[beat]

hey guys,
I'm lookin at finally building my room. What I have to deal with is a space that is 18 x 24ish. I can of course fudge that alittle in either dimention. I know that room height is of utmost importance but to get it any higher than 8' is cause for me to do alot of digging....I'm fine with that really exept that in order to plumb the place I will have to create a step up to the working kitchen/bathroom due to gravity..the place will have a killer 20 foot drop off on the west face so the water has to travel east. Which is another thing. The structure will be strawbale with 4 walls and possibly a divider wall for a drafting table and an inlaw style bathroom/kitchen thingy. The idea behind that is that if I ever move I want to sell the place as though it had an extra liveable space...with a killer view to boot if I decide to add windows!! So in a nutshell I'm looking for optimum dimentions for an audio room and possibly trying to sqeeze another room into the 24-28 length. So it maybe a 1 or 2 room building.

any ideas or input will be appreciated. thanks.
beat

[ScottMayo]

hey guys,
I'm lookin at finally building my room.

No, you fool! That way madness lies!  Noooo!        

What I have to deal with is a space that is 18 x 24ish. I can of course fudge that alittle in either dimention.

Seriously, having been through this exercise (and through it, and through it...) I suggest you bring in a consultant if you can afford it and want really good acoustic results. (Disclaimer/Shameless plug: yes, I offer this service.) If you can't, here's some advice which is worth more than you paid for it.

It's useful if the room dimensions you pick aren't nice neat multiples of any one number. For example, if you used the whole 18x24, those are both multiples of 6, and that leads to nasty modes. On the other hand, you want the room to be as large as you can manage, because that pushes the modes down towards the bass range, where they are less irriating. 18x23x8 isn't too bad, for example.

Is this critical? No. You'll have modes in any case and you'll want to treat the room anyway. But you'll have less treating to do if you keep the numbers from being neat multiples.

If you are doing the building yourself, you can get some advantage from a non-rectangular room: angled walls, if done right, can be a help. They can also be a tremendous pain, it's easy to get wrong, and I wouldn't even consider it if you are bringing in contractors to put up walls. They'll screw it up. These guys have t-squares. They use them. Things come out square.

Consider up front if you extensive want sound proofing - and if it's worth doubling the cost of the project, which is a good rule of thumb.

Don't even start planning until you know how much space you need to break off for te other rooms. Doing a lot of planning and then realizing you need to squeeze another 2' out for something else is intensely frustrating. This is the voice of experience talking.

You can have a killer room in 13x17 - and you won't have to crank the music as loud. Big is nice, but it's not worth losing extra living space over, most of the time. Unless you want a world class hall, and then you want to dig deeper (in the ground and in your wallet), because you want more than 8' for a ceiling. 17x24x11 would be pretty good....

[JoshK]

FWIW,

I heard a seriously top dollar professionally designed room and I felt it did more harm than good.  It was multi multi thousands of dollars I am guessing.  Same company as did Scott's.  Not to poke holes or cause fights, but in this room I heard there was zero bass, hardly even any midbass to speak of, and this isn't even what "we are used to" phenomena, it was seriously non-existant.

Acoustics is a much debated subject for which we know very little. You have to ask yourself what is your end goals?  What do you wish to accomplish and for how much?  I *personally* feel you are better off talking to csero than to a pro room consultant.  I have heard both the expensive and supposedly SOTA vs. the inexpensive and more educated (sorry my assessment) conclusion.  You already know my choice.

Josh

[ScottMayo]

FWIW,

I heard a seriously top dollar professionally designed room and I felt it did more harm than good.  It was multi multi thousands of dollars I am guessing.  Same company as did Scott's.  Not to poke holes or cause fights, but in this room I heard there was zero bass, hardly even any midbass to speak of, and this isn't even what "we are used to" phenomena, it was seriously non-existant.

In fairness, I did a fair bit of adjustment on the plans I got - it was the training ground for Obsession consulting. The room doesn't lack bass (at the moment I'm playing _The Wall_, so I'm speaking from a very lively experience). In fact after I replace my current sub with a pair of VMPS Largers, I might put a little more bass absorption in.

You definitely have to decide what your goals are before throwing money into consulting, though. I'm cheap, as befits a small operation, but if someone is a DIY/tweaker who likes nothing better than a few weeks of dragging around traps and speakers, I agree they don't want to go the consulting route. The real value of consulting is avoiding the Big Mistakes - sinking money into construction, and then learning that a 12x12x8 room wasn't a good idea; or spending hours trying to adjust the mid-bass response without ever realizing that unbroken modes along the ceiling are the big problem. (I learned the hard way about putting a metal eyeball light at the 63Hz mode point - at least I was able to fix that with a tube of caulk and a brick!)

