Hardwood vs. MDF for a DXT-MON build.

[antennaguru]

20 years or thereabouts ago I built a small bookshelf loudspeaker from Visaton units in an passive radiator config ( AL130 midwoofer) in solid birch. They were drop dead gorgeous, with heavy radius all around 25 or 30 mm thick walls, professionally made by a small kitchen and wc cabinet maker to furniture grade standard. He assured me the wood would be perfectly dry and wouldn't budge, got it treated/lackered inside and out for ease of mind, plus bituminous sheets lining the inside.You know where this is going...they were always in my living room, subjected to little temperature and moist variation, but after some 7 or 8 years the humitidy that somehow had crept in and out, caused too much of different expansion rates along the wood grain (unavoidable) and too much stress accumulated and the whole thing begun to disjoint...Don't do it, or get Sonus Faber to do it for you if you must, I tried to be as carefull as I could and got burned...( then again, I didn't build them myself...)

Birch is oddly considered a hardwood, but as someone who has a lot of experience with burning it in fireplaces and woodstoves I can tell you it is actually rather soft and burns very fast compared to say Oak - which is indeed hard. In the woods there is a joke that when you plan to cut down a Birch tree it falls over on its own before you can get the saw running - cuz it's scared. In fact you see a lot of Birch trees fall each storm, on their own. Whenever I buy seasoned firewood I never accept much Birch content in the load because Birch burns too fast, and I always demand as much Oak content as I can get in the load of firewood. Lately that's been easy due to all of the Gypsy Moth attacks we've had on the Oaks.

 

[BostonJack]

20 years or thereabouts ago I built a small bookshelf loudspeaker from Visaton units in an passive radiator config ( AL130 midwoofer) in solid birch. They were drop dead gorgeous, with heavy radius all around 25 or 30 mm thick walls, professionally made by a small kitchen and wc cabinet maker to furniture grade standard. He assured me the wood would be perfectly dry and wouldn't budge, got it treated/lackered inside and out for ease of mind, plus bituminous sheets lining the inside.You know where this is going...they were always in my living room, subjected to little temperature and moist variation, but after some 7 or 8 years the humitidy that somehow had crept in and out, caused too much of different expansion rates along the wood grain (unavoidable) and too much stress accumulated and the whole thing begun to disjoint...Don't do it, or get Sonus Faber to do it for you if you must, I tried to be as carefull as I could and got burned...( then again, I didn't build them myself...)

Thanks for sharing that. Unfortunately I have 4/4 walnut and 8/4 Brazilian cherry (aka tigerwood) sitting in my apartment coming to moisture equilibrium, so the die is cast. I think my approach is going to be to 1. consult with some local cabinetmakers, as making boxes isn't exactly a new thing and they might know tricks that work and 2. really pay attention to surface treatment.

I did find one builder online who had speaker cabinets produce small cracks after moving from a dry climate to a humid coastal climate. He provided pictures (sorry don't have the link handy) and was able to repair by injecting epoxy, so some small cosmetic damage but not terrible result.

Just to summarize, I wasn't able to find any data that solid hardwood is worse than MDF in acoustic terms. I appreciate your advice and I'm going to steer in the direction of taking whatever mitigating steps I can.

So here is a strange case: I've owned a couple of small sailboats: J30 with teak rub rails and hand rails, 26' Columbia with custom teak cockpit seats, and a 1960's Lightning with mahogany, ash, and mahogany plywood decks (entirely wood construction). All three boats were exposed to marine environment, dry stored in winter New England climate, basically put through a torture test for years. Result: no cracking or failed joints. What's going on there?

Teak is a very resin rich wood (as is Brazilian cherry), so that is part of the story. Mahogany and ash are not.

What was your failure? Joints failing? Cracks across panels? What adhesive was used on the joints, do you know? Thx.

 

[BostonJack]

Thanks for sharing that. Unfortunately I have 4/4 walnut and 8/4 Brazilian cherry (aka tigerwood) sitting in my apartment coming to moisture equilibrium, so the die is cast. I think my approach is going to be to 1. consult with some local cabinetmakers, as making boxes isn't exactly a new thing and they might know tricks that work and 2. really pay attention to surface treatment.

I did find one builder online who had speaker cabinets produce small cracks after moving from a dry climate to a humid coastal climate. He provided pictures (sorry don't have the link handy) and was able to repair by injecting epoxy, so some small cosmetic damage but not terrible result.

