The "Real" Story, MDF vs Ply vs Particleboard (1 Viewer)

[Dan M~]

There is a current thread discussing this issue, but I started a new thread to assure visability of this often asked question.
In the February 2002 AudioXPress Magazine, author Jim Moriyasu has written the article " Panal Damping Studies: Reducing Loudspeaker Enclosure Vibrations". Mr. Moriyasu created a test fixture to compare panel materials such as MDF, Plywood (7-ply Birch) and particleboard. He also test the effects of Acouta-Stuf, Black Hole 5, Deflex, sand filled panels, lead sheet and many other combonations of woods and plys.
So if you are interested in this topic, get a copy of February AudioXPress.
AudioXPress published by Audio Amateur Inc. Peterborough, NH. Cutomer Service (888)924-9465 or (603)924-9464. E-Mail [email protected] or online at
http://www.audioXpress.com
Enjoy and lets discuss what you think of the study.
-Dan

 

[Jack Gilvey]

I don't suppose you could paraphrase the conclusions, Dan, I'm very interested in this subject. Think I'm gonna try some Baltic birch in my next box, lovely stuff, although the sheets I looked at where no-void 13-ply or something like that, not regular 7-ply.

 

[Randy G]

Oh DO keep us in suspense until we run to the store to peek at the last paragraph of the article.

 

[Brian Bunge]

Randy,

I think the author is the same person that did the extensive study on how enclosure shape and baffle thickness affected midrange performance a year or so ago. If I remember correctly, a perfect cube wasn't nearly as bad as he'd expected it to be!

If this is the same author, it should be an interesting read! I got my issue yesterday. I'll have to read the article this weekend!

Brian

 

[Dan M~]

I have only read through the article once and need to re-read it to make my own conclusions (There are 78 waterfall and/or SPL graphs included in the study that need to be studied ). I hope that can happen this weekend. In the mean time,from the conclusion...
"This study suggests a well-built enclosure should incorporate CLD (Constrained layer dampening... edit DM) or multiple-thickness panels, extensive bracing, and some method of isolating the woofer from the enclosure."
"The study,while finished for the present, resembles an unfinished book."
From my quick read of the article, here are the high points:
Comparison of MDF vs Particleboard vs 7 Ply Birch:
"These results suggest none of the three materials tested to be significantly better then the other."
Multiple layers of material, i.e. 2 layers of MDF:
"... the first two modes are at the same frequency but are attenuated by 3-5dB compared to a single layer of MDF."
"...show further gains by tripling the thickness: the first two modes remain the same frequency but are reduced by 5-6dB."
Appling damping material to surface, or extensional damping i.e. Black Hole 5, Deflex, lead, sand-filled panel, etc.:
"...suggest that most methods of extensional damping are capable of reducing secondary resonances but are ineffective when dealing with primary resonance mode."
CLD, using many differing combos:
"In all cases, however, decay times of resonances remain stubbornly high."
Bracing:
"... bracing can reduce resonance modes by up to 5- 10dB."
"Decay times... remain unchanged, however."
"Thus, a well designed enclosure should feature extensive bracing."
Woofer isolation (i.e. de-couple the woofer from the baffle to reduce mechanical transfer):
"... it appears that a gasket made of some resilient material and some thickness can reduce these troublesome vibrations."
He also looks into Wellnuts, the rubber lined threaded inserts to help decouple the driver. They look effective.
The author spent over a year doing the test and there is a tremedous amount of data to go through.
From my first reading it looks like the bracing and thick panels did the most to reduce box vibration. Decoupling the driver and extensional damping or CLD may not be worth the efforts/expense.
Too bad Sonotube couldn't be worked into the testing!!
-Dan

 

[Wes Nance]

This interests me, as I'm going to be building a sub that's supposed to be double layer 3/4" MDF, which for a 3ft^3 sub will undoubtably be very heavy.

I'm intrigued with using some other material that is possibly thinner/lighter, yet would offer similar dampening characteristics to MDF without some of the bulk.

Would the baltic birch plywood be a suitable substitute? How does it's price compare to MDF? (I have no experience here)

Also, with finishing details, would you be able to successfully use round over bits on the edges/corners on plywood and use wood veneer?

What do you think?

Wes

 

[Jack Gilvey]

Thanks you for taking the time to do that, Dan. I wish a no-void ply had been included, marine or BB.

Also, with finishing details, would you be able to successfully use round over bits on the edges/corners on plywood and use wood veneer?

