In general, the Access Factor (AF) at any frequency is related to the Attenuation (A) at that frequency by the following formula:
AF = lO-(NlO)
How To Use The Access Factor
In order to explain how to use the Access Factor, let us recall the definition of sound absorption coefficient, used to charac- terize the sound absorption efficiency of an acoustic treatment. We saw in Figure 12 that a 1-inch blanket of glass fiber material absorbs about 20% of the incident sound energy at a frequency of 250 Hz, about 65% at 500 Hz, about 87% at 2000 Hz and about 99% at 4000 Hz. On the other hand, a 6-inch layer absorbs about 99% of the incident energy at all frequencies.
All of these numbers assume no covering over the sound absorptive material. But when we cover the material with perforated metal, we must expect some degradation of the sound absorptive efficiency. The amount will depend on the frequency, of course, but also on the choice of the perforated metal.
The Access Factor is a measure of this degradation: it describes how much "access" the sound wave has to the underlying acoustical treatment. If the Access Factor is 1.0, there is complete access and 100% of the sound energy can get through. But if the Access Factor is 0.50, then only half the sound energy can pass through; the other half is reflected from the surface of the sheet and never reaches the acoustic treatment at all.
Therefore, in order to find the effective sound absorption efficiency of an acoustical material covered with perforated metal sheet, we simply multiply the sound absorption coefficient of the basic material at each frequency by the corresponding Access Factor for the metal sheet.
For example, suppose that we cover the 1-inch glass fiber material mentioned above, having a coefficient of 0.99 at 4000 Hz, with a perforated metal sheet having TI = 1500, corresponding to an Access Factor at 4000 Hz of 0.82. Then the effective sound ab- sorption coefficient of the combination is 0.99 x 0.82 = 0.81. The perforated covering has degraded the absorptive performance of the original material at 4000 Hz by 19 percentage points. Of course, perforated sheet with a TI of only 1500 is a poor choice for this application in the first place. The whole point of the acoustical design in the "transparency approach" is to find a sheet with as high a value of TI as possible, consistent with the other requirements of the project.
It is clear from the figures given above that if we choose a sheet with acceptable transparency at 10,000 Hz (that is, small A and high AF), then everything is much better at the lower frequencies.
