# Bending 3 mm plywood



## h.g. (22 Aug 2015)

I am looking for curved panels with a radius of 150 mm and the stiffness of flat 18 mm plywood sheets. 1.5 mm 3 ply aircraft plywood looks good but is too expensive. 3 mm 3 ply marine plywood is just about affordable but is the radius too small? Table 1-2 on page 1-25 [of an old aeroplane manual I do not have permission to link to] suggests "thoroughly soaked in hot water" would work whereas 10% moisture would not. So the radius looks to be close to the limit. Does anyone have good or bad experiences of soaking plywood to make tight radii?


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## PAC1 (22 Aug 2015)

Yes I have steamed ply to form a bullnose step for stairs. It does need to be well steamed I also did it over a number of curves to bring it down to the required curve otherwise it could split


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## h.g. (22 Aug 2015)

PAC1":2fcfrgcv said:


> Yes I have steamed ply to form a bullnose step for stairs. It does need to be well steamed I also did it over a number of curves to bring it down to the required curve otherwise it could split


Thanks that is encouraging. I presume it was a tighter radius than 150mm? and what thickness was the plywood? It was normal plywood and not the flexi-ply with the soft core?


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## PAC1 (22 Aug 2015)

Yes ordinary ply and about 100mm radius


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## 7sheds (22 Aug 2015)

I have seen some very attractive curved panels for boat interiors made from 9 or 12mm ply by making saw-cuts from the reverse using the same technique coffin-makers use for side panels.. They can be made very strong by filling the grooves with a thickened epoxy as they are bent. For added strength one can coat the inner face with a layer of veneer using a vacuum bag (Polythene, parcel tape & vacuum cleaner) or if it is not visible, a layer of glass fibre cloth.


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## worn thumbs (22 Aug 2015)

It depends on the ply to a large extent.If you buy the stuff with paper thin face veneers and 85% of its thickness in the thick core veneer,prepare to be disappointed.If you buy 3mm gaboon with all veneers of similar thickness maybe it will work.A quick way to find out is to take a small piece and pour boiling water straight from a kettle onto it and then bend it.If your ply isn't WBP grade it will probably fall apart.

If you follow the advice about using a vacuum cleaner,prepare for a spell in the doghouse as you will probably burn it out without a flow of air to cool the motor.A proper vacuum pump can normally be had from ebay for around £45 and may be a good investment for domestic harmony.


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## h.g. (23 Aug 2015)

7sheds":zoxcm89x said:


> I have seen some very attractive curved panels for boat interiors made from 9 or 12mm ply by making saw-cuts from the reverse using the same technique coffin-makers use for side panels.. They can be made very strong by filling the grooves with a thickened epoxy as they are bent. For added strength one can coat the inner face with a layer of veneer using a vacuum bag (Polythene, parcel tape & vacuum cleaner) or if it is not visible, a layer of glass fibre cloth.


Thanks for the suggestion but the curved panel needs to be as stiff as 18 mm flat plywood. The requirement comes from the need to keep the lowest resonant frequency of the structure above a particular frequency where it will not get driven by vibration. I don't think I have much wiggle room.


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## ColeyS1 (23 Aug 2015)

Slightly off top but you can get flexi ply









Not sure if it would be rigid enough after gluing to make 18 mm though .......

Coley

Sent from my SM-G900F using Tapatalk


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## h.g. (23 Aug 2015)

worn thumbs":2gyjeaf9 said:


> It depends on the ply to a large extent.If you buy the stuff with paper thin face veneers and 85% of its thickness in the thick core veneer,prepare to be disappointed.If you buy 3mm gaboon with all veneers of similar thickness maybe it will work.A quick way to find out is to take a small piece and pour boiling water straight from a kettle onto it and then bend it.If your ply isn't WBP grade it will probably fall apart.


Thanks for the warning. I was tempted by the 3mm(ish) £10 sheets at the local builders merchant but eventually accepted I would have to pay more like £30+ a sheet for marine ply to get the required stiffness.


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## h.g. (23 Aug 2015)

ColeyS1":4f1req1q said:


> Slightly off top but you can get flexi ply
> [...]
> Not sure if it would be rigid enough after gluing to make 18 mm though .......


I am fairly sure it would not be stiff enough on it's own but bonded to an inner and outer layer of something stiff it might work.


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## ColeyS1 (23 Aug 2015)

Don't think you mentioned yet, but what are you using it for ? I'm curious 

Sent from my SM-G900F using Tapatalk


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## h.g. (23 Aug 2015)

ColeyS1":373ne1cr said:


> Don't think you mentioned yet, but what are you using it for ? I'm curious


It is the low frequency section of a prototype (quick and cheap!) loudspeaker. If the lowest frequency resonances can be pushed to frequencies above the highest frequency handled by the driver they will not get excited and the cabinet will remain quiet. Unfortunately, a near miss is a big fail because the lowest resonance will then sit at a frequency where music has the most energy, the ear is fairly sensitive to cabinet noise and in a cabinet with almost no damping. One generally needs to sacrifice stiffness to get significant amounts of damping (e.g. constrained layer damping) so one tends to go all in for one or the other and in this case it is stiffness.


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## Droogs (23 Aug 2015)

Could you not make a mould and build it up yourself from thin strips/boards of solid?


