Case Study

High-end Cinema Extension

Problem: In this case study of a high-end home cinema that is primarily used for critical 2 channel listening, there were some notable room resonance slurring from about 40-55 Hz. The original acoustic design filled the room with Tube Traps, which couldn’t quite tame the annoying low frequency anomaly. While characterizing the room, it was noted that the ambient noise floor was also troublesome from both the HVAC and projector.

Prior acoustic treatment

Prior treatment layout

Finished FRP system

Solution: An onsite visit allowed actual characterizing of the room and system interactions. To address the bass problems, A/V RoomService recommended lengthening the room 18” in order to smooth out the coincident room modes. The new dimensions allowed for much better mode distribution as well. We also optimized the speaker/listener positions, and introduced our Frequency Response Panel (FRP) system. To address the noise floor problems, we took measurements of the HVAC system for modeling and made several recommendations to lower the noise of the system, which was a distraction whenever it turned on or off. We also moved the projector behind the new rear wall and built a noise rated port window (incorporating special optical glass panes for video color integrity) to eliminate the fan noise.

Result: Below are actual before and after treatment results. As can be seen in the articulation graph below (fig. 1), the troublesome bass frequencies have been eliminated. Overall, bass response is now linear and fast to start and stop. The Noise Criteria graph (fig. 2) shows the improved drop in the noise floor, which indicates a > 5 dB SPL improvement in dynamic range for the room, which also means higher resolution in low-level details. Finally, the room now sounds very natural and articulate due to the FRP system, which attenuated all first order reflections by about 15 dB, smoothed out room modes by an additional 4.4 dB from 325 Hz. and down, and controlled reverberation times to an average RT-60 of 0.25 seconds from 260-3,740 Hz. (fig. 3). Voicing was included a second time when the client changed speakers in the system from Wilson Audio to Dynaudio.

“Norm led me through a process that has transformed my room acoustics and the performance of both my 2 channel and home theater system to the levels I had dreamed of achieving...and beyond. On his first visit he correctly diagnosed my problem as a combination of room acoustics and speaker placement. I was so impressed with the results of his first visit that I implemented all of his recommended changes which included extending the length of the room by 18" to eliminate a nasty 50-60Hz resonance and replacing my extensive tube trap treatments with a full room FRP treatment. He also made important recommendations that led to me making HVAC modifications to reduce noise”.

Fig. 1 Bass slurring elimiated

Fig. 2 HVAC noise floor lowered

Fig. 3 Reverberation times controlled

Click to enlarge graph

Reverberation is probably the most recognized characteristic of a room’s sound. Reverberation is the acoustic energy in the space that lingers on after the sound stimulus has been removed. Each room’s reverberation times at each frequency are as unique as a signature. Ideally, we want the reverberation times to decay at the same rate across the audible bandwidth, and within a time window of about 0.25 - 0.35 sec. This allows for neutral sound conditions. An exception is for frequencies below about 100 Hz., where we need slightly longer decays in order for our brain to make sense of the difference between what our ears hear vs. what our eyes see. Lack of reverberation control results in masking of low-level details, loss in dynamic range, soundstage, timbre and articulation.