| ADDENDUM | |
| 4.0 Absorptive Diffraction Grating
The scattering, diffusing action of distributed sound absorption has long been known (13). If absorbing or reflecting surfaces are in a regular pattern, the diffraction grating aspects of scattering are developed. Sound on a picket fence is split into two parts, one reflecting and the other transmitting. Both parts exhibit diffraction grating effects. The picket fence is a transmission type diffraction grating. If pickets are filled with a sound absorbing material, then only the reflective diffraction effect is developed. If instead the pickets are absorptive, then only the transmissive diffraction effects are observed. QSF rooms use the reflective component of the absorptive diffraction grating. |
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| 4.1 Lumped Parameter Absorption
The sound traps that have been used for diffraction wall work are 1/2 round, tubular shaped. Their interiors are hollow; their curved surface is of highly compressed, fine filament fiberglass. The acoustically resistive surface (R) in conjunction with interior volume (C) establish an effective RC acoustical circuit. This is a “high pass” sound absorber whose lower frequency cutoff is set by the value of the RC time constant. In addition to the two lumped acoustic parameters R and C, each trap has a “limp mass” reflector (L) buried in its outer surface. Thirty to fifty percent of the trap’s surface is covered with this strip that reflects 400Hz and above. The strip is usually centered on the trap. High frequencies are reflected off the strip while the lows pass through it, to be absorbed. |
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| 4.2 Diffraction Tests, Normal
Incidence
4.2A (see below) Test Setup shows the speaker mounted to the ceiling of a testing room and surrounded with 6 inches of absorption to a radius of 3 feet. This damps the ceiling image to give a sharp spike delivery. The 1/4 inch mic is 4 feet above and parallel to the floor, the ceiling is 8 feet. 4.2B (see below) shows the hard surface reflection. The floor return is nearly identical in timewise character to the direct signal, except that it is about 10dB lower in sound level. The direct signal passes by at 3.2ms and the floor bounce returns at 10.2ms. The ever present spike at 13.5 to 14ms is an extraneous reflection. The expanding point source wavefront accounts for a 20Log 12/4= 9.5dB reductio | |
A
series of tests easily show the results of the distributed reflective
and absorbing surfaces. A Techron-12 Frequency Sweep from 100 to
30K gives a 13ms Time Window, in which the ETC is taken.