Enclosure shape?

[fouchagalaga]

Most subwoofers I see have a cubic and symmetric shape. I was wondering if an enclosure with the same volume but with more of a rectangular shape like in the picture below would have the same sound or if it really has a big impact on the way the driver will push air. What would happen if I build the enclosure like that?

[Glowbug]

I wouldn't worry about it

[dave_gerecke]

The biggest difference you will find by making an enclosure that isn't square, or near square, is that there will be less likelyhood of the cabinet resonating. However, if you brace the cabinet enough that the resonances of the enclosure are above the frequency band of the subwoofer, this will effectively do the same as making the box non-square. The best option would be to do both, make the enclosure with dimensions that aren't multiples of each other, and then brace it well.

Peace,

Dave

[planet10]

At the frequencies involved the shape of the cabinet has little internal effect.

dave

[Don Hills]

There is one main rule for sub enclosure shape:
The longest dimension must be less than half a wavelength at the highest frequency the sub will be reproducing.
For example, for a 100 Hz crossover, the box needs to be less than about 1.7 metres long.
This is only likely to be a problem if you're building a long, thin box to fit in some constrained space such as behind the sofa.

[head_unit]

Originally Posted by fouchagalaga

Most subwoofers I see have a cubic and symmetric shape...

That's because normal people want subwoofers as tiny as possible, so manufacturers make the smallest box possible behind the driver, which is therefore cubic.

For subs, the shape doesn't matter. The strength and bracing and stuffing of the box make the difference.

[LineArray]

Originally Posted by Don Hills

There is one main rule for sub enclosure shape:
The longest dimension must be less than half a wavelength at the highest frequency the sub will be reproducing.
For example, for a 100 Hz crossover, the box needs to be less than about 1.7 metres long.
This is only likely to be a problem if you're building a long, thin box to fit in some constrained space such as behind the sofa.

I agree to what you say but the answer to the question
"how clean should my subwoofer play in the lower midrange"
depends on the slope of the crossover too.
Think of a bandpass subwoofer with 12db/octave slope elecrically
plus 12db/octave acoustically: Resonances in the Box can be audible
above crossover frequency very well (which is bad).

Resonances in lower midrange (caused by vibrating cabinet walls or
aspect ratio of enclosed air volume) can attract unwanted
attention to the subwoofer.

My personal rule of thumb is to have the lowest resonances of
the subwoofers cabinet walls at least 2 octaves higher than the
desired crossover frequency. Make the walls stiff enough to achieve
that, the more compact (low aspect ratio) the shape is, the less
bracing is needed.

Knocking the cabinet with a rubber hammer at different positions
and spectrum analysing the sound has proven good practice to me.

Same for the shape dependent modes: Above say 40cm
---
340/400/2=0.42 m is half wavelength at 400Hz,
wavelength gets even s h o r t e r in stuffed cabinet
---
for the longest side one should think seriously about
incommensurable aspect ratio, which may be advantageous
against structural vibration too.

Stuffing is essential, even BR subwoofers should be stuffed
lightly, but with a means to keep stuffing material away from
the port (cords or net).

Furthermore i would avoid mounting a driver symmetrically
in any given cabinet when possible. Mode excitation
is in almost all cases more evenly distributed when the
driver is offset (incommensurable).

Yau can neglect all these issues but in the end there
is a difference between an enjoyable loudspeaker and a boom
box. "The big difference" is made up from the multiplicative
effects of those small differences.


Cheers