The mathematical formulas corresponding to the curves in this nomogram are, respectively:

A(10KHz) = -22.S6log log (TI) + 0.008vn- + 13.79 dB and
AF = 10-(N¹⁰).

The first formula is valid for values of TI up to 50,000, but only for a frequency of 10,000 Hz; the second formula is generally valid for any frequency for which the value of A is known.

Example 8

Suppose you have a perforated metal sheet with TI = 4000, used as a covering for a sound absorptive glass fiber blanket. What is the effect of the covering? Enter the lower horizontal scale of Fig. 23 at TI = 4000, move upward to strike the lower curve, then move left to find an attenuation at 10,000Hz of 1.8 dB. Next, move to the right from the first intersection point to intersect the upper slant line, then upward from this point to the horizontal scale to find an Access Factor of 0.66. With this sheet covering an absorptive material, you will realize only 66% of the intrinsic absorption performance of the glass fiber material at 10,000 Hz.

NOTE: See Appendix C for an important technical qualification to the use of the Access Factor, as prescribed above.

NOTE: Full-sized, clean versions of Figs. 21, 22 and 23 are included in Appendix D at the back of this booklet, to be copied and used as worksheets.

Table 1 presents calculated values of the TI, the Attenuation (A) and the Access Factor (AF) at 10,000 Hz, for a group of the most commonly manufactured perforated metals.