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15" bass box - with a difference ...

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slaz View Drop Down
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    Posted: 06 January 2021 at 1:46am
Well - had the idea for this for quite a while now ...

Needs a bit of pre-amble - but I'm guessing quite a few in here are not short of time to read/muse/laugh etc. in these lockdown times:-)

So - I'm 65 years old - bit of back trouble - and I live on the 3rd floor of a tenement block with no lift.
Heh. Thing is - I also live very close to Victoria Park (nice) - where I've been playing out for quite a while with various sizes of battery power speakers/setups.
So right now my "senior" battery setup consists of a single passive FBT Maxx 4 (4 ohm version  - which is rare-ish) powered with a  Bassface 4ch class D car amp. This works very nicely - the 12" FBT only gets about 150W or so (2ch of the amp running 4R bridge) - but gives a very good account of itself. Sounds very hi-fi - except of course for the fact that bottom octave is pretty much missing. Now in the context of playing outdoors from battery power - it actually sounds OK - most folks just take it and enjoy as is .... but not me of course.
Now the logistic involved with playing over the park is - got a bodged up trolley thang that carries the FBT + about 20Kg of 12V lead batteries. I can _just about_ get the whole shabang down the narrow stairs - 3 or 4 journeys on my tod  - but I do sometimes get help from a younger/fitter neighbour.

I've actually been offered (FOC) an oldish Tannoy 15" bass box of some sort - haven't seen it yet. Suspect it might be a bandpass box from the description. But of course its made from ply or MDF (might be from an install) - and I stand feck-all chance of safely getting that up/down the stairs. The driver might well be decent though ....

Aaaanyway .... here's the idea.

I happen to have a 8' x 4' sheet of this stuff (or very similar).

http://www.cxdinternational.com/boards/support-backing-board/aluminium-honeycomb-centered-panel-glass-fibre-bdhexg7195?returnurl=%2fboards%2fsupport-backing-board%2f

(sorry dunno why that doesn't seem to hyperlink so here's a pic)



Came from  a former flatmate who worked for a well-known museum - it got used as backing for displaying pictures/artworks. The piece I have has some very hard-to-remove gunk on it, not a problem methinks. So - well it should be obvious where I'm going with this.

If this could be successully cut into a 15" bass box it would feckin _awesome_ - absolutely the difference between having a bass box of sorts - and just not having one at all, as all the normal boxes are impossible for me re stairs/logistics.

So the idea I  have is :-

Just a conventional bass reflex box - as compact as possible. I'm not trying to play really low ... 45Hz with a bit still there at 40 would be fab. Also, I'm obviously not trying to flap trousers, the box ain't gonna get more than about 150-200W or so. I'm really just aiming for a "taste of bass". Nothing radical here then.

But the material _is_ a bit on the radical side. As per the web link, you can see the price :-)

I have in mind to make a five-sided box from the honeycomb stuff - and make a normal baffle - from 15mm ply I guess.
The problem with the honeycomb stuff (HC) is its just about imposible to get a clean cut with any kind of saw I think ... but a mate over the road had a go with a small test piece - to butt 2 bits together perpendicular style (butt-joint) - seemed to work OK - so I'm hoping a 15" box is possible/viable.

I've yet to work it out, but I think it should be possible get the weight down to around 14-15Kg using this HC stuff - I can definitely manage that on the stairs - the FBT 12" box is 13Kg. Might even be less if I can scrape up enough down the back of a sofa for a neo driver.

I'm currently to trying figure out - with the constraint of not making any cut-outs/holes in the HC:-

1) What to use for some kind of edging strip to cover the exposed edges of the HC.
2) How to achieve some kind of corner protection.
3) How to accomodate a removable plwood baffle.
4) How to achieve some kind of internal bracing.

I'd be very interested to hear from anyone who knows this HC stuff, has worked with it in any way, or has any informed opinions on how to construct. I figure on mounting the speakon panel on the front baffle - a bit strange I know.

