# shape of a sub box, does it matter?



## cutra

I'm just in the middle of a new install in my brother's car. 
The box i'm making is going to house 2 12'' subs. 
The space i'm working with is 37 inches wide and i'm making it to fit the space.
The height is 16'' and the depth is 10''. for a total of aprox 2cubic feet for the two subs. 

question, does the shape of the box affect the performance or does it matter at all?

Pro's let me know please...


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## BEAVER

I've heard mentioned that you should try to include at least one angle in the design to eliminate the possibility of standing waves inside the enclosure, but the general consensus around here is that the shape doesn't matter.


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## cutra

ok thanks.
are there any rules on how far the back of the box is internally from the sub's magnet?
right now it's only 1.5 inches away...


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## BEAVER

That's plenty. 1/2" is plenty, really. 

You just need to leave an area open equal to the diameter of the pole vent.


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## cutra

BEAVER said:


> That's plenty. 1/2" is plenty, really.
> 
> You just need to leave an area open equal to the diameter of the pole vent.


Thanks alot buddy, I was worried about it after the fact...
My brother will be happy....

Back to work now.....hehe.


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## Candisa

That angle isn't that important in sub boxes, how lower the frequency, how longer the space needed for letting a wave stand. The longest dimension of the box is 37", you need a frequency of 300Hz+ to let a wave stand in that space, so you don't have to worry about it for subs 

Really, take a look at my project, you can see a couple of pictures of the enclosure I'm working on. If shape was that important, I wouldn't have made my enclosure the shape it is 

greetz,
Isabelle


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## BoostedNihilist

While the math might work for the 300 hz comment in theory, *you can* still develop standing waves in an enclosure with parallel sides. I always angle one side of my flat boxes (fiberglass is different, not usually many parallel sides in a GRP enclosure, and find that the angled enclosures sound nicer. Whether or not that is a result of a lack of standing wave I don't know, but they do sound nicer.

An example of this.. take an enclosure in your trunk, fire it through the seats, then turn it around towards the back of the car, it gets louder! The majority of this gain is because of constructive interference (otherwise known as a standing wave) damping plays a role too but not as much as the constructive interference. If you calculate the wavelength of the 50 - 80hz you are producing, you will find that the wave will not fin in a car period... yet a standing wave can develop... it's not hard to follow this logic through to the internals of an enclosure...

Also, I like to have as much room as possible between the magnet and the sub wall. The reason being, the air port is right there, and clean flow out of the vent will make a difference in sub performance.


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## mvw2

If you do straight faces, you might consider fiberglass on a couple walls instead of an angled face.

There's probably some minor truth in box shape affecting sound, but I don't know the level of influence. If you were really concerned about what happens, it might be useful to model wave propagation through a box with some sort of fluid dynamics software. It would be neat to see a time study of the wave movements within an enclosure.


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## Mr Marv

I built a perfect cube along with a couple of "odd" shaped enclosures of the same volume and measured the responses with the same sub. Interestingly enough they were nearly identical below about 100hz  and most importantly _they sounded the same_ IMO.


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## chad

Mr Marv said:


> I built a perfect cube along with a couple of "odd" shaped enclosures of the same volume and measured the responses with the same sub. Interestingly enough they were nearly identical below about 100hz  and most importantly _they sounded the same_ IMO.


Maybe it has something to do with the size of the box and the wavelengths of frequencies below 100 cycles


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## 60ndown

standing waves cant stand up in sloping boxes.


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## mvw2

Yeah, they keep falling down.

Well, I know there's been a variety of discussion on this in bits and pieces on many forums. I do agree that for the most part, it really makes little to no difference for subwoofer use. However, I would still assume differences, even if minute, do exist based on design. The interesting side-effect is >100Hz frequencies. For this, wave lengths start to get relatively short. Some of us do cross at higher frequencies and/or use shallower slopes where the sub may in fact be playing audible and constructive sound up at 200Hz or even 300Hz. In this case, should one start worrying about box design? Or is everything potentially bad about high frequency use and box shape fixable with just a little polyfill or fiberglass matting?


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## BoostedNihilist

A 100hz wavelength can not fully propgate inside a car either. The wavelength of a 100hz tone is 3.43 meters per cycle. Well, this means that inside a vehicle any information your ears interpret as sound is actually only a small reflected part of a wave which cannot ever fit in a car. Yet, in cars, standing waves do develop in this frequency range. The logic stands inside a subwoofer enclosure... there can be standing waves. 

This is just simple physics. Air is matter and therefore has mass. Now, remember sophmore physics? If you drop a ball straight down in a vacuum (no, not the one you used to simulate your weekend adventure) with no other factors (ball rotation, windage etc) it will bounce straight back up (a 'standing' wave). Now, if you drop the ball on a 45 degree slope with no other factors (as outlined above) the ball will bounce off at 45 degrees (no standing wave). The basic principle is, when one object intercepts another stationary object, that object will deflect away at the same angle as the angle of incidence... Well, if it happens to be propogating onto a parallel surface, you *will* develope a standing wave, so long as there is a constant frequency. The wavelength does not matter because it is the air matter that is developing the standing wave. This is more a fluid dynamics principle than an acoustics principle.

Why bother if the difference is miniscule? Well, perhaps in an every day system you're right, 
don't bother. However, in a high end sq system, it is those miniscule differences which add up to an overall difference in sound quality.