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## h.g. (23 Aug 2015)

Droogs":2taxq58i said:


> Could you not make a mould and build it up yourself from thin strips/boards of solid?


Solid wood is usually avoided with speakers because it moves too much. Although stiff in the direction of the strips I suspect such a construction would be insufficiently stiff in the circumferential direction because of the need for something soft to accommodate movement between the strips?

PS Or have I misunderstood your suggestion?


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## Droogs (24 Aug 2015)

i menat make your own ply in the shape of the mould and use UF glue


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## h.g. (24 Aug 2015)

Droogs":3gd8r5v5 said:


> i menat make your own ply in the shape of the mould and use UF glue


Yes that is the intention. The structure has an internal framework to make it stiff and so there will be something to bend the ply around without the need to construct something separate to do the job. Had I been able to afford very thin ply things would have been straightforward but I am looking to use 6 or 7 sheets of cheaper thicker ply that requires heat and water to bend to the radius.

Having not done it before I have some concern that by bending the ply to the desired radius and not beyond it is going to spring back too much when drying. In which case I guess I will have to construct something to bend around. Should I be concerned?


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## PAC1 (24 Aug 2015)

If the dimension is critical then hot water bending is not going to work without permanent formwork or good non creep glue.


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## Beau (24 Aug 2015)

Kerf some 18mm ply. Have done this for a friends horn loaded speakers. Don't think the radius was as tight as 150mm but there was no problem with rigidity as the sides were glued to the sides of the cabinet. 

Not mine but gives you the idea http://p10hifi.net/FH/images/dallas-2-k ... jpg?105,84

Google "kerf ply speaker building" and you will get lots of different builds some of which are very tight radius's.


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## h.g. (24 Aug 2015)

PAC1":3m1valb7 said:


> If the dimension is critical then hot water bending is not going to work without permanent formwork or good non creep glue.


Thanks. There is a permanent internal framework that the bent plywood will be glued to but the outside will be free. What type of glue would you recommend?


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## h.g. (24 Aug 2015)

Beau":3u8w6ffy said:


> Kerf some 18mm ply. Have done this for a friends horn loaded speakers. Don't think the radius was as tight as 150mm but there was no problem with rigidity as the sides were glued to the sides of the cabinet.
> 
> Not mine but gives you the idea http://p10hifi.net/FH/images/dallas-2-k ... jpg?105,84
> 
> Google "kerf ply speaker building" and you will get lots of different builds some of which are very tight radius's.


Thanks for the suggestion but I doubt it will be stiff enough in the circumferential direction (although I am not stating this with high confidence given the wide range of glues). The cabinet is designed to work in a particular way which requires a higher than normal stiffness.


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## woodbrains (24 Aug 2015)

Hello,

The whole point in plywood, is that it is designed to remain flat! Wanting to do something with a material that is designed to do precisely the opposite to your intention is just daft, IMHO. Use bending plywood with constructional veneers, or aero ply between the laminations and what ever face veneers you choose on both outside faces. This will make a very stiff structure that will stay in shape and not require much, if any an understructure. 3 layers of 5mm bending ply is good. 

You could use 2mm MDF.

Mike.


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## profchris (24 Aug 2015)

How big is the structure? If it's domestic loudspeaker size then you have two options.

1. 3mm ply can be bent over a hot pipe - I've done that to make musical instrument cases. A pipe around 2 inch diameter in a vice with a heat gun blowing through it works well. It takes practice to bend precisely, but it's not that hard.

2. Cheaper 1.5mm ply is available via eBay - you can usually pick up 10 12 inch square sheets for about £15 delivered. These could be bent and glued cold.

If stiffness is the important criterion then I don't think you'd need to go to 18mm thick. I'm reasonably confident that a piece of 3mm ply bent to a 150mm radius might well hold my weight if I stood on it (if the ends were anchored), because curves are vastly stiffer than flat, and I'm sure that a two layer (6mm) instrument case would do so.


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## h.g. (24 Aug 2015)

woodbrains":1lr6hop6 said:


> The whole point in plywood, is that it is designed to remain flat! Wanting to do something with a material that is designed to do precisely the opposite to your intention is just daft, IMHO. Use bending plywood with constructional veneers, or aero ply between the laminations and what ever face veneers you choose on both outside faces. This will make a very stiff structure that will stay in shape and not require much, if any an understructure. 3 layers of 5mm bending ply is good.


Thanks for the input but I thought I was proposing to use the same construction method as that for wooden aeroplanes: a wooden frame and a plywood skin although in my case the skin uses more layers. I would like to use 1.5mm 3 ply aero plywood but cannot afford it (unless someone knows where I can get it cheap?) and so I am looking at 3 mm sheets of 3 uniform plys plus more messing about. High stiffness is a requirement which is why there is a frame and the more straightforward options of MDF, flexiply, kerfing and the like have been set aside. An attractive surface finish is not important because it will not be visible.


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## h.g. (24 Aug 2015)

profchris":fk7v9zra said:


> How big is the structure?


It is about 1' x 1.5' x 2.5' and is the size of a domestic loudspeaker.



profchris":fk7v9zra said:


> 1. 3mm ply can be bent over a hot pipe - I've done that to make musical instrument cases. A pipe around 2 inch diameter in a vice with a heat gun blowing through it works well. It takes practice to bend precisely, but it's not that hard.


