Prediction models for Sound Reduction index

There are many ways to model the Sound insulation of constructions.

A simple model of a single construction (usually thin single leafs) can be seen below.

In the frequency range below the critical frequency the Sound insulation is mass controlled. In the frequency range above the critical frequency the Sound insulation is stiffness controlled and the resonant transmission due to the structural modes dominates the transmission.

When figuring out what model to use when predicting and calculating Sound insulation, remember to weigh the strengths and weaknesses of each model.

Prediction model for Single Thick constructions

A general idea of the Sound transmission loss Can be seen below, which illustrates how Thick single constructions behave based on its Wall dimensions (length and height). The structural modes f11 and f22 being the dominant modes at lower frequencies can have a significant impact on the transmission loss.

This Can be a very crucial piece of information regarding the transmission loss of the Wall, as the dimensions of the Wall Will directly dictate the structural modes.

This is something to keep In mind, when designing for good transmission loss at low frequencies.

Just remember that more precision In the calculation model, usually Will require better input data.

Final observation, fs is the cross over frequency from bending waves to shear waves.

Prediction model for Double wall constructions

How about another schematic for modelling Sound insulation?

This time it is about Double leaf constructions:

On the sketch below is seen a general prediction for double leaf construction in regards to its Sound insulation.

F0 is the mass Air mass resonance frequency, fd the crossover frequency of the cavity which indicates the shift from low to high frquency behaviour.
The critical frequencies of the two plates are shown with fc1 and fc2.

Of course this is a general model and will not fit every scenario.