[whubbard]

Hi Everyone,

I've been going around asking a lot of questions, and I now have a topic that will hopefully help others in the future, although I do think it has probably been touched on before.

Cabinet Materials.
I.   What are all your experiences with different cabinet materials?
II.  What about cabinet materials in full range, open baffle, single driver, small speakers, big speakers?
III. How much does the physical structure of the speaker design matter vs. the materials used?

I hope we can get a good discussion going.

-West

[bunky]

Hey west,I am swaping a barely used Rega Apollo cd player for a pair of Green Mountain Audio Europa monitors. the cabinets are cast from Q-stone synthetic marble. I too am interested in this topic and would like other AC members to chime in.thanks....WCW III

[JimJ]

I've used MDF & birch ply, mainly.

There are some speaker designs that apparently use cabinet resonances in their design, but I've always tried to deaden the cabinet as much as possible to take it out of the equation.

Ply sure finishes with less work than MDF, though Painting MDF to a uniform finish can be a pain...

[JLM]

Cabinet design (physical structure) is by far a bigger factor than material selection (in most cases).  For instance if you choose to build garage sized bass horns, they’d better be built of something much more rigid and massive than what’s needed for a sealed standmounted monitor.

One of the easy ways of achieving rigidity is by the use of bracing.  How much a material deflects is a factor of the square of the square of the unbraced length, but only to the square of the material thickness.  Obviously boxes are much more rigid structures than unbraced baffles/wings. 

By far most of the energy is in the bass frequencies, so the bigger the driver the much bigger issue of cabinet/panel resonance.  OTOH most extended range (single) drivers are better termed mid/tweeters, as they don't produce gobs of bass.

Open baffle designs have no internal pressure, but typically are extremely poorly braced and have relatively low mass.  Newton's law (conservation of momentum) dictates that when the cone (and air in front of it) moves forward, the baffle/overall speaker must move back (m1 x v1 = m2 x v2). 

Sound pressure energy levels inside a speaker cabinet due to backwaves are 100s/1,000s of times higher than in the room.  Not surprisingly then, some have measured more sound emanating from the cabinet than the drivers.  So rigidity, mass, and uniformity of material(s) are essential to keeping the speaker as an accurate reproducer (not producer) of sound.

Some (such as those who prefer use of baltic plywoods, etc.) like "hearing" the cabinet (like a violin), but that adds the same coloration to everything you hear.  Combining materials of different resonant frequencies drops the resonant frequency of the composite, therefore requiring more energy to excite it into a resonant mode.  Using homogenous materials eliminates concern for voids or differing strengths in different directions that affect resonance and rigidity.

Another factor, not often addressed in sealed or ported designs, is the reflectance of the backwave.  As cone materials are not acoustically opaque, any sound generated in the cabinet can reflect back out through the cone.  This is especially prominent where the back wall of the cabinet is parallel to the front wall (driver mounting baffle).  Again, cabinet design is a bigger factor than material selection.

[Daygloworange]

I can't add much more, so I'll echo JLM's post.

As mentioned, there is what happens outside the box, and what happens inside the box. A speaker enclosure is not a musical instrument. The ultimate goal is for it to be acoustically dead.

Density/Mass, and stiffness of the enclosure are two very important and simple to implement criteria for controlling what happens outside the box (enclosure resonances). The stiffer you can make the enclosure, the higher the resonant frequency. The higher the resonant frequency of the enclosure, the more excitation it takes to get it to resonate sympathetically, therefore less amplification of resonances.

The use of dissimilar materials (AKA composites) aids performance by damping any remaining resonances even further by either raising resonant frequency, or converting soundwaves to heat.

Controlling backwave reflections inside the box are also very important, and sometimes difficult to implement due to enclosure size and cost constraints. There are alternative methods to control backwave energy from radiating back against the driver and creating non linearities.

Diffraction of soundwaves outside the enclosure is another important factor to minimize as well.

Even in a small 2 way speaker can you yield significant performance improvements in performance. A number of years ago, I built new enclosures for a pair of inexpensive Tannoy nearfield monitors for my studio. The original enclosures were built with a combination of 5/8" and 3/4" MDF and particle board, with a cardboard front port, and weighed 9 lbs apiece (sans drivers and crossovers). I overbuilt some new enclosures with 1 1/2" and 2 1/4" MDF while retaining the exact same internal dimensions. The weigh was 32 lbs per enclosure (sans drivers and crossovers). A large radius roundover was used to minimize diffraction, and the front firing port was replaced with a flared PVC port.

Then I A/B'd them side by side in my studio, feeding them the exact same mono sound source and by going back and forth by alternating which speaker got played via mutes on the mixing board.

I was not prepared for the differences heard. Clarity was much improved. The integration of the woofer and tweeter seemed smoother overall. The transition was much more transparent. Bass performance became much better defined and much clearer. Interestingly, the excursion on the woofer became larger with testing low frequency content. The latter might be due to the increase in width of the front baffle(by use of much thicker enclosure material) affecting the baffle step of the woofer.

