Sound waves pass through your traps/ absorbers *twice*. in and back out again. So you are exposing the surface you have twice. Having an air gap lowers the effective frequency you can manage for a given thickness of foam. AFAIK there are no differences across frequencies.
Muhammad, M., Sa’at, N., Naim, H., Isa, M., Yussof, H., Yati, M., 2012. The effect of air gap thickness on sound absorption coefficient of polyurethane foam. Def. ST Tech. Bull 5, 176–187.
(from the paper)
The main function of acoustical materials is to reduce the acoustic energy of sound waves that pass through it.
This can be performed using resistive materials that consist of porous structure which change sound energy into heat.
This can happen when energy is changed due to frictional forces between sound waves and the cell walls in the pore structure (Fang et al., 2007).
In other words, the further the distance sound waves travel through the medium of porous structure, the higher the amount of sound energy which is dissipated.
In practice, there are two methods used to install acoustical materials; either using an air gap or not.
By introducing an air gap behind an acoustical material any residues of incident sound waves that are not absorbed or transformed into heat energy after transmission through the acoustical material will face additional resistance through the air medium.
The mechanism of sound waves dissipating in the air medium is known as the Helmoltz resonance effect.
It excites some of the sound waves’ frequencies, causing them to oscillate at greater amplitude. When the energy reaches the maximum level, the sound waves become weak due to friction with air particles, which converts sound energy into heat (Zhang et al., 2012; Norton & Karszub, 2003; Crocker, 2007).
Previous studies show that the combination of acoustical materials and air gap thickness has significant impacts on the sound absorption coefficient.
See Figure 7 for a graph showing sound absorption coefficient and wave frequency for the different air gap thickness sound absorbing characteristics of polyurethane foam (the paper is on researchgate if you are interested)
Their results showed that the air gaps reduced the sound absorption capability at high frequencies of sound waves.
Both methods, with and without air gap, were not able to absorb the low frequencies of sound waves, particularly below than 250 Hz.
The implementation of combination polyurethane foam with air gaps was able to enhance the sound absorption coefficient in the medium frequency range without changing the thickness of acoustical materials.
ciao
gr
