Sealed box shape

[fritosaregood]

Does the shape of a sealed box make a difference on how it plays?

for example you have 2 different boxes, both are 1 cubic foot internal volume.

box 1: inner dimensions are 12"x12"x12"

box 2: inner dimensions are 16"x4"x27"

Both boxes are the same volume but one is long and skinny while the other is a cube. well there be any difference in performance.

[n8ball2013]

I heard a long time ago to stay away from anything that is a perfect cube. I cant remember the exact reason but it has to do with standing waves inside the box .

[fritosaregood]

would having both filled with polyfill eliminate standing waves?

[Boon]

would having both filled with polyfill eliminate standing waves?

Sorta but not really at the same time.

Mind you to get a standing wave in a small box you have to be playing like 300hz+ - I wouldn't worry about it with subs unless you're building a huge-ass box.

[fritosaregood]

Sorta but not really at the same time.

Mind you to get a standing wave in a small box you have to be playing like 300hz+ - I wouldn't worry about it with subs unless you're building a huge-ass box.

it might be like 8 boxes 1 cube each

[Brianrj90]

The optimal ratio is ~.62:1:1.62 according to acoustical engineers(the ones who know what they're about)

[kirill007]

Both box examples will sound the same, because it's a sealed box.

You can make a sealed box whatever form you want to, if it has the same internal volume it will sound the same.

Only with a ported box you wouldn't want a perfect cube or some funky shapes, with a sealed box it doesn't matter.

[Brianrj90]

Both box examples will sound the same, because it's a sealed box.

You can make a sealed box whatever form you want to, if it has the same internal volume it will sound the same.

Only with a ported box you wouldn't want a perfect cube or some funky shapes, with a sealed box it doesn't matter.

You're right when you say box shape matters more for ported, but it still does matter for sealed. A rectangular enclosure is better than a square, and a trapezoid is even better than that. The perfect enclosure is a sphere do to it having no parallel sides if I'm not mistaken. Dampening materials such as deadener or pollyfill will probably be more important than shape though. And to be honest you probably wont notice much difference between the shapes, but IMO you should just build the better box unless you have a good reason that you can't.

[Brianrj90]

Not sure if I'm breaking the rules by doing this, if I am please delete my post.

I found this interesting read on MP3car.com written by a guy who at least seems to know what he is talking about. Also he is a bit of an ass hole; just a warning. Coppied from CoolBlueGlow of MP3car.com:

In answer to the question "does the shape of the cabinet matter" Yes it can matter a great deal, if you know enough to use it to your advantage - or you can just keep stuffing those boxes full of peanuts, fiber fill, black hole, or whatever the latest and greatest whiz-bang stuff the car audio store is trying to sell this week.

First you need to know a wee bit about what goes on in an undamped resonant space (your sub cabinet or any finite baffle transducer cavity.)

Of course, you are aware that the acoustical power of your bass transducer compresses and rarifies the air in your vicinity (including your eardrums and the neighbor's too, along with his windows and furniture and so forth) If you think about it, it also compresses and rarifies the air inside the sub box, right? Yes, but that's NOT all that's going on in there. There is another neat little gremlin in the box called "resonance". Now, resonance is word derived from Herman Von Helmholtz's coined term -"resonator"...Its' taken from his work "On the Sensation of Tone", which is the definitive work on the topic for almost a hundred years, by the way. You might check it out, but be warned that it is pretty tough sledding.

Anyway, to understand reasonance on the cheap, here's an easy illustration. Ever blow on a bottle of your favorite beverage and make it whistle? THAT'S a resonator. We engineers still sometimes call them "Helmholtz resonators". Guess what, transducers in finite space cabinets do about the same thing and that resonance is related both to cabinet volume AND shape. (negating the internal resonances of the materials themselves - which, compared to cabinet surface resonances and volume resonance, are puny variables.) With all due respect to the other posts here, within reasonable limits, the wall-material is the MINOR factor in cabinet resonance, compared to the acoustical albedo of the internal surfaces at their resonant peak frequencies. Sorry guys...that's just the way it is in acoustical engineering world where the real heavy lifting related to the audio sciences gets done. Yes, I know-your megadon 1500 made out of andalusian spruce sounds "awesome", yes, I know you made a sub out of eleven layers of marble and it was divine...yes yes YES! I've heard it all before. Hate to break it to you, but the resonance of your andalusian spruce is called a sympathetic secondary resonance. It induced by the energy of the primary resonance of the fundamental resonant peak of the cavity contained by said spruce! No primary? no sympathetic. So...

Now back to reality - the resonance of any rigid finite baffle sealed cabinet is primarily and overwhelmingly determined by the internal dimensions of the reflective surfaces as they sympathetically resonate at the frequency represented by that dimension (or that frequency's partial.)

