Page 280 - Master Handbook of Acoustics

FIGURE 12-26 Measured absorption of polycylindrical absorbers of various chord and height

dimensions. (A and B) Only empty poly data are available; the broken lines are estimated absorption
when filled with mineral wool. (C and D) Both empty polys and polys filled with mineral wool.
(Mankovsky.)

Poly Construction

The construction of polycylindrical absorbers is fairly simple. A framework for vertical polys is
shown in Fig. 12-27, mounted above a structure intended for a low-frequency slat absorber. The
variable chord dimensions are apparent, as are the random placement of bulkheads so that cavities

will be of various volumes, resulting in different natural cavity frequencies. It is desirable that each
cavity be essentially airtight, isolated from adjoining cavities by well-fitted bulkheads and
framework. Irregularities in the rear wall can be sealed with a nonhardening acoustical sealant. The
bulkheads of each poly are cut to the same radius on a bandsaw. Sponge rubber weather stripping
with an adhesive on one side is stuck to the edge of each bulkhead to ensure a tight seal against the
plywood or other hardboard cover. If such precautions are not taken, rattles and coupling between

cavities can result.

FIGURE 12-27 The construction of polys in a motion picture sound mixing studio. Foam rubber anti-

rattle strips are applied on the edge of each bulkhead. Also note the random spacing of the bulkheads.
(Moody Institute of Science.)

The polys of Fig. 12-28 use 1/8-in tempered Masonite as the poly skin. A few hints can simplify
the job of stretching this skin. In Fig. 12-28, slots of a width to snugly fit the Masonite are cut along