Calculating The Dimensions Of The Tuned Absorber To Give The Desired Resonance Frequency
Having determined the desired frequency fR of maximum absorption for the tuned absorber, the next step is to calculate the required dimensions for the various elements, in order to make the absorber resonate at the desired frequency.
For this purpose, we use the nomogram on p. 24, where:
fR = resonance frequency (Hz);
h = distance between the perforated sheet and the solid wall, in inches (see sketch, p. 22);
e = effective "throat length" of the holes; it is given by: e = t + 0.8d, where t is the sheet thickness and d is the hole diameter, in inches;
p = percent (not fractional) open area of the sheet.
For round holes, staggered: p = 0.9 (d/b)2 x 100%;
For round holes, straight: p = 0.8 (d/b)2 x 100%,
where b is the on-center spacing of the holes and d is the hole diameter, both in inches.
[See below, p. 30, for an Important Note qualifying this design procedure.]
