Home theater room acoustic treatment
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:
Here are two 3D views on my room produced in CARA. First is front view towards the
The second view is of rear part of the room
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
According to the Everest book, reverberation time is
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.
For measurements I bought a copy of
ETF 5.x software
RadioShack SPL meter
($25) and made measurements of the untreated room:
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.
I installed 2” and 1” panels
and ASC Tube Traps
from Silent Source
Measurements after installing bass traps and fiberglass panels
- Four 13" diameter full round (48" high) tube traps in corners and 11"
full round 48" high traps on top of them.
- Two 2´x4´ and one 2´x2´ 2" thick panels below the screen
behind center channel speaker and subwoofer.
- Three 2&squot;x2’ 1" panels on one wall in different colors (imitating piece
- Three similar 2´x2´ 1" panels on another wall.
- One 2´x4´ panel on yet another wall.
- Five 2´ foam corner traps and one 4´ corner traps to fill some corners
between wall and ceiling.
- Two 2´x4´ 1" thick panels on the back wall which I am planning
to replace by two or four 2´x2´ RPG diffuser panels.
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
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:
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