What is structure-born sound?
Acoustics is the science of sound – more precisely, it is the science of vibrations and oscillations in solid, fluid, and gaseous media. Sound can be described as the propagation of local pressure fluctuations in elastic media. These pressure fluctuations are caused by a mechanical deflection of atoms from their equilibrium position.
Sound is classified by the medium in which it propagates. Unlike air- and liquid-borne sound, structure-born sound propagates in solid structures. The propagation speed depends on the medium. In many cases, air-born sound is a result of structure-born sound radiation.
In general, a distinction is made between periodical and impulsive signals. The frequency of a signal describes the number of periodically repeating events within a specific amount of time. The number of these events per second are characterized by the unit Hertz [Hz]. The reciprocal value of the frequency is the period, which means the duration of one event.
Acoustic emission events occur from sudden deformation of a material under stress. A distinction is made between tensile stress, compression stress, and shear stress, which usually occur at the same time. Outside influences, e.g. thermic, acoustic, or mechanic stress, lead on to elastic or plastic deformation.
Joseph Kaiser already determined in 1950 a proportional correlation between the stress on metallic materials and the acoustic activity arising from that. Acoustic emission is the measurable output of the propagation of acoustic waves resulting from mechanical deformation. This may occur for example from sudden crack events. These are elastic waves arising from structural change in material under stress. The resulting crack disturbs the equilibrium of the material’s field of tension. This disturbance is compensated trough an elastic wave with a steep rise and spherical propagation through the medium. These waves cause every point of a solid to oscillate around its position of rest. The oscillation is a continuous conversion of potential energy to kinetic energy and vice versa. Essentially there are transversal bending waves (perpendicular to the body’s axis) and longitudinal waves (along the direction of the body’s axis). In addition, there are also torsion waves which result from twisting the body.
Measuring acoustic emission is an excellent mean for condition monitoring. Structure-borne sound sensors measure the acoustic emission directly at the source without interfering or damaging components. Acoustic emission usually occurs in ultrasonic frequencies between 150 kHz and 500 kHz thus needing sensors with an accordingly broad bandwidth for detection (e.g. iMPactXS). Along monitoring cracks and fractures structure-borne sound sensors can also be used to determine an “acoustic fingerprint” of the component or process being monitored. This enables the recognition of even minimal deviations and thus improving the quality management.