Resonance inside boxes with parallel surfaces is just an acoustical fact of life. So...ready to go outside the stupid zone, where everyone stuffs the sub cabinet full of turkey feathers? Ready to stop doing just like the kid who flunked out of math and now works at the stereo store tells you? Ready to apply your brain plus hard acoustical science? Have you chosen?

Great! Let's explore! First the principles -

Just for fun, let's imagine a 12" square cabinet, with a 10" transducer - built from say...MDF. No ports, no stuffing, just a transducer and a box. Let's say you've actually read the spec sheet and see that the transducer is rated at 88db efficiency 1watt/1meter. So, what do you suppose will happen when I put 1 watt worth of white noise power into our 1 foot cube sub? Why out will come something like 88db of white noise conforming to the performance curve of the transducer as the spec sheet rates it, right?

If you think so, then you don't understand resonance. You see, transducer is exciting air inside a 1 foot cube which has an internal fundamental resonance of 644 Hz. Each pair of resonant surfaces (top-bottom, side-side and to a lesser extent, front-back) will resonate at a particular frequency, in this case, 644Hz. Why? because parallel reflective surfaces bounce the acoustical energy back and forth between them...but not all the energy, a resonant cavity will re-concentrate a portion of all available energy at the acoustical wavelength that is best represented by the distance between those two parallel reflective surfaces! (plus, in descending order of efficiency, the harmonics) Are you starting to get it? Do you see how the idea that the shape of a sub cabinet (or any cabinet) can make a HUGE difference in the sound makes sense? Great, you're getting somewhere.

So returning to the foot-cube-sub...that 1watt of white noise in our 12x12x12 cabinet will have what we engineers call an emphasis peak at 644 Hz. That emphasis peak is generated (at the expense of energy stolen from other frequencies - there is no free ride) by the acoustical coupling of the transducer's output to the resonant peak of the cabinet.

So your sub is hotter than a pepper - at 644 Hz. Not exactly earth shaking...644 HZ is about the fundamental frequency of the second mounted tom on a drum kit. But at least you're not wasting that energy as heat with your horse feather stuffing!

Now, armed with your basic knowlege, let's imagine you get very ambitious and decide to make another sub cabinet. This one is bigger though. It is 36 inches on a side! Put the same signal at the same power through the same 10" transducer and you will now see a resonant bump of 12 db at 196 Hz. Why? Because the resonance of a 36 inch cavity is 196Hz. Ah...that resonance again!

If you made it twice again as big - 72 inches, can you guess what the resonant frequency will be? Multiple choice

1.) 19.6 Hz

2.) 12,000 Hz

3.) about 90 Hz

If you chose 3.) congratulations, you're learning.

You may notice that the calculated resonant frequency has roughly halved, though it is shifted upwards slightly. Why? As the cabinet gets larger, the relative displacment of the transducer motor itself becomes less significant, causing a slight shift up towards absolute values in the resonance. (the transducer motor is a smaller percentage of the total affected volume)

Now, let's make a sub 144 inches...wait, that's bigger than my car. So what gives? How do those guys get those gut thumping lows out of cabinets that fit in cars (sort of fit, anyway) For that matter, how come my semi-cheap little 18 inch square sub in a cube can still thump my car. That sure isn't 600 and something Hz I'm hearing, is it?

You are right, and there is a simple reason. While your little 18" square sub WANTS to resonate at 410Hz the sub builder stuffed it full of horse hair or fiber fill or whatever the heck they thought was best or found was cheapest that day...and that stuffing is absorbing the 410 Hz resonant energy and turning it into heat. (second law of thermodynamics) Since shorter wavelengths are more prone to absorption than long ones, the energy resonant at 410 Hz is wasted more efficiently by the stuffing than it's first sub-harmonic. See, there is what we call a lower partial at 205 Hz, and another one at 102.5 Hz, and yet another one at 51.25 Hz, and so forth, each is less affected by the absorption, though of course, each lower partial is down 3 db in acoustical power compared to the the one right above it, because each wave is progressively less of the total acoustical energy. Do you see why? The 18 inch resonant cavity can contain one 410 Hz wave, but only 1/2 of one 205 Hz wave, and only 1/4 of a 102.5 wave, and so forth. The total energy is therfore reduced proprtionally.

So your little sub box can sort of make thumps...but are you starting to understand why people sell you 1000 watt amps to get that low bass thumping? It is the stereo shoppe's version of Dr. Seuss's Star Bellied Sneeches. Wrong shaped, cheap to build boxes stuffed full of power robbing horse feathers or whatever need MORE AMPS... and who's going to sell you those amps? Why your friendly neighborhood stereo sneech!

Ports - We're not going to talk about ports here - except to say that ports are nothing more than resonators. Just like the hole in the top of your favorite beverage bottle is a port. Same principles apply. Ports are wonderful high-Q (narrow bandwidth = high Q) resonators, and I'm going to presume that you can do the math to get there on the port part of the design I'm about to hint at.

