[JLM]

I dinked around yesterday with the acoustical design for my listening room in the new house we're building.  The room is 12' - 11 1/4" x 20' - 11 1/4" with 8'-0" ceilings (1 x 1.618 x 2.618 ratio) and will be well isolated from the rest of the house with insulated staggered stud walls, suspended/insulated drywall ceiling, lined fiberglass ductwork to the room, and weather sealed/insulated door.  Walls will be drywall.  As the room is in a basement I'm somewhat limited as to flooring options and being in cold Michigan I don't want vinyl or ceramic.  That leaves engineered wood (with or without rugs) or carpet.  Note that real wood and  laminated/simulated wood flooring products from leading manufacturers are not recommended for below grade use.

I used a relatively simple program I found here:

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

The program allows for user inputed room dimensions and absorption coefficients of the selected materials to determine the RT60 (time needed for a 60dB decay of reverb for a specified frequency).  Ideally RT60 should be between 0.35 and 0.50 seconds.  Lower numbers and the room is too "dead".  Higher numbers the room is too "lively".  Using ball park absorption coefficients from the site and another site I first determined how the bare room would behave at 125, 250, 500, 1000, 2000, and 4000 Hz:

Wood floor:  RT60 ranged from 0.06 - 2.18 seconds  (this is awful)

Wood floor with large rug:  0.35 -1.12 seconds (much better)

Carpeted floor:  0.43 - 1.31 seconds (surprisely no better)

(In all cases the values are highest around 500 Hz.)  


Going to the RealTrap site:  http://www.realtraps.com/data.htm  I fudged in the values of various combinations of their products in different combinations of locations and found that 10 traps worked the best.  Wall versus corner placement would only really affect frequencies below 100 Hz.  Wall versus ceiling placement made little difference, but obviously first reflection concerns aren't addressed within this modeling.  Mondo versus mini and mini versus micro traps behaved as advertised.  IMO they need different names for the mini and micro traps as neither are small, in fact 10 traps are almost hard to fit in the room and still leave room for a mininal amount of furnishings while holding on to a residental (not totally freaky) look.  This is only somewhat important as the room is mine and I'm a, "Form should follow function," kind of guy.  Here's the summary numbers for the best combinations I found:

Wood floor with rug with 6 mini + 4 micro:  0.34 - 0.50 seconds

Carpeted floor with 10 mini:  0.32 - 0.49 seconds

It surprised me that the carpeted scenario seemed more difficult to find a good solution.  Added furnishings (mostly wood and metal in this case) will affect the numbers and help diffuse reflections.  Using a nearfield setup (68" equalateral triangle as per Cardas) its relatively easy to position 10 traps to account for first reflections, although I'd put traps on the front, not back wall as RealTraps recommends preferring to go the LEDE route for depth of soundstage.  BTW the 20 millisecond delay RealTraps specifies to minimize poor imaging is about twice what most recommend.  This translates into a 22 foot longer reflected signal path compared to the direct signal path.  In my case the backwall reflection is about 25 feet long versus less than 6 feet for the direct signal.

So it looks like I'll be shopping for carpet in the room as its warmer/cheaper and the price of RealTraps are nearly the same for 10 mini versus 6 mini + 4 micro.

[Ethan Winer]

JM,

Just a couple of comments:

Most acoustics experts agree that even the best room prediction software gets you only partway to the right answer. Until you actually build the room and measure it, such software gives at best an estimate.

> the 20 millisecond delay RealTraps specifies to minimize poor imaging is about twice what most recommend. <

20 milliseconds is generally accepted as the threshold where echoes "fuse" with the direct sound and affect imaging. When the delay time is longer the echoes can be more easily distinguished. The confusion here may be between distance and round-trip distance. If a wall is 10 feet behind you the round trip distance is 20 feet, or about 20 milliseconds.

--Ethan

[DSK]

JLM,
Most "experts" seem to suggest that the RT60 measurement is appropriate for auditoriums but not for small rooms like home listening rooms.