Just to summarize, I wasn't able to find any data that solid hardwood is worse than MDF in acoustic terms. I appreciate your advice and I'm going to steer in the direction of taking whatever mitigating steps I can.

So here is a strange case: I've owned a couple of small sailboats: J30 with teak rub rails and hand rails, 26' Columbia with custom teak cockpit seats, and a 1960's Lightning with mahogany, ash, and mahogany plywood decks (entirely wood construction). All three boats were exposed to marine environment, dry stored in winter New England climate, basically put through a torture test for years. Result: no cracking or failed joints. What's going on there?

Teak is a very resin rich wood (as is Brazilian cherry), so that is part of the story. Mahogany and ash are not.

What was your failure? Joints failing? Cracks across panels? What adhesive was used on the joints, do you know? Thx.

Oh, yeah. Birch is kind of a special case. Plus, one use of Brazilian cherry is for outside decking. its so resin-rich that it will stand up to years of weather with just a periodic stain and cleaning.

 

[digitalfrost]

One thing you could try is to seal the inside seams with acrylic sealant. Do not use silicone, the vapors can damage rubber surrounds. I simply use my finger to make it smooth, but a coin works as well.

 

[BostonJack]

One thing you could try is to seal the inside seams with acrylic sealant. Do not use silicone, the vapors can damage rubber surrounds. I simply use my finger to make it smooth, but a coin works as well.

Good thought. I'm considering using epoxy (WEST system or System 3) as surface treatment inside and out, then spar varnish over the epoxy outside. This is standard on a lot of marine work and results in wood encapsulated in epoxy which should be pretty darn stable.

 

[carbidetooth]

Birch is oddly considered a hardwood, but as someone who has a lot of experience with burning it in fireplaces and woodstoves I can tell you it is actually rather soft and burns very fast compared to say Oak - which is indeed hard. In the woods there is a joke that when you plan to cut down a Birch tree it falls over on its own before you can get the saw running - cuz it's scared. In fact you see a lot of Birch trees fall each storm, on their own. Whenever I buy seasoned firewood I never accept much Birch content in the load because Birch burns too fast, and I always demand as much Oak content as I can get in the load of firewood. Lately that's been easy due to all of the Gypsy Moth attacks we've had on the Oaks.

Hardwoods and softwoods are classified as such because they have certain characteristics, but surface hardness is not really one of those. In general, hardwoods have leaves they shed and softwoods don't. Think conifers...pine, fir, larch, etc.

 

[antennaguru]

Hardwoods and softwoods are classified as such because they have certain characteristics, but surface hardness is not really one of those. In general, hardwoods have leaves they shed and softwoods don't. Think conifers...pine, fir, larch, etc.

That makes sense that it's related to evergreens versus deciduous trees.

 

[carbidetooth]

One thing you could try is to seal the inside seams with acrylic sealant. Do not use silicone, the vapors can damage rubber surrounds. I simply use my finger to make it smooth, but a coin works as well.

I assume you mean seams in box joinery. While commonly done, I question the value. It adds almost nothing in the way of structural strength, so its perceived purpose appears to be sealing. But what exactly is being sealed? A leaky enclosure? I'm curious as to your reasoning, please enlighten.

As to silicone, I suspect what you're referring to is a smelly curing agent used in some (not all) silicone caulks. If so, that's acetic acid and once cured and smell is gone, it's benign. I fail to see how it would affect a rubber surround, presumably installed well after curing has taken place.

 

[carbidetooth]

Thanks for sharing that. Unfortunately I have 4/4 walnut and 8/4 Brazilian cherry (aka tigerwood) sitting in my apartment coming to moisture equilibrium, so the die is cast. I think my approach is going to be to 1. consult with some local cabinetmakers, as making boxes isn't exactly a new thing and they might know tricks that work and 2. really pay attention to surface treatment.

I did find one builder online who had speaker cabinets produce small cracks after moving from a dry climate to a humid coastal climate. He provided pictures (sorry don't have the link handy) and was able to repair by injecting epoxy, so some small cosmetic damage but not terrible result.

Just to summarize, I wasn't able to find any data that solid hardwood is worse than MDF in acoustic terms. I appreciate your advice and I'm going to steer in the direction of taking whatever mitigating steps I can.

