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Old 28th August 2008, 11:41 AM   #21
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While I stand behind my choice of MDF/cement board sandwich, I have been beaten into submission. I am now using plywood for my commercial speakers. It seems that the market equates MDF with "cheap" and is not interested the the real reasons for this choice of material.

Anyway, there is another argument going on: Should the speaker cabinet contribute to the speakers sound or should the cabinet be absolutely silent. My vote is for the latter. My logic is that a HiFi system should reproduce exactly what is on the recording.

I suppose that it is possible to build a cabinet that adds to the driver output equally at all frequencies, but I doubt that it will ever happen. Plywood and real wood cabinets that add to the driver output do so at high frequencies that produce "brightness". This synthetic brightness is perceived as "life". Have we had this discussion before?

I used to build cabinets out of a 3/4" MDF -- "Liquid Nails" -- 1/4" cement board sandwich. Any sound that escaped from the cabinet walls was inaudible at the SPL's common to single-driver speakers. I am now building cabinets out of 1" plywood. Again, any sound escaping from the cabinet walls is inaudible. I have played speakers made of the two materials side by side. I can here no difference. Mission accomplished.

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Old 28th August 2008, 09:34 PM   #22
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Quote:
Anyway, there is another argument going on: Should the speaker cabinet contribute to the speakers sound or should the cabinet be absolutely silent. My vote is for the latter. My logic is that a HiFi system should reproduce exactly what is on the recording.

I suppose that it is possible to build a cabinet that adds to the driver output equally at all frequencies, but I doubt that it will ever happen. Plywood and real wood cabinets that add to the driver output do so at high frequencies that produce "brightness". This synthetic brightness is perceived as "life". Have we had this discussion before?

Bob [/B]
Definitely see your point of view about controlling cabinet resonances. My personal thought is that I would like the cabinet to contribute to the sound, ONLY IF it contributes in a positive manner. (Just like any other component in the chain) If what some wood is producing is "brightness, but perceived as "life", that's okay if it provokes a positive response from the listener and is enjoyable to listen to. For me, eliciting an emotional response from the listener is more important than having a dead accuracy. Much the way a famous concert hall may actually acoustically enhance an orchestra sonically, compared to in the practice room or anechoic chamber. It may not be "accurate", but it still induces goose bumps and engages emotionally.
We all have different tastes in music and how we like it presented. Another good example is the 300B tube. Probably very far from accurate, but many people like them because it sounds "good" to them.
And how do we know when our system reproduces "exactly what was on the recording" unless we mastered the recording?
We've all probably heard our favorite recording played on various systems and it sounds different depending on the system, but also the room. How does one know when you've heard exactly what was on the recording?
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Old 29th August 2008, 06:17 AM   #23
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My goal is to make a cabinet that does not add anything of its own.

dave
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Old 29th August 2008, 08:12 AM   #24
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I'll have to say that I've heard speakers, such as the original Advent that seemed to add a certain life to the material. It may have simply been younger ears but it seemed that the cabinet...sang the material to me? Like it was an instrument... Maybe it was simply, as you say, it dissapeared and let the music come out. However you might as well say that you want the driver to not add anything either or the amplifier, or the microphone... it's all fine but what you really want is to have a realistic rendering of the music. If the cabinet has to work to do that then so be it.

Can you have a hardwood cabinet? A cabinet made of glass or metal or mdf? Do they all not add something or is it just that you have to pick the least intrusive material? Why not make a speaker out of foam rubber or out of a bed pillow?

Can you make a pillow like that? I guess that's a silly idea. How would you get to sleep? Ha ha ha.
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Old 29th August 2008, 11:13 AM   #25
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Originally posted by EmergencyDpt

Can you have a hardwood cabinet? A cabinet made of glass or metal or mdf? Do they all not add something or is it just that you have to pick the least intrusive material? Why not make a speaker out of foam rubber or out of a bed pillow?
Having spent a long time trying to quieten down my guitar amplifiers ear bursting volume when overdriven - I was just thinking exactly the same thing.

Maybe a lead lining backed up with something to give it some more rigidity. I believe professional sound proofing usually has barium doped rubber in it - since it's so dense. I have some of said sound proofing and it is indeed very nice! Albeit, a little pricey. It's impressive what you can achieve in terms of proofing given some thought and cash.

Personally, I prefer the clean approach to audio. If I want it to sound of something, I can add tone filters and other stuff to alter the sound. Getting rid of it is a much bigger pain.
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Old 29th August 2008, 12:06 PM   #26
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My goal is to make a cabinet that does not add anything of its own.
Same here. However a lot of people somehow ASSUME that MDF contributes least to the sound. Or at least less than plywood.

I've read somewhere that MDF is actually quite a "reflective" material. And therefore if a mdf baffle is not otherwise damped it will at certain frequencies be very "alive" and will reflect that back to the driver conus

I've read somewhere that this adds a certain muddling and or glare to the midrange (voices) especially and that is the reason some prefer something like plywood. This is ofcourse anecdotal. But OTOH I don't think anyone has gone and done scientific studies on the effect of mdf/plywood on sound.

But obviously there are many types of plywood available.

A simple test would be to shout at a piece of mdf from close range. I've done this and it comes back quite unpleasantly
I think partly because of the stiffness of mdf but also because it is almost perfectly smooth. Whereas plywood will have real wood grain and little pores and un-even-nes and even a sort of "softness"

For a subwoofer mdf might be more suitable. Or maybe plywood on the inside laminated with mdf for the outside is best for a normal speaker.

