Structure-borne sound is sound that propagates in a solid . This includes phenomena as diverse as vibrations and earthquakes , the transmission of vibrations in buildings, vehicles, machines, etc. or the ultrasonic waves used for material testing .
In addition to normal stresses, a solid can also absorb shear stresses . That is why two different types of structure-borne sound waves can propagate in solids, which propagate independently of one another:
The speed of sound is influenced by
- the density
- the hardness of the sound
- the Poisson's ratio
- the modulus of elasticity (longitudinal waves) and the shear modulus (transverse waves):
The propagation of sound in thin components such as panels and beams is of interest for technical applications . These components are limited solid bodies with a surface free of shear stress. This leads to the coupling of longitudinal and transverse waves, which creates other types of structure-borne sound waves.
The most important are the type of wave bending waves in which bending - deformations occur. The speed of sound of these waves is significantly lower than that of the longitudinal and transverse waves and it is frequency- dependent ( dispersion ). On the other hand, bending waves usually transport significantly more sound energy than longitudinal or transverse waves, and they are also the main cause of airborne sound being emitted .
An exception is structure- borne sound transmitted into the cranial bones , which can be perceived directly by the inner ear ( bone conduction ). This mechanism does not depend on the functioning of the middle ear . This is in bone conduction hearing aids and for the diagnosis in sound conduction disorders used.
Pickups on musical instruments
Contact microphones, structure-borne sound microphones or structure-borne sound pickups for musical instruments are acceleration sensors which usually use the piezoelectric effect to convert the acceleration into electrical signals. The sensors work in the frequency range that can be perceived by humans and are mounted on the surface of the instrument. This means that the structure-borne noise of an instrument is recorded where the surface is accelerated. The signal of an instrument picked up with a contact microphone can be very similar to the signal of a microphone, which picks up the airborne sound. To do this, it must be placed on the instrument in a suitable area.
The surface vibrations of individual parts of an instrument (soundboard, eardrum, body, etc.) can clearly differ audibly, although they come from the same instrument.
The greatest advantage of contact microphones, for the purpose of recording close to airborne sound, is that there is usually less feedback and crosstalk.
The recording and analysis of structure-borne noise plays a major role in technology. The acoustic properties of motor vehicles or the technical condition (wear of bearings, critical conditions during machining) of machines and generators can be checked.
Furthermore, crack formation and material failure can be registered. One example of this are break-in sensors on window panes. The pickups used for this work, as with musical instruments, on the basis of the piezoelectric effect ( piezo effect ). The evaluation of the electrical signals obtained includes the finding of characteristic frequency components and sound amplitudes.
Mechanical clocks can be checked and adjusted on the timing machine with a structure-borne sound sensor.
In principle, seismometers also register structure-borne noise, but with longer wavelengths. With the help of natural or artificial seismic events, one is able to determine the location and type of the events as well as to carry out investigations of the earth's interior. The basis for this is the different propagation speed of the longitudinal and transverse waves as well as their reflection and diffraction on layer structures.