Designing a new box for an old sub ? - diyAudio
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Old 18th October 2009, 08:12 PM   #1
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Default Designing a new box for an old sub ?

My wharfedale sw-15 has a box that is sealed but the box is not that well built and rattles a lot.
I was thinking of designing a box that would be like a bench with low profile that can be used to put amplifier and other things on.
The dimensions is not set in any way but a length of about 2meter (6-7 feet) and a height and width that is similar to the diameter of the 15" woofer.
I do not know any parameters of the woofer but the cone seems very light and not that high xmax.
The rear magnet is not that huge.
I could build a sealed box, what do you think ?

Pelle
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Old 19th October 2009, 10:13 AM   #2
volare is offline volare  New Zealand
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Default To vent or not to vent

Hi Jakobsson,

The box design will affect the performance of the driver hugely. A few things to consider are the Qts of the driver (how controlled it is by the magnet and mechanical spider components). Traditionally low Qts drivers say 0.25-0.38 are suited to a vented box design, and drivers with Qts over say 0.6 are better put in a sealed box.

I looked up the model number and came up with this active system Amazon.com: Wharfedale Topaz SW-15 300-Watt Subwoofer (Single ): Electronics

Given the original configuration, the sealed box is recommended, as a vented box may cause the amp to be stressed when it gets a huge back EMF and sudden current draw to control the drivers operation. Also xmax would suggest that the driver in a vented box might suffer mechanical problems by being uncontrolled and bottoming out with high excursions. Then there is the added complexity of tuning a vented box depending on the T&S parameters.

Ok so lets assume you go for a sealed box, assuming you make the box suitably reinforced (cross bracing and edge reinforcements) you could look to double the box volume (Vs) to improve the bottom end extension. Note here that a rigid box of the same internal dimensions will improve any distortion or colourations you are having from sloppy box design. In terms of using it for an amp stand, that really depends on the system you have, I don't recommend putting the CD/DVD player on it as the vibration will make them unhappy

I have a large sub with vented twin 18" drivers in what looks like a small bed (1600mmx400mmx600mm) and even with all the reinforcement and 25mm thick walls it still has vibrations that make ornaments dance across the surface (one day I will get around to upholstering it the lounge ottaman iI promised my wife).

All that said making boxes is a load of fun, from the description of the driver I'd make another sealed box to house your baby. Increasing the depth and length of the box will reduce the rear wave reflections (some high density foam will help too).Also check the amp module for any loose parts as these might be the source of the original rattles, you could try reseating it in an audio clay and checking the mounting screws for a tight fit.

hope this helps

Cheers Richard
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Old 19th October 2009, 11:57 AM   #3
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He Pelle,

Standard subwoofers of max 500euro/dollar are made of reasonably poor materials that resonate more then most materials used in diy subwoofers (solid heavy wood, HDF and bitumex with good spikes/feet). So rebuilding a cheap sub with better materials for the cabinet would be an improvement...

But without any parameters, it would be best to measure then from the bass-unit. Or if this is not an option, rebuild the original bass-cabinet with good wood/plywood or MDF/HDF. And brace it well. Keep the same volume (calculate some less with bracing off-course...) And buy a bass-port that has about the same diameter, which is adjustable. So that you can adjust the tuning of the port to your needs. You could think of making an seperate case (in the sub) for the amp to be fitted in. And to make the baffle extra thick... And so on.

To build an closed box, the bass-extension would probably be sacrificed (way earlier roll-off of the low octave, but less sharp maybe, and less efficiency). Most sub-woofers that are closed design have a unit that has fitting thiel/small parameters. For example I've got two sub on a Reckhorn A402, one for the bass-slam of my surround set, an 10" Aluminum sub-woofer ported with loads of Xmax. And one for Vifa MA26WR09-04 in a closed box (seems to be one of the few with the thiel/small parameter fit for a 10" closed sub under 200euros) for the lowest octave for music, where I don't really need a lot of decibels/power.

When you're not sure; wharfedale must have at least designed the sub for a reason and a tuning. So best guess is that the driver is best suited for BR, not closed, unless you measure it, or test it in one way or the other... But you could end up returning to their design, only maybe wanting to tune it a bit different.

I would expect a decent sub to contain at least a tune-able bass-port for adjusting a sub to its environment, but thats just my opinion, and I shouldn't complain; I allways build my own subs, it's one of the easiest speakers to be diy-ing... When you've got all parameters of the unit
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Old 19th October 2009, 05:35 PM   #4
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If i would just give away the sub and get myself a new driver are there any designs that are nice ?
I guess everything comes down to the driver before any cabinet suggestions can be calculated or recommended.
Is a 15" or 2 12" what i should look for when selecting drivers for some horn construction ?

Pelle
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Old 20th October 2009, 04:26 AM   #5
volare is offline volare  New Zealand
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Hi Pelle,

now you ask a harder question... what sort of bass do you want? Tight and punchy or loud and boomy? in terms of spl the cone area of a 15" is approx 2x12". You will need the T&S parameters to make an informed decision here.

depending on the Vas of of the drivers one 15" driver may give simular performance and bass extension as 2x 12" in a larger box. you can then play around with the tuning of ports etc

A horn loaded sub's bass extension will be proportionate to the aperture of the horn (i'd have to look up the formula for this but 1/4 wave for 20Hz is approx 5ft (150cm). Horn loaded subs tend to be large (we call them bass bins here in NZ), they can achieve some great spl, but like all subs need a bit of care in the contruction and mounting of the driver and bracing of the box.

Materials wise plywood is good for forming the horn flare, we have also used HDF/MDF sheet aswell with drywall as a laminate. As suggested above bitumen based products can help with the mass, we have used barium lead sheets in a bitumen backer, simular to those used for automotive damping of the pressed steel body parts. Then used good high density foam, on the enclosure side, (foam should be dense but still allow air to pass through it, ie not closed cell type).

Driver wise there are plenty of options with the usual suspects (Jbl, JL audio, vifa, etc).
you will need to decide on what characteristics you want form the bass output to start narrowing down the T&S parameters.

Wiki has a good page explaining each of the parameters - Thiele/Small - Wikipedia, the free encyclopedia

things to consider
Large Vas = large box to get max bass extension
Low F3 = better bass extension
Higher Spl can afford more flexibility in box design as you can tune it to optimal bass output with a lower F3 than in free air with ports etc
lower Qts's = greater control (with the right amp = more accurate bass response)

You can start the design by considering price, size, bass accuracy, spl and then make your way from there.

cheers Richard
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