Home theater room acoustic treatment



Room modeling

For room modeling I purchased a copy of CARA 2.1 room acoustic modeling software ($75). The software is good enough for simple room mode calculation, modeling frequency response, reverberation and speaker positioning. It has a library of construction, furniture and acoustic materials which you can extend with your own ones. Library of speakers is fairly limited though. It is possible to add your own but then you have to know a lot about innards of them, such as crossover frequencies and such. You can accept defaults and that is what I did, but I am not sure how much it affects precision of the model. The software produces reasonable results, however, its major drawback is that it only takes into account absorption coefficients and has not diffusion coefficients. Hence RPG Diffuser will look like absorption panel, not as diffuser.

My room is L-shaped, approximately 20 feet long by 15 feet wide in the widest section. Here is the floor plan:

Listening room floor plan

Here are two 3D views on my room produced in CARA. First is front view towards the projection screen:

Room view (front, towards the projection screen)

The second view is of rear part of the room

Room view (rear part)


Analysis

I’ve purchased Alton Everest "Handbook of Acoustic" which is very readable and does not require much beyond basic knowledge of physics, waves and such. It contains good recommendations for listening rooms as well as for studios with examples. According to the book, the floor carpet is actually pretty bad idea since it introduces too much absorption at high frequencies and you can’t also treat ceiling since then the room becomes too "dead". Which CARA software confirms by showing that best deal is a hardwood floor with a rug on it and some ceiling and wall treatment. However, removing carpet was not in my plans, hence I decided not to mess with ceiling and live with early ceiling reflections. I decided to concentrate on reducing echo and improving bass control.

According to the Everest book, reverberation time is

   
RT = 0.049*V / S

where RT is reverberation time, V – room volume in cubic feet and S is absorption coefficient in sabins. Typical reverberation time for smaller rooms should be < 0.5s. My room is 20x15x8 ft = 2400 cu ft. Hence I need S = 240 sabins. 1” fiberglass panel has absorption coefficient around 1.0 at 500Hz. Room has carpeted floor and some furniture and drapes which yields ~150 sabins. Hence it seems that I need ~100 sabins of absorption or 100sq ft of rigid fiberglass or 25 1” 2x2 ft panels. Fiberglass tends to absorb more at high frequencies so I needed some dedicated bass traps and corner fillings which would add to the overall absorption coefficient. I decided on about 15 panels and some tube bass traps and corners for better bass control. ASC Tube Traps are absorptive at low frequencies and reflective (diffusive) at high frequencies.

Measurements

For measurements I bought a copy of ETF 5.x software ($150), RadioShack SPL meter ($25) and made measurements of the untreated room:

Untreated room impulse response


Untreated room low frequency response


Untreated room low frequency spectral decay


Untreated room reverberation time


As expected, the room showed very uneven low frequency response and tons of reflections. 2 ms reflection on the impulse response graph was most probably early reflection from ceiling and 14ms one was reflection from rear wall. I believe there was no floor reflection at least at high frequencies because of the carpet. Reverberation time showed significant boom which was confirmed by the long LF decay time. Read: too much LF echo or boom.

Treatment

I installed 2” and 1” panels from Acoustical Solutions and ASC Tube Traps from Silent Source


Measurements after installing bass traps and fiberglass panels

Treated room impulse response


Treated room low frequency response


Treated room low frequency spectral decay


Treated room reverberation time


Impulse response looks better, no peak at 14ms and fairly good diffusion at 10ms and beyond. I am not sure what that peak in reverberation time at high frequencies is about. It might be just microphone error since RadioShack SPL meter is not good beyond 10KHz.

It is a bit funny that ETF shows some increase in the reverberation time in the mid frequency region. I have no explanation for that.

LF response was still not what I hoped for so I added Behringer parametric equalizer ($229) to the subwoofer channel and managed to get more even bass response:

Treated room reverberation time with low frequency equalization applied

Treated room reverberation time with low frequency equalization applied


Speakers on carpet and Speakers on tiles

Finally I placed left and right speakers on spikes on ceramic tiles which sit on the carpet. I did the same to the subwoofer. This leveled frequency response in 150-200Hz area.


Low frequency response with speakers standing on carpet with spikes

Low frequency response with speakers on spikes on ceramic tiles