Case Study

Resonating Cinema Seats

Problem: In this case study, we had no problem until the very end. We designed and built the cinema (mainly used for 2 channel listening) from scratch including floating riser, ceiling and walls, HVAC, electrical, acoustically rated door, Frequency Response Panel system, etc. The client is an ear, nose and throat surgeon who really enjoys music. This room, at the client’s request, was the first room of the new house to be completed. We performed onsite voicing and everything was going great and I was just about ready to pronounce the acoustic treatments and all calibrations completed. During voicing I had used a non-cinema chair to easily move about, but when I placed the custom made seat into position, there was a 90 Hz. “muddyness” in the playback!? Both the Doctor and I could hear it. I did not hear it a minute ago, nor did any of my measurements indicate it. Going on a hunch, I switched back to the chair, and the problem was gone. I moved the cinema seat back, and there it was again. What was really odd was that neither one of us could feel the vibration on our body, yet I was able to confirm that the seat resonated at about 90 Hz. What was happening was bone conduction resonating our skull and/or middle ear. The doctor, being a specialist of the ear, completely understood why the problem existed, but of course was very upset about its presence.

Solution: After taking apart the seats, we could see a spring system inside the cushions that was likely the culprit. Using a tone generator through the playback system, it did sing. It was actually the metal suspension bars held by the springs that resonated. After a run to the hardware store and experimenting with different materials, we found a winner. Cutting up pieces of commercial-grade rubber flooring mats, and placing them in-between the springs and against the metal suspension bars stopped the resonance. A cheap and easy fix!

Seat before treatment

Seat after treatment

Prep for FRP acoustic treatment

FRP before fabric

Finished FRP

Result: Below are the actual before and after treatment reverberation times. As can be seen in the graph below, the room now sounds very natural and articulate due to the FRP system. Note the huge before and after differences between the acoustically treated room vs. the untreated.
·  FRP is effective down to 63 Hz. and is only 2.5” deep
·  Controls room modes, first order reflections, reverberation times and flutter echo in a linear, tunable fashion
·  Conceals with an acoustic stretch fabric system
·  Fitted on-site by professional installers

Unique room reverberation time signature before FRP acoustic treatment.

Controlled reverberation time signature after FRP acoustic treatment

Click to enlarge graph

Reverberation is probably the most recognized characteristic of a room’s sound and the most difficult to control. 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 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.