Having said that, my room's measured (not estimated) RT60 is around 0.7-0.8 secs and is only *slightly* too lively. The room is 19'w x 24'd x 9'h. I'm currently trying to come up with WAF friendly treatments to bring it down to 0.5-0.6 secs. I would not want to go lower than that. Most listeners believe my room is fine as is, but I find that the occasional (especially female) voice on loud and sustained notes becomes ever so slightly glassy (due to lack of inner detail typically caused by excessive RT60). The effect is not caused by freq response as this measures +/-3db from below 20hz up (3rd octave measurements), nor by direct reflections as the impulse response graphs have no significant peaks.

I listened to a SACD system in a very well damped room (don't know what it measured but would guess around 0.3-0.4 secs) and it was absolutely lifeless. Surprisingly, imaging and soundstaging weren't very good either.

So, if you have the luxury of building/furnishing a dedicated listening room from scratch, my suggestion would be to target an RT60 of around 0.7 secs then flavour to taste with acoustic treatments after that.

Good luck!!

[JLM]

Ethan,

Yes, this is just a ballpark excercise.  It's biggest shortfall was a lack of data for construction materials under 125 Hz.

I consider you an expert in these matters, so I'll have to adjust up the numbers I've seen.  Yes, I was accounting for round trip travel but in my case the round trip travel of the backwall reflection is nearly 20 feet longer than the direct.


DSK,

Thanks for the advice, best to go one step at a time.  The room will double as an office with a desk, a couple of file cabinet, drafting table, and bookshelves to the back of the room.  It would be cheaper/easier to achieve say 0.6 seconds.  I'll have to run the analysis later, but maybe two mondo traps in the front corners would do it with carpet.

[JLM]

DSK,

At what frequencies were you measuring RT60?  I'm finding it very hard, even in a dedicated room, to start out anywhere near ruler flat RT60 values of 0.6 seconds or above at all frequencies.  Note that at 125, 2000, and 4000 Hz RT60 is already around 0.4 seconds, so adding anything absorbent will only decrease RT60 below what you're recommending.


For example, starting with drywall walls/ceilings and carpeted floor yields lively mid/bass:

125 Hz  0.43 seconds
250       1.09
500       1.31
1000     0.75
2000     0.48
4000     0.44


Adding two mondo traps in the corners is about the best I could do, keeping with your suggestion of starting out with RT60 = 0.7 seconds:

125 Hz  0.35 seconds
250       0.78
500       0.78
1000     0.59
2000     0.42
4000     0.38


I know this is over analysis given the lack of good data, but I'm a measure three times, cut once kind of guy.  Just trying to pick out (hopefully cheap) materials for the room to get it right the first time.

Thanks for any help.

[DSK]

Hi JLM,
The ETF software measures full range, then you can view the various graphs (freq response, waterfall plots, impulse response, reverberation times etc etc) afterwards and change gate times and other settings etc.
I took the readings when I was testing frequency response and optimising speaker/sub location. I have not spent any time reading up the ETF doco to ensure that the way in which I took the measurements was optimal and that the results are accurate. I just re-open the saved result file, click on the "Reverb" graph, select RT60 (RT20 & RT30 are also available), click on Update and voila ...an RT60 graph appears.

So my RT60 graph is full range (20hz to 20khz) but the RS SPL meter is not good enough to be useful in the upper frequencies, especially on ribbon speakers.

The graph suggests:

0.9   secs @ 200hz
0.8   secs @ 320hz
0.75 secs @ 400hz
0.67 secs @ 511hz
0.6   secs @ 1023hz
0.65 secs @ 2047hz
0.6   secs @ 4094 hz
0.5   secs @ 6499 hz
0.46 secs @ 8188 hz
0.7   secs @ 10316 hz

There are some bumps below 200hz, suggesting that my room could do with more bass traps (though there is no boominess or overhang) or that the measurements are inaccurate in that range, and the chart goes straight up above 10khz suggesting that the RS SPL meter is inappropriate in this range.