Thanks. That sounds exactly like what I am after. Did you use the cheap 3 mm ply with 2 thin outer layers and a thick core or the more expensive 3 mm ply with uniform thickness?



profchris":fk7v9zra said:


> 2. Cheaper 1.5mm ply is available via eBay - you can usually pick up 10 12 inch square sheets for about £15 delivered. These could be bent and glued cold.


Thanks again. I had seen a few of these but could not find anything large enough. My panels are about 650 x 850 mm. I have sent a couple of emails asking about larger sizes but with no response yet.



profchris":fk7v9zra said:


> If stiffness is the important criterion then I don't think you'd need to go to 18mm thick. I'm reasonably confident that a piece of 3mm ply bent to a 150mm radius might well hold my weight if I stood on it (if the ends were anchored), because curves are vastly stiffer than flat, and I'm sure that a two layer (6mm) instrument case would do so.


The requirement for stiffness is not really to do with strength but a need to raise the lowest resonant frequency higher than it would normally be in speaker using flat panels.


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## profchris (25 Aug 2015)

I used the cheap ply - this was only an instrument case after all. But the size of your panels seems too big for a hot pipe. 1.5mm ply is expensive in large panels, it's only the offcuts which you can get cheaply.

If it helps, I've also bent 3mm MDF over a hot pipe (dry bending only) and that works OK too, and I suspect you could bend it over a form using a heat gun. But I doubt that layering up MDF would be as stiff as you want.

You might try the experiment of attempting to bend 3mm ply over a form using a heat gun - try a narrow 150mm panel using a big saucepan as your form. Clamp one end to the saucepan, clamp the saucepan down somehow (handle in vice?), heat the ply where the bend is to start (you want it hot enough so you can't rest your hand on it, but short of scorching!) and ease it into shape, moving the heating area as you go. You will need to go slowly, applying only light pressure, or it might bend unevenly (you can actually feel the wood turn plastic with a bit of practice).

If this works you could build a form the right size and go for it. This would also tell you how many 3mm layers you'd need - my guess is two or three.


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## woodbrains (25 Aug 2015)

Hello,

Are you sure you are not putting too much empasis on the strength of the material you want to use. I've built speaker boxes before, and have an interest in such. I don't believe the system resonance can be altered by the rigidity of the box material. I think you need the material to a) not vibrate at the sound volume the speaker is used at and b) not have any voids of loose plies within the structure and c) not have any unintentional holes in it. Resonance is modified by the compliance of the air in the box and size shape and length of any ports in the box. In fact some enclosures like labyrinths are non-resonant. As long as the speakers are mounted on a strong enough baffle the air envelope does not know whether the box is made from MDF, ply or solid timber. In fact it is a good idea to line the insides of the box with a yielding material anyway such as bitumastic sheet.

For this reason I would avoid saw kerfing, as you already have disqualified. But I would also avoid trying to bend ordinary plywood. If it can be done at all it will only be a slight bend and you will find the pieces will 'saddle' which not only is not the curve you want, but it will layer up with voids, as each piece will be slightly different to another and not nest.

Bending ply will be fine for what you need, and since you need to know the radius of the curve to be able to calculate the internal volume accurately, bending ply will give a more accurate shape, without spring back. Trying to bend plies that are too thick will result in a lot of spring back. 

Mike.


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## h.g. (25 Aug 2015)

profchris":3fbuyc18 said:


> But the size of your panels seems too big for a hot pipe.


A flue pipe perhaps but I had thought to bend a sheet of metal and heat that possibly over the internal frame for the speaker or a bit tighter depending on the degree of spring back.



profchris":3fbuyc18 said:


> You might try the experiment of attempting to bend 3mm ply over a form using a heat gun - try a narrow 150mm panel using a big saucepan as your form. Clamp one end to the saucepan, clamp the saucepan down somehow (handle in vice?), heat the ply where the bend is to start (you want it hot enough so you can't rest your hand on it, but short of scorching!) and ease it into shape, moving the heating area as you go. You will need to go slowly, applying only light pressure, or it might bend unevenly (you can actually feel the wood turn plastic with a bit of practice).


Thanks I hadn't considered hot and dry but if it works it should be quicker and less messy. Well worth an experiment and can always fall back to hot water and towels and/or steam if not.



profchris":3fbuyc18 said:


> If this works you could build a form the right size and go for it. This would also tell you how many 3mm layers you'd need - my guess is two or three.


Tapping the speaker with a hammer and looking at the frequency spectrum of the response will show me if I need more layers. The BBC used to use 9mm plywood to get LOW resonant frequencies in some of their famous old monitor designs (they did one or two other things as well to get low resonant frequencies).


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## DennisCA (25 Aug 2015)

I got some 4mm birch plywood and it bends like heck in one direction. with unsteamed ply I made this from 4 laminations for my kids:






I didn't specify bendy plywood or anything, but that's what it does.


