Considerations for hardwood amp cabinets

[John Sonksen]

So I recently got into a discussion on another forum with some folks debating making a guitar cabinet out of solid hardwood. There was someone asking if it had been done before and a lot of people responding that it wasn't a good idea. I felt like there was a lot of specious information being thrown into it to justify their positions, with kernels of truth here and there. I'm interested in a couple things that were brought up and figured this would be a good place to ask since Boogie actually makes hardwood shells for combo's, and there are likely some folks around here who have them.

First off, one argument that someone was making is that plywood is stiffer than solid wood, something I know is not true but surely you could find harder plywoods and softer hardwoods to have this argument work. But for our purposes I'm going to use Koa as an example of hardwood and baltic birch ply as the plywood, which is nearly the stiffest plywood you can get. The reason this was brought up was that people were saying that the inferred flexibility of hardwood would make the cabinet flex and therefore cause problems with volume and resonance apparently. Firstly, plywood, even baltic birch is not stiffer than most hardwoods and certainly not stiffer than koa. Plywood is stronger in a couple ways, but stiffness is not one. Hardwood is stiffer when flexed across the grain than plywood, but it really doesn't fare well when flexed parallel to the grain. This is one of the main reasons why plywood is found in things like skateboard decks and hockey sticks, but as far as guitar amp cases I don't think it's an important consideration because this force is not being applied here.

Another consideration for the superiority of plywood is the uniformity of material dimensionally as a new material and over time. Plywood doesn't change as much as hardwood with respect to environmental influence, temperature and humidity. Hardwood becomes acclimatized to it's environment even after it's been cured in the initial drying processes either kiln or air dried. Rapid changes in humidity can have drastic effects on hardwood, especially after it is joined to other pieces since the expansion forces may oppose each other and magnify problems like cracking and checking. This could be an argument for plywood over hardwood as a material of choice for cab construction. I do think though that this is only a consideration if proper care isn't given to the amp cabinets in storage, transportation and exposure. I would think that taking such an amp out to the local bar or club for a gig isn't going to have a drastic effect on such and amp, while going from a home in the SW to one in the south in summer could have a huge effect. This could probably be mitigated somewhat by using a road case for all of the time the amp isn't in use, and dehumidifying materials. For these reasons though I'd give the nod to plywood as being more user friendly.

Lastly, and this is the one I'm really interested in, someone brought up the idea that if you're going to make an amplifier case from hardwood, you should also "tune" the case. In the discussion I had talked about how hardwood, being a contiguous material could propagate vibrations and even amplify them, whereas plywood because it is made up of opposing layers would suppress such vibrations. So I'm wondering, does anyone know if Boogie tunes their combo cases? I'm trying to imagine how exactly you'd do that, not from a practical standpoint so much, but how you'd tune something to inhibit vibrations across the entire spectrum of guitar notes, not to mention additional wave formations brought on by the use of effect pedals and feedback.

I'm curious because of how adamant some were that cases should never be built of hardwood, yet some of the most beautiful amps I've ever seen are made of hardwood, as well as by one of the most revered names in the business. If it was really such a fussy thing, or so difficult, would Boogie be doing it? Also, has anyone here had Boogie replace their original cab with a hardwood? I'm interested in finding out what's involved. Do you have to send the amp to them to do it, or can you order the shell and have a local boogie tech do it? Is tuning the shell really part of the process? I nearly replaced the shell on my Mark IIc+ a few years back and decided to just keep it stock, but I hadn't considered much of any of these factors back then.

[Halgeir Wold]

Google Fargen amplifiers. Beautiful cabs, some of them.
You may also look at the ampgarage, buy you may have to register to see the pictures..

Tuning an ampcase ??? Hm.. there sure are plenty of myths in the guitar amp camp..........

[Clay Schaeffer]

Arguments I've heard for using plywood is that it is acoustically pretty dead, relatively tough (split resistant), comes in large sheets to make fabrication easy, and is relatively cheap. A beautiful rosewood veneer might give the best of both worlds.

[Rodger Knox]

For speaker enclosures, MDF used to be the material of choice, for both it's uniformity and density. Baltic birch plywood was a close second, it has the advantage of being more resistant to moisture and is tougher, important considerations for a mobile cab.
Personally, solid mahogany appeals to me.

edit: As for "tuning", that would refer to a sealed box with ports (bass reflex design), not so much for an open back. Speakers are usually designed to work in one or the other, a few will work in either.

[Steven Wilson]

I've built a few hardwood cabinets for amps without any problems. My Kustom Studio model from the 70s hase I solid oak cab, my original Crate had a pine crate for a cab.

[Will Morrison]

Gerald Weber of Kendrick Amplifier fame in his fourth book talks about building cabinets out of every hardwood he could find. It's a pretty interesting part of the book. He came to some very interesting conclusions. He said that there are woods that he would NEVER build cabinets out of after doing this test, maple and mahogany being at the top of the list. He decided that OLD pine, if one can get hold of it, is an excellent choice, new pine, not so much. He also said that lots of exotic hardwoods are NOT good, but that canarywood gives an incredible tone, and about 3db more volume than any other wood he tried.

I've got a slab of canary, but it's not big enough to make a cabinet out of. I keep thinking of a mandolin or mandola set of sides and back, and see what it sounds like. It would definitely be pretty, canary is beautiful. I'm jut wondering if it would be that much louder, of if that only works in amp cabinets.