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## MIAaron

BoostedNihilist said:


> A 100hz wavelength can not fully propgate inside a car either. The wavelength of a 100hz tone is 3.43 meters per cycle. Well, this means that inside a vehicle any information your ears interpret as sound is actually only a small reflected part of a wave which cannot ever fit in a car. Yet, in cars, standing waves do develop in this frequency range. The logic stands inside a subwoofer enclosure... there can be standing waves.
> 
> This is just simple physics. Air is matter and therefore has mass. Now, remember sophmore physics? If you drop a ball straight down in a vacuum (no, not the one you used to simulate your weekend adventure) with no other factors (ball rotation, windage etc) it will bounce straight back up (a 'standing' wave). Now, if you drop the ball on a 45 degree slope with no other factors (as outlined above) the ball will bounce off at 45 degrees (no standing wave). The basic principle is, when one object intercepts another stationary object, that object will deflect away at the same angle as the angle of incidence... Well, if it happens to be propogating onto a parallel surface, you *will* develope a standing wave, so long as there is a constant frequency. The wavelength does not matter because it is the air matter that is developing the standing wave. This is more a fluid dynamics principle than an acoustics principle.
> 
> Why bother if the difference is miniscule? Well, perhaps in an every day system you're right,
> don't bother. However, in a high end sq system, it is those miniscule differences which add up to an overall difference in sound quality.


It's cuz you only need a quater of the wave to propagate.


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## mtxflorida

i have two 12's in a weird shaped box that i designed, it is 3/4" mdf about 37"long, 16" tall and 14" deep with 2 sloped walls and filled with the polyfill, somewhat like this /__\ with a sub on each slope faceing out, i was wondering what oppinions anyone has on this particular box, i have looked for simular box designs online but have not found anything, i seem to think it bumps pretty good and so do people who have rode in my ford taurus stationwagon


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## t3sn4f2

BoostedNihilist said:


> While the math might work for the 300 hz comment in theory, *you can* still develop standing waves in an enclosure with parallel sides. I always angle one side of my flat boxes (fiberglass is different, not usually many parallel sides in a GRP enclosure, and find that the angled enclosures sound nicer. Whether or not that is a result of a lack of standing wave I don't know, but they do sound nicer.
> *
> An example of this.. take an enclosure in your trunk, fire it through the seats, then turn it around towards the back of the car, it gets louder! The majority of this gain is because of constructive interference (otherwise known as a standing wave) damping plays a role too but not as much as the constructive interference. If you calculate the wavelength of the 50 - 80hz you are producing, you will find that the wave will not fin in a car period... yet a standing wave can develop... it's not hard to follow this logic through to the internals of an enclosure...*
> 
> Also, I like to have as much room as possible between the magnet and the sub wall. The reason being, the air port is right there, and clean flow out of the vent will make a difference in sub performance.


Seeing that this happens because the forward firing wave is more then 6 feet ahead in time then the wave that has to bounce off the trunk and come back. How then is this comparable to what happens in a box where waves are funneled away from the cone in phase and all hit and reflect off the enclosure walls at the same time.


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## digitalhifi

CAUTION: MATH HEAVY

Yes, the shape certainly can have a huge effect on the sound of the system. Let's say we use a standard rectangular box...from JL audio's recommended enclosures for the W7 - V = 0.0531 m^3, Surface Area (S) = 1.13 m^2, 16 in X 15.75 in X 19.25 in. (You have to convert everything to m for this to work!). Now this is a subwoofer so we only care about standing waves in the range 20-80Hz, so we define this as a center frequency (fc) of 50Hz with a range (df) of 30 Hz. Plugging this into a modified form of the wave equation and mode calculator : N = df/f * [ 4(pi)V * (fc/c)^3 + (pi*S)/2 * (fc/c)^2 + 4*(W+H+D)/8 * (f/c) ] c is the speed of sound (~343 m/s) and N is the number of modes present in the specified frequency range. Since we can only use integers for room modes we get ~3 modes in the subwoofer range. Just because the box is too small to allow the full wave to propagate, does NOT mean zero standing waves exist. However just because we know 3 modes exist, we have no idea of their location or whether or not they will cause positive or negative interference with the sub. It all depends on the wave propagation and its effect on the subwoofer cone. Let me know if you're interested in this kinda stuff or not.


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## chad

Nice post loved the numbers, BUT as you stated there is not nearly the room for full wave prorogation within the enclosure but in fractions of a wavelength. Sure, I can did that but what is not stated is how little these fractions have on the final result of the enclosure and how much if any of these artefacts are audible over the intended output of the sub woofer enclosure.

The question was not if there were fractions of a wavelength available, the question was if the geometry mattered, in which i still have to say, no.


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## AdamTaylor

i hope not










actually this is probably the best sounding box i have made


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## chad

AdamTaylor said:


> i hope not
> 
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> 
> 
> actually this is probably the best sounding box i have made


In theory that's great! because you have greatly reduced the number of parallel surfaces!


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## AdamTaylor

chad said:


> In theory that's great! because you have greatly reduced the number of parallel surfaces!


wasn't by choice, thats what i had to do for it to fit in my 350z


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## chad

AdamTaylor said:


> wasn't by choice, thats what i had to do for it to fit in my 350z


That took some beers!


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## JDMRB1ODY

I was wondering the same for my box I'm considering. I want to build a square box in my spare tire well and extend it out the sides of my trunk. Like a big T shape. I need the space in the bottom for enough air space. Only thing I gotta watch is figuring out total net airspace,


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## e36bumpin

Does the shape matter if it is going to per ported or sealed? or does it not matter all around?


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## imomm

shape of the box is not as important as it looking at your subwoofer spec and designing it to the characteristic of the subwoofer. If you design a too small or too big and not tune it right...it's not going to perform its best and then you think the sub is crappy. 

Port length and opening is more important. It will affect the quality of the sound


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