12.1.2.5-6 Reverberation Time and Sound Absorption

Where oral communication is a consideration, the acoustical treatment in the control room should be sufficient to reduce reverberation time below the applicable limits shown in Figure 12.6.

Additional Information: Excessive reverberation in rooms and at workstations should be controlled by applying materials with a high coefficient of sound absorption to various room surfaces, such as coverings on the floor (e.g., carpets), acoustical tiles on the ceiling, and sound absorbing treatments on the walls (e.g., acoustical panels or thick drapes). Wherever possible, these sound absorbing materials should be rated by Noise Reduction Coefficient. Many manufacturers of such materials specify the Noise Reduction Coefficient of their products. The Noise Reduction Coefficient method measures the Sabine sound absorption coefficients ('”Sabine alphas”' or '”aSAB”') in octave bands from 125 Hz to 4,000 Hz. To generate a single-number rating, the Sabine alphas for 250, 500, 1,000 and 2,000 Hz are averaged to compute the Noise Reduction Coefficient. Theoretically, a Noise Reduction Coefficient = 0 indicates perfect acoustic reflection (i.e., no acoustic absorption), and a Noise Reduction Coefficient = 1 indicates perfect acoustic absorption (i.e., no acoustic reflection). However, because of the averaging method employed, Noise Reduction Coefficient values slightly greater than one are sometimes observed. For example, ordinary drywall may have a Noise Reduction Coefficient of 0.2, while sound-absorbing ceiling tiles may have a Noise Reduction Coefficient between 0.9 and 1.0, or slightly higher than 1.0. The Noise Reduction Coefficient is being replaced by the Sound Absorption Average, but many commercial sound- absorbing products are still rated in terms of the Noise Reduction Coefficient. ASTM C423a (2009) contains more information on the Noise Reduction Coefficient method. 1472G

 

Figure 12.6 Acceptable Reverberation Times

(Source is Figure 39 in Mil-Std-1472G, 2012)