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Ideal Cabinet Construction?


Guest Peter the Greek

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Guest Peter the Greek

Hello,

 

I am possibly getting ahead of myself, as I am not sure when I'll find the time for this, but this topic has been niggling me for a while so was hoping for some direction (I have a pile of drivers for a 3 to 4 way build and also subs that need to find themselves into boxes)

 

It would seem that common knowledge is to build cabinets out of a thick board material, say mdf/ply, and have lots of bracing. Then apply damping (some sort of rubber membrane) along with something to mop up high frequency artifacts.

 

I have become pretty handy at casting things in concrete and was contemplating building cabinets using that method and then still applying damping as appropriate. If I understand correctly, Terry's speakers are concrete and @jkn if talking of using concrete in part for THIS build (though I am concerned in that case about the concrete shrinking as it cures and it coming away from the internal mold, ableit only likely to me small - potential for "rattle").

 

Then I came across THIS thread on DIY audio and wasn't too sure. They quote he work of Harwood specifically, which if I surmise correctly, a light weight box is better as it means the cabinet resonances are in the bass region and are less (not) audible.

 

BUT, if using concrete moves the resonances up into the mid range, surely they're easily damped with a combination of a rubber membrane glued to the inside, covered in something like high density insulation (25-50mm thick)

 

Anyway, what are peoples thoughts? concrete? plywood? should the sub cabinets be different to the LCR cabinets?

 

I am not concerned about practicalities, we moved a 900kg free standing bench in easily enough, a few speaker cabinets are not a concern.

 

Cheers,

Peter

(relative newcomer to the DIY speaker space, ok total NOOB...)

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1 hour ago, Peter the Greek said:

Ideal box

  • Cabinet is an appropriate shape (diffraction)
  • Sound from the back of the cone does not reflect inside the cabinet and return to the cone
  • Walls do not vibrate (and create sound of their own on the outside)
  • Walls do not allow sound to pass through them

 

Concrete would be very suitable, aside from all the other obvious drawbacks.   It is likely overkill for the above.

 

Summed response of the drivers (which depends on their location in space, and crossover filters) .... is so much more important.

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1 hour ago, Peter the Greek said:

They quote he work of Harwood specifically, which if I surmise correctly, a light weight box is better as it means the cabinet resonances are in the bass region and are less (not) audible.

It's a complex balance between preventing the outside of the cabinet moving and making it's own sound ......   and dissipating the vibration energy somewhere else - ie. not back into the driver.

 

The idea of a floppy cabinet, the directs all the energy into becoming vibration at low frequencies ..... may avoid the issue of energy returning to the driver.   It's very hard to generalise these things, because it depends on so many specifics - eg. how the driver is mounted, shape/dimensions of the cabinet.

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There is a thread somewhere in which Terry documented his cabinet build. It wasn't concrete. Going from memory here, he used an MDF skeleton and troweled on plaster.

 

An ideal box is never a combination of already-chosen drivers and the first design you come up with. If you really want ideal, you need to build prototypes and test multiple drivers. At least one of those two things - either the box design or the drivers (and preferably both), needs to be flexible.

 

If you're committed to the drivers, then I'd suggest that you build prototypes and experiment with fill, bracing and the baffle design. Keeping in mind also that drivers vary in their sensitivity to the shape of the baffle.

 

Sometimes you can get away with jumping right in with an uber box but there is always the risk that problems can emerge that you didn't expect. It might not actually result in the ultimate in performance.

 

Keep in mind that you might be able to move an ultra heavy box into your room, but before that you need to be able to handle it in the workshop. Everything becomes exponentially more difficult with weight - building it, painting it, testing it. Moving it into the room is the least of your concerns. If you can't actually take proper measurements for the crossover because it's too heavy to elevate, then that's a serious compromise.

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1 hour ago, Red Spade Audio said:

An ideal box is never a combination of already-chosen drivers and the first design you come up with. If you really want ideal, you need to build prototypes and test multiple drivers. At least one of those two things - either the box design or the drivers (and preferably both), needs to be flexible.

This.

 

Perhaps OT to "what's the best cabinet construction" ..... I often say to people that "redesigning" a speaker, eg. where the drivers are already chosen, or the cabinet/driver layout is already chosen .... is actually much harder, than a blank sheet of paper.   The reason is that it removes degrees of freedom in the design process  (to move a driver, or replace a driver, etc.)

 

If you put the drivers in a box/baffle ... and then think about designing the crossover - you have backed yourself into a corner.   Especially if you are building a passive network (ie. you don't have delay in your toolbox).

 

Build a super easy terrible looking test box out of cheap wood... and get the driver positions and crossover right .... and do the "super cabinet" last.

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Guest Peter the Greek

Sweet, thanks guys, understood (my naive enthusiasm was getting in the way). I'll mock up something when time permits.

 

Need to get my dsp's working with windows 10....just ordered a new usb to serial cable.

