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Old 5th February 2011, 05:14 PM   #21
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Probably. According to Scott, this adds the final touch.
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Old 5th February 2011, 05:23 PM   #22
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yes it does.

So, I would personally go with ply. I don't have access to Baltic birch and Marine ply is very expensive...so I use Oak ply from Lowes. Sometimes I get lucky and get really good quality sheets with more than just 5 layers. But it depends on what they have handy. MDF is better for Multi speaker systems IMHO. Single drivers do not produce much resonance into the wood so ply is perfect, especially when bracing. Of course, it depends on the size of the driver. Having built the same system both in ply and MDF, the ply, to me, sounds warmer. It resonates differently than the MDF and thus produces a more softer quality to the sound. As you know, describing the sound you hear into "words" sometimes is very subjective and difficult to transpose from one individual to another, unless both individuals have some similar experiences. But nontheless...my last build contained BOTH ply and MDF.

MDF is a pain in the *** to work with...dust everywhere etc. What I did this last time around was use ply for the front, back, top and bottom of my cab...and MDF for the sides. I also braced the inside on all for planes (front, back, and sides...basically a "cross" with a frame around it so as to provide breathing space. I like this build MUCH better than either the full MDF of full ply builds.

Last edited by aptquark; 5th February 2011 at 05:26 PM.
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Old 5th February 2011, 05:46 PM   #23
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What is you theory behind the MDF sides? What was your bracing made out of. I can get high quality 7 layer ply in any species, but it is not without voids. From my understanding, this is the primary reason for using Marine ply or MDF
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Old 5th February 2011, 06:10 PM   #24
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Originally Posted by Scottmoose View Post
Well, a vented box tends to have a double-humped impedance curve, but that doesn't necessarily mandate Eq.

Oh hell. Generalising like made, BR, DBR & MLTLs should be much of a muchness in terms of presentation, but in all cases it depends on the alignment used. You pick whatever is most appropriate for the job in hand.
Dear Scottmoose:
Taking this opportunity on Alpair 10 driver, I have a doubt about the Alpair 12 Pensil box.
I see in the Pensil Alpair 12 plans the rectangular port use a 1.4 ratio, not the 1.618 Golden Ratio.

My question is the 1.4 ratio better than 1.618 on sound quality??

I wonder if for a 12 inches FR driver, in a big box, 45cm x 51cm x 118cm = 270 litres, what are the best ratio for this port? 1.4 or 1.6??

I apologize the out of topic question. Thanks for any help,
Gustavo
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Last edited by FullRangeMan; 5th February 2011 at 06:13 PM.
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Old 5th February 2011, 06:22 PM   #25
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If I may interject, the advantages of ply over MDF is personal preference. Baltic birch is the king of ply due to its perfect layering and no surprise gaps like some other plys. Its expensive as hell though. If you are not too picky about looks..then other plys will work just as nicely. MDF will have less resonance than plys, and because of this will tend to make the sound more "hard" vs "soft" that the plys tend to exemplify. Thats why my last build was made with a combination of ply AND MDF.
Wrong. MDF is not less resonant than a decent grade of plywood, it is arguably more so, as it is not as stiff, and rather denser. This tends to put its resonant BW into the cabinet's own operating band, where there is the maximum amount of energy available to excite it. Substantially more damping is thus necessary to push the resonances to a sufficiently low level to be inaudible (roughly a -35dB threshold).

All materials have different MOE specs. & therefore need to be used somewhat differently from each other. As a general ROT though, the lower the cabinet's BW stretches, the stiffer the build material should be, to push panel resonance up, above this region, to a point where the panels need little, if any, damping.

Last edited by Scottmoose; 5th February 2011 at 06:27 PM.
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Old 5th February 2011, 06:24 PM   #26
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Originally Posted by Scottmoose View Post
Wrong. MDF is not less resonant than a decent grade of plywood, it is arguably more so, as it is not as stiff, and rather denser. This tends to put its resonant BW into the cabinet's own operating band, where there is the maximum amount of energy available to excite it. Substantially more damping is thus necessary to push the resonances to a sufficiently low level to be inaudible (roughly a -35dB threshold).

All panels have different MOE specs. & therefore need to be used somewhat differently; that goes for different plywoods as well as solid wood, plastics, ceramics, glass, metal or anything else.
Does this mean i was correct about your endorsement of Marine grade plywood over MDF
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Old 5th February 2011, 06:43 PM   #27
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Originally Posted by Scottmoose View Post
Wrong. MDF is not less resonant than a decent grade of plywood, it is arguably more so, as it is not as stiff, and rather denser. This tends to put its resonant BW into the cabinet's own operating band, where there is the maximum amount of energy available to excite it. Substantially more damping is thus necessary to push the resonances to a sufficiently low level to be inaudible (roughly a -35dB threshold).

