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Old 7th September 2003, 03:19 AM   #11
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could someone also tell me whether it matters if i change the dimensions to suit my room conditions? still keeping the volume consistent of course but would doing so alter any resonating features (anything??) of the enclosure?

if i was to use a tube vent though, what would the required size of it be?

thanks again,
lach
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Old 7th September 2003, 10:26 PM   #12
Bose(o) is offline Bose(o)  Canada
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As long as the volume of the enclosure's interior is kept the same, I don't see a problem. However, if you make the depth of the enclosure too small you don't allow the rear-wave work to it's fullest use {especially in a vented enclosure}. Too small, meaning less than three inches behind the driver.
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Old 8th September 2003, 12:46 AM   #13
PRR is offline PRR  United States
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> whether it matters if i change the dimensions to suit my room conditions? still keeping the volume consistent of course but would doing so alter any resonating features (anything??) of the enclosure?

As far as the bass response goes: you can adjust the relative dimensions over a very wide range.

A cube would use the least plywood, if you could get plywood in any size.

In a wide-range woofer, you almost never want a literal CUBE. Each dimension has a resonance, typically upper mid-bass or mid-range. If all three dimensions are the same, that resonance gets very strong, and even packed stuffing may not mute it. I would compute a cube, then make one side 0.8X and another 1.25X (1.25 by 1.0 by 0.8) because that gives a nice spread between dimensions and spreads the resonances.

In a true SUB-woofer, not getting any upper bass, a cube or near-cube is often not a problem.

In practice you want dimensions a bit under 16", 24", etc, to make the most use of a 4x8 sheet.

If your room dictates otherwise, do what fits.

When the long dimension gets to be 2 or 3 times longer than the short dimension, you tend to be buying a lot of lumber and you also tend to need a lot of bracing on the long sides.

You get in trouble if you go VERY long-and-skinny: don't make one dimension 10 feet because it will start to work like an organ-pipe instead of a bottle resonator. (There is a whole separate school of theory for long-skinny pipes; it is not as developed as the "box" types and I doubt it has any core advantage, though it does have attractions.)

The internal volume is important but not real critical. See MCP's plot of two boxes, one TWICE as big as the other. Above 50Hz there is "no" difference (+/-0.5dB), and the biggest difference is only 3.5dB and about one note wide.

However the internal volume is critical when you try to calculate the PORT. Especially when you get to very over-sized boxes. You can use excess volume and tune-up the low-end by leveraging that excess volume, but the more leverage you use the more fussy the tuning becomes. I've never had any luck predicting port sizes, I always resort to tests. Run a sweep tome 20-80Hz and put your ear close to the port. If you sweep down from 80 to 20, you will hear sound from the woofer cone, then that fades and is overwhelmed by sound from the port, then that fades. The maximum port-output frequency is the tuning, and should match the number in your bass-reflex program or formula.
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Old 8th September 2003, 06:20 AM   #14
mcp is offline mcp  United States
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Some bass is very airy, like bandpass subs. Some gives good punch but lacks the vibration. Yet others are able to "throw" longer distances. Apart from all these, there is also a question of room interaction and the quality of the driver.

A good approach is to get a test box done cheaply and listen to it with different tube lengths. Effectively, you will be tuning the sub to your room. If it is to your liking, proceed to make a proper box. Otherwise, make another test box and more listening. If still not to your liking, consider changing woofers.
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