It seems that I can't find 3/4 '' (19 mm) MDF here, no wonder, is 16 mm ok for a 1mx20cmx20cm enclosure, with a 17 cm, 50 W driver? Theoretically it will go down to 30 Hz (-3dB). It won't be used at very large power, will it be rigid enough? Should I do bracing?
I could extend the question and ask what is the rule of thumb for choosing the thickness of the MDF based on enclosure parameters.
I could extend the question and ask what is the rule of thumb for choosing the thickness of the MDF based on enclosure parameters.
If you can buy/afford 19mm Baltic Birch, this would be the preferred material. If not, a double thickness 16mm cab will be slightly more rigid/damped than the 19mm BB. In either case, a single brace near the driver is sufficient.
If 16mm is used, then since the driver should ideally be ~35.92cm down from the top, a brace above and below the driver will provide sufficient rigidity for it. If the driver is placed near the top, then a brace below it and another one ~2/5 of the way down from it is sufficient.
GM
If 16mm is used, then since the driver should ideally be ~35.92cm down from the top, a brace above and below the driver will provide sufficient rigidity for it. If the driver is placed near the top, then a brace below it and another one ~2/5 of the way down from it is sufficient.
GM
GM,
I don't understand what you mean by "double thickness".
Since plain 19 mm MDF isn't available, so much for birch. At least I suppose, I'll ask around.
The driver will be placed near the top (about 25 cm).
And concerning the bracing itself, since I haven't given it too much thought until now, how should it be done? Is there a preferred way of doing it? How should the material be chipped out? Should the holes be round or square? Oh, I sound like a little boy boring his parents with too many questions, sorry
I don't understand what you mean by "double thickness".
Since plain 19 mm MDF isn't available, so much for birch. At least I suppose, I'll ask around.
The driver will be placed near the top (about 25 cm).
And concerning the bracing itself, since I haven't given it too much thought until now, how should it be done? Is there a preferred way of doing it? How should the material be chipped out? Should the holes be round or square? Oh, I sound like a little boy boring his parents with too many questions, sorry
Build the cab, then glue another layer of MDF on top of it. I recommend making the driver hole larger in the base cab, with the proper size cutout on the top layer, or vice versa if you want the driver flush or inset. Better still, make them the proper size and after the second layer is installed, bevel the inner 'baffle' out at ~45deg to allow the driver to 'breathe'.
For such a small cross sectional area (CSA), I would just use some relatively small diameter dowels or similar sized scrap hardwood to form a 'X' brace, glued or screwed together where they cross. Or you can make a 'shelf' brace cut out to mimic these in ~total open area, though using free form irregular shapes are preferred to reduce any standing waves of any amplitude from forming/summing as happens when four equal squares/rectangles or all the same size diameter holes are used. If other than a round material shape is used, then I recommend covering them with some damping material.
"I could extend the question and ask what is the rule of thumb for choosing the thickness of the MDF based on enclosure parameters."
I see I missed this Q on the first response and do not have time right now to go into all the considerations. Anyway, it deserves a thread of its own, though before posting one, please search the archives, I bet it has been covered more than once.
GM
For such a small cross sectional area (CSA), I would just use some relatively small diameter dowels or similar sized scrap hardwood to form a 'X' brace, glued or screwed together where they cross. Or you can make a 'shelf' brace cut out to mimic these in ~total open area, though using free form irregular shapes are preferred to reduce any standing waves of any amplitude from forming/summing as happens when four equal squares/rectangles or all the same size diameter holes are used. If other than a round material shape is used, then I recommend covering them with some damping material.
"I could extend the question and ask what is the rule of thumb for choosing the thickness of the MDF based on enclosure parameters."
I see I missed this Q on the first response and do not have time right now to go into all the considerations. Anyway, it deserves a thread of its own, though before posting one, please search the archives, I bet it has been covered more than once.
GM
OK, cleared the bracing issue, next question.
Here comes a translation problem. In Romanian they call this PAL (translation is something like wooden agglomerated board), but I'm not sure it is the same thing as plywood. And you can imagine that those who sell it don't know either. If PAL = plywood, it is strange, because MDF is supposed to be harder and heavier than plywood, but this PAL thing isn't. It is quite weak and light. I tried to break a 5 cm scrap and it broke in an instant without much resistance. It doesn't seem quite right for speaker building, doesn't it?
Since plywood is recommended over MDF (at least I think so, judging from what I have read here), hmm... Sorry, my question is: how can I tell if this PAL thing is the same with what you call in English plywood? How can I tell for sure?
Here comes a translation problem. In Romanian they call this PAL (translation is something like wooden agglomerated board), but I'm not sure it is the same thing as plywood. And you can imagine that those who sell it don't know either. If PAL = plywood, it is strange, because MDF is supposed to be harder and heavier than plywood, but this PAL thing isn't. It is quite weak and light. I tried to break a 5 cm scrap and it broke in an instant without much resistance. It doesn't seem quite right for speaker building, doesn't it?