 

I'm going to build something that will allow pretty much any speaker to be measured 4-5m in the air. Weight is a non issue...well up to say 1t or so. 

 

Good to know concrete isn't a bad idea. 

 

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18 hours ago, Red Spade Audio said:

There is a thread somewhere in which Terry documented his cabinet build.

It be here.

I'm about to leave for work, but I'll add my tuppence ha'penny later.

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8 hours ago, audiofeline said:

Didn't Tannoy have a concrete enclosure speaker system a long time ago? 

I have heard it spoken of, but never had any confirmation of it.

There were speaker systems manufactured  in Melbourne under the brand name of " Bard " that were fitted with Tannoy  HPD speakers and also Altec 604 co-axials. These used a concrete front baffle .

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I typed out a long reply on Friday at lunch, but it disappeared into the ether.....

 

Based on long experience and testing, I've got a few simple rules. The first two I stole from a company who's speakers I generally don't like, but the ideas are excellent. Both come from B&W. The first is matrix bracing. I've settled on 150mm centres for main speakers and 250mm or so for subs, depending upon the size of the enclosure. There is no need to use the same thickness for the bracing; for example 18/19mm external walls can be braced with 12 or 15mm. CD grade construction ply is excellent for the braces and relatively cheap. The corners are the strongest naturally, so save some internal volume by nipping off the corners. I also find it easiest to cut all the horizontal braces and rip off some strips of the correct height for the verticals, then trim them to fit. I'm going to try some of the bituminous paints to line the interior next time, but am doing that on a hunch and it's efficacy is not yet proven to me. It's pretty cheap though. Cutting fancy shaped holes in the braces and radiusing is a waste of time and effort as it will have no benefit acoustically.

 

Number 2 is the shape of the midrange enclosure. Again B&W here with the Nautilus tubes. Whilst the tubes are theoretically best, you can get a worthwhile improvement by making them square / rectangular CS and achieving length by bending them up, down or bifurcating horizontally. Stuff them lightly near the driver, more densely further down the 'tube'.

 

Ports I prefer on the bottom of the enclosure because they're generally ugly and pointing them towards thick carpet can attenuate some midrange spill. I also like to stuff ported enclosures to some extent, but it depends on the design, and the location of the driver(s) and ports. The area behind the driver to the ports must be kept clear. Again, depending upon the design, I almost always use cotton fabric to hold the stuffing where I want it to stay. It's cheap as at Spotlight. Matrix bracing also gives you plenty of places to staple and hot glue the fabric into place. If you really need a sketch, I'll make one later.

 

I'm not convinced with matrix bracing that BB ply is worth the extravagant $ asked in Australia. If you have the money to burn go ahead. I use ply and MDF.

 

The rest is down to the design and thinking about it 10x before cutting. If I think of anything else, I'll add it later.

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Thanks Dave.

Another thought WRT ports. One of the problems is getting enough port volume into smaller bookshelves or monitors to achieve the required tune and port air velocity. Why not incorporate them into the stand, as they'll usually need a stand anyway? Or make them a floorstander and even if you don't use the increase in internal volume, you can use it for a hidden port?

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Guest Peter the Greek
1 hour ago, A9X said:

Number 2 is the shape of the midrange enclosure. Again B&W here with the Nautilus tubes. Whilst the tubes are theoretically best, you can get a worthwhile improvement by making them square / rectangular CS and achieving length by bending them up, down or bifurcating horizontally. Stuff them lightly near the driver, more densely further down the 'tube'.

Many thanks for the detailed reply, much food for though.

 

Regarding the midrange, I am a little confused about what you've said here. I presume the Nautilus internal volume is a spiraled tube of sorts? so what you're saying is insert internal baffles of a similar nature, albeit with square/rectangle shapes?

 

Here is a pick of what I believe it'd look like inside. You're saying, do this, but squared off?

 

image.jpeg.cf9de426234ef1e756549cbda453d67f.jpeg

 

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1 hour ago, Peter the Greek said:

Regarding the midrange, I am a little confused about what you've said here. I presume the Nautilus internal volume is a spiraled tube of sorts? so what you're saying is insert internal baffles of a similar nature, albeit with square/rectangle shapes?

Basically the second dot point from my post up above.

 

You are trying to prevent the back wave of the driver from returning to the cone.... so can make a kind of tapering tunnel, with bends or curves in it .... it can be square/rectangle if that makes it easier to construct.    Aside from rules of thumb to do with wavelength vs dimensions of the box/tunnel - then the geometry is not super important.

 

1 hour ago, Peter the Greek said:

Here is a pick of what I believe it'd look like inside. You're saying, do this, but squared off?

Yes, or even just to the point where the tunnel has turned 90 degrees.... no need to go to massive extremes.   You just trying to absorb and/or divert the energy away from returning to the driver.

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Guest Peter the Greek
5 minutes ago, davewantsmoore said:

Yes, or even just to the point where the tunnel has turned 90 degrees.

