Ideal room dimensions for bass?

[mlah384]

I plan on building a new home and was wondering if there was an ideal room size for a theater? Are there certain dimension ratios that are ideal to avoid nulls? Or is that not possible because of too many variables? I guess a circular room would be best but that's not realistic? Or am I looking at this all wrong? I guess it's a "build the system for the room" vs "build the room for the system" type of thing?

[Ricci]

Well there is the old school golden ratio. If we are talking ideal you would have none of the large surface boundaries be parallel with each other but that is usually difficult to pull off.

[Infrasonic]

You got it. It's either 'build for the system' or 'system for the room'. Many ways to approach it really there is no perfect solution unless you find one that meets all of your needs.

 

If I were making a room, I'd build it up to the specifications that I need and then work the system into it.

[Shredhead]

You can try hitting this guy up over PM on youtube for some ideas.  Just don't tell him you are planning to use multiples of large subs or he'll lose his mind.

https://www.youtube.com/user/AcousticFields/featured

[SME]

The answer to this is that it definitely depends.

 

Let's start at the bottom end.  Your ULF performance with sealed subs depends a lot on the transmission loss (TL) of your walls.  The TL varies with frequency, and with typical construction, TL is generally a lot higher for bass than for high frequencies. The lower the TL for ULF, the more room gain you'll see and higher SPL you'll get for the same displacement.  A lower TL also has the benefit of allowing more bass energy to escape outdoors or to the rest of the house.

 

On the flip-side, low TL makes acoustic problems worse for frequencies where the wavelengths are twice the size of the longest dimension or smaller.  Lossy walls help reduce the magnitude of reflections that cause comb filtering and buildup of unwanted room resonances.

 

So what to do?  That depends on your level of effort.  First and foremost, you'll want to plan to install a large amount of bass absorption in your room.  The difference in sound quality is staggering, and you'll appreciate it whether or not you're sitting in your sweet spot.  If you can't or won't install bass absorption, the you should avoid rigid walls (i.e., concrete, especially underground) as much as possible, unless all you care about is ULF. 

 

Room ratios are typically recommended based on modal analysis that considers an idealized rectangular room with infinitely rigid walls.  That model will be inaccurate unless you are in a concrete bunker, and the model also "hides" the influence of early reflections, which can also be very harmful.  It's at least reasonable to expect the "1st order" modes to be relevant, especially for the longest room dimension.  It is also good to avoid ratios that share (or approximately share) common multiples.  Square rooms are not good, nor are rooms that are 20 x 10 (share 20) or 15 x 10 (share 30).  Don't forget about the ceiling height too.  Room resonances below 80-100 Hz or so are the most important to try to avoid because they are much harder to mitigate using acoustic treatment.  It is possible to absorb bass at lower frequencies, but it often requires solutions that involve a lot more time and/or expense.

 

To control bass in the modal region up to 100 Hz I suggest a combination of multiple subs, DSP, and careful placements of both the subs and the listeners to achieve optimal results.  Sub placement recommendations based on modal analysis are not necessarily appropriate because they may ignore harmful early reflections.  Placements in corners are typically very good because they minimize these reflections.  Near-field placements can also be helpful by increasing the level of direct sound but multiple coincident reflections arriving at the same time can still be a problem.  Often corner placements will be superior for deep bass while near field is superior for mid bass.  Placement of either subs or listeners in the middle of the room should be avoided unless you are using DSP to do something clever, such as is done for Double Bass Arrays.

 

Other things that don't really work are non-parallel walls or any non-rectangular geometry.  Non-parallel walls can be somewhat helpful for preventing slap echo, but slap echo is entirely a high frequency phenomenon.  They have their use in studios where they are carefully engineered to direct specular reflections beyond the sweet spot so that the sound energy can be retained (for acoustic liveness) while ensuring sufficient time elapses before that energy is returned to the listener for it to not adversely affect imaging for tonal quality.  In home theaters, one generally desires less liveness and better sound quality outside of a single sweet spot, so slap echo is best treated using absorption or diffusion.

