Increase of Low Frequency Absorption by Means of an Air Space between a Porous Material and its Perforated Facing

Abstract

The increase of low frequency sound absorption of porous materials by means of perforated facings has been discussed by Bolt. Sepmeyer and Leonard have presented reverberation chamber measurements which show some discrepancies between theory and practice. Need for material with large absorption for a band of low frequencies led to an investigation of these discrepancies. Acoustic tube tests were made on two facings of 38-inch plywood with 12-inch holes, having 9 and 36 holes per square foot. The backing material was 3 inches of glass wool batt, which had an R/ρc of about 1.3. The first facing increased R/ρc to about 10, while the other facing resulted in values for R/ρc of about 4. With the first facing the peak absorption was only about 40 percent of the calculated value, and with the second facing about 70 percent. The increase in R/ρc indicated that the flow resistance of the sample had increased because the exposed area of the porous backing had decreased. It was decided, therefore, to investigate the effect of an air space between the facing and the backing material. It was found that the air space could be increased to reduce R/ρc to approximately that for the backing alone, and that this spacing gave values of absorption in good agreement with the calculated values. Design criteria for estimating the minimum spacing for facings with varying percent of open area will be presented.