So here is a strange case: I've owned a couple of small sailboats: J30 with teak rub rails and hand rails, 26' Columbia with custom teak cockpit seats, and a 1960's Lightning with mahogany, ash, and mahogany plywood decks (entirely wood construction). All three boats were exposed to marine environment, dry stored in winter New England climate, basically put through a torture test for years. Result: no cracking or failed joints. What's going on there?

Teak is a very resin rich wood (as is Brazilian cherry), so that is part of the story. Mahogany and ash are not.

What was your failure? Joints failing? Cracks across panels? What adhesive was used on the joints, do you know? Thx.

Something to bear in mind. All wood and wood composites move with changes in temperature and humidity. It's not a removable characteristic but is mitigated to some degree in composites because they are "modified" in their cell structure. This results in more stability.
The best one can hope for is slowing down movement of water vapor in and out of solid wood. Wood finishes, epoxy, caulk, glue all do this to some degree, but Mother (nature) will have her way in the end. The most integrated structures stay that way because they took the inevitable movement into account.
Can you build with solid wood? Certainly. But you nor I will ever defeat Mother. Which makes construction of a thing somewhat more demanding, assuming longevity is one of the goals. Your sailboat didn't break itself apart because it accommodated movement is some way or another.

 

[BostonJack]

Something to bear in mind. All wood and wood composites move with changes in temperature and humidity. It's not a removable characteristic but is mitigated to some degree in composites because they are "modified" in their cell structure. This results in more stability.
The best one can hope for is slowing down movement of water vapor in and out of solid wood. Wood finishes, epoxy, caulk, glue all do this to some degree, but Mother (nature) will have her way in the end. The most integrated structures stay that way because they took the inevitable movement into account.
Can you build with solid wood? Certainly. But you nor I will ever defeat Mother. Which makes construction of a thing somewhat more demanding, assuming longevity is one of the goals. Your sailboat didn't break itself apart because it accommodated movement is some way or another.

Yes, very good point. That's why I'm going to consult with a couple of cabinet makers. The lightning, BTW, was mainly epoxy encapsulated. My telescope, which I build using strip-planking technique is epoxy encapsulated. If you couldn't mitigate expansion/contraction all those 200+ year old antiques would be piles of sticks. A lot of salt water boats are built using hardwood in layers with epoxy as adhesive and surface treatment. Essentially a modern wood/epoxy composite. I think there are solutions, perhaps not perfect ones.

 

[antennaguru]

Yes, very good point. That's why I'm going to consult with a couple of cabinet makers. The lightning, BTW, was mainly epoxy encapsulated. My telescope, which I build using strip-planking technique is epoxy encapsulated. If you couldn't mitigate expansion/contraction all those 200+ year old antiques would be piles of sticks. A lot of salt water boats are built using hardwood in layers with epoxy as adhesive and surface treatment. Essentially a modern wood/epoxy composite. I think there are solutions, perhaps not perfect ones.

We have a large grandfather clock in our colonial dining room that was built in 1770, keeps perfect time today, and its hardwood woodwork still looks great after hundreds of years of use and moving between several homes. They used some sort of varnish back then on the wood.

I have no fear of using solid hardwood for speaker cabinets and have built them in the past from solid Walnut, Oak, and Teak. They all held up great over time.

I dislike veneering although I certainly can do it, and have done it for furniture building/repairing, because it's too hard to get nice rounded corners - and eventually a hard veneered corner gets bumped and is harder to repair than when I've used solid lumber.

One thing I tried that made for very easy and sturdy hardwood speaker cabinet construction using solid Oak was biscuit joinery - with lots of glue and clamping. I always make sure to use a round-over bit in my router on the inside (back of the panel) of where the cone drivers mount to not disturb the airflow from the back of the drivers. You can see that on the back of the front panel below, next to my biscuit slot cutter. Once the glue is cured I then use my round-over bit in my router to soften every exterior corner of the cabinet and then use a Jitterbug sander before applying finish. This one is a small cabinet for 4 small full range drivers in a short line array.

 

[BostonJack]

We have a large grandfather clock in our colonial dining room that was built in 1770, keeps perfect time today, and its hardwood woodwork still looks great after hundreds of years of use and moving between several homes. They used some sort of varnish back then on the wood.

I have no fear of using solid hardwood for speaker cabinets and have built them in the past from solid Walnut, Oak, and Teak. They all held up great over time.

I dislike veneering although I certainly can do it, and have done it for furniture building/repairing, because it's too hard to get nice rounded corners - and eventually a hard veneered corner gets bumped and is harder to repair than when I've used solid lumber.

