What room shape for ideal acoustics?

[Fidelity4Me]

After reading many articles and forum discussions here and elsewhere (HomeTheaterHiFi.com is nice), I am considering building a “performance first, aesthetics second” dedicated listening room and home theatre in my basement. I have relatively free rein (restricted by total dimensions and modest budget) to design and build this small space as an acoustically-calculated rectangle, amphitheater-like trapezoid, or just with slightly off-kilter walls in an attempt to avoid standing waves and room resonances. I intend to employ at least a few absorbers and diffusers, carpeted floor, drop-tile ceiling and careful placement of the speakers in order to best manage the overall sound. Ultimately, I hope to have a total of 5 HT recliners (2 moveable in front with three built together and raised 10 inches behind) with 6.1 sound (Rotel 1065, a Velodyne DD-15 and B&W CDM 9NTs, SNTs, and CNT), tight light control and some form of projector. I would love for this space to be an optimized compromise for 2-channel and multi-channel audio for one (me, with the single recliner in the absolute sweet spot) but useful for up to five HT viewers so I can share the fun.

I am looking for any experiences, suggestions, warnings, favorite measuring devices (I have the Radio Shack SPL meter, AVIA HT setup, a few MP3 audio test files and Rives Audio Test CD 2), or resources regarding room shape and resulting acoustics. I have a decent workshop and would rather save money by going the DIY route than simply calling in an expert team (at least at first). Luckily the DD-15 offers fairly flexible bass management with its parametric EQ and spectrum analysis, but I want to attempt to manage all of my audible frequencies. My fear is that I may take the walls slightly out of parallel and create an incalculable and untamable acoustic nightmare with modes galore and a timbre like the inside of brass instrument’s bell. Thank you in advance for your advice and comments!

Paul

 

[Mudcat]

How big a screen? Give me a few days

 

[Fidelity4Me]

Mudcat,

I believe that the screen would likely be in the 8' range or smaller.

I did some suggested reading on the Harman website regarding Floyd Toole's white papers on room acoustics, treatments, and modes. Good stuff, that. I also experimented a little with reflection control last night. I placed pillows and quilts strategically around my speakers and behind my listening couch. There was a marked tightening of images (Norah Jones was only 2 feet wide, not 5 feet wide as she always was before). I suppose I always considered "smoothing out the soundstage" sort of a good thing from speakers, being pleased with the illusion of "widening the sweet spot" in my naiveté. The newfound pinpoint sound sources last night were refreshing, although it will take some time to get used to the effect. Ahh, absorption! That's what it does! It is also clear from this experiment that even though it is angled down properly to the listening position, my center channel on top of my TV (the most convenient spot), being 6 feet above ear level, is killing my multichannel soundstage. I suppose exposing problems with speaker placement validates the "improvement" that absorption has caused in the room.

Although my current space is not ideal (shared living room with no wall into kitchen/dining room, shed cathedral ceiling, 5 oversized wood blind covered windows, etc.), I think I will start there by procuring recommended "Owens Corning Select Sound Black Acoustic Board" and some acoustically transparent black fabric. In this shared room, a set of moveable panels to place behind and beside the main speakers and one larger panel behind the couch might work well to deaden the reflections a bit. I could set them up when I want to listen or watch movies more "accurately" and they would not interfere aesthetically otherwise. I'm now thinking of copying the rounded grill shape of the B&Ws and framing the homemade panels in routed solid cherry to give them a more furniture-like and uniform audio appearance. I can do all that window-dressing after I prove the effectiveness of just the raw fiberboard in-situ. Although I am limited to only one subwoofer, the DD-15 has measurably and effectively, I believe, allowed me to position it ideally and tame the worst of my room modes.

So perhaps as a starter I'll just spend a few bucks on some improved seating (maybe eventually that projector) for now and manage the sound as best I can in the most convenient room available. That doesn't mean the dedicated room is dead, perhaps just postponed for additional experimentation.

An additional question regarding measurement, though. I bought the Rives Test CD 2 because it is weighted to work with the Radio Shack SPL meter. Toole indicates that 1/3-octave intervals are insufficient to truly equalize a room. He also suggests (in my interpretation) that there is little to be gained from equalizing mid and high frequencies, just control reflections and resonances with absorption. Is there an effective and relatively inexpensive way to measure a more complete audible spectrum? Should I get a series of 1/10-octave sine wave frequencies as MP3s off the 'net and methodically measure and graph the responses? Perhaps I should just be content to equalize below 200 Hz with the Velodyne's parametric EQ and not even bother measuring the higher frequencies?

I continue to be open to and thankful for your suggestions. Thank you again!

Paul

 

[Mudcat]

(Norah Jones was only 2 feet wide, not 5 feet wide as she always was before).

I never saw Norah Jones. I did get to see Grace Smith sign God Bless America at a Flyers game once. See was about 5 feet wide (and growing).

 

[Rip Van Woofer]

Rectangular.

Angled walls do not eliminate room modes and standing waves, but merely move them about in increasingly unpredictable ways.