All materials have different MOE specs. & therefore need to be used somewhat differently from each other. As a general ROT though, the lower the cabinet's BW stretches, the stiffer the build material should be, to push panel resonance up, above this region, to a point where the panels need little, if any, damping.
So stiffness of a material is more determinate of resonance characteristics than density?
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Old 5th February 2011, 06:47 PM   #28
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Originally Posted by buzzforb View Post
What is you theory behind the MDF sides? What was your bracing made out of. I can get high quality 7 layer ply in any species, but it is not without voids. From my understanding, this is the primary reason for using Marine ply or MDF
My choice for different materials is mainly to provide different resonant characteristics for the cabinet. I wanted more ply overall then MDF in the build...so I chose the sides to be the only part of the cab to utilize MDF. The bracing is the same 3/4 inch ply I used for the cab. Also, the oak ply I used had no observable voids when cut. Thats not to say that it was void free. But possibly very minimal to having any. The whole "void" thing is blow out of proportion IMO. Unless you have a really crappy quality ply...few voids will NOT make a difference in performance. At this point in our hobby...paranoia and perfectionism take the forefront for some of us rather than reality.

Last edited by aptquark; 5th February 2011 at 06:53 PM.
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Old 5th February 2011, 06:56 PM   #29
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Quote:
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If I may interject, the advantages of ply over MDF is personal preference. Baltic birch is the king of ply due to its perfect layering and no surprise gaps like some other plys. Its expensive as hell though. If you are not too picky about looks..then other plys will work just as nicely. MDF will have less resonance than plys, and because of this will tend to make the sound more "hard" vs "soft" that the plys tend to exemplify. Thats why my last build was made with a combination of ply AND MDF.
I can refute every one of these points except possibly the 1st. Construction design can take advantage of materials strengths & weaknesses. The designs i do assume plywood and do not work as well with MDF.

The following applies to boxes made with flat sheet material, and ugnores any gains made by constrained layers of multiple panels.

1/ Baltic Birch is not the king of plywoods. It is a very good utility grade material, and ignoring the fact that we could get MDF scraps for free, not much more $$ than MDF (at least here).

You are looking for a material with high stiffness. Of 2 materials of the same stiffness, the ligher one is usually preferred.

Good plywood (typically many plies, and void free) is much stiffer than MDF. Because of the layers -- each layer boundary has an impedance change -- damping is also higher in a plywood (a good example here is the higher performance of the 6 layer MDF plywood in the parts express curved cabinets -- althou cheap MDF, the layers make its sonic performance better than a similar thickness of monolithic MDF.

The best plywood we have used is block core/stranded outer layer, bamboo ply -- an order of magnitude pricier than ply.

Not all plywoods (or MDF for that matter) are created equal

2/ all materials resonate. With good plywood you can make these resonances higher in frequency & higher Q.

The latter has been shown to be less audible (see Toole). The former thou is VERY powerful. The amount of energy available to excite a resonance is inversly proportional to the square of the frequency. Further, one can argue, given certain assumptions, that the actual likelihood of exciting a resonance is inversly proportional to the 4th power of frequency (think about that, the implications are huge)

It is much easier to make a cabinet with higher frequency, higher Q resonances.

Let me put it this way: MDF tends to ouze out time smeared low level wide band resonances that obscure low level information, thus impairing a loudspeakers downward dynamic range.

Gievn that one of the hallmarks of the Mark Audio drivers is their very high downward dynamic range. MDF boxes obscure that capability. They can still sound good, but you won't be giving them the opportunity to perform at their best.

My personal opinion is that MDF is not suitable as a primary material for making speakers.

And if you need to do a "cheap" build, chipboard is better than MDF.

dave
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Old 5th February 2011, 07:14 PM   #30
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Dave,
My local hardwood store offers both 7 ply domestic lumber plywood and two different marine grade plywoods, birch and meranti. From what i have read, meranti is the higher quality choice. The BB they offer is not the standard 5' x 5', but a marine specific ply. It is only $20 cheaper than meranti, so i am not considering it, but the 7ply domestic plywood is a very nice product and is half the price of the meranti. If you think the meranti will yield superior results, i will use it. I am somewhat dependent on the opinions of others since this my first build. Thank you everyone for the input.
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