I'm not going to do an elaborate sales job here - it's not the forum for that. Consulting works in the right circumstances - and it's sometimes much cheaper to plan the acoustics in advance, than it is to go after it, ad hoc, after the fact. But no, it's not a magic bullet for everyone.

[andyr]

hey guys,
I'm lookin at finally building my room. What I have to deal with is a space that is 18 x 24ish. I can of course fudge that a little in either dimension. I know that room height is of utmost importance but to get it any higher than 8' is cause for me to do alot of digging....

Hi beat,

You've got some valuable pointers from Scott & Josh but, having been though the exercise of building my "listening room" many years ago, I might be able to add another 2c worth!    To my knowledge (and from the comments of various people who have come round to listen, over the years), there are no major sonic defects with the room.

Firstly, you said 8' high ... is that because you have another room on top?  Is there any chance you can put in a pitched "/\" ceiling?  This is good becuse it stops/minimises vertical reflections - particularly the first reflection point from the speaker up to the ceiling and down to your ears!

Secondly, you said straw-bale walls.  This might look great but I'm not sure that it will do much for your bass response ... but I'm afraid I really don't know.  Scott might like to comment but, in my experience, the best bass comes from walls which are made of something solid - like clay bricks or concrete blocks.  Then the bass energy gets reflected ... whereas, with the straw, it might just go straight through?  

I have a spreadsheet on room dimensions which I picked up off the Net (I think it originated out of THX).  If you'd like to PM me and give me your email address, I'll send it to you ... I presume you'd want the "foot" version, not the metric version?  Then you can play around with various options.

Regards,

Andy

[ScottMayo]

Straw bales absorb an amazing amount of sound - even being pretty good at low frequency absorption, unless they get very very dry. When plastered over lightly, they provide decent bass absorption, while the thin coat of plaster reflects and diffuses a little HF. I've known rooms that could have thrived on that exact recipie.

For a long time I wondered about all the tales of young lovers sneaking off to the hay loft. I mean, scratchy and dusty isn't so good, right? But after spending time in one, I began to see the wisdom of the thing - noise is well absorbed, and sometimes, well, that's important.

You can do worse for a bass trap, than a stack of bales in your corners.

[andyr]

Straw bales absorb an amazing amount of sound - even being pretty good at low frequency absorption, unless they get very very dry.

You can do worse for a bass trap, than a stack of bales in your corners....

Hi Scott,

Shirley we've got two different scenarios here?  

I could understand in a concrete-block room - which contains the bass - a stack of hay bales in each corner would be a very good bass trap.

However, walls made of hay bales would absorb ALL the bass ... so you'd hear very little.  Just as a listening room which was a tent (with canvas walls) wouldn't be much good??

Regards,

Andy

[ScottMayo]

Hi Scott,

Shirley we've got two different scenarios here?  

Don't call me Shirley.

However, walls made of hay bales would absorb ALL the bass ... so you'd hear very little.

It doesn't actually work that way. The bass you want to hear is the bass that travels straight from your speakers to your ears. Unless you put a bass trap in front of your speakers, there is nothing a bass trap can do to interfere with that "good bass", since it travels along that straight line. You'll hear plenty.

There is also bad bass, which bounces off the walls and gets to your ears indirectly. A little of this does no harm and adds some fullness. More than a little is bad. The problem is that it leads immediately to modes - the bass reinforces or cancels itself all over the room as the reflections continue to bounce, leading to some very unpleasant problems.

If someone is a bass hound, modes might be acceptable. You can get 20db peaks involving your favorite frequency with room modes, and it can be a rush. A 40Hz mode can make HT a blast. But it's not accurate; if you want accuracy, you need to cancel the modes. And the way to do that is to absorb any bass reflection that is eventually going to reach your ears.

A lot of rooms have some natural bass trapping - insulated walls eat some, windows port some away, furniture absorbs a good amount. Most rooms don't have enough, though, which is why so many audiophiles find bass traps are one of their first really effective upgrades.

Are walls made of straw "too much" bass trapping? Some people say you can't have too much. I'd argue that maybe you can, but the cure is real simple - put up some 5/8th in sheetrock over some of the bales and retest. Mind you I wouldn't try it with maggies or any other speaker than like having their rear wave reflected back as reinforcement, but for a lot of gear it might not be so bad.