I'm particularly sensitive to any glare or sizzle and on a few louder female vocal notes I hear a very slight silveriness which I believe is lack of detail due to excessive reverb in that frequency range or a hot spot reflection. The RT60 doesn't show any spikes in this range, and moving a large urn (that I use to block right side wall first reflection) slightly had a positive effect, so I'm contemplating a WAF friendly treatment (probably mobile) to treat that side. I'll then reassess whether I feel that the room is slightly too lively or whether it was simply a little upper midrange emphasis from the right side wall reflection.

Not sure whether this is of any help to you  

[JLM]

DSK,

Thanks for the info.

Seems like your room and mine are quite different to start with.  That provides some improved perspective and reinforces a go slow approach and see what I experience.

[BrunoB]

I listened to a SACD system in a very well damped room (don't know what it measured but would guess around 0.3-0.4 secs) and it was absolutely lifeless. Surprisingly, imaging and soundstaging weren't very good either.
 ...

I am not sure what a dead or life room is and why one is better than the other to reproduce recorded music. My room is extremely damped - I have layers of foam on all walls. Is my room a lifeless one?  At least to my ears, it is very musical. Imaging and soundstaging are outstanding.

Bruno

[John Casler]

JM,

Just a couple of comments:

Most acoustics experts agree that even the best room prediction software gets you only partway to the right answer. Until you actually build the room and measure it, such software gives at best an estimate.
  .

Hi Ethan,

I would also mention that it seems that "many" software programs are developed for Acoustic Spaces that will actually serve as venues.

Even though they "claim" they can be used for HT and Audio, the applications are "not" nessessarily set up to reduce, room interaction but to "use" it.

From a purist standpoint this is sometimes a not to accurate presentation.

What I mean by that is, measuring and relinquishing to reflections, or even diffusions, is not true to the recording.

If the room "is" used, it should be for the "lower frequncies" which need PLENTY of help and management. (Your products are badly needed for most)

While the laws of physics do not change, purpose and application do.

In a live venue, the purpose "IS" to "USE" the room or environment to enhance the acoustics of the perfromance.

In an "audio reproduction" space, the idea is to "reduce" environmental interaction to certain portions of the signal and "manage" others.

Acoustic management for "audio recreation" is similar to front projection HT video.  The greatest realism is realized when "reflected" light is attenuated or eliminated as much as possible.

Brightness (video) is not measured, by adding in room reflected light, nor is it considered in calibration, other than to "increase" relative brightness to compensate/overide a highly reflective room.

Much of that same concept applies to Audio.

[JLM]

I've been inside a couple of anecohic chambers and it's NOT the kind of environment you'd want for listening.  They were very %      " and unnatural experiences, like listening in outer space.

BrunoB,

As discussed on previous threads, foam is only effective at higher frequencies.  Don't know your room and don't doubt what you're saying.

John,

IMO %      " (inert) speaker cabinets are the best as you wouldn't want whatever colorations you'd be after in a cabinet design to be added to everything.  But can a room add "color"?  Are you advocating for uniform RT60 across all frequencies?  

IMO unamplified music played in a real venue is the only meaningful standard (versus say rock music in a studio).  If one picks a particular hall as having the best acoustics, how does one       his listening room to sound like that hall during playback?

[ctviggen]

I don't think you'll ever be able to make a room sound like a concert hall, because real rooms are typically rectangular and concert halls aren't, and real rooms are comparatively tiny.  By the way, an excellent book that discussed these concepts is the Master Handbook of Acoustics.  It discusses pretty much everything about acoustics, even how the ear functions.  See:

http://www.amazon.com/exec/obidos/tg/detail/-/0071360972/qid=1105623438/sr=1-1/ref=sr_1_1/103-0072553-7090254?v=glance&s=books