Acoustic mass law
The first rough approach to calculate sound insulation for a single leaf panel is the so called acoustic mass law. According to this theoretical model the sound insulation increases with the surface density (kg / m2) and the frequency according to following formula:
Acoustical mass law for a single wall:
For a given material the mass law is valid in a limited frequency range, but other effects reduce the sound insulation at certain frequencies.
A principle diagram for the sound reduction index over the frequency is shown below:
The resonance frequencies depend on the size of the vibrating panel. In most cases the fi rst resonance frequency f11 results in a clear dip in the sound reduction, while higher resonance frequencies do not become that visible in the sound reduction curve.
The critical frequency, where the speed of sound in air is equal to the propagation velocity of the bending waves in the panel, creates the second serious dip in the sound reduction curve.
The two characteristic dips of the sound reduction, at the resonance frequency and at the critical frequency, can be very important when relating to the dominant frequencies from the source.