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## h.g. (25 Aug 2015)

woodbrains":1e4l106m said:


> Are you sure you are not putting too much empasis on the strength of the material you want to use. I've built speaker boxes before, and have an interest in such. I don't believe the system resonance can be altered by the rigidity of the box material. I think you need the material to a) not vibrate at the sound volume the speaker is used at and b) not have any voids of loose plies within the structure and c) not have any unintentional holes in it. Resonance is modified by the compliance of the air in the box and size shape and length of any ports in the box. In fact some enclosures like labyrinths are non-resonant. As long as the speakers are mounted on a strong enough baffle the air envelope does not know whether the box is made from MDF, ply or solid timber. In fact it is a good idea to line the insides of the box with a yielding material anyway such as bitumastic sheet.


Sound is radiated by the drivers (wanted) and from the cabinet (not wanted). My concern is reducing the latter sufficiently to be inaudible in the presence of the former. If you have a separate enclosure for the low frequency woofer/s which is isolated from the enclosure for the midrange and tweeter then a high stiffness can push the lowest resonant frequency of the woofer cabinet above the highest frequency handled by the woofers and so the resonances will not got driven. If there are no resonances then the cabinet requires very little damping which is a significant win because effective damping has a significant cost. Subwoofer cabinets are usually designed like this but I am wanting to do it for a woofer cabinet which is pushing things a bit but hopefully not too much. 

The midrange cabinet needs to be designed differently because it will contain cabinet resonances in the passband. A high stiffness will not do much that is beneficial and it is high damping that will lower the resonant peaks. Which is just as well because effective damping tends to come at the cost of a reduced stiffness.


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## woodbrains (25 Aug 2015)

Hello,

Just make the box from thicker material. Stiffness goes up markedly with quite moderate increases in thickness. It seems to me that finding out the resonant frequency of the plywood or whatever would be virtually impossible, anyway, since it would vary with size for any given thickness. An 8 by 4 sheet is less stiff than a 4 by 4 sheet of the same thickness. Just make the box thicker for the woofer cabinet by as much as you think is more than you need. Maybe use 5 layers of 5 mm bendy ply. Otherwise there is no way around using the expensive materials! 

Alternatively, if you can design out the curve and make the enclosure from flat panels. Makes construction easier and less expensive. Why are you wanting a curve? Is it to prevent standing waves? If so a many sided polygon will do as well, especially if it isn't parallel to the speaker baffle. 

Mike.


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## h.g. (25 Aug 2015)

woodbrains":tz0xmv5m said:


> Just make the box from thicker material. Stiffness goes up markedly with quite moderate increases in thickness.


Stiffness goes up but so does mass and, unfortunately, the resonant frequency scales only with the square root of the stiffness/mass ratio so we get an increase but not by anything like the amount the stiffness has gone up. 



woodbrains":tz0xmv5m said:


> It seems to me that finding out the resonant frequency of the plywood or whatever would be virtually impossible, anyway, since it would vary with size for any given thickness. An 8 by 4 sheet is less stiff than a 4 by 4 sheet of the same thickness.


The resonant frequencies can be predicted using Finite Element Analysis with the CAD model of the structure. There are one or two things which lead these predictions to being close rather than precise but they are normally good enough to answer most things. Once the structure has been built then tapping it with hammer, recording the 'ping' it makes and then creating the frequency spectrum for that 'ping' will show the frequencies of the resonances. 



woodbrains":tz0xmv5m said:


> Alternatively, if you can design out the curve and make the enclosure from flat panels. Makes construction easier and less expensive. Why are you wanting a curve? Is it to prevent standing waves? If so a many sided polygon will do as well, especially if it isn't parallel to the speaker baffle.


The woofers are in the side of the cabinet and the sound needs to pass around to the front without encountering any sharp edges which would cause it to diffract and act like a second source which interferes with the direct sound. In addition it is part of the waveguide on the tweeter.

Approximating a curve with a series of straight sides would work better than a single sharp edge but not as well as a curve at the frequencies handled by the tweeter. At lower frequencies there probably wouldn't be much difference. If I had a CNC machine to play with then it might be easier than bending some plywood but by hand my guess is bending plywood will be easier.


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## woodbrains (26 Aug 2015)

Hello,

Bendy ply is lighter than birch ply.

Unless you find a compromise, you can't make what you want. All design is compromise. Always. You can't bend 3mm ply, so what are you going to do, no one here can make it happen by talking about it, that is for sure. Just make a box solid enough that does not vibrate. All your calculations are just so you can make a box that does not vibrate, however much you make a song and dance about it. Calculations do not make a speaker sound good, making it sound good does. Just do it, and good luck. 

Mike.


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## h.g. (26 Aug 2015)

woodbrains":1nttphij said:


> You can't bend 3mm ply, so what are you going to do, no one here can make it happen by talking about it, that is for sure. Just make a box solid enough that does not vibrate. All your calculations are just so you can make a box that does not vibrate, however much you make a song and dance about it. Calculations do not make a speaker sound good, making it sound good does.


Thanks for the input but I think we may be at cross purposes somewhere. I may not be experienced in how to do it, which is why I am posting to the forum, but I am pretty confident one can bend 3 mm plywood if it is wet and heated. I agree it will not bend cold and dry to the radius I want and flexiply possibly will but flexiply is unlikely to be stiff enough to do the job required. A near miss with the stiffness is a big fail because the resonances will then get strongly driven, in a box that is effectively undamped and at a frequency where the ear is relatively sensitive.