It can be a pain, but I've rounded over ply. Practice on scrap, first, trust me.

 

[Dan M~]

BUMP
Anyone else read this article yet?? I would like to hear your opinions.
-Dan

 

[MichaelGomez]

I don't know what they had to say but there is some good info on the www.carsound.com There, Richard Clark, showed that the difference in wood doesn't matter. As long as it is braced so that it doesn't flex. The tinly little holes don't matter. You can take a drill and put a hole in the baffle and not alter the sound.

 

[Doug Fraser]

I read the artical last night. It is long and detailed.

The take aways that I got are as follows....

The thicker wood is better - i.e. multi layers of MDF

Isolating the woofer from the front baffle is good. - using either rubber (or similar elastomer) between the speaker and baffle. Even better is having a baffle that isolates the mount for the speaker from the regular baffle.

Traditional thinking using constrained layer construction has little value - i.e. sand between layers of wood.

There seems to be some merit in using a glue that stays flexible when cured to join the 2 layers of material to increase the thickness of the material used to make the speaker.

Internal bracing is very very good.

Based on this artical for my next speaker projects I will consider two 3/4" layers of MDF, lots of internal shelf bracing and I will investigate isolating the driver from the baffle and North Creeks glue.

On an associated issue of types of wood here is my experience using MDF vs hardwood (Cherry).

I posted the following on another thread (sorry for the repeat here).

--------------------------

WRT to using MDF or hardwood, here is what I have found.

I built bookshelf speakers using 3/4" Cherry and 3/4" MDF. The cherry units where made for the L, Centre, R and Surrounds. The MDF ones where to be mounted between ceiling joists for the rear channels and no one would see them, thus the use of the MDF.

The Cherry units use lock miter joints, the MDF units biscuit joints. Other than the wood and construction tequniques they use the same drivers, same crossover, same stuffing, same grills, same terminal cups, same internal wiring etc.

I have done considerable critical AB listening (with the speakers beside each other, before mounting them in the appropriate location for HT use) and have concluded that there is no audible difference.

From a wood working perspective MDF does not crack, expand, shrink whereas, depending on humidity variation, Cherry could. This may lead to problems in the future. Cherry is very nice looking, exudes quality while MDF, is well, MDF.

Cherry is expensive, MDF is relatively inexpensive.

Since these were bookshelf speakers they is not that much energy being pumped into the boxes from the speakers. I have set my speakers to small on my Integra 9.1 receiver, therefore frequencies are rolled off at 80 Hz (THX spec). What I am trying to say is that there is no 15" woofer moving large amounts of air and perhaps causing the case to flex and/or resonate. Also, the dimensions of the case are small so there is no large unsupported panel areas that can flex and/or resonate easily.

My conclusion is that if you want your bookshelf speakers to be a piece of fine furniture and are willing to put in the extra effort in constructing them then use hardwood, otherwise use MDF. With my bookshelf speakers there is no audible reason to use cherry or other hardwood just esthetics.

----------------

Regards,

Doug

 

[MichaelGomez]

I have never built a home speaker before so I am curious. Why use 2 layers of 3/4" MDF and bracing? I have on the other hand build car sub boxes. Here we tend to use 1 layer of 3/4" MDF (sometimes smaller if we want to be lighter). I am using 2 12" Rockford Fosgate subs getting 500 RMS a piece via a RF Power 1000a2. When I had it metered I was hitting 141.5 dB at the windshield. So I am assuming that there is a lot of pressure in the box (it's sealed with one dividing brace). So back to the question, what kind of home speakers require bracing in addition to 2 layers of MDF. Doesn't that add a lot of weight as well as expense that doesn't need to be there?

Please educate me because I am thinking of entering the world of DIY speakers. Sounds like fun and I already enjoy doing the car.

Mike

 

[Doug Fraser]

Michael:

You should get a copy of the magazine article referenced above to appreciate the complexity of this topic.

Basically, there are excitation modes (1st order, 2nd order, etc vibrations) that are set up in speaker cabinet walls. The purpose of the article was to identify these modes and to see if there was a way to reduce or stop them. Of course the premise is that these excitation modes have an effect on the sound.

The conclusions from the article are summarized above.

It was clear that the thicker the material (i.e. 2 sheets of 3/4" MDF glued together) and significant bracing reduced the vibration in the cabinet walls and thus...improved sound quality.

Doug