Dome/Hemispherical room.... blessing or curse?

TRADERXFAN · « **on:** 10 Oct 2009, 01:33 am »

I wondering about a monolithic cement dome for a detached, dedicated audio room.
http://www.monolithic.com/topics/domes

Would this shape be good/bad or indifferent for sound quality?

-Tony

richidoo · « **Reply #1 on:** 10 Oct 2009, 03:01 am »

http://www.silcom.com/~aludwig/Physics/Main/SphCavRes.htm

With acoustic treatment according to the measured frequency response it should will be fine, but no better than a rectangular box for modes. Speakers will need to be a little further out from the "wall" because volume at the edges is reduced by tilting wall. The floor/wall corner can be used for bass traps. The center point on the floor might prove to be an interesting point for a tuned bass trap corresponding to the wavelength which equals the radius of the dome. Instead of treating one point per wall for 1st reflections, there will be a long curved strip to be treated if you sit on one side and speakers on the other of the center point. If you sit on same side of room with speakers, then the first reflection strip will be behind you, giving reflected sounds later than direct - so no problem. But that would require a pretty large dome.

srlaudio · « **Reply #2 on:** 10 Oct 2009, 05:33 am »

I believe it would be a horrible choice. The concave surface is one of the primary things to avoid in good room acoustics. I have been involved in several acoustical consulting jobs to fix the effects of a concave surface. A combination of thick absorptive panels and large scale 1-D diffusers has reversed the bad reflection into a good one.

JLM · « **Reply #3 on:** 10 Oct 2009, 09:18 am »

I have extended experience in two geodesic domes. They have weird acoustics (that are poorly documented and probably haven't been studied much).

Standing in the center can be amazingly resonant.

OTOH they also act like parabolic reflectors in that a sound on one "side" is easily heard on the opposite "side".

My guess is that a simply round room would have similar characteristics, but less pronounced.

Overall, both would be horrible choices IMO.

pbrstreetgang · « **Reply #4 on:** 10 Oct 2009, 09:59 am »

Single driver or Coax right in the center ceiling. Go to Frys and stand under those parabolic domes in the cd section- pretty neat

bpape · « **Reply #5 on:** 10 Oct 2009, 12:20 pm »

Quote from: srlaudio on 10 Oct 2009, 05:33 am

I believe it would be a horrible choice. The concave surface is one of the primary things to avoid in good room acoustics. I have been involved in several acoustical consulting jobs to fix the effects of a concave surface. A combination of thick absorptive panels and large scale 1-D diffusers has reversed the bad reflection into a good one.

I couldn't agree more. They're awful to work with.

Bryan

Speedskater · « **Reply #6 on:** 10 Oct 2009, 04:53 pm »

In Salt Lake City, the Mormon (LDS) Tabernacle has a doomed roof. It was not a good room for listening to such a highly skilled musical group.

TRADERXFAN · « **Reply #7 on:** 10 Oct 2009, 05:43 pm »

Well thanks for the feedback. Good stuff.

I have been in auditoriums that have angled side walls, with a concave rear wall. Is this approach detrimental for sound reproduction?

What if it was laid out inside a dome, like this?

black dots are speakers
red dot listener

-Tony

JackD201 · « **Reply #8 on:** 10 Oct 2009, 07:08 pm »

Extremely unpredictable behavior with lots of filtering unless the curved surface is heavily damped as well as faceted with diffusive elements. Both are expensive, difficult and are no guarantee.

It reminds me of that JVC speaker driver. Yes you can spend years and lots of dough to make a diaphragm out of sake soaked wood, but really, what for?

bpape · « **Reply #9 on:** 10 Oct 2009, 07:14 pm »

The angled side walls in relation to the listener are a good thing as they push reflections back past the listener easier. On the other hand, it can provide a ton of room gain too depending on how close to the front corner the speakers are.

The rear wall is still an issue. If it must be that way, you'll need to kill it, use a lot of diffusion, or likely a combination of both.

Bryan

jtwrace · « **Reply #10 on:** 10 Oct 2009, 10:30 pm »

Bryan,

What would be ideal shape room then?

Like most here, I've been to A LOT of live shows at many different venues. They were all different sizes / shapes but the one that I walked into and said to myslef, this will not sound good is the best I've heard with the exception of Lincoln Center.

bpape · « **Reply #11 on:** 11 Oct 2009, 01:46 pm »

Ideally, if you had unlimited space, you'd have a truncated triangle (the correct geometry term escapes me right now) where the front and rear wall are parallel but the side walls are splayed out front to rear. Ceiling would be higher in the rear than in the front. At least 1" per foot per side of taper on the side walls and 2" per foot on the ceiling since we can't do the floor.