All speaker boxes vibrate. Good speaker boxes use the appropriate combinations of mass, damping and stiffness to minimise the cabinet vibration caused by the drivers. How to do this effectively changes with frequency which makes the construction of a good cabinet an interesting challenge. Solid in the sense of thick walls is a widely held belief among speaker DIY folk but it is not particularly effective at midrange frequencies. A church bell for example is pretty solid but it would not make a good speaker cabinet.


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## Beau (26 Aug 2015)

Speaker cabinets are complicated old things. Was privileged to be shown around the Heybrook speaker factory some years ago by it's founder. They had done a lot of research into the best materials for the conventional box and chipboard was the preferred material as it did not "ring" .They did not rate solid wood, ply, or any of the MDF offerings. Sorry no help for your project but understand that there is more to it than strength.


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## Racers (26 Aug 2015)

Well h.g. we have given you some answers but you don't seem to like them!

My speakers are MDF boxes with plastic feet that rock about but sound very good indeed (they retail for about 4K) so there is more than one way to skin a cat.

Why do you need the sound directing to the front? my speakers fire upwards at about 45 deg.

Pete


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## h.g. (26 Aug 2015)

Racers":3qdyzcpk said:


> Well h.g. we have given you some answers but you don't seem to like them!


I don't know why you would think that. I came to the forum somewhat uncertain about bending plywood and have been given advice that has both set my mind at rest and also given me one or two things to try which, if they work, will be better than what I was initially intending to do. Good stuff.



Racers":3qdyzcpk said:


> My speakers are MDF boxes with plastic feet that rock about but sound very good indeed (they retail for about 4K) so there is more than one way to skin a cat.


The design of commercial speaker cabinets is a balance of a wide range of concerns. A bit like a family car being a lot more difficult to design than a racing car where the objectives are much better defined. In my case the speaker is a bit unusual in being an on wall design to be placed against a light partition wall which will not take it's weight and would act like a sounding board if the speaker vibrated like a conventional speaker. The fun lies as much, if not more, in the challenge of getting things to work well rather than the construction of the speaker itself. 



Racers":3qdyzcpk said:


> Why do you need the sound directing to the front? my speakers fire upwards at about 45 deg.


If I understand your question correctly, the speakers are designed to be conventional in the trade off between imaging and spaciousness. The directivity is smooth with a beam width that is a touch wider than a good studio monitor.


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## Racers (26 Aug 2015)

I have problems with speakers that shout directly at you I much prefer Semi-omnidirectional speakers.

I can understand why you wouldn't want the wall exciting.

The best mod I ever did to an old pair of speakers was to paint the inside with car underseal it damped the cabinets very well.

Pete


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## sploo (26 Aug 2015)

I used to be heavily into loudspeaker (and sub) building, but it's been quite a few years, and memory of specific values/frequency/material/diffraction issues has faded - so, it's entirely probable some of my info may be wayward, but...



h.g.":67h10ul3 said:


> It is about 1' x 1.5' x 2.5' and is the size of a domestic loudspeaker.
> ...
> The requirement for stiffness is not really to do with strength but a need to raise the lowest resonant frequency higher than it would normally be in speaker using flat panels.


Linkwitz recommends that you don't have any panel area larger than 4"x4" unbraced, in order to push the resonant frequency high enough such that (at those frequency levels) there won't be enough energy to excite them. If you're going for a bent lamination then I assume you might require some bracing/skeleton inside the cabinet. Could you get anywhere near that level of bracing, and then likely make the resonance issue irrelevant?




h.g.":67h10ul3 said:


> Sound is radiated by the drivers (wanted) and from the cabinet (not wanted). My concern is reducing the latter sufficiently to be inaudible in the presence of the former. If you have a separate enclosure for the low frequency woofer/s which is isolated from the enclosure for the midrange and tweeter then a high stiffness can push the lowest resonant frequency of the woofer cabinet above the highest frequency handled by the woofers and so the resonances will not got driven. If there are no resonances then the cabinet requires very little damping which is a significant win because effective damping has a significant cost. Subwoofer cabinets are usually designed like this but I am wanting to do it for a woofer cabinet which is pushing things a bit but hopefully not too much.


IRC The issue is usually that the frequencies produced by a mid driver are the ones that tend to excite the panels on a larger cabinet. Therefore, building a small mid/treble cabinet and an isolated larger woofer cabinet prevents that transmission. AFAIU the frequencies produced by a woofer are usually too long/low to excite all but a really huge panel (even though there's plenty of energy). If I read your comments above correctly then you're trying to stiffen the woofer cab to protect it from woofer frequencies?



h.g.":67h10ul3 said:


> The woofers are in the side of the cabinet and the sound needs to pass around to the front without encountering any sharp edges which would cause it to diffract and act like a second source which interferes with the direct sound. In addition it is part of the waveguide on the tweeter.


I can't recall the radius vs frequency formula, but you do tend to need some pretty large radii to alleviate diffraction at anything but the highest frequencies. I'm just bringing that up in a "how practical will it be to get a large enough radius?" question.

I'd very much like to see any designs or finished boxes though - I'm still very much "in" to speaker builds, even though I don't tend to do them myself these days.