Also interested in what folk think is a suitable driver for this. The P-Audio E15-300S looks like a candidate - small power handling, but plenty for this box. Nice price too from Blue Aran, but not especially light weight. Bear in mind I won't have fancy EQ or anything available (well I might consider getting one of those Thomann mini-DSP things. ).
Whats the simplest, smallest 15" reflex design I could use in this context ?

OK - ramble over - for now :-)

TIA.








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imageoven View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote imageoven Quote  Post ReplyReply Direct Link To This Post Posted: 06 January 2021 at 9:39am
Interesting design challenge 👍🤔

Would a rivet into the outer skin be viable?

I imagine a silicone sealant (or similar PU etc) is going to be needed to seal the edges so could be used for construction too. Sikaflex 512 is used widely in the motorhome industry for bonding Ali. It is expensive though and people report good results from cheaper alternatives

Ali extrusion for the corners?

Some flightcase profile s may be big enough and could help with making a removable baffle 
Keep pushing on, things are gonna get better.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote infrasound Quote  Post ReplyReply Direct Link To This Post Posted: 06 January 2021 at 10:26am
Rivets have really poor pullout in these, unless you have the "proper" inserts (hella expensive, and AFAIK only suitable for thicker Aluminium skins)


Alongside a suitable epoxy. I can't remember which we used, but it was 3m and really not cheap. I'm sure RS do a slightly more reasonable equivalent. It could be worth getting one with gap set glass beads (~150um) if the 90 degree angle doesn't support this by geometry. Also, primer if needed and clean the joints properly with IPA and very fine grit before even thinking of gluing.

Bear in mind you have to have continuous adhesion along the full length of both sides (inside and out). Not following this will just result in the composite tearing itself apart quickly. Your edges need to be completely rigid in every plane.

Another alternative is to use two 90 degree aluminium angles inside and out. Getting the correct single extrusion (as linked above) is a bit of a challenge in itself.. Rivets can be used but purely to help keep in position during cure. They must be within the glued area to prevent point loads.

Bracing, you can use the same composite, but at 90 degrees. 

Baffle wise, you can use the same material, but have a tapped ring (ideal for load spread) or separate nuts with washers for the woofer bolts. Unless you want to swap woofers about, no reason to have MDF (ugh)

The alu ones have very poor damping, don't know about GF. It could be worth adding some small squares of bitumen to the centre of the panels to see if it makes a difference. This can be done to the outside, and will of course start to weigh quite a bit. Hopefully they will made naff all difference.

Oh, and go Neo if you're making this. Worth extending the weight savings if you're using such a fancy material. I'm a big fan of the B&C Neo drivers and the CL series are hilariously light (and cheap!), with really respectable BL/QTS.

Good luck! A bit jealous that you have such a big sheet of it, it's lovely that it's only half the weight!




Edited by infrasound - 06 January 2021 at 10:28am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JBK Quote  Post ReplyReply Direct Link To This Post Posted: 06 January 2021 at 10:35am
https://www.facebook.com/rootikalvibeshifi

http://jbkaudio.fr

https://www.youtube.com/playlist?list=PLKB8xPHqsVywM2EgtOaMjdgfIUf_-kFdV
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Post Options Post Options   Thanks (0) Thanks(0)   Quote infrasound Quote  Post ReplyReply Direct Link To This Post Posted: 06 January 2021 at 10:41am
Thanks Smile
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Post Options Post Options   Thanks (0) Thanks(0)   Quote doober Quote  Post ReplyReply Direct Link To This Post Posted: 06 January 2021 at 7:33pm
Originally posted by slaz slaz wrote:


I've actually been offered (FOC) an oldish Tannoy 15" bass box of some sort - haven't seen it yet. Suspect it might be a bandpass box from the description. But of course its made from ply or MDF (might be from an install) - and I stand feck-all chance of safely getting that up/down the stairs. The driver might well be decent though ....





A friend of mine collects old Tannoy stuff, he was actually looking for plans to make a Tannoy bandpass type cab. Would you be able to post some pics when you see the box? It might be just what he's after.