[andyr]

It doesn't actually work that way. The bass you want to hear is the bass that travels straight from your speakers to your ears. ...

Hi Scott,

Well, you're the expert ... I'm just an amateur!  However, I thought I'd read somewhere that in a "listening room" which has lots of open doorways, you do not hear as much bass as when you close all these doors (if they're sealed well).  Something about a closed room "containing" the bass?

Now this has obviously nothing to do with bass which comes straight to your ears from the speakers but I agree, it could be this bass is "bad" bass coming from room modes.  I dunno!!  

Regards,

Andy

[_scotty_]

I would suggest a room whose dimensions are based on a Fibonacci series
or the Golden section. http://www.mcs.surrey.ac.uk/Personal/R.Knott/Fibonacci/fib.html
Using room dimensions which are based on a Fibonacci
series minimises the occurance of resonance problems at low frequencies.
Re-enforcement  of resonant nodes as a by-product of room dimensions being even or odd multiples of one another does not readily occur.The number 8 as in 8ft. for a ceiling height is basically
the one number you say that you really don't want to change because it will involve more digging.  If we take 8 as the start of our Fibonacci series this is
8x1.618 =12.94ft.  12.94x1.618=20.94  These numbers would give you an
ideal room size of approximately 13ft.x21ft. with an 8ft. ceiling.
Staw-bale construction appears to be quite lossy according to this source with a TL of 59.4dB http://listserv.repp.org/pipermail/strawbale/2000-February/027731.html
A conventional frame construction is rated at 36dB
http://tinyurl.com/8q7ba
The layer of plaster or stucco on the walls necessary for the structure to pass your local firecodes will probably result in near conventional performance at bass frequencies.  By all accounts external noise intruding into your listening room should not be a problem.
Scotty

[Russell Dawkins]

For the record, here are the details of my mixing room dimensions. I was able to build the room from scratch into an unfinished basement. The one very odd dimension is floor to ceiling where I was limited by the existing (low) ceiling height, so it is low at 6.5 - 7'. The variance is because the floor has a slight slope to allow for water drainage.
The main innovation is the curved side walls, intended to break up bass standing waves. They are convex with a 14' radius. The overall dimensions are 20' wide by 24' long. There is a bass trap behind the speakers created by a false wall stood out 8.5" from the real wall. Between and behind the speakers there is an opening 13' wide and 4'9" high from the ceiling to 2' from the floor with 2 layers of 3" fiberglass batts filling the space between the studs and arranged so there is space left behind for them to vibrate and absorb bass energy. The plate above the false wall has openings cut into it so bass energy can further bleed into the space between the ceiling and the floor above, increasing the cubic volume of the bass trap. The whole trap is covered with the equivalent of grill cloth.
The inward curved side walls are completely covered in open shelves (the carpenters hated me!) which hold records, magazines, spare equipment, etc. - diffusive but not particularly absorptive. The entire room is perfectly symmetrical from the front to 14' back, at which point it becomes highly irregular, with an L shaped opening to the left rear and various other shapes. My listening position is roughly 6' from the front wall, the midfield monitors (Tannoy Ardens, soon to be SP Tech Timepieces) are about 4.5' away and about 2' from the wall which is entirely bass trap out to 1.5' either side of the pair which are about 5' apart. My nearfields - Harbeth HLP3s - are about 3' away.
The room works surprisingly well. Bass damping is surprisingly good and there are virtually no bass nodes in evidence. I can play bass warble tones or sine waves and walk around the room and hear very much the same sound everywhere but extreme positions, like against the wall or in the corner. The only criticism I can make is that it is a little dry, so I have to be sparing of reverb added to mixes for the result not to sound too reverberent in the typical living room. Clients sit about 8' behind me and here a very similar sound to what I'm hearing.

[beat]

Thanks for all the tips guys.
When I decided to go with the strawbales it initially had nothing to do with acoustics other than sound leakage (pissing off neighbors during a late night work session) and insulation factor. It seems that I have enough to work with if I don't divide it up into two rooms. ScottM, I am doing all the work myself so getting what I want out of it isnt an issue. I was advised to keep the walls square to each other to help eliminate guess-work if a problem arises..I can see both sides of that issue. . With the roof I was planning on doing kind of the skylight thing with recesses cut into the ceiling. I would like to get that THX stuff from you andy..I'll pm you. Scotty, thanks for the links I have alot to chew on with that.

best regards,
beat