David Coate Consulting (DCC) performed reverberation time measurements in Small Army’s conference room. Excessive reverberation in this space is causing teleconferences via the speakerphone to have unacceptably poor speech intelligibility. “Reverberation” refers to the combined effect of multiple reflections within a space. Reverberation increases with increasingly hard/reflective surfaces and size of the room. In general, speech intelligibility decreases with increasing reverberation time.
The RT60 (reverberation time) is the time in seconds for sound levels to decay 60 decibels (dB). Very reverberant spaces, such as gymnasiums, may have RT60 values of 3 to 5 or more seconds. Recommended criteria for conference rooms is 0.6 to 1.4 seconds, in the mid-frequencies.
Acoustical Test Results
Using a high performance sound system to excite the space with pink noise bursts, the RT60 was measured at 9 locations within the conference room. The data was processed in octave frequency bands using a real time spectrum analyzer. Figure 1 shows the results of these tests. Existing conditions are labeled “untreated RT60” which reveals that the maximum RT60 occurs in the mid-frequencies at 1.2 seconds. While this value does fall within the acceptable range for conference rooms, it is on the high side which probably explains the difficulties with teleconferences via speakerphone. For people seated at the conference table further away from the speakerphone, the reverberant room sound may be sufficiently high to compete with the direct sound of their voices. This would in turn result in varying degrees of unintelligible speech.
Acoustical Model and Recommendations
Conference room dimensions, architectural finishes, and measured RT60 data were used to develop a computer model to replicate the existing acoustics. Next, various acoustical absorption treatment options were auditioned in the model to reduce the reverberation time to an acceptable level.
The “Treated RT60” curve in Figure 1 corresponds to the addition of 180 square feet of 2” thick Owens Corning 703 semi-rigid fiberglass. This translates to 18 5’x2’ panels, which would be covered with acoustically transparent and fire-rated fabric.
As can be seen in Figure 1, the resulting RT60 would be about at or below 0.6 seconds which is at the low end (more favorable) of recommended values for conference rooms. This should result in substantially improved speech intelligibility.
Ideally, the panels should be placed somewhat randomly on all reflective surfaces (i.e., all surfaces besides the floor). However, assuming that approach may not be practical or desirable from a visual perspective, other placement options probably will not cause noticeably different results.
For many retrofit acoustics projects like this one, visual aesthetics often may be at odds with the acoustics. For example, commercially available fiberglass panels in the quantity needed may compromise the visual appearance of the conference room. However, it is quite possible to hide the acoustical panels behind acoustically transparent fabric. Figure 2 shows an example of this where the ceiling appears to be plaster or GWB, but actually is stretched fabric with acoustical treatment behind the fabric.
Figure 2. Example Fabric System
Other completely different treatment approaches, such as spray on acoustical plaster are possible. This approach has certain visual appearance advantages. For example, DCC has recently recommended SonaKrete on acoustical design projects with very good results. Please note that an acoustical model was not developed for this project using this particular treatment approach.
As discussed, before installing acoustical treatments it will be important to verify that the speakerphone is not part of the problem. This could easily be done by conducting a test with the speakerphone located in a different space with less reverberation. Care would need to be taken to attempt to replicate the various talker to microphone distances that occur in the conference room. On that subject, once treatments are installed, it may be necessary to obtain better microphone coverage along the conference room table by adding supplemental microphones.