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  I've now built one! -- my comments 
 
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Roy Wall Sep 17, 2005, 02:17pm EDT Reply - Quote - Report Abuse
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I used 25.4mm MDF and changed the internal dimensions to suit: reducing the size of the amplifier box to take the Hypex DS2.0 amp. which is relatively small.

I basically used hand-tools to build the box, using a "no nails" grab-adhesive to butt-join the panels (this glue is better for filling gaps than normal PVA wood glue).

The finished weight was 84 pounds (about 38 kilo?) - pretty heavy. Nevertheless, I played the "Thunderstorm" track from SoundCheck CD, and the box moved quite a bit. (This track is a ridiculous test for any subwoofer, I would think).

The performance seems great (I am still testing). It's a real eye-opener as to what i have been missing over the years. I highly recommend this subwoofer.

I'm using the subwoofer to "fill-in" below my hi-fi speakers' response (-3dB at 60Hz). I've had my speakers about 25 years (they are original KEF Concorde III's, Type SP1111), and they have always had a noticeable, but not huge, bass-resonance, which is quoted as 68Hz for a later version of the same speaker, and is probably about the same for mine. I am now working on building a fourth-order Linkwitz-Riley filter to add into my hi-fi amp (another antique: an original Mission Cyrus One, Mk.1, which, classically, has no balance control!). I've designed the crossover frequency quite high: 150Hz, so that the signal to the main speakers should be well-down at 68Hz, hopefully, effectively eliminating the bass-resonance in these speakers.

Thanks for the design Sander,

Roy


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trevor powell Feb 17, 2006, 05:08am EST Reply - Quote - Report Abuse
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>> Re: I've now built one! -- my comments
Magic article. I have been searching for months for the right plan and was about ready to through in the towell. I am going to build this! I have ordered the drivers and they should be here shortly. Went looking for 22mm mdf today. Guess what. I can only get 18mm or 25mm here in Australia. No 22mm mdf. Just wondering if 25mm for the internal braces would stuff up the volume measurement, the accoustics etc. Sander used 22mm. Roy Wall used 25mm. Roy could you tell me why you used 25mm? Couldn't you get 22mm? Also the mitre corners i would consider to be a worry when it came time to assemble. Also what is the view in the long term of the glued only construction no screws. Will it hack the pace two, three years down the track. Forgive me if i sound negative.
Trevor.

Roy Wall Feb 17, 2006, 08:50am EST Reply - Quote - Report Abuse
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>> Re: I've now built one! -- my comments
Hi Trevor,

I used 25.4mm MDF 'cos I couldn't find 22mm and didn't want to drop down to 18mm.
As I said, I used butt-joints for the construction. I tested the joints using some scraps of MDF and using a "grab", "No-Nails" adhesive in the joints. They tested okay even tho the failure under test-stress (bang with hammer!) results from the MDF being a "layered", laminated, material, i.e., the joint fails 'cos a thin layer of MDF peels away. But the joints are pretty strong. I guess mitred joints would be stronger 'cos they're effectively end-grain to end-grain. But with hands-tools only, you'd have to be a damned-fine craftsman to get the thing to fit together with mitered joints. You could increase the strength of butt-joints by peppering the non-"end-grain" surface of the butt-joint with indentations made using, say, a 12.7mm drill bit. This would ensure that if the joint failed, it would have to rip a thicker "lamination" off the surface of the MDF. I didn't do this simply 'cos I tested simple butt joints and thought they seemed to be strong enough.

I used 50.8mm countersunk chipboard screws to hold all joints together since I didn't have big-enough cramps to cramp any of the joints. I used filler to hide the external screws. Beware the laminate structure of MDF: if you put a 50.8mm chipboard screw straight into MDF end-grain, the MDF will split, so drill holes first the size of the screw core diameter.

As I said, tho I used 25.4 mm (and not 22mm) MDF, I kept the external box dimensions the same as in the design but I moved the internal, vertical baffle back in order to maintain the design internal volume (30 litres?). I could do this 'cos I used a physically-smaller amplifier than that used in original design.

Another change I made to the basic design was to recess the amplifier front-panel so that the various control-knobs and connectors did not stand proud of the back of the box. Another change was to mount the driver and radiator straight on the front-panel, i.e., no recess that would require use of a router in order to machine the recess.