This rids you of most early side reflections, ceiling reflections, and width and height axial mode issues. Absorbtion mixed with diffusion on the rear wall and broadband bass control in the acute rear corners and behind the front speakers to deal with SBIR issues (unless you prefer diffusion and are using a dipole speaker.

Bryan

Browntrout · « **Reply #12 on:** 11 Oct 2009, 09:12 pm »

Looks like a brilliant idea and loads of fun.
I would mount the speakers on the ceiling either side of centre parallel with the floor and facing down. The listening position would be lying on my back in the middle of the floor looking up, if I had the choice I would have a skylight fitted so as I listened I could look through the dome into the wonderfull night sky. Spectacular idea, no?

Tone Depth · « **Reply #13 on:** 12 Oct 2009, 12:04 am »

What about a Quonset Hut shape?

Quote from: TRADERXFAN on 10 Oct 2009, 01:33 am

I wondering about a monolithic cement dome for a detached, dedicated audio room.
http://www.monolithic.com/topics/domes

Would this shape be good/bad or indifferent for sound quality?

-Tony

bpape · « **Reply #14 on:** 12 Oct 2009, 12:09 am »

Almost as bad but it only focuses in one dimension.

Bryan

JLM · « **Reply #15 on:** 12 Oct 2009, 10:17 am »

Cardas calls the ideal "retangular" shape a golden trapagon:

http://www.cardas.com/content.php?area=insights&content_id=36&pagestring=Room+Setup+10

The auditorium design is done to promote reinforcement from back in the day before electronic amplification to allow for larger venues. There is nothing like hearing a thunderous male or piercingly loud female voice filling a large auditorium. Its amazing. The shape is basically what loudspeaker designers would call a waveguide.

Probably the most ideal shape is inside headphones with appropriately recorded (biaural) material. Biaural recordings are specifically made for headphone use and the imaging is amazing.

Russell Dawkins · « **Reply #16 on:** 12 Oct 2009, 09:25 pm »

I think a dome or a section of a dome is the most dangerous shape in acoustics. There is a church locally that was converted into a small theater with a ceiling consisting of barrel vault sections and a dome in the center. Everything sounds appalling in there. The sounds changes radically every 4 feet with first one instrument then another spotlit.
Lying on your back looking at the stars through a skylight is the best way I can imagine to enjoy music, but not under a dome, thank you very much!

I designed a little music studio for a friend who was a skilled carpenter and thus could entertain wild shapes. What we ended up with had a pentagonal floor plan with walls splayed out and an angled and eccentric roof with exposed beams.
There were no parallel reflecting surfaces. It sounded really good - resonant, but without favoring any particular frequency.
In my own basement studio, I had the side walls built convex (to the inside) with a 14 foot radius, to eliminate lateral standing waves. The width of the room behind the speakers from corner to corner is 20' and in the center of the curve it's 16'. The length of the curved wall part of the room is 16' and the overall length of the room is 22'.
So, it would seem that curved surfaces convex to the room interior are as good as surfaces concave to the interior are bad. The first scatters the reflections while the second focuses them.

jqp · « **Reply #17 on:** 12 Oct 2009, 09:46 pm »

Quote from: JLM on 12 Oct 2009, 10:17 am

Cardas calls the ideal "retangular" shape a golden trapagon:

http://www.cardas.com/content.php?area=insights&content_id=36&pagestring=Room+Setup+10

Very interesting - thanks for the link

brj · « **Reply #18 on:** 28 Oct 2009, 02:54 pm »

Quote from: bpape

Ideally, if you had unlimited space, you'd have a truncated triangle (the correct geometry term escapes me right now) where the front and rear wall are parallel but the side walls are splayed out front to rear. Ceiling would be higher in the rear than in the front.

Frustum?

Russell Dawkins · « **Reply #19 on:** 28 Oct 2009, 03:17 pm »

Quote from: Browntrout on 11 Oct 2009, 09:12 pm

Looks like a brilliant idea and loads of fun.
I would mount the speakers on the ceiling either side of centre parallel with the floor and facing down. The listening position would be lying on my back in the middle of the floor looking up, if I had the choice I would have a skylight fitted so as I listened I could look through the dome into the wonderfull night sky. Spectacular idea, no?

All except for the dome part!

Refinements would be:
- lying on a recliner instead of the floor
- having a grid of fine threads across the skylight so the movement of clouds could be tracked during the day.