Flexible acoustic wave

Professional Definition

A bending acoustic wave is an elastic wave that propagates in rods, beams, and plates, causing their alternating bending.

In wave physics, the displacements of the medium's particles occur perpendicular to the direction of wave propagation. Unlike waves in air, bending waves are characterized by dispersion – this means that their speed depends on frequency (high tones travel through the material faster than low tones).

For an acoustical engineer, this phenomenon is crucial, as it allows the determination of the so-called coincidence frequency. This is the moment when the speed of the bending wave in the wall equals the speed of sound in air, resulting in a dramatic drop in the acoustic insulation of the partition.

Acoustics in Simple Words

Imagine you are holding a long, thin metal ruler. When you lightly strike it on one end, you will see the bend travel along it, causing the ruler to wave side to side like a snake. This is a visualization of a bending wave.

Another analogy is beating a carpet or waving a sheet of metal. The material does not compress or stretch (as it does in air), but physically bends up and down.

In buildings, it is this “waving” of walls, ceilings, and pipes that makes them act like large speaker membranes. When a neighbor strikes the floor, it excites bending waves in it, which travel through the structure and “push” sound straight into your apartment.

Summary

• Structural vibrations: A bending wave relates to solid bodies of a certain stiffness (gypsum boards, concrete ceilings).

• The phenomenon of dispersion: The speed of the wave changes with frequency, complicating insulation calculations.

• Significance in construction: This is the primary mechanism for the transmission of impact noise (footsteps, moving furniture) and the formation of acoustic bridges.