If I made another one, I'd do a couple of things different. Basically I'd brace the back-panel more. This could be done by making a four-sided box that would fit behind the back-panel cut-out so that it would both brace the back-panel and provide, say, 12.7 mm edges on which the amplifier panel would sit (I hope you can follow this, it's a bit hard to explain without pictures.) Also, above the amplifier I would include a vertical brace inside the sand/lead-shot box. This is because I have noticed that the back-panel of the box seems to vibrate a bit more than the other external box panels.

BTW, I painted the box satin black using a small sponge roller that gave an atrractive "hammered" effect. I used shiny zinc-plated rounded-head screws (16 needed) to screw the driver and radiator to the front-panel, but I later changed these to matt-black screws 'cos the shiny screws made the units look like boat steering-wheels (helms), and I just couldn't justify this "nautical " look.

send me your email Trevor and I will send you some piccys of my finished subwoofer. roy "at" kencomp.net.

trevor powell Feb 17, 2006, 05:59pm EST Reply - Quote - Report Abuse
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>> Re: I've now built one! -- my comments
Thanks for that Roy good stuff. I had sort of decided that mitre joints were not the go. I have had a quick sqiz
at your second post and will read again to absorb your revisions. I wonder where sander got the 22mm mdf?
Although the 25mm sounds like a better bet with all the movement you say you are getting . I would love to see your pictures. My email address is powell99@gmail.com or powell99@internode.on.net.

Regards Trevor

Roy Wall Feb 18, 2006, 06:44am EST Reply - Quote - Report Abuse
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>> Re: I've now built one! -- my comments
Hi Trevor,

I've yet to take piccys of my finished subwoofer, so hang on a bit for them.

I should emphasize: my local timber merchant said that they only stocked 18mm and 25mm MDF. I checked elsewhere and it was the same case. But it's not 25mm but 25.4mm, i.e., an "imperial" inch. I didn't realize this until halfway thru construction, but it didn't really matter.

I guess that on the European continent proper, they have 22mm MDF, but it doesn't seem to be common or available in GB.

The sequence of construction is quite important if you are using a "grab" adhesive and butt-joints like I did. As opposed to ordinary PVA wood-glue, a "grab" adhesive can easily add something like 1 mm to a joint. This is useful 'cos, with hand-sawing (which is what I did), you need a glue that can fill gaps, and ordinary PVA wont do this.

So, given that I wrongly assumed the MDF to be 25mm thick and not 25.4mm, and because of unpredicted build-up of glue thickness, my final box ended up 353mm wide and 351.5mm deep instead of the designed 350 x 350mm. Because 25 doesn't equal 25.4, aim to make the final box dimensions 350.8 x 350.8 x 600.8mm (tho it will end up slightly bigger 'cos of accumulated glue-thicknesses). So the build-sequence is as follows (NB: use 50mm chipboard screws, pre-drilled, to clamp all joints together. I removed all the externally-observable screws once the glue had set, and then filled the holes):

1) From your sheet of MDF (you will need a complete "8ft x 4ft" sheet 'cos a half-sheet is just not quite enough) cut-out the following pieces that are 300mm wide: the vertical bulkhead (300 x 550mm); the horizontal brace that has the two 100mm dia. holes (I made these holes about 103mm 'cos of increased thickness of material); the horizontal brace that forms the bottom of the "sand box"; the back-panel {300 x 550mm}); but don't yet finally cut the top and bottom panels to length (which will eventually be both nominally 325 x 300mm) Cut these to fit, later. (So, in fact, your first bit of sawing will be to cut an 8ft length of MDF that is 300mm wide, and then to cut individual pieces from this strip).

You will, of course, have re-calculated the dimensions of the horizontal braces 'cos both the increased thickness of the MDF and mounting the drive unit un-recessed affects the 30-litre volume if you keep the box external dimensions to 350.8 x 350.8 x 600.8mm. I just found my calculations that show that the original 119mm dimension was increased by me to 140mm. This would reduce the sand-box volume, so you have to drop the rear horizontal brace (the one with the 20mm hole) to increase the sand-box volume, and you can only do this if you don't need 373mm of height in the amplifier box.