If you were to A/B these 2 speakers blindly, you would think they were 2 different speakers. No joke.

Much like the body of a musical instrument has huge impact on the quality of sound, so does the the enclosure of a speaker. It has almost as much impact as driver selection of the speaker, if not more in some instances.



Cheers

[WGH]

And I will echo both JLM's and Daygloworange's posts, the less resonant the enclosure, the better the speaker will sound. I have added internal bracing to the last 2 sets of speakers I have bought. The most recent speakers, Von Schweikert VR2's, were transformed from pretty good to very good. Imaging and clarity were the first improvements I heard, and when bass material is present the sound is deeper and more forceful.

I like doing a knuckle rap test on a speaker side, if it sounds hollow then the speakers are definitely effecting the sound. A rap on the 8" x 16" top of my VR2's now sound very close to the same rap on the 38" x 16" sides.

VR2 cabinet bracing diagram

[BrianP]

My best results, especially with larger boxes, have come from laminating dissimilar materials. An outer shell of void-free birch ply enclosing an inner shell of MDF or particle board, laminated together with a glue that dries flexible. And then plenty of internal bracing. Plywood "shelf" braces that connect all cabinet sides, cut out to allow plenty of air flow through them, have worked pretty well.

There are all kind of fancy (and pricey) materials available to line the inside of cabinets to add mass, absorb vibration, and control reflections, standing waves, etc. Some cheaper "non-audiophile-market" alternatives include dense closed-cell foam mats (like those made for people who stand on hard floors long hours at work), carpet scraps, acoustic ceiling tile, roofing felt, asphalt-laden plastic sheets used to dampen car panel vibrations, sheets of rock wool, and various DIY goops (such as lead bird shot mixed with an adhesive) that can be painted/troweled on.

Some panel resonances are still inevitable. The trick is to push them UP in frequency, DOWN in dB relative to the output of the drivers, and damp them out to reduce their ringing time. 

[sbrtoy]

There are many different ways to skin a cat. Some speaker companies utilize mass as described with MDF and Ply being the preferred materials for this method due to the cost/performance ratio of such materials.  Some like Wilson use composite materials that they claim are less resonant than even very thick MDF panels.  It is hard to argue their point about other materials being potentially superior to MDF, however if you have checked their prices lately you definitely pay for these composites as well.  

Magico has been using aluminum in a lot of their designs which seems somewhat silly to me, but many report their speakers sound excellent so I guess as always implementation is a big part of the final result.  

A properly braced MDF box utilizing internal dampening material can be made to perform very very well.  Our upcoming monitors are 1" MDF all around and at 9"Wx14"Dx17"H they weigh in at 27 pounds each raw without drivers or other components. They perform extremely well with very low coloration of the bass or midband when combined with internal constrained layer dampening.

[TheChairGuy]

Cabinet materials and construction is a HUGE matter.  If you do little else right in the making of a speaker, and only make it heavy for it's size and inert, you'll probably still get a decent sounding product.

If you read thru Frank van Alstine's Audio Basics newletter written 25+ years ago (available as free resources on his site, avahifi.com), he instructs one on filling the inner cavity of a speaker with a 1/4" of Plast-i-Clay modelling clay with tremendous effect.  Today, some of the better designs employ thicker mdf or hdf (yesterday's designs used particleboard much of the time), or layered birch and/or Black Hole 5 or other damping products on the interior walls.

I've layered the inside of my last 3 (modestly priced) speakers with the Plast-i-Clay, added steel shot bags to the bottoms, added0.125" sheets of (amazing) EAR Isodamp to the (outer) rear and bottom panels and, on my fave speaker the Linaeum Tower speakers, even added somewhat controversial Tekna-Sonic speaker dampening aids (to substantial enough benefit).

Inert and damped cabinet walls is an amazingly positive piece of speaker design...and one in which you can do many things to improve upon the original, constrained budget that your speakers were originally designed to 

Regards, John

[Don_S]

http://www.cerioustechnologies.com/Speakers/m-speakers.html

Ballistic ceramic enclosures as well as liquid ceramic filled tubes.  Does that qualify as unique?

[mfsoa]

I use a modified knuckle test. Put your ear up to one side of the cabinet as you rap the other.

On my VR4JRs, they sound super inert and well braced with the standard rap, but with your ear up the cabinet they definitely "Toooonn"
for about a second.

I'm sure that some additional bracing/damping would, as WGH found, really improve the speaker.

[Daygloworange]

On my VR4JRs, they sound super inert and well braced with the standard rap, but with your ear up the cabinet they definitely "Toooonn"
for about a second.

I'm sure that some additional bracing/damping would, as WGH found, really improve the speaker.

Yes, both would aid in that. I did some tests with well braced enclosures using a (resonance control) foam product that contains a damping layer and found that it aided quite a bit in damping the ringing.

http://www.audiocircle.com/index.php?topic=32448.20

Some like Wilson use composite materials that they claim are less resonant than even very thick MDF panels.  It is hard to argue their point about other materials being potentially superior to MDF, however if you have checked their prices lately you definitely pay for these composites as well. 

