The sections of duct between the test section and the reverberation room are constructed of dual-wall ducts with a cavity depth of 2.25 inches. The inside and outside walls of the ducts are constructed of 18 gauge sheet metal and the cavity is filled with fiberglass. Silencers with lengths of up to 10 ft can be tested. There are test sections in both the supply air and the return air ducts. The sound source for the ASTM E477 test consist of three speaker canisters that are mounted directly to the duct walls of either the supply or return air duct.
Pink noise is directed into the equalizer.
The crossover network is adjusted with crossovers at 300 Hz and 3,000 Hz. Sound energy at frequencies between 300 Hz and 3,000 Hz is directed through the other channel of the stereo amplifier to the mid-range speaker. Sound energy above 3,000 Hz is directed through the monaural amplifier to the high-frequency tweeters. Two piezometric rings, one placed ten feet upstream of the test section and the other placed ten feet downstream of the test section, are used to measure the static pressure drop across a duct silencer or other duct element that is being tested. The static pressure drop and the velocity pressure are continuously monitored by means of differential pressure transducers that are connected to digital meters.