2) Having assembled and glued-together the 3 pieces mentioned above, you can now cut-out the 6 brace-pieces that are all 75mm "wide". Now, when you assemble and glue these 6 pieces to the assembly, you are likely to build-up some errors due to glue thickness, so its best to predict, in advance, allowances for glue-thickness (say 1mm). It doesn't really matter if you have got errors of up to, say, 1mm 'cos you can easily bridge a 1mm gap with grab adhesive. Having done all this, you will have an assembly of 9 pieces.

3) As I said before, I would add further braces to the design. I would add a vertical brace that effectively divides the sand-box in two (so you will then need 2 x 20mm holes to allow filling with sand); and a 4-sided box that surrounds the amplifier. I used a Hypex DS2.0 amplifier which is about 195 x 225 mm, so the 4-sided box would be roughly 220 x 250mm externally, and the back-panel cutout would be, say, 198mm x 228mm, i.e., leaving about 3 mm "play". This allows the amplifier to be recessed (you will see this in one of my piccys), i.e. the amplifier control knobs do not "stand proud".

4) Cut-out the top and bottom pieces. These are nominally 325.4 x 300mm but you should adjust the 325.4 dimension to be the same as that of the internal assembly that you have built. Assemble and glue the top, bottom and back pieces to the assembly ensuring that the front edges of the top and bottom line up with the front edge of the brace that has the two 100mm holes in it.

5) Cut out the 2 side panels to a nominal 325.4 x 600.8mm adjusted to fit. Assemble and glue these to the assembly ensuring that they are flush at the front of the box.

6) The front panel is nominally 600.8 x 350.8mm but cut it out slightly too large (1-2mm) so that you can sand-off any overlap later. Glue the front panel to the assembly.

7). Having now assembled all the MDF pieces, cut out the holes for the drive unit and passive radiator. Interestingly, the hole is 242mm in diameter, which is a bit weird since the drive unit is supposed to be 10 inches, and 10 inches equals 254mm! In fact the actual cone is a only just over 7 inches 'cos of the massive roll-surround that allows for cone excursions of plus and minus 20mm! The units are called XLS 'cos this stands for "eXtra Long Stroke".

8) Sand down the box to get a good finish, and don't forget to drill a hole for the wires from amplifier to drive-unit, sealing the whole later, once the wires are in.

9) As you will see from my piccys, I radius-ed some external corners for aesthetic reasons. I went for a 25mm radius, done by hand 'cos you'll have trouble finding a router-bit with such a large radius. I rounded the cornes by first doinf a 45 degree chamfer. I then chamfered the schamfers, i.e., added 22.5 and 67.5 degree chamfers to the 45 degree chamfer. The final curve is obtained by using a relatively log strip of abrasive paper and, holding each end, sort-of "see-sawing" it around the corner (it's damned difficult describing this!). Whatever you do, the corners "must" have a minimum radius of 3mm 'cos paint thins-out at sharp corners, offering little protection to damage. You can determine the size of the chamfers needed to rough-out the big 25mm radius by drawing a 10 x scale drawing of the radius and measuring the width of the 45, 22.5 amd 67.5 chamfers from this.

You should expect to take MANY hours studying the design and my notes before you put-saw-to-MDF. Get back to me if you have any queries, I could well have made mistakes in the above. I recommend downloading the big version of the design drawing and then make marked-up changes to it according to adjusted dimensions, changing mitre-joints to butt-joints (you should be able to work-out the direction of the butt-joint overlaps from my above notes, damn-it! I might-aswell spell this out now: The front panel overlaps all other panels, the side panels overlap the top, bottom and rear panels, the top panels only overlap the rear panel, the rear panel is fully over-lapped by the other panels.)

You might think: why has Roy gone to all this trouble writing these instructions? The reason is that it took me 3 weeks solid to build the subwoofer, and I don't think you should "re-invent the wheel". But it's truly worth the effort.

Piccys to follow once I've worked out how to connect my new digital camera to my PC,

Roy

PS had trouble with both of your email addresses, so pls email me directly with a working address

trevor powell Feb 19, 2006, 12:18am EST Reply - Quote - Report Abuse
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>> Re: I've now built one! -- my comments
Thanks Roy. After reading your email i think i will get the exact mdf
size that i can get and as you say redraw the plan exactly as the new
dimensions will be. I have a mate who is a cabinetmaker so i will pick
his brain for his perspective on building the sub. I cant wait to see
your pictures. I would like to know what you think of the sub with 5.1
movies and music in comparison to other subs you have owned or have
heard. Thanks for your indepth anaylisis i appreciate it.