But I think it might be wrong to assume mdf should be the speaker baflle material of choice because it contributes LESS to the sound. It might actually contribute MORE to the sound and in a more negative way...because we are so sensitive to the midrange. Whereas I ASSUME plywood is worse at bass/low freqs.
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Old 29th August 2008, 02:26 PM   #27
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It's an interesting question. AFAIK, plywood (assuming BB here) is not inferior to MDF in the LF. I can see Bob's MDF / cement-board laminate being far superior to 3/4in MDF alone though.

GM & I were discussing this recently & he put me right on certain aspects -I'd noted that to ~match the stiffness of 3/4in BB ply you need ~ 1 1/4in - 1 1/2in of MDF. I'd also incorrectly suggested that this increased thickness would help push the panel's natural resonance downward, below the operating BW of the cabinet, but (correctly, thank goodness ) that some feel that the price would be too high due to the thicker panels retaining too much energy.

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FYI, stiffness (its MOE spec) goes up at the cube of thickness and while MDF has a fairly wide range of acceptability between particleboard and HDF, ~527,000 psi is the most published spec I've found along with 3/4" (18-19 mm) no void plywood at ~1,800,000 psi, so in theory we can get by with a bit less thickness for MDF, though of course when in doubt, thicker is preferred, with sheer weight being the limiting factor:

t2 = t1*(m1/m2)^0.33

Where:

t1 = existing thickness
t2 = new thickness
m1 = MOE of t1
m2 = MOE of t2

0.75*[(1800/527)^0.33] = ~1.125" thick

I didn't quickly find my materials density chart, but doubling mass (2x panel thickness) only lowers its Fs 0.707x, so it will take many inches thick MDF to lower a cab's Fs well below a LF cab's BW to damp out its harmonic structure, putting its density on a par with concrete.

With good reason since the lower the cab's material Fs, the greater its amplitude and the wider its resonant BW (lower Q), ergo it takes much more damping to push it below the estimated -35 dB required to ensure it can't be sensed by humans. If you stop short of this damping ideal, then the material's storage/release of energy can seem to 'suck the life' out of an ultra-wide BW driver as it alters the tonal balance and clarity similar to the way early first reflections comb filter with a speaker's response.

If starting with a much higher Fs material though, it can be pushed much higher with bracing and since stored energy falls at 1/f it requires much less damping to make the cab acoustically benign in an ultra-wide BW app, so the lower the cab's BW, the stiffer a material should be to push its Fs as high as practical, ergo from a technical POV MDF is best limited to either narrow BW cabs that can be damped enough without 'sucking the life' out of the driver's HF BW or HF BW only cabs.

Exceptions to this guideline are cabs designed to either lower a driver's effective Qts or damp spurious resonances as some 'fullrange' and horn designers have been known to do with varying degrees of success, but I don't recommend them for the casual DIYer.

As always though, YMMV.

GM
Of course, not all plys are equal; we were discussing BB at the time. There are plenty of other suitable types; maple / apple plys, marine (assuming it's void free) etc., but they'll need treating differently. In fact, I suspect that last is a major cause of problems -people forgetting to acount for the specific properties / requirements of their chosen build material & simply building a box in the same way, whether out of MDF, spruce ply, solid oak or whatever.

Moving away from MDF / plys, and working from the perspective that I don't want my cabinet panels to add any of their own contributions, I see no objections against good hardwoods from the POV of their resonant behaviour or stiffness, as, taking a quick glance around the 'net, there are plenty which have excellent properties in this regard, sometimes far in advance of man-made sheet materials. So a solid wood cabinet, although frequently associated with adding coloration does not necessarily have to do anything of the sort. TC made a point too about the importance of a wood's grain structure in its resonant behaviour which seems logical enough.
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Old 29th August 2008, 05:40 PM   #28
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Quote:
Originally posted by Bas Horneman
Same here. However a lot of people somehow ASSUME that MDF contributes least to the sound. Or at least less than plywood.

I've read somewhere that MDF is actually quite a "reflective" material. And therefore if a mdf baffle is not otherwise damped it will at certain frequencies be very "alive" and will reflect that back to the driver conus

I've read somewhere that this adds a certain muddling and or glare to the midrange (voices) especially and that is the reason some prefer something like plywood. This is ofcourse anecdotal. But OTOH I don't think anyone has gone and done scientific studies on the effect of mdf/plywood on sound.

But obviously there are many types of plywood available.
I have found MDF oozes a continuous low level grunge that reduces the downward dynamic range.

Due to its homogeneous nature MDF can also be quite transparent at some frequencies. It also needs to be sealed as it is not air tight.

Plywood does come in a huge number of varieties -- an even broader spread than the available qualities of MDF. Even sticking to one brand (BB in our case) you have variability. But just being plywood helps. Even MDF plywood -- Bill Perkins of PEARL was not happy with MDF so he came up with a self-made 7 layer MDF plywood and the new curved PE cabinets (& i understand some of the newest Polk) use a multilayer MDF plywood on the sides (6-1/8" layers for the PE cabs. This is quite an improvement on MDF alone.

dave
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Old 29th August 2008, 06:33 PM   #29
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Could you tell me what hardwoods are preferable? A friend of mine told me that his cabinet makers all use plywood because the solid wood will change dimentions over time. I'm torn between the desire for a real wood cabinet and longevity.

The lumber store near me has rows and rows of (expensive) hardwood boards. I spent two hours caressing them. I actuall bought sample peices to put in my room. I want to look at them every day. Bird's eye maple in addittion to the wonderful birds eyes, has a marvelous changing sheen like a gemstone when you turn it in the light. This property is shared by some of the other hardwoods but is totally lost when they are varnished,or otherwise finished.
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Old 29th August 2008, 07:39 PM   #30
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I have seen an interesting discussion about this that concluded and had some pictures explaining how the "best" was to use a dual layer.... something like MDF and plywood, I can't remember the ratio's... anyway the damping was supposed to take plase in the glue layer as a type of shearing force generating heat...
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