 

FWIW, I've been planning a dedicated room build for a long time and am currently looking at building a room whose walls and ceiling are as stiff as reasonably possible.  I have often seen it claimed that "high mass" is required for low bass TL, but I believe this is mistake and it is "high stiffness" that is essential.  It should be no different for a room as it is for a subwoofer box.  The problem is to achieve sufficient stiffness for surfaces that are much larger than those used in a subwoofer box and without internal bracing.  I have not yet researched strategies for achieving this.  Underground concrete walls are likely pretty stiff on their own, but the ceiling might require steel framing, which could get real expensive real fast.  For control of modes and reflections, I plan to place bass transducers all around the room with each on a separate DSP output and MIMO filters to optimize the full room response of the subs alone and their integration with the mains.  I also intend to use generous bass trapping and diffusion.

[mlah384]

Don't bass traps have to be incredibly thick? Like 5 ft or more of insulation stacked, etc?

 

I thought about underground basement theater, but I've read where people regretted it after doing it.

 

Also, ceiling height is probably a new avenue to tackle now that Atmos is here? Kind of throws a wrench in everything? Can't be too tall or too short? ha

[SME]

No, bass traps don't have to be incredibly thick.  That's a myth arising from the misunderstanding about how they operate.  So-called velocity-based absorbers are most effective where the velocity is highest, which is 1/4 wave length from a wall.  However, if the absorber is resistive enough while providing a good impedance match, it will absorb sound quite well much closer to the wall.  Something like 1/12 or 1/16 the wave length is usually good.  Furthermore, the typical rockwool / fiberglass boards used for traps don't act exclusively as "velocity-based absorbers".  They actually resonate with pressure fluctuations in the 100-250ish Hz range, depending on thickness.  The glue that binds the fibers as well as the friction between individual fibers damps out these vibrations.  Lastly, a vinyl membrane can be installed on the front, and depending on its mass and wall distance, low frequency absorption can be greatly enhanced in a narrow region of the spectrum while keeping the depth reasonable.

 

With that said, it's a lot harder to get good bass absorption below 80 Hz or so.  It's not impossible, but you won't find many commercial products that are capable and will likely need some sort of custom solution.  This route is not for the faint of heart.  Even with "5 feet or more of insulation", you must be mindful of the flow resistivity of your materials and how well the impedance of the installed product matches that of the air.  Too much flow resistivity, and the sound will reflect off the face of the trap.  Too little flow resistivity, and most of the sound will pass through the absorber without being absorbed.  Generally, thicker traps need materials with less flow resistivity.  There's several great threads on the Gearslutz forum on these subject, even though they are unfortunately littered with many posts from many confused people.  This subject is not easy.  My reference on the subject is chock full of math from cover to cover, and it is still quite incomplete.

 

At the same time the longer wave lengths and inability to localize sound below 80 Hz makes it much easier to address acoustics issues by using multiple subs, possibly in conjunction with intelligent DSP.  I have not personally validated the double bass array idea I pointed to above, but the theory behind it is reasonable to me.  I believe it's more likely to succeed if the the room is rectangular and all the walls are very rigid, as would be the case in an underground setup.

 

I can definitely see some people regretting the underground theater option, both because of the resulting acoustic problems and because of the tactile inertness of a concrete slab floor versus a suspended wood floor.  At least the latter problem can be solved easily by building wooden risers.  The acoustic problems can be solved too, but you must plan for this in advance and be prepared to invest a lot of time, money, and/or effort into the process.

 

As for ceiling height, higher is better.  In addition to the need to get those ceiling speakers as far away from the listeners as possible, you also want extra room for treatments, if you plan to install them.

[CommanderLake]

 

Ignore the mode on the right the room has bare walls and floor atm, I only have an SPL meter with a dBC weighting available and the correction in REW only works down to 7Hz.

This is my new IB setup in a purpose built soundproof concrete cinema room with a sand filled door, I calculated the dimensions at 3 x 2.5 x 2.2 meters with the realtraps room mode calculator.

The listening position is in the very center of the room and the UM18's are in the corners of the room face down with a concrete enclosure under the floor thats over 5000 liters, the same response can be achieved with 2 subs in oposite corners.

[MemX]

IB sub response.png

 

Ignore the mode on the right the room has bare walls and floor atm, I only have an SPL meter with a dBC weighting available and the correction in REW only works down to 7Hz.

This is my new IB setup in a purpose built soundproof concrete cinema room with a sand filled door, I calculated the dimensions at 3 x 2.5 x 2.2 meters with the realtraps room mode calculator.

The listening position is in the very center of the room and the UM18's are in the corners of the room face down with a concrete enclosure under the floor thats over 5000 liters, the same response can be achieved with 2 subs in oposite corners.

 

Welcome, glad you found this place!

 

Start a build thread for your room!