What changes should I make to this cabinet?

[quickshift]

Hello

I hope you don't mind but I wanted to ask what changes I should make to this cabinet.

It'll be made from 25mm MDF for the most part, the front will be 3 layers of 25mm MDF which will be CNC'd and bonded together.

I have decided to try using 1 inch cylindrical wood for the bracing which I'll drill a hole through the center of and use a piece of threaded steel which will pass right through the whole cabinet. The nuts will be countersunk into the cabinet and filled over.

I also want to use steel rods to attach the woofer to the cabinet on 4 of the 6 fixing points.

An idea for coating the internal surfaces is to use a layer of polystyrene beads (beanbag filling) about 10mm thick which I'll either mix with latex sealing paint and paint on or just glue on. It depends how the beads behave as to which method works best.

I'll use bitumen backed foam for the mid enclosure and the tweeter will be sealed into the front baffle.

What do you think?

 

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Pic 2

 

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Pic 3

 

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Would you believe....... pic 4

 

[quickshift]

Oh, also, this is a close up of the profiled bit. I don't know if this will work well or not but it looks nice IMO

Ignore the bump at the top, I have only been using 3DS for a couple of days and I'm not very good with it yet.

I'll smooth the top bit off IRL.

 

[wintermute]

Personally I'd make the mid cabinet deeper rather than tall and shallow, should help reduce reflections back through the cone. Try and use the golden ratio for it, if you can do so without too much difficulty and make the longest dimention the depth.

Also with 75mm thick baffle, you will need to flare the cutouts for the drivers (on the inside) so that the drivers can "breathe"

Tony.

 

[quickshift]

Thank you for your advice.

It shouldn't be too difficult to alter the ratio of the cabinet <<reaches for calculator>> and flaring the cutouts will be easy enough.

 

[soongsc]

Thank you for your advice.

It shouldn't be too difficult to alter the ratio of the cabinet <<reaches for calculator>> and flaring the cutouts will be easy enough.

Maybe considering the Golden Ratio might help keep internal resonances to minimum.

If the baffle is built up of multiple layers of MDF, you might want to see how each layer is cut out. If you do it layer by layer, you need some alignment holes to keep them in place when doing assembly. Also each layer will have a different bit, it's going to be a little tricky. If you CNC it after the layes are combined, you might actually need a round headed bit to carve the edges out, lots of machines hours. $$$ You could also rough cut each layer first, combine them, then carve the edges. The tricky part seems to be where the concaved edges turn into round edges.

The way Thiel does the edges makes it easier to manufacture.

 

[quickshift]

Yes, good point, thank you. I'm adding alligning holes to the final version so I can use dowels to make sure the layers are alligned properly when I bond them. I'm planning to use my car as a weight when I'm bonding the layers - I'll jack a wheel up and lower it down onto the baffle - that should be heavy enough. I'll pop up a couple of pictures of it in action.

The CNC people say they will need to use a ball cutter so it works out at about £150 per baffle. I thought about changing the design to the curves are less complicated but I like the design as it is and I want to try to make something I'm really proud of with no compromises.

 

[soongsc]

Yes, good point, thank you. I'm adding alligning holes to the final version so I can use dowels to make sure the layers are alligned properly when I bond them. I'm planning to use my car as a weight when I'm bonding the layers - I'll jack a wheel up and lower it down onto the baffle - that should be heavy enough. I'll pop up a couple of pictures of it in action.

The CNC people say they will need to use a ball cutter so it works out at about ?50 per baffle. I thought about changing the design to the curves are less complicated but I like the design as it is and I want to try to make something I'm really proud of with no compromises.

If you use the ball cutter, you will get better results after glueing them together, this will iliminate the need for alignment dowel pins which adds additional work in the process. When you glue them tocether before CNC, you will only need guides on the sides.

 

[quickshift]

If you use the ball cutter, you will get better results after glueing them together, this will iliminate the need for alignment dowel pins which adds additional work in the process. When you glue them tocether before CNC, you will only need guides on the sides.

That's useful, many thanks.

I'll have a chat with the CNC people about that.

 

[kelticwizard]

An idea for coating the internal surfaces is to use a layer of polystyrene beads (beanbag filling) about 10mm thick which I'll either mix with latex sealing paint and paint on or just glue on. It depends how the beads behave as to which method works best.

How did that part work out?

 

[Al.M]

I would prefer not to use steel rods (maybe ringing problems?), instead use MDF bracing with large holes cut out, placing the MDF vertically from top to bottom / side to side. That way you get more surface contact and less chance of walls vibrating.

 

[Cloth Ears]

With such a thick baffle, have you thought about time-aligning the drivers within the baffle? So your tweeter & mid might be sitting on only 1 25mm sheet, while the woof is on 3 (so the full 75mm).

I agree with wintermute that you'd probably want to have the mid enclosure deeper, rather than shallow. Possibly you might also look at having it offset to one side of the cabinet as well - rather than having them all aligned down the centre of the front baffle.

 

[quickshift]

Re: Re: What changes should I make to this cabinet?

How did that part work out?