It's most likely a phenolic based panel that they use. They name them product X and product Y. It's probably 2 different densities, and yes phenolic is very dense. Probably at least 3 times as dense as hardwood based MDF for any given thickness. I have some 19 mm thick 4'x8' sheets of phenolic and the stuff is extremely heavy, and very hard to machine. It's very expensive as well. It is no doubt a fantastic material to use based on it's density.

On a speaker the size of the Alexandria X-2's, the raw material costs alone for the enclosures would be several thousands of dollars. Machine time to process the panels would be several thousands of dollars, let alone assembly with special glues and fixtures, and the paint and finish process.

Cheers

[Wind Chaser]

Instead of using MDF, which is relatively inexpensive and easy to work with, I opted to do something radical… make the Jordan recommended 8 liter mini monitors out of stainless steel; a far more expensive and difficult task.

Why stainless steel? While it is possible to make a good enclosure out of MDF, stainless steel is vastly harder, stiffer and denser. In short the result is extremely low distortion, excellent bass and midrange tone with the absence of coloration.

I sold them last year to someone who much preferred them to his Rogers LS3/5a.

[Daygloworange]

Why stainless steel? While it is possible to make a good enclosure out of MDF, stainless steel is vastly harder, stiffer and denser. In short the result is extremely low distortion, excellent bass and midrange tone with the absence of coloration.

Given the thickness of the stainless used there, and depending if there was any crossbracing, they would in fact not be stiffer than 3/4 MDF, and would be prone to ringing. The real benefit in this design would be the curved sides add stiffness to the panel via preloading, and  the curved sides and rear of the enclosure on the backwave of the driver, also the minimal baffle diffraction outside of the enclosure at higher frequencies. But the high resonance of the sides would be detrimental unless the sides were damped with another material.

Cheers

[Wind Chaser]

Given the thickness of the stainless used there, and depending if there was any crossbracing, they would in fact not be stiffer than 3/4 MDF, and would be prone to ringing. The real benefit in this design would be the curved sides and rear of the enclosure on the backwave of the driver, also the minimal baffle diffraction outside of the enclosure at higher frequencies. But the high resonance of the sides would be detrimental unless the sides were damped with another material.

The interior was damped with 1/8" foam matting.  I beg to differ on 3/4" MDF being anywhere even remotely close as stiff as 10 gauge stainless steel.  And no, there isn’t any cross bracing.

[Daygloworange]

The interior was damped with 1/8" foam matting. 

Was the foam just cut and stuffed in there? If so, that's not doing anything for damping.

I beg to differ on 3/4" MDF being anywhere even remotely close as stiff as 10 gauge stainless steel.

Density and stiffness are 2 different things. Often people think that higher density automatically equals higher stiffness and strength, which it doesn't.

Buy a 4'x8' sheet of 10 gauge stainless steel, and buy a 4'x8' sheet of 3/4" MDF. Try bending both and get back to me on which one is stiffer. 



Cheers

[Wind Chaser]

Your assumptions are merely assumptions.  You’d have far more credibility and the benefit of knowing what you are talking about if you had built them out of both materials.

[Daygloworange]

Your assumptions are merely assumptions.  You’d have far more credibility and the benefit of knowing what you are talking about if you had built them out of both materials.

You're assuming they're assumptions. I work with metals quite often.

I have the benefit of knowing. Thanks. 

Get back to me on that test BTW.

Cheers

[Wind Chaser]

DGO,

That you "work with metals quite often" isn’t tantamount to fabricating speakers out of metal.  Make that same speaker out stainless steel and 1”MDF and compare the two.  Until then, whatever you think you might know pertaining to how they sound or what it takes to dampen them is neither scientific, nor objective or useful.

[JLM]

The geometry of enclosure can make huge differences.  A sphere is the most rigid shape, but again the back waves in a sealed sphere would reflect right back out through the cone or at least push back on the cone.  Both effects result in smearing.  A rectangular sealed box would be little better, but would possibly add vibrations from the less stiff flat walls.  Ported designs relieve more of the internal pressures, but are typically bigger (possible addition wall vibrations) and more importantly allows some of the out of phase back waves come directly out.

Front loaded horns are theoretically ideal, but have practical limits size/bass extension.  And because of the compression loading, the walls of front or rear loaded horns must be extremely stiff (most aren't, therefore the typical horn colorations).  Rear loaded horns also share that nasty out of phase back wave issue with ported designs.

Tuned quarter wave pipes and transmission lines offer the best solutions IMO.  The typical taper directs the backwave away from the driver(s), proper tuning keeps the back wave in phase, and the pressures are much lower than in horns.  Transmission lines tend to extend bass deeper than pipes.  These designs tend to be a bit more complex and larger, but not nearly as much as the naysayers would lead you to believe.