Page 19 - Winter 2009

Fig. 6. Transmission loss for 10.16 cm and 30.5 cm thick panels (5.95 m2 specimen area).
Fig. 4. Outer chamber wall install in a perimeter channel.
with Johns Mansville 30.5 cm thick R-30 fiberglass insula- tion. The installation of the chamber walls can be seen in Fig. 5. The inner chamber walls were constructed in a similar fashion to the outer chamber walls. They have the same 16- gauge outer skin with a 2.54 cm thick gypsum board lamina- tion. Since the panels are thicker, the cotton fiber fill is 27.94 cm thick. The main difference between the two panel designs is the inner skin, which is made of 22-gauge perforated steel with 23% open area consisting of 1.59 mm holes spaced on 3.18 mm staggered centers. By perforating the inner skin, the absorptive properties of the fill material can be used in con- junction with the wedge system. With this system, greater low-frequency performance can be achieved with shallower
wedges. The dimensions of the working area of the chamber (area inside the wedge tips) are 6.1 by 6.1 by 5.4 m (L x W x H).
To verify the performance of the design, the outer wall panels were tested in accordance with ASTM E90 (Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements) in ETS- Lindgren Acoustic System’s reverberation chamber suite. The transmission loss of these panels appears in Fig. 6.
A structural mount was installed in the ceiling directly above the test specimen location in the center of the floor. A second turntable can be attached to this mounting point to rotate a microphone array around a test specimen when rotating the test spec- imen is not feasible.
The inner chamber is lined with wedges that are made from 48.26 cm deep monolithic melamine foam with a base that measures 20.32 cm by 60.96 cm. They are installed in alternating banks of three giv- ing each bank a 60.96 by 60.96 cm foot- print. Each wedge is secured using ETS- Lindgren’s patented wedge clip system. The wedge clip system holds the base of each wedge 10.16 cm off of the inner panel skin, thereby creating a 10.16 cm airspace. The
Fig. 5. Inner and outer chamber walls during construction.
18 Acoustics Today, October 2009