I started building yet, I got slightly sidetracked building a 4 ch gainclone to drive the speakers and I’m still waiting for a few parts. I should be able to start at the weekend with a bit of luck – the woofers should be here today or tomorrow and I’m going to order the MDF and other drivers as soon as they arrive. I managed to find some 30mm Brazilian Mahogany sheets on Ebay to use as the front of the front baffle that should look great.

I still want to use a square sided cabinet but will try using the poly beads mixture to create internal angled sides to reduce standing waves. I’m thinking about trying a diamond shape so the sides narrow towards the top and bottom.

Al.M – I have wondered if steel rods would be the best things to use – I was also thinking about using a hollow nylon rod with the steel rod running down the middle but I can’t find anything suitable off the shelf. I wanted to try to avoid using MDF bracing because of all the flat surfaces but it’s the tried and tested method that works well I guess. After all, I don’t want these to just be a test bed for ideas, I want them to sound good when I have finished too. I may build a small test cabinet using steel rods although I’m not sure how I would test if the rods were ringing. Maybe wooden dowel would be better but I wouldn’t be able to apply any tension.

Cloth Ears – I hadn’t thought seriously about time aligning the drivers, I guessed that because I would have to set them back into the baffle I would end up with either a stepped front baffle or kind of waveguides round the mid and tweeters which may detrimentally affect the sound. I did look at offsetting the mids and tweeters so they are at different distances from all sides, one of the first drafts had a kind of Yin shape with the drivers running along the top swirl (that’s not a great description) but I was worried it might look daft IRL.

I have slightly changed the dimensions now so that it'll be 6 inches deeper and an inch narrower which will also make more space to allow slightly steeper internal angling of the poly bead mixture.

I can’t wait to start building these as it goes, having almost all the stuff I need just sitting around is slightly frustrating.

 

[Vikash]

Hi,

This quick standing waves calculator might be of use.

I think you will get far more structural strength from comprehensive asymmetric shelf bracing from say plywood. Angled bracing is even better and can potentially help break up internal standing waves. You can also incorporate this to support the drivers rigidly from behind.

With a 75mm thick baffle you will have to do something special to let the drivers breath from behind. I would suggest not laminating the sheets across the whole face but only strips on the edges. Even then, you should route out breathing space in the 25mm baffle for the mid in particular.

 

[quickshift]

Hiya,

That standing waves calculator is useful, many thanks.

I was going to cut out a sort of cone shape from the 2 rear sheets of MDF and slightly into the front sheet at 45 degrees and round off the edges.

Am I going a bit OTT with a front baffle that thick? I don't want it to do more harm than good.

I'll do some experiments with the bracing. If I can cut say 4" holes close together and round off all the edges to there aren't really any flat surfaces I would be happy using that I reckon. I have something similar in my current speakers and the boxes are pretty dead.

I would like to try a few new things but not if they won't work properly.

I also thought about using fuel cell baffle foam soaked in carbon fibre resin to basically fill up the cabinets but that might be a bit ambitious. I reckon it would give reasonable results if used with some additional bracing - normal foam soaked in fibreglass resin sets pretty solid but obviously takes up all the volume - fuel cell foam only takes up about 5% of the volume if that IIRC.

Oh dear, more stuff to experiment with...

 

[Vikash]

Am I going a bit OTT with a front baffle that thick?

IMO, yes. A good bracing strategy will do a far more than just a thicker baffle. 18mm MDF for the baffle is plenty and then brace then hell out of the internal enclosure. If you want the cool side cut outs for looks and smooth out edge diffraction then I would just add strips to the side to carve (CNC or whatever) out. Kinda like this: http://www.vikash.info/audio/mtm_floorstander/build_day02.asp

Also consider mounting your tweeter off center.

Some ideas for bracing:
http://www.vikash.info/audio/mtm_floorstander/build_day06.asp
http://www.vikash.info/audio/xls10/construction.asp

Oh dear, more stuff to experiment with...

That's half the fun!

 

[quickshift]

Hmm, well I don't want to build unnecessarily thick baffles so I'll try 18mm MDF with the 30mm mahogony or maybe just the mahogony. I'll have to change the design <crys> Oh well, is only exists on the pooter so that's no big deal.

Cheers for the links, I have bracing like that in my current speakers but it has lots of smaller holes with rounded edges and it works very well. Nice job . You have loads of weights! <<makes note not to upset Vikash >>

I guess if I'm changing then design I can offset the tweeter and mid now and make a deeper mid enclosure. I could make cylindrical ones actually with layers of circles of MDF..

Anyway, I have ordered the MDF now so I can start the box when I have tested the bracing. I have 2 unbraced boxes with 8" woofers in which I can use to try the bracing types out.

Yup experimenting is good fun, now if someone could only invent something that disolves PVA easily.....

 

[quickshift]

Progress

Right.

Getting there.

I have gone for loads of 18mm MDF bracing.

I experimented with a few different types of stuff to mix the poly beads with and found PVA works best. It sets solid but slightly flexible and is easy to work with.

I mixed the beads with the PVA in a bucket and literally put the mixture on with my hands, just patting it down into place.

I have both cabs done with the bead mixture - I used about 1 cubic foot of beads and 1.5 litres of PVA.