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## h.g. (26 Aug 2015)

sploo":1f681yxa said:


> Linkwitz recommends that you don't have any panel area larger than 4"x4" unbraced, in order to push the resonant frequency high enough such that (at those frequency levels) there won't be enough energy to excite them. If you're going for a bent lamination then I assume you might require some bracing/skeleton inside the cabinet. Could you get anywhere near that level of bracing, and then likely make the resonance issue irrelevant?


Pushing resonances high works if the driver creating those frequencies is elsewhere and not exciting the cabinet. The energy to excite the resonances comes from the drivers and will be just as much present at midrange and high frequencies as at bass frequencies. What is different is that it takes less movement of a panel to create a given sound pressure level at high frequencies than at low (e.g. large woofer and small tweeter).

The inside of the cabinet is braced with unsupported areas slightly larger than 4"x4". Extensive bracing does not make resonances quieter, it raises the frequency of the lowest ones which may or may not be desirable depending on how the noise is being tackled. The BBC for example aimed to lower the frequency of the resonances in their old monitors. What reduces the size of resonances is damping. Exactly on a resonance it is the only force that reduces the motion because the forces due to stiffness and inertia, although large, cancel each other.



sploo":1f681yxa said:


> If I read your comments above correctly then you're trying to stiffen the woofer cab to protect it from woofer frequencies?


Sort of. If the resonances are above the passband of the woofer then the largest force (of stiffness, damping and mass) is stiffness. The cabinet is "stiffness controlled" and so making it stiffer reduces the motion making it quieter. If you were to make the cabinet heavier keeping the same stiffness the motion would be largely unaffected. 



sploo":1f681yxa said:


> I'd very much like to see any designs or finished boxes though - I'm still very much "in" to speaker builds, even though I don't tend to do them myself these days.


I have a CAD model but want to check things first. The design seems a reasonably straightforward thing to do and yet it doesn't seem to be done. The obvious reason for that is that I have missed something and it doesn't work well but I am hoping the reason is a less probable one.


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## sploo (26 Aug 2015)

h.g.":hk856382 said:


> Pushing resonances high works if the driver creating those frequencies is elsewhere and not exciting the cabinet.


I don't quite follow that? Surely if the bracing pushes the resonances high then it works just as well if the driver is in another cabinet (that's transferring energy to that cabinet) or is directly mounted in that cabinet. Obviously the energy will be higher with the driver in the cabinet, but if the cab is braced sufficiently then there should be relatively little resonance in the lower frequencies (due to the bracing).



h.g.":hk856382 said:


> The inside of the cabinet is braced with unsupported areas slightly larger than 4"x4". Extensive bracing does not make resonances quieter, it raises the frequency of the lowest ones which may or may not be desirable depending on how the noise is being tackled.


Indeed. But the question surely is: if you've removed a lot of resonance issues, are the remaining resonances a genuine problem?

I ask that question because - ignoring the downright crazy audiophool stuff - there's a lot of effort in the audio world that seems to be spent on reducing problems that aren't actually detectable by the human ear (measurable doesn't mean audible).


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## h.g. (26 Aug 2015)

sploo":1zrxn847 said:


> I don't quite follow that? Surely if the bracing pushes the resonances high then it works just as well if the driver is in another cabinet (that's transferring energy to that cabinet) or is directly mounted in that cabinet. Obviously the energy will be higher with the driver in the cabinet, but if the cab is braced sufficiently then there should be relatively little resonance in the lower frequencies (due to the bracing).


Bracing stiffens the cabinet and pushes the frequency higher at which resonances start to occur. It does not reduce the sound pressure level of those resonances at the listening position which I suspect is what you are expecting to happen. At high frequencies a panel needs to deflect less to be equally loud as at low frequencies. To see that consider how far a large woofer has to move to be equally loud as a small tweeter. In practise they tend to be roughly the same level but we are able to perceive resonances below the signal easier at higher frequencies and so the net result is likely to be to make things slightly worse not better although not by a great amount.

So what you need to do to make things work well is put the woofer and midrange in separate cabinets that are isolated from each other and build those cabinets differently. The woofer cabinet needs to be stiff so that lowest resonances are in the midrange passband and will not get driven because the midrange is elsewhere. The midrange cabinet needs to be heavily damped and less stiff so that the lowest resonances are in the woofer passband and not driven and the higher order resonances that are in the midrange passband are strongly damped. 



sploo":1zrxn847 said:


> But the question surely is: if you've removed a lot of resonance issues, are the remaining resonances a genuine problem?
> 
> I ask that question because - ignoring the downright crazy audiophool stuff - there's a lot of effort in the audio world that seems to be spent on reducing problems that aren't actually detectable by the human ear (measurable doesn't mean audible).


Yes I am removing resonance issues in the way outlined above and I am doing it a pragmatic way without using 2" thick mdf, excessive bracing and similar favoured by many DIY speaker enthusiasts. There are a small number of audibility studies of cabinet resonances around which show, not surprisingly, that audible levels are around 30 dB down in the same ball park as other audible grunge.

Audiophile hardware is designed to get bought by audiophiles. If audiophiles want, for example, multiple binding posts for biwiring/biamping or whatever then speakers that do not have them are not going to get bought. It doesn't matter that biwiring/biamping or whatever isn't detectable. It doesn't matter that the manufacturers know this. A manufacturer needs to provide multiple binding posts because that is what the customer wants.

