The sound absorption coefficient improves with frequency since at high frequencies the wavelengths reduce and for a given sample the velocity is a maximum at the in­terface. Better sound absorption occurs when the incident velocity is at a maximum at the interface. Thus to improve the low frequency sound absorption either the thickness of the sound absorbing material can be increased or an air gap between the material and the rigid backing can be provided. Usually in many applications where low frequency absorption is to be improved an air gap is usually provided as shown in Fig. 6.8. Figure 6.9 shows the effect of air gap on the sound absorption coefficient of a 50.8 mm jute fiber with a density of 348.2 kg/m3.