and the surface reflects the laser beam sufficiently. Laser vibrometers detect structure-borne noise optically at one point and are therefore independent of ambient noise. The signal is recorded without contact. Laser vibrometers with a high sampling rate are available for precise vibration measurement, which can be used for a high frequency range or speed range.

Laser vibrometers are used, for example, to test rotating parts such as ball bearings or dental drills. Laser vibrometers can also be used if no mechanical contact is possible, e.g. when retrofitting existing test benches.

A component oscillates in its resonant frequencies by a corresponding excitation (e.g. hitting). The vibrations are determined by the material properties and geometry of the component. If the properties of the component change, the resonance frequencies also change. This physical effect is used in testing technology (sound testing) for quality monitoring. 

Natural frequencies of components are measured via structure-borne or airborne noise. The physical effect is used here that a component that is stimulated to vibrate (e.g. by an external impact) vibrates in its natural frequencies. 

Natural frequencies can be measured on coated brake discs using acoustic resonance analysis (or FRF). Determining the natural frequencies after coating makes sense, since the natural frequencies can change after coating.
In general, a number of specified natural frequencies (resonance frequencies) are monitored during the production of brake discs at various stages of the manufacturing process. 

The measurement and testing of natural frequencies in series production makes sense, as this allows continuous quality control of the individual components to be carried out and, in particular, documented. Changes in the process are detected at an early stage and countermeasures can be initiated. 

Selected natural frequencies of brake components are specified as quality features in the production drawings and are used for series monitoring. The manufacturer must prove and document that the components (e.g. brake discs or brake pads) correspond to the given specification. The monitoring is usually carried out as a 100% control, but can also be checked in random samples depending on agreement.

The natural frequencies are already agreed and defined by the manufacturer in the development phase to match the overall design of a vehicle platform. The aim is to prevent resonance during braking and to minimize noise. The specified natural frequencies are monitored during the manufacturing process as a quality feature using acoustic resonance analysis (or FRF). 

What is a CAQ system? The term CAQ stands for Computer Aided Quality Assurance. CAQ is part of quality management in a company and is implemented with CAQ software or a CAQ system. It accompanies the entire value-added chain from product development through production to additional company processes for quality assurance. A CAQ is divided into various CAQ functions and CAQ modules that support processes, automate workflows and provide analyzes and reports at the push of a button.