In the 1970s hi-fi enthusiasts used to give a lot of weight to small differences in completely inaudible levels of total harmonic distortion of a 1 kHz sine wave into an 8 ohm resistor when deciding which amplifier to purchase. Today audiophiles make fun of such foolish consumers but then base their purchasing decisions of DACs on small differences in jitter that are even further below audibility thresholds. In both eras the manufacturers were perfectly well aware of the irrelevance to real world performance but they are businesses giving the consumer what they want to buy.


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## profchris (26 Aug 2015)

Reading all this (mostly mysterious to me!) a thought occurs which might be stupid, but ...

What about an inner and outer skin of 1.5 mm ply (or aluminium, or plastic) with expanding foam to fill the gap? That should be rigid with high damping, but also give you a smooth reflective surface. Easy to build, compared with all that bending.


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## h.g. (27 Aug 2015)

profchris":2t76vl2p said:


> What about an inner and outer skin of 1.5 mm ply (or aluminium, or plastic) with expanding foam to fill the gap? That should be rigid with high damping, but also give you a smooth reflective surface. Easy to build, compared with all that bending.


If things work out the next one may well use composite panels, double curvature and be more CNC orientated. Single curvature was a compromise to ease construction and was not in the initial paper design. I did briefly consider honeycomb but I need to rout recesses for the drivers, a conical shape for the waveguide, blending chamfers, a lot of tapped holes for movable panels (it is something of a prototype and I do not know the optimum configuration),... By hand this seemed more straightforward with plywood compared to making a lot of insets for a composite panel.


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## SteveF (27 Aug 2015)

what about just glue up a load of boards flat and shape by hand?
you would not need many and would not have to be full size boards as you will be removing a lot of them in the shaping?

Steve


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## sploo (27 Aug 2015)

h.g.":10tt8uou said:


> Bracing stiffens the cabinet and pushes the frequency higher at which resonances start to occur. It does not reduce the sound pressure level of those resonances at the listening position which I suspect is what you are expecting to happen.


Nope - I understand the concept of pushing the resonance higher vs reducing sound pressure level - my question was whether there would be enough energy at those higher frequencies to be audible.

Now that said...



h.g.":10tt8uou said:


> So what you need to do to make things work well is put the woofer and midrange in separate cabinets that are isolated from each other and build those cabinets differently. The woofer cabinet needs to be stiff so that lowest resonances are in the midrange passband and will not get driven because the midrange is elsewhere. The midrange cabinet needs to be heavily damped and less stiff so that the lowest resonances are in the woofer passband and not driven and the higher order resonances that are in the midrange passband are strongly damped.


I was meaning make "the" cabinet (or all cabinets) braced sufficiently to push resonances higher (thus it wouldn't matter where the drivers were). However, building separate cabinets targeted for the specific resonance issues, makes good sense and is a pragmatic bit of engineering.




h.g.":10tt8uou said:


> Yes I am removing resonance issues in the way outlined above and I am doing it a pragmatic way without using 2" thick mdf, excessive bracing and similar favoured by many DIY speaker enthusiasts. There are a small number of audibility studies of cabinet resonances around which show, not surprisingly, that audible levels are around 30 dB down in the same ball park as other audible grunge.
> 
> Audiophile hardware is designed to get bought by audiophiles. If audiophiles want, for example, multiple binding posts for biwiring/biamping or whatever then speakers that do not have them are not going to get bought. It doesn't matter that biwiring/biamping or whatever isn't detectable. It doesn't matter that the manufacturers know this. A manufacturer needs to provide multiple binding posts because that is what the customer wants.
> 
> In the 1970s hi-fi enthusiasts used to give a lot of weight to small differences in completely inaudible levels of total harmonic distortion of a 1 kHz sine wave into an 8 ohm resistor when deciding which amplifier to purchase. Today audiophiles make fun of such foolish consumers but then base their purchasing decisions of DACs on small differences in jitter that are even further below audibility thresholds. In both eras the manufacturers were perfectly well aware of the irrelevance to real world performance but they are businesses giving the consumer what they want to buy.


Indeed. It's a world that contains more nonsense and non-science than just about anything else I know :wink:

Random other thought; I'm looking into 3mm sheets of PETG plastic for a forming/bending application. Have you considered using something like that instead of ply? Lots of different engineering issues granted.


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## h.g. (27 Aug 2015)

SteveF":45xslmlt said:


> what about just glue up a load of boards flat and shape by hand?
> you would not need many and would not have to be full size boards as you will be removing a lot of them in the shaping?


It is an option and had I stuck with the original double curvature or decided bending ply was going to be a problem it is what I would most likely have done. It is a fairly popular method for small scale manufacture of expensive speakers using a CNC machine to cut the stacks and was how the local woodworking firm suggested they would make the original paper design with double curvature panels.


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## h.g. (27 Aug 2015)

sploo":2ts9g4ps said:


> my question was whether there would be enough energy at those higher frequencies to be audible.