One thing I tried that made for very easy and sturdy hardwood speaker cabinet construction using solid Oak was biscuit joinery - with lots of glue and clamping. I always make sure to use a round-over bit in my router on the inside (back of the panel) of where the cone drivers mount to not disturb the airflow from the back of the drivers. You can see that on the back of the front panel below, next to my biscuit slot cutter. Once the glue is cured I then use my round-over bit in my router to soften every exterior corner of the cabinet and then use a Jitterbug sander before applying finish. This one is a small cabinet for 4 small full range drivers in a short line array.

Yes. Your clock sounds beautiful. My sons inherited a pre-Revolution grandfather clock and a governor Winthrop desk. The clock had its surface finish restored by a antiques specialist in Boston. Its sitting in a climate controlled storage unit now, as they don't have a suitable place for it. Those are one of a just a few really old pieces from my father's side of the family. Bunch of "swamp Yankees".

I think it is totally possible to have a beautiful set of hardwood speaker cabinets. I am veneer adverse, a careful but not very expert woodworker, and an engineer by trade and training so worrying over details is second nature.

Love your photos! I have to biscuit join walnut to get 9.5" width pieces. Contemplating half-blind dovetail joints for the main joints (router cut). That will require a consult with a real cabinet maker as I'm not sure of the disadvantages.

DXT-Mon plans include a mitre on the back of the baffle, as you describe.

 

[tvrgeek]

Do not forget high density boards. Like MDF and "dead" but much stronger, more precise machining.
I like MDF but I usually paint my speakers. I saturate them with poly resin to get a nice sealed surface for finish and use either epoxy or powered resin glue as they don't shrink like PVA does. A PVA glues joint will sink in and show years later. There is quite a trend in woodworking for more superglue.

Just like in fine furniture, solid wood is a no-no as it will move. Always. If you try and defy Mother Nature, you will find out she wins every time.

 

[antennaguru]

Yes. Your clock sounds beautiful. My sons inherited a pre-Revolution grandfather clock and a governor Winthrop desk. The clock had its surface finish restored by a antiques specialist in Boston. Its sitting in a climate controlled storage unit now, as they don't have a suitable place for it. Those are one of a just a few really old pieces from my father's side of the family. Bunch of "swamp Yankees".

I think it is totally possible to have a beautiful set of hardwood speaker cabinets. I am veneer adverse, a careful but not very expert woodworker, and an engineer by trade and training so worrying over details is second nature.

Love your photos! I have to biscuit join walnut to get 9.5" width pieces. Contemplating half-blind dovetail joints for the main joints (router cut). That will require a consult with a real cabinet maker as I'm not sure of the disadvantages.

DXT-Mon plans include a mitre on the back of the baffle, as you describe.

Thanks, and yes when my parents bought that clock from an antique dealer (over 50 years ago) that antique dealer wouldn't sell it to them without first visiting their home to see where the clock would be placed. It needed to be placed on a sturdy flat level floor and they had a nicely slate floored entry room which the dealer approved of. Eventually the clock became mine and our dining room has traditional colonial wide pine plank flooring, various widths up to 18".

Woodworking was something my father taught me, even though I was educated and worked in electrical engineering for my career. I do still enjoy building with my hands as there is a special satisfaction it gives me.

 

[egellings]

Hardwood, being stiff, will have rather High Q resonances. The internal damping of MDF is likely to result in a lower Q material that will not have such peaky resonances. In a subwoofer, this is not likely to matter, but in a full range speaker, it might.

 

[BostonJack]

Yeah, the people who deal in really beautiful antiques are working for both love and money and don't want to see good examples get damaged. The clock is really LOUD. Its from the era where the movement was shipped over from England and the cabinetry was built by local colonial craftsmen. I'm glad those pieces are still in the family and I've convinced my sons that having stewardship of them is a privilege.

 

[BostonJack]

Hardwood, being stiff, will have rather High Q resonances. The internal damping of MDF is likely to result in a lower Q material that will not have such peaky resonances. In a subwoofer, this is not likely to matter, but in a full range speaker, it might.

solid point. its not density, as the woods I have are denser than the median MDF (there's a lot of variation). stiffness might come into play.
however, the bismuth/hardboard/bismuth treatment is going to damp a lot. and nobody makes a fuss (including the designer) about substituting
Baltic birch plywood for MDF. we will see.