Sweet, thank you. Understood

 

.....the Mrs advises a new dining table is in order before new speakers....plenty of time for planning I suppose!

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Dave answered very well WRT Nautilus, but I was thinking of the three spiky tubes out the rear, not the snail shaped enclosure for the LF driver, though the theory is the same.

B&W's page on the subject.

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  • 4 months later...
On 19/07/2018 at 11:16 PM, audiofeline said:

Didn't Tannoy have a concrete enclosure speaker system a long time ago? 

I have heard it spoken of, but never had any confirmation of it.

 

There was someone in Australia who made concrete speaker boxes (Yes, the full box is made of concrete using mould) to fit Tannoy HPD385 drivers. Only less than  500 pairs ever made. Not sure how many still around. 

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The only (part) concrete cabinet, Tannoy HPD 385 model  that I know of   was built by Gerry Burkett  under the 'Bard' brand name. It was claimed that the voice coils of the bass section had been replaced with silver coils. I believe that what the modification meant  was that a voice coil with an aluminium former replaced the original type. Only the front baffle was made from concrete.

With concrete as a material, moulding a near complete cabinet would require a filler/binding material such as  fibre glass mesh to reinforce it and to prevent it ringing. 

Edited by VanArn
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36 minutes ago, VanArn said:

The only (part) concrete cabinet, Tannoy HPD 385 model  that I know of   was built by Gerry Burkett  under the 'Bard' brand name. It was claimed that the voice coils of the bass section had been replaced with silver coils. I believe that what the modification meant  was that a voice coil with an aluminium former replaced the original type. Only the front baffle was made from concrete.

With concrete as a material, moulding a near complete cabinet would require a filler/binding material such as  fibre glass mesh to reinforce it and to prevent it ringing. 

 

I have read about concrete front baffle before. And it's good to capture more information from you. 

 

There were cabinets made with concrete fully. Never believed 'a full cabinet made of concrete' until I saw one in person. 

 

I will try to get more info on this.

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Rocla have a unique method of spraying concrete into or onto a mould using fibre glass as the binding material: this results in a very strong ,lightweight product.. They made a few trial speaker enclosures many years ago. Normal concrete  would present problems  of strength unless reasonably thick and unfortunately would result in a very heavy enclosure. It is possible to use aereated concrete ( Hebel)  in block or slab form for a d.i.y. box and Garry Cawsey  has  made  specials using this lightweight material. The only drawback with this technique is that the walls are  thicker and then the box will then  be larger externally. 

Concrete enclosures using polystyrene foam as a filler have also appeared in the past.

Edited by VanArn
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4 minutes ago, VanArn said:

Rocla have a unique method of spraying concrete into or onto a mould using fibre glass as the binding material. this results in a very strong ,lightweight product.. They made a few trial speaker enclosures many years ago. Normal concrete  would present problems  of strength unless reasonably thick and unfortunately would result in a very heavy enclosure. It is possible to use aereated concrete ( Hebel)  in block or slab form for a d.i.y. box and Garry Cawsey  has  made  specials using this lightweight material. The only drawback with this technique is that the walls are  thicker and then the box will then  be larger externally. 

Concrete enclosures using polystyrene foam as a filler have also appeared in the past.

 

Do you know how the sound of concrete cabinets are compared to original Tannoy cabinets? Got a chance to listen them in  person?

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 I only ever heard the Bard speakers using the HPD385 units in a shop in High Street, Armadale sometime in the late 1970,s. that is  far too long ago to recall anything about the quality of the sound reproduction, nor was I able to make a comparision with a conventional pair. I am not a fan of the  Tannoy speaker cabinets as they are  compromised  if you use  T & S parameter criteria, for  vented designs.  The Arden is the model that I typically come across and the problem of fatigued surrounds and  bad repairs means that care is needed in a listening test as often a less than perfect speaker is being judged. The imported English manufactured cabinets while being nicely finished and attractive in appearance ( if you like a traditional look ), tend to  be of a  flimsy construction, especially against the versions made by John Woodhead for an earlier Tannoy agent.

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Here is the photos of concrete cabinets sitting in my garage. I can see that they are made for the same drivers I have, Tannoy HPD385. 

 

Dimensions are

Width: 70cm

Depth: 41cm

Height: 95cm

 

Weight: I believe they weigh around 150kgs each. 

 

20181223_102959.jpg

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On 24/07/2018 at 12:21 PM, A9X said:

... Cutting fancy shaped holes in the braces and radiusing is a waste of time and effort as it will have no benefit acoustically...

I don't know if I would necessarily agree. If a panel is left solid versus one that has big holes they can't have the same acoustic affect, especially if braced in three directions.

To me I would be inclined to think that a series of triangles inside a square would provide the greatest strength for the least amount of interference.

This in my mind would have a different affect to the same thing being solid.

 

Bracing.jpg

Edited by Silent Screamer
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