Regards Trevor

Roy Wall Feb 19, 2006, 07:44am EST Reply - Quote - Report Abuse
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>> Re: I've now built one! -- my comments
Hi Trevor,

Yes, it's well worth re-drawing the subwoofer drawing (I just used "Typex" on
the original drawings and then made ammendments). I found that I had to
study the drawing quite a bit to build-up a mental picture of the internal
bracing. I think that my extensive construction notes should also be
studied so that you have the construction sequence in mind at all stages of
construction (and with any modifications that you want to make). Basically,
it's a protracted process of construction: you have to glue a bit, wait, say
24 hours for the glue to get well-set, then glue the next bit, and so on.

I have not ventured beyond stereo hi-fi 'cos I listen mostly to older music
that is restricted to mono or stereo. So I know nothing of 5.1.

I have no experience of having listened to other subwoofers. But i do have a
very critical ear for bass frequencies. My traditional stereo speakers,
which I am now using only for their high- and mid-range response, date from
the late '70's when bass-resonance was a marked feature of most hi-fi
speakers (at least in my price range). A close friend of mine for years had
the best speakers of anyone I know: they were 4-way units standing about
5-feet high and using the KEF B139 bass unit and a transmission line (folded
horn?) loading. But my subwoofer just blows these away and my friend, when
he was last visiting, had to admit that I now have a highly-impressive
stereo system.

So I know bass-resonance when I hear it (the "boominess" that many critics
refer to) and this subwoofer has no trace whatsoever of "boominess": it has
a very "clean" response.

The subwoofer has more than lived-up to my expectations. It actually creates
new problems 'cos lots of past recordings seem to have cared little about
frequencies less than say 60Hz. For example, I played an old blues CD only
to find that there was massive rumble: the subwoofer cones were visibly
moving something like full excursion at low frequencies of about 5 to 10Hz.
I had to copy the CD and, using GoldWave editor, effectively remove all
frequencies below 20Hz using GoldWave's wonderful filtering facilities.

Recent recordings seem to be more careful about what they contain at the
bottom end, and there are some damned-fine bass lines coming from them.

I must say that before I built the subwoofer I was a little skeptical. I
thought: will the effective shifting of my system's low-frequency response
from 68Hz down to 20Hz be really worth the effort? After all, it's only 48Hz
difference. But one has to look at it in octaves. The range of hi-fi is
roughly 11 octaves: 20Hz to 20.48KHz. The subwoofer is adding about 1.6
octaves to the bottom-end of my system, and this 1.6 octaves is really
noticeable. It's really surprising, for instance, what the subwoofer adds to
merely the sound of an ordinary acoustic guitar. These added 1.6 octaves do
not simply connect to your ears: they hit you in the chest so it really
helps in reproducing the feelings that I remember form those late '60's rock
concerts. Ah, those were the days!

I recommend the Hypex DS2.0 subwoofer amplifier even tho the speakers could
handle more power, but I don't need any more volume for my living-room. The
DS2.0 is a class D amplifer (if that means anything to you), and I can't
fault it except that I wished that it had had about 12dB more sensitivity on
the line inputs. I've built an active crossover-filter into my old Mission
Cyrus One hi-fi amplifier which uses a high crossover-frequency of 150Hz (in
order to avoid the bass-resonance at 68Hz in my old KEF Concordes. The
danger with such a relatively-high crossover-frequency is that one may start
to get into "directionality", i.e., one can detect where the bass is coming
from. This is perhaps the only criticism that I now make of my whole
system: there is, perhaps, a trace of directionality, but I have put the
subwoofer roughly between the two stereo speakers and this helps a lot.

Just an odd thought: You have to add extra mass to the passive radiator, as
Sander states. But I recall that he painted his lead-disks black, which is
weird 'cos you can't see them in the finished item. I got the lower-mass
version of the XLS 10 passive radiator (264gm?) and had to add quite a bit
of mass to it (to bring it up to 600gm total?). I melted some scraps of lead
and plumbers-solder, that I found lying around, in a discarded coffee tin
on my gas-stove. I let it solidify and then pealed the tin away leaving a
disk roughly 3 inches in diameter and a quarter-inch thick.

Roy




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