 

[puppet]

When you lay up hardwood into panels such as a table top it's important to do the edges properly. Don't mill them straight. What you want is a slightly convex curve to the board in the length. When you lay them out there should be a gradual space between each board while they touch at the ends. looks like this ... [x()x()x()x()x] ... the "x" is each board. When done correctly you should be able to use one clamp across the center of the panel for glue up. The reason for this is that the ends of the board loose moisture at a different rate. When you taper them it imparts tension on the panel so that when the ends shrink the panel will go back in balance and the ends won't crack.

You also want to have all of the panels orientated with the grain running the same way in the box. That way all of the panels move in the same direction .. across the grain. Never position an edge to an end for a corner joint. Always end to end for corners, best for joinery. Dovetails make good corner joints as well as a simple finger (box) joints. The latter can be cut on a table saw. Remember end grain has no glue value. You have to produce a joint of some kind to get long grain involved for a glue surface. Biscuits and dowels are a waste of time for edge gluing solids as well. What I don't like about biscuits is that over time they will telegraph through the surface of the panel. They are too hard and wood shrinks while the biscuits don't. Dowels are better in this regard, being round, but are still a waste of time for edges. Tests have shown that edge gluing solid stock is plenty strong without any additional material(s) added in (biscuits/dowels).

Tage Frid had some excellent books on joinery and solid stock fabrication.

... and there are many species of birch fellas. Paper birch is soft ... other birch are as hard as white maple.

 

[BostonJack]

When you lay up hardwood into panels such as a table top it's important to do the edges properly. Don't mill them straight. What you want is a slightly convex curve to the board in the length. When you lay them out there should be a gradual space between each board while they touch at the ends. looks like this ... [x()x()x()x()x] ... the "x" is each board. When done correctly you should be able to use one clamp across the center of the panel for glue up. The reason for this is that the ends of the board loose moisture at a different rate. When you taper them it imparts tension on the panel so that when the ends shrink the panel will go back in balance and the ends won't crack.

You also want to have all of the panels orientated with the grain running the same way in the box. That way all of the panels move in the same direction .. across the grain. Never position an edge to an end for a corner joint. Always end to end for corners, best for joinery. Dovetails make good corner joints as well as a simple finger (box) joints. The latter can be cut on a table saw. Remember end grain has no glue value. You have to produce a joint of some kind to get long grain involved for a glue surface. Biscuits and dowels are a waste of time for edge gluing solids as well. What I don't like about biscuits is that over time they will telegraph through the surface of the panel. They are too hard and wood shrinks while the biscuits don't. Dowels are better in this regard, being round, but are still a waste of time for edges. Tests have shown that edge gluing solid stock is plenty strong without any additional material(s) added in (biscuits/dowels).

Tage Frid had some excellent books on joinery and solid stock fabrication.

the curve technique you describe is mentioned in the Router book I have. your account is crystal clear.
"Woodworking with the Router" by Bill Hylton (his take is more for grain pattern matching, not dimensional changes)

do you think that this end vs. body taper applies to really short panels? the largest will be 12" x 10.5" (grain along 12")

similarly, if the case is treated with surface epoxy, including on the end grain is the moisture change in the piece going to be
enough to bring this shrink/expand effect into play? Bear in mind that the epoxy has low enough moisture permeability to be
used to barrier coat boat hulls that last with decades of exposure in salt water (which polyester resin does not).

what are you thoughts on using epoxy as the glue, especially in dovetail joints? I've seen fine furniture makers advocate for epoxy.
you can't unglue things for repairs, however!!

I hope I don't sound argumentative as I really do appreciate the advice. I'm in gathering information stage now.

 

[puppet]

Cabinetmakers have used regular wood glue(s), be they hide or resins, for centuries. If your completed loudspeaker cabinets will be used to get you across the Atlantic, go with the epoxy
Don't forget, some of these makers are building for harsh commercial environments. They buy their adhesives in bulk and frankly begin to use them for everything they might make. Nothing wrong about it but one has to bare in mind that some adhesives will have adverse effects on certain woods. They might stain and discolor. Always good to check this stuff out first.

To add: old wood working techniques have been lost to some degree, which is a shame. Today manufacturing is geared toward production. Hand tools aren't used as they once were. You're building a custom piece. The skill level you have will make or break your project. If you don't want it to fly apart, crack some books on the older techniques. I worked at this for over 40yrs ... took the first 10yrs to find my ass with both hands.