Thanks
Blahblahblah
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Post Options Post Options   Thanks (0) Thanks(0)   Quote slaz Quote  Post ReplyReply Direct Link To This Post Posted: 08 January 2021 at 12:25am
Thanks for the input guys ...

Well I got around to digging up the HC stuff from the shed ... and it seems my memory of what I have is a bit screwed up ..... I don't have a complete 8 x 4 sheet of HC. but I have 2 pieces :-

hexalite 14mm thickness

1) 38" x 41.5" 965mm x 1055mm - few small holes (weighs 3.3Kg)
2) 34" x 52"   863mm x 1320mm - covered with material/foam underneath + timber edging.

Altogether thats about 1.5 sheets :-)  .... but may not be optimal in terms of making a cut sheet.
Should be plenty to make a compact 15" box though.
The 1st piece with holes - the holes can be easily gummed up with something (the holes don't go right through - just one layer of the GF).
The 2nd piece needs a bit of attention re removing the material covering, foam and timber edging, but should be in good nick apart from a few holes like above.

I'm pleased with the weight - its about 30% of the weight of 18mm birch ply I think. My proposed 5-sided box should come in at about 5Kg I think - but thats without front baffle, bracing, grill etc.

I've been using this beyma plan (with cut list) as an example/rough guide for size/shape etc.


I'd use cylindrical port(s) (plumbers plastic pipe - 50mm or so ? )instead of the triangular ones.
I'd likely bleat for help a bit with WinISD/box tuning etc. .... some TS params I don't understand properly.

The B&C 15CL76 looks like a little honey for this - thanks for that heads up - and yes of course you're right about using neo .... was hoping to get the driver for <100 quid - but not _too_ expensive.
I'm guessing these are probably meant for stage wedges etc. - so likely designed as bass/mid really rather than specific bass, but should work well for about 40Hz-80Hz or so ... no ?
Actually, my FBT Maxx4 uses B&C neo drivers - so thats kinda nice and neat :-)

I'll almost certainly come back for tips on type(s) of glue/epoxy etc. and stuff like order of assembly of the panels.

Got a bit more of a clue (I think) on how to deal with the edges. Will come back with my proposed details and sketches etc. .... but this ain't gonna be a next week type of schedule ... I'll probably be relying on a mate over the road for tools etc., but ATM he's very covid wary so can't do anything for a while anyway. Can't use the box till summer time anyway, so now is the time to start planning it.





Edited by slaz - 08 January 2021 at 12:29am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote slaz Quote  Post ReplyReply Direct Link To This Post Posted: 08 January 2021 at 1:02am
Originally posted by doober doober wrote:


A friend of mine collects old Tannoy stuff, he was actually looking for plans to make a Tannoy bandpass type cab. Would you be able to post some pics when you see the box? It might be just what he's after.

Thanks


Well I want to have a look at this box myself first obviously - figure out how to get inside it and check out the driver. Apparently someone has made a bit of a mess of the input panel (prob trying to connect it to a hi-fi amplifier or some such), so might need some attention.

For sure I'll post up some pics - the whole thing might be up for grabs - not sure yet.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote doober Quote  Post ReplyReply Direct Link To This Post Posted: 10 January 2021 at 5:00pm
That's great thanks.
Blahblahblah
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fsurfer Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2021 at 8:59am
Lovely finish
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JonB67 Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2021 at 12:08pm
Id be trying to take advantage of the face material being basically grp epoxy. 

Id use a thick epoxy to bond (basically glue) the 90 degree edges and let it set. Id then us glassfibre reinforcing tape (like chopmat but on rolls) and do the internal corners. 

Should then be strong enough that all you need on the outside is a cosmetic capping. 

Just make sure you key and prep the faces well and it should be really strong. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JonB67 Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2021 at 6:27pm
Id also try cutting it with a router. A sharp bit should leave you a nice clean cut. Just cut it about 5 mm away with something else, jigsaw or grinder then tidy to finish dimensions with the router
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