Anyway, Gerald says that this is your best choice for a serious sounding cabinet, and he must believe it, because he does offer a serious boutique amp that has a canarywood cabinet.

[Halgeir Wold]

I'm sorry, but any wood that would generate extra dB's must be from outer space...the guy clearly needs to read up on physics.....
For some strange reason, guitar amp electronics seems to be even worse tha HiEnd HiFi in terms of peculiar myths... if at all possible....

[Will Morrison]

As I wasn't there for his tests, I can't verify what he's written, but he does won a fairly well known amplifier company, and has been building the things for well over 20 years. He's also got enough equipment to measure the output before he writes it down in his fourth book. I would write to him and ask for specifics, don't take it up with me. My experience with him is that he's pretty good about writing back if you have questions and ask nicely.

[Halgeir Wold]

Maybe I came across somewhat harsh, which was not my intention at all. Mr.weber is also known for making claims about his products that are smetimes rather unbelievable.
Since power amplifying woods from outer space does not exist, and 3 dB is a power doubling, the claim is impossible. He may have measured a higher resonance peak at some frequency, but then the energy must be taken from other frequenscies in a wide band signal.... on a single tone it is impossible.
I think we can leave MR.Weber and his claims alone.....
Peace....

[Keith Howell]

It is possible to talk of power gains in terms of passive components like speakers and antennae by relating them to an omni-directional equivalent.

ie if you measure the power output in the omni-directional case then substitute the component with the directional component you could well read a "power" gain. This is because more power is being directed in the direction of the measuring point and less in other directions.

[Halgeir Wold]

I've been working in RF and general electronics since '75, so I do know fairly well what you mean. I've also done a fair bit of loudspeaker design on the side, so I'm fairly well aquainted with loudspeaker theory, as well as fiddling with various aspects of audio. I've been building amplifiers and audio gadgets for over 40 years, - meaning that I do have a fairly good command of audio electronics in general.
The only loudspeaker designs that to some extent will increase a speakers efficiency beyond its basic data are horns. This claim deals with a given species of wood increasing the basic efficiency of a speaker, which is a physical impossibillity. Guitar amplifiers and HighEnd Audio are the two technological areas which unfortunately are crammed with unsolicited claims of rather fantastic merit, very often beyond the capabilities of physics.
As an example, - In some circles dealing with guitar amps, component properties are taken far beyond both technology and physics and well into voodoo land....

[David King]

It's my understanding that "voodoo" is much more efficient at conveying economic benefit from the fool's party to the party known as technobabble generators.

[Keith Howell]

"As an example, - In some circles dealing with guitar amps, component properties are taken far beyond both technology and physics and well into voodoo land.... "

I agree entirely. I too have worked with electronics and RF (Microwave) since 1975 but then went over to the dark side (software)! Some of the rubbish expounded in guitar and generally in audio is amazing. Usually, it is just ignorance of valid scientific principles but sometimes it is willful distortion of the truth mostly by commercial considerations.

I was definitely not trying to validate Mr Weber's claims as I have not read any of them but merely trying to point out a valid principle, possibly what he was eluding too but not using the correct engineering terms. (or perhaps deliberately obfuscating them?)

[Will Morrison]

Sorry for taking so long to get back here. Let me put forth a speculation. Could it possibly be that the canary wood is NOT amplifying so much as it's getting out of the way of the speaker and letting it actually put out what it's capable of? Is it possible that other woods are actually pulling down the efficiency of the driver in certain areas that the canary wood doesn't? I have no idea, I'm just asking.

As to Gerald and his claims, we DO have to remember he's from Texas...

[David King]

Will, here's what I have to say about it. The canarywood cab probably has a resonant peak that's close to the frequencies where our hearing is most sensitive. There could and often will be some additive effect close to and at resonant peak. At other frequencies, especially right above the resonant peak there will be significant dampening of the sound. If what you want to hear is in the right place on the spectrum then yes you believe, and correctly so, that the signal is louder than it was before but it hasn't technically been amplified. Think of it in terms of a horn loudspeaker like the old Klipsch monsters or those plastic iPhone cradles with the horn built in. That said it's definitely not a horn either.

[Dave Sayers]

I'm presently building a Madamp MK 15 valve amp and I'm building a header cabinet and speaker cabinet to hold a Celestion gold. I am going to build the cabinet out of European Redwood, which I know is not a hardwood, but it is reasonably priced. If you can afford it then go ahead, I think you will be pleased with the result. I very rarely ask for advice nowadays but tend to look at what is on sale. If they can sell it then I can build it.

[Halgeir Wold]

Hi guys.... I think the idea about resonances in hardwood cabs is as close as we can get, but still the energy needs to come from somewhere. If a resonance is excited, it will steal energy from the main output, making the frequency response even more uneven than the most popular guitar speakers are from the beginning,- exactly what makes these speakers popular in the first place.
Having built loudspeakers for HiFi and some PA since the early 70s, I seriously think that making cabs out of hardwood has little more too it than good looks, which of course can be important in itself. The vast majority of the best sounding speakers of all types so far, be it HiFi PA or guitar, still used or uses MDF, plywood or plain jane pine...