Cabinet structure MDF vs HDF

twinter

Member
2008-06-13 12:00 am
If you are building a moderate size two system, of say 12 liters or less, use any of the three. Their will be some differences, though not major. Brace well and apply some damping. If your are building a 3 way tower, then things change.

All three materials, HDF, MDF, and plywood (Baltic Birch) do not have significant internal damping, despite the anodotal comments on the internet stating otherwise. A damping factor of 0.1 (Q of 10) is consider good damping, with critical damping at a damping factor of 2.0 (Q of 0.5). Laminated bamboo appears to be stiffer and have greater internal damping, though be aware Ascend Audio mentioned concerns of warping with certain lamination configuration.

See the link for stiffness (Young's modulus of elasticity) and damping factors of some materials:

http://audioqualia2.webs.com/rawdata.htm

Some testing of different baffle materials:
HSI :: van der Hoff Sound Innovations :: Luidspreker specialist (requires translation)

How am I aware of these properties? I am currently in the process of building a three way tower speaker, approximately 41" tall. After years of reading numerous comments about MDF and Baltic birch plywood, I built the cabinet walls of 1" MDF, with a 2" thick front baffle (2 layers), with significant 3/4" Baltic birch plywood bracing. Using the low tech nuckel rap test, the enclosure seemed to ring like a bell at numerous points (a little exaggeration here). I borrowed an accelerometer and a computer with FFT analysis software (no CSD software), and found sharp resonances from 330 to 800 hertz. The Baltic Birch internal braces were even resonating at 400 Hz. This matches what others have posted on the internet for MDF and plywood baffles using Cumulative Spectral Decay (CSD) plots.

I applied epoxy resin with fiberglass and carbon fiber woven roving, epoxy resin with powdered limestone, and extensional damping material to the interior walls. These were efforts to further stiffen and damp the enclosure walls, with some improvement noted though not as much as desired. A good learning experience. Next time will be different.

DIY speaker building has progressed significantly in the last decade. Computer programs (and the associated equipment) such as LspCAD, SoundEasy, and others have allowed us to design crossovers with good amplitude and phase characteristics. In the last several years, harmonic order distortion profiles have be come available to assist in objective driver selection. Hopefully, in the future cabinet and baffle resonances & damping will be more objectively measured and discussed.
 
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Good information twinter, It's definitely a topic deserving of further attention.

Any idea how much of a reduction in resonant frequencies there could be had by doubling up on all sides and not just the front baffle? Assuming the two layers are closely bonded and free of any voids.

My thoughts for a project later this summer include a very large 3-way system and I've tentatively planned on using 3/4" Baltic birch for the exterior and bonding it to an interior layer of 1" MDF. This coupled with heavy corner bracing making the inside almost octagonal. A plus is that due to their size each cabinet will actually be two separate cabinets.
 
Laminated bamboo appears to be stiffer and have greater internal damping, though be aware Ascend Audio mentioned concerns of warping with certain lamination configuration.

We had some issues in an early build, but not since.We prefer using the stranded bamboo ply, it is much stiffer.

dave
 

twinter

Member
2008-06-13 12:00 am
Rob41, take a look at the constrained layer damping configuration, yielding very efficient damping. This is with stiff inner and outer layers sandwiching a 2mm to 4mm soft damping layer.

Extensional damping is much less effective, composed of a stiff layer and a thick damping layer. For effective extensional damping, the damping layer is 1/2 to 1 times the thickness of the stiff layer. Note the attaching adhesive in this case is an epoxy or similarly stiff glue, differing from the flexible adhesives for constrained layer damping.

More informational sites:

Soundown - peace and quiet for architectural, marine, & industrial applications
E-A-R Specialty Composites

The Noise Manual, 5th Edition, pages 305 - 314
The Noise Manual - Google Books
 
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How about the multi-core materials? Has anyone used the material with alternating mdf and ply cores?

We are experimenting with it. a recent build was with 3/4" MDF-ply-MDF with 1/2" solid cedar laminated to the outside. It worked well but we have not done a comparable enclosure to compare it with. 2nd from the right

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dave
 

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twinter

Member
2008-06-13 12:00 am
I would not recommend less stiffness, unless possibly for a mid-range enclosure. The stiffer panels resonate at high frequencies, though of decreased amplitude, and are usually easier more succeptible to damping, especially above 1000Hz.


From John Atkinson of Stereophile Magazine, Measuring Loudspeakers, Part 2, Page 7:

"However, looking at the behavior of the 300 or so loudspeakers that I have measured, several common factors emerge from the auditioning that correlate with the presence of strong cabinet resonances between 100Hz and 500Hz. (Remember that other "objective" factors will also contribute to the same subjective perceptions.) The clarity in the lower midrange can be disappointing. Tenor instruments like cello or trombone lack clarity or acquire a "woody" character. The bass can sound muddy, diffuse, one-note, blurred, or lacking in power, rather than tight, articulate, and extended, as it does in real life. Music can seem to drag, in rhythmic terms. Male voices can "boom" and female voices "hoot" at some frequencies and not others, with the result that the little inflections of tone that are characteristic of real voices become diluted. Centrally placed images, particularly of voices, can smear toward the speaker positions at some frequencies."


Measuring Loudspeakers, Part Two Page 6 | Stereophile.com


Other good links:
Identifying Legitimately High Fidelity Loudspeakers: Myths & Facts about Cabinets — Reviews and News from Audioholics
A Detailed Look at the Importance of Proper Loudspeaker Cabinet Bracing — Reviews and News from Audioholics
Lowering Mechanical Noise Floor in Speakers — Reviews and News from Audioholics
Lowering Mechanical Noise Floor in Speakers Pt 2 — Reviews and News from Audioholics
 
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