The rate of energy pumped into the cabinet is the product of the reaction force on the driver frame to the accelerations of the cone and the velocity in direction of that force by the cabinet. Broadly whatever is required to be done on the air to generate a given sound pressure level is equally done on the cabinet in reaction. So yes there is broadly the same proportion of energy available to generate a matching sound pressure level by the cabinet as the driver. Note that the relative amount of energy at high and low frequencies is not relevant, it is the amount relative to the driver that is relevant.

Having said that, in practise it is easier to control vibration at higher frequencies than low and so they tend to be less of a problem. Damping is more effective and, for example, a tweeter with a reasonably heavy frame does not need the mass of the cabinet to react against in the way a woofer usually does and soft rubber grommets could be used to isolate the vibration of the body of the tweeter from the cabinet. 



sploo":2ts9g4ps said:


> I was meaning make "the" cabinet (or all cabinets) braced sufficiently to push resonances higher (thus it wouldn't matter where the drivers were).


With normal construction and materials it is possible to push the resonant frequencies into the midrange passband but not into the tweeter passband and certainly not above the tweeter passband. A speaker cabinet will resonate at audible frequencies and the task is to make those resonances sufficiently low in level to be inaudible. 



sploo":2ts9g4ps said:


> Random other thought; I'm looking into 3mm sheets of PETG plastic for a forming/bending application. Have you considered using something like that instead of ply? Lots of different engineering issues granted.


Not at this stage. I would expect plywood to be stiffer, lighter and often cheaper when bought as sheets. Plastic mouldings have a significant presence in studio monitors and so it is more a question of the cost and effort of the tooling rather than the material. They can be designed to have significantly more damping than most wood products which is useful.


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## sploo (27 Aug 2015)

h.g.":21mur551 said:


> The rate of energy pumped into the cabinet is the product of the reaction force on the driver frame to the accelerations of the cone and the velocity in direction of that force by the cabinet. Broadly whatever is required to be done on the air to generate a given sound pressure level is equally done on the cabinet in reaction. So yes there is broadly the same proportion of energy available to generate a matching sound pressure level by the cabinet as the driver.


Good point. I guess there's potentially also a larger radiating area of cabinet than driver (though off the top of my head I don't know how that would affect the unwanted resonances).



h.g.":21mur551 said:


> Having said that, in practise it is easier to control vibration at higher frequencies than low and so they tend to be less of a problem. Damping is more effective and, for example, a tweeter with a reasonably heavy frame does not need the mass of the cabinet to react against in the way a woofer usually does and soft rubber grommets could be used to isolate the vibration of the body of the tweeter from the cabinet.


Linkwitz tends to mount mid/bass drivers by the magnet in order to reduce transmission to the baffle. Would that be helpful/effective in your application?




h.g.":21mur551 said:


> Not at this stage. I would expect plywood to be stiffer, lighter and often cheaper when bought as sheets. Plastic mouldings have a significant presence in studio monitors and so it is more a question of the cost and effort of the tooling rather than the material. They can be designed to have significantly more damping than most wood products which is useful.


It (PETG) is something I've only just come across myself (no experience with working with it) but just wondered. Most of my builds were MDF, with the occasional bit of ply. I'll admit I've generally gone for the heavy and well damped approach, as I've not had need to create something lighter.


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## h.g. (27 Aug 2015)

sploo":qs4ls5ol said:


> Linkwitz tends to mount mid/bass drivers by the magnet in order to reduce transmission to the baffle. Would that be helpful/effective in your application?


Yes for the woofers because they are in force cancelling pairs. Unfortunately for reasons of cost in the first build I have opted for modestly priced midwoofers that do not have a way to bolt to the frame on the magnet and so I will simply use metal rods between the normal mounting holes on the frame. Not ideal and one of the things to be addressed in the next version if there is one.

Yes for the midrange because it sits behind a waveguide that isn't deep enough to take bolts for rear mounting. However, the chosen midrange also doesn't have a means of bolting to the frame around the magnet. This has forced mounting from the front in the conventional way with a bit too much wood blocking the cone at the rear and, worse, the need for a conical ring to be inserted after the driver to fill the gap from the lip of the cone to the wall of the hole required to insert the driver. An unwanted complication caused by the need to be able to assemble the speaker!


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## sploo (27 Aug 2015)

h.g.":1pag10nl said:


> Yes for the woofers because they are in force cancelling pairs. Unfortunately for reasons of cost in the first build I have opted for modestly priced midwoofers that do not have a way to bolt to the frame on the magnet and so I will simply use metal rods between the normal mounting holes on the frame. Not ideal and one of the things to be addressed in the next version if there is one.


It's a good approach. I've built dual "boxer" style subs and I'm always amazed how little vibration you get with opposing pairs - even with large excursion.



h.g.":1pag10nl said:


> Yes for the midrange because it sits behind a waveguide that isn't deep enough to take bolts for rear mounting. However, the chosen midrange also doesn't have a means of bolting to the frame around the magnet. This has forced mounting from the front in the conventional way with a bit too much wood blocking the cone at the rear and, worse, the need for a conical ring to be inserted after the driver to fill the gap from the lip of the cone to the wall of the hole required to insert the driver. An unwanted complication caused by the need to be able to assemble the speaker!


I've previously used a split 'O' bracket made from good plywood, which bolts together and the compression holds the magnet. Not practical with all drivers granted, and it indeed makes putting everything together more of a hassle.


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