David Coate Consulting (DCC) analyzed various interior acoustical aspects of the planned Dover Community Center, in Dover, Massachusetts.
DCC created an acoustical model of the Community Room which is based on the planned architectural finishes and corresponding surface areas as well as the room volume. The baseline treatment scenario consists of 459 sqft of 5/8” thick Armstrong Acoustabuilt, placed in a band around the room. Table 1 shows the input data and calculated Reverberation Time (RT60). The RT60 is the time in seconds it takes for reverberation to decay 60 decibels. Excessive reverberation can interfere with speech and cause other undesirable acoustical effects.
Table 1. Community Room Reverberation Time for Baseline Treatment
Figure 1 shows the baseline treatment option compared with Speech Interference (SI) criteria. The analysis shows that the baseline treatment option would not be sufficient to reduce reverberation below the SI criteria.
Figure 1. Community Room Reverberation Time for Baseline Treatment
Next, DCC modified the model to increase the surface area of Acoustabuilt to a total of 1260 square feet. This modification would cause the reverberation time to meet the SI criteria at frequencies 500 Hz and above. Reverberation at lower frequencies would cause the room to sound somewhat “boomy” but not interfere with speech since speech typically peaks in the mid-frequencies.
An acoustical model for the Fitness/Dance room was developed using the previously described methodology. The model assumes that the ceiling would be comprised of the 5/8” Acoustabuilt treatment, and with no Tectum treatment on the walls. Tectum was under consideration for wall treatment if necessary. Figure 2 shows the results of this analysis indicating that reverberation in this space would be under control with this acoustical treatment. The peak at 250 Hz is due to all surfaces having poor absorption at that frequency.
Figure 2. Fitness/Dance Room Reverberation Time
Flanking Paths Since a modest sound system would be used in this space for fitness, there is some concern for paths of that sound reaching the conference room and flex rooms via a flanking path through the attic. DCC modeled this assuming the 5/8” Acoutibuilt ceiling, a 5/8” GWB layer, 13” airspace 50% filled with mineral wool, and ¾” plywood on top of the rafters. Table 2 shows the model results and Figure 3 shows the result graphically. Figure 3 shows that the sound levels would be at NC 30, which is relatively low, but perhaps audible since it is above the threshold of hearing. Note also that the sound system sound pressure level, measured by DCC at another fitness room, is relatively high. Sound levels for this project could be set to be below this level to maintain the estimated level in adjacent rooms.
Table 2. Fitness/Dance Flanking Path Model Results
Figure 3. Estimated Fitness Room sound system level in Flex Room
Flex Rooms and Conference Room Since Figure 3 shows that the Fitness room sound system might be audible in the flex rooms and conference room, recommended treatments would include a 5/8” GWB ceiling with ACT beneath (to control room reverberation).
The recommended moveable partition (Modernfold Model Acousti-Seal #931) Transmission Loss is STC 50. The bulkhead for the partition should also be at STC 50 which is expected with four layers of 5/8” GWB with a four-inch airspace and cavity 50% filled with mineral wool.
Recreational Room (Gym)
The Recreational Room (Gym) was modeled using the previously described techniques, assuming TorisA (2.5” acoustical decking), GWB walls and linoleum on concrete floor. Figure 4 shows the results of this analysis which shows that the expected Reverberation Time is at or below the SI criteria, except in the high frequencies.
Figure 4. Recreational Room Reverberation Time
The wall separating the gym from the Parks and Recreation offices is planned to be 12” concrete masonry unit which is assumed to be hollow core. Figure 5 shows sound levels from the gym sound system in the offices assuming the same relatively high measured sound system level used in the previous analysis.
Figure 5. Gym Sound System Levels in Parks and Recreational Offices
The resulting gym sound system level in the offices would be relatively high (NC 40) with the 12” CMU. While the generous 12” separation provides good sound isolation, the hollow core blocks are relatively lightweight (compared with solid blocks) and the assumed sound system level is also relatively high. Figure 5 also shows the results of adding another 2x4 wall layer assuming 5/8” GWB and the cavity 50% filled with fiberglass batt. This results in a sound system level of NC 25 which is low and possibly masked by HVAC noise in the offices.
COA Offices Since there are no major noise producing activities adjacent to the COA offices, the walls of the offices should be specified at STC 50 which could be achieved by a wall construction of four layers of 5/8” GWB with a four-inch airspace and cavity 50% filled with mineral wool. Similarly, expected noise levels from flex room activities above are expected to be low, so a floor/ceiling assembly with similar construction to the walls should achieve STC 50. Acoustic ceiling tiles should achieve sufficient reverberation control for the COA offices.