Yes, there is a better way (TRUMPET FANFARE)- but it requires some for-real engineering. I mean the kind you fell asleep for during geometry and physics class. It requires that you understand acoustics a bit, plus have some math skills, patience with carpentry, and the guts to go against the parking lot wisdom of the ignorant masses. You will have to go out and find the resources. They're out there - Theile and Small's work on resonances is still a standard work. Helmholtz is good background too. There are plenty of resonance calculators for free on the web, too. A college physics book and your little sister's t/i 83 calculator can come in handy as well.

O.K. I'm not going to do all the work for you, but here are the critical hints...

Imagine two surfaces that are parallel on only one axis. Think of a "V". Can you visualize that these two surfaces are in fact a wide-band resonant cavity? Each single axis parallel surface represents a small amount of opportunity for resonant coupling, expressed over a range of dimensions that create a "bandwidth resonator". Got it? If one end of the V resonates at 400 Hz (and 200 and 100 and 50, in partials) then the other larger end might resonate at 500 Hz (and 250 and 125 and 62.5) are you starting to get it. Use this design reality to your advantage to TUNE your sub cabinet to deliberately resonate at the partials of the desired efficiencies.

1. determine the desired bandwidth performance of your sub cabinet. Be realistic. Determine the bandwidth of the low frequency spectrum of the music to which you listen. (this is for music, right? not noise and bragging rights in the parking lot wars...right?) Do NOT get into an idiotic contest over how low can you go. Kick drums are NOT 5 Hz, o.k? A five string electric bass' lowest note is 30 Hz. BE REALISTIC.

2. Now build your bass cab around an entire set of strictly one-axis asymetrical surfaces which dimensionally represent the range of desired resonances. (or their first subharmonic partials - if you want it to fit in your vehicle). You'll have to do some figuring to determine these exactly. DO NOT ALLOW ANY TWO AXIS PARALLEL SURFACES.

3. Build, brace, seal and wire but DO NOT DAMP THE CABINET (yet)

Now install your transducer in your asymetrical cabinet, put your cabinet in your car, and carefully and slowly sweep the cabinet with a calibrated sweep sine wave. You'll find that the cabinet is now capable of producing vastly more acoustical output in the desired frequency range than exactly the same transducer / amplifier combination that is suffering from the stuff it like a Christmas turkey syndrome that they recommended at the stereo shop. As you sweep the performance bandwidth, you'll note the out-of bandwidth frequencies that still jump out at your ears. Don't fret - these are the frequencies that require still require what we call "critical damping". We'll get there, but DON'T damp yet. Now, do your Theile/Small parameter calculations and install a properly calculated port. Leave it movable, for by-ear tuning adjustments...don't just cement it in at the recommended dimension. Even the best calculators don't have adequate input from you to account for every brace, the vagarities of transducer dimensions, etc.

NOW, sweep it again - See how your port has changed the swept efficiencies of the various frequencies? fine tune the port to the desired primary LF resonant point. (the window thump point, for your brainless types :-)

No, DON'T GLUE IT YET!

Now, noting the swept frequencies you wrote down (you did write them down, right?) and those undesired upper resonances (the ones above your design's preferred bandwith output) calculate, build, and install inside your cabinet parallel absorptive surfaces placed at those precise dimensions which will absorb that resonance and only that resonance. Clever designers with multiple resonance issues may be able to use both sides of an internal damper plate and place a single damper at the distance representative of the two critical frequencies. These dampers will be free-standing flat surfaces with highly absorptive surfaces. They'll representing a percentage of your total cabinent interior surface area which remains undamped. These tuned dampers will absorb the stray resonances (and ONLY the stray resonances, without wasting acoustical power by the stuff the turkey method the stereo shoppe wants you to use so they can sell you some more amplifiers, and wire and terminals and really cool neon glow tubes, etc.) Congratulations - you now have a critically damped subwoofer cabinet.

Listen again. You should be close to happy. You may still need to install a small amount of additional damping, to make up for the corners you cut in your cyphering. Remember non-critically applied damping wastes power! Avoid it.

Properly executed, this method can produce 9-12 db gains in the desired performance range...all other things being equal, without a single watt of additional electrical power. Remember 6db is a doubling of acoustical power! Put that in perspective, 6db of gain requires a ten-fold increase in electrical power. Does the work on a critically damped sub cabinet sound more appealing now? Imagine how much more thunder you can make with a tuned-resonant critically-damped cabinet!

Science is fun!

Cheers,

CBG

Edited October 21, 2009 by Brianrj90

[EXOcontralto]

Damn that's a long ass quote..

I like to believe that everything will affect the sound to a degree -

but in this case - Sealed is sealed - As long as the box fits the specs cube wise - the sub would'nt know the difference, even in a box shaped like Dolly Porton Boobs.