Carefully read the Rives articles here. Buy yourself a copy of The Master Handbook of Acoustics and study it (it is written for laypersons, not experts). Take a look at the room acoustics article on Siegfried Linkwitz's site (www.linkwitzlabs.com). Read Floyd Toole's whitepapers on the Infinity Speakers site (oh, you already did!).

Once you have done so, you will see that it is not necessary to sacrifice aesthetics for acoustics, nor resort to odd room shapes. On the contrary, many attractive and traditional interior decorating and architectural features such as coffered ceilings, wainscoting, bookcases, carpeting, drapery and furniture can be used as acoustical features as well.

The sound quality of mid to high frequencies, according to Toole and others, is primarily dependent on the quality of your speaker. Some audible (and annoying) frequency anomalies will of course result from reflections (more precisely, as reflections meet and either multiply or cancel each other) but not as large as those caused in the bass by room nodes; the main problem in mids/highs will be an excessive resonance and echoes that smears and muddies the sound (and eventually gives you a headache!)*. Kind of like what you get in a bare room or in the bathroom, but not as exaggerated.

I've been thinking of building a room in the basement myself, and going DIY for the same reasons. Mine would be strictly music, though. Keep us posted so I can learn from your mistakes!

And the thought of a five foot wide Norah Jones is very disturbing BTW...

*The behavior of sound in rooms is fundamentally different in bass vs. highs and mids, with the dividing line somewhere around 200 or 250Hz. It's in the book...

 

[Jaycan]

RV Woofer
Didn't you at one time have "The Audio Perfectionist" Richard Hardesty on your wisdom page?

 

[Rip Van Woofer]

RV Woofer
Didn't you at one time have "The Audio Perfectionist" Richard Hardesty on your wisdom page?

No. I looked at his stuff and mostly didn't like it. I still shill shamelessly for Peter Aczel's "The Audio Critic", however, in spite of their erratic publishing schedule.

Back to basement listening rooms: have you looked at the listening room examples on the Rives site? In one of them they show a basement room under construction where they nailed arrays of angled 2" x 10"s (?) up between the main floor joists for a louver effect, and there seems to be fiberglas board behind those. Then a conventional looking suspended ceiling covered all that up. Looks like a comination of absorption and diffusion treatment. I'd love to know more about the hows and whys of that but I'm sure they won't give away their secrets so I haven't asked. Anyway, take a look. It's pretty interesting.

And, hey, if Mr. Rives is feeling generous...

 

[Fidelity4Me]

Gentlemen (loosely defined),

Thank you for all the suggestions. This subject will keep me out of trouble and in my woodshop for a while.

I found a limited source for Owens Corning Black Acoustical Blanket (100' x 48" x 1") asking $150 a roll. I bought one and he has two left (Southern or Southwest Insulation here in Denver - he even has common carrier contacts). I have it currently unrolled and placed vertically like a little 4' curtain all around my living room and it has deadened everything wonderfully.

I may be back to the idea of acoustically treating this odd room rather than dedicating another room. My current thoughts (I do seem to be so terribly fickle these days) are to create folding 2- or 3-panel absorbers (like Japanese room dividers) that can be readily removed when I am not listening or watching "critically." An exposed 1"x3" 45-degree-chamfered poplar frame surrounding 2 layers of fiberglass (totalling 2" thick) encased under a 1/2" roundover-routed decorative edge under a black fabric covering to make it all look official and more furniture-like than just some covered insulation.

The insulation has had a dramatic effect, which I would consider an improvement in the sound: very precise imaging. The downside is that if you are even inches out of the stereo sweet spot the image deteriorates rapidly. I think that 4 carefully placed screens instead of a fully-enclosed curtain of insulation will return enough reflection to the room to be a better compromise between ultimately selfish imaging and sharing with others.

I'll keep playing in the shop and checking back here for ideas. Thank you for the help thus far. I just hope everyone else has as much fun playing with this stuff as I do. My audiophile friend keeps asking, "Is it worth the effort: does it sound better?" I simply respond, "It's different, it's fun, and I'm having a great time."

 

[behringer]

Attic for music room

I could clear up the attic and furnish it to make it comfortably liveable as a music room Curious if the angled roof sides with plywood forming what I think is a 60 degree angle would make for better sonics than a regular box room. Any opinion out there?

 

[Iain]

Recommended reading

Try getting a copy of 'The Master Hand book of Acoustics' by F. Alton Everst

It is a excellent book covering everything you'll want to know about acoustics. It even mention that the piont at which modes stop being such a problem is about 300Hz for most rooms...

or... f= 11,250 * Sqrt(rt60/V)

-Iain

 

[Rip Van Woofer]

I could clear up the attic and furnish it to make it comfortably liveable as a music room Curious if the angled roof sides with plywood forming what I think is a 60 degree angle would make for better sonics than a regular box room. Any opinion out there?

Don't know if it would be better; I've heard pro and con on cathedral ceilings. But nothing you coudn't fix if it turned out to be a problem. Go for it.

That's what I hate about these newer houses with the engineered roof trusses: no useable attic!