It is a common practice to leave an air gap between the absorber material and the hard backing wall in applications such as suspended acoustical ceilings, although the practical use is relatively limited.68 The highest NAC will be achieved when the distance between the absorber and the wall is odd multiples of a quarter wavelengths for the sound frequency of concern. This phenomenon is due to the fact that the incident waves and the reflected waves will have a phase difference of 180°. Con­versely, when the air gap is a multiple of half wavelengths, the airspace becomes totally ineffective as the incident and the reflected waves will be in phase.17 Air gap behind the material increases NAC substantially in the low frequency range at the cost of high frequencies.68 This effect is seen in Fig. 5.10. Jayaraman61 found that the air gap caused an increase in the absorption in the frequency range between 500 and 4500 Hz. He did not find a significant difference in NAC values for fibrous mats with 5-mm air gap compared to those with 10-mm air gap, whereas the maximum peak is at a lower frequency, that is, a greater quarter wavelength for the greater depth of the gap. There are useful design charts in Attenborough and Ver17 to predict the NAC of absorbers separated from the wall with an air gap with the knowledge of air gap distance, flow resistivity of the absorber and sound frequency.