Since plywood is recommended over MDF (at least I think so, judging from what I have read here), hmm... Sorry, my question is: how can I tell if this PAL thing is the same with what you call in English plywood? How can I tell for sure?
When viewing the edge/end of plywood you'll see distinct layers of material glued together. Standard plywood may have 6 layers, higher quality plywood will have 9 or more layers all made of real wood, not sawdust or chips. It is very difficult to break by just applying pressure and will splinter rather than break.
Plywood is preferred (at least by me) over MDF.
I'd tend to build a well braced single layer box over a non-braced double-layer box. bracing should be non-symetric and run the long direction (dividing the 20 cm panels into even narrower sub=panels (ideally not rectangular). If the bracing also ties the box walls togther that is good too.
dave
I'd tend to build a well braced single layer box over a non-braced double-layer box. bracing should be non-symetric and run the long direction (dividing the 20 cm panels into even narrower sub=panels (ideally not rectangular). If the bracing also ties the box walls togther that is good too.
dave
Re: brief translation
Placuta surpriza Deci de asta mi-a spus vecinul ca n-a auzit de MDF in viata lui. O sa reformulez intrebarea cu PFL...
Sorry about that, I said "that's why my neighbour hadn't heard about MDF in his life" (the local acronym seems to be PFL). Some use MDF, some PFL. OK, so that scrap wasn't plywood, neither MDF, it was some weak agglomerated (or is the right word sintered?) board.
OK, I'll search for plywood too.
Ioan said:
Placuta surpriza
Deci de asta mi-a spus vecinul ca n-a auzit de MDF in viata lui. O sa reformulez intrebarea cu PFL...Sorry about that, I said "that's why my neighbour hadn't heard about MDF in his life" (the local acronym seems to be PFL). Some use MDF, some PFL. OK, so that scrap wasn't plywood, neither MDF, it was some weak agglomerated (or is the right word sintered?) board.
OK, I'll search for plywood too.
Tomorrow I'll take my bike and pay a vist to all the lumber yards around. Now at least I can tell apart particle/chipboard from MDF! I'll be more confident when going into an argument with those know-it-all guys.
I'll ask for baltic birch plywood too. What minimum thickness should I look for?
I'll ask for baltic birch plywood too. What minimum thickness should I look for?
19mm is considered the acceptable minimum. The more plys, the better. In my area it's 11, though I can order 13 ply.
Normally, PB is limited to helping damp high Q systems, but material rigidity is a function of thickness, so once the MOE (modulus of elasticity) of the material and a reference is known, then an equivalent thickness can be calculated.
19mm (~0.75") 11ply BB has a MOE = ~1.8m psi and the local 0.75" PB (there's many grades) = ~1.9k psi, so for this particular PB to ~match the stiffness of the BB:
0.75*(1.8/0.19)^0.33 = ~1.58" (~40.28mm) thick.
GM
First of all, I found 18 mm MDF, and it's pretty cheap too! It's about 6 US dollars a square meter, just of curiousity, what quality do you get for this price in the US?
Second of all, no succes with baltic birch.
Third of all... Because I adhere to the theory that humans weren't meant primarily for cummunicating through words, I'd like to see some pictures of proper bracing I'm not into woodworking. Actually I won't touch any woodworking tool during the building of my speakers, I'll rely on more experienced people to do the cutting/assembling/finishing. At least veneering horrifies me from what I've heard about it, not to mention that I don't have/afford the tools. But I want to know how to instruct those who will build the cabs, since they don't have speaker building experience, they're just woodworkers. Now, more questions:
1. Is using a layer of 16 mm MDF for baffle reinforcement, coupled with solid MDF bracing worth it? I mean, from the point of view of the difference in audibility. Using two layers of 16 (or 18) mm MDF for all the walls will probably exceed by budget
2. I'm thinking about rounding the vertical front edges, or at least chamfering them. At what radius should they be rounded for it to worth the effort? And, is the difference between rounding and chamfering audible? Chamfering should be way easier, at least so I've been told.
3. Should I flush mount all the drivers or it is only important for the tweeter?
More question will come as I progress, probably
PS: this should be renamed into "the random cab building questions thread", I think many are disappointed with the newbie questions I keep asking
Second of all, no succes with baltic birch.
Third of all... Because I adhere to the theory that humans weren't meant primarily for cummunicating through words, I'd like to see some pictures of proper bracing
I'm not into woodworking. Actually I won't touch any woodworking tool during the building of my speakers, I'll rely on more experienced people to do the cutting/assembling/finishing. At least veneering horrifies me from what I've heard about it, not to mention that I don't have/afford the tools. But I want to know how to instruct those who will build the cabs, since they don't have speaker building experience, they're just woodworkers. Now, more questions:1. Is using a layer of 16 mm MDF for baffle reinforcement, coupled with solid MDF bracing worth it? I mean, from the point of view of the difference in audibility. Using two layers of 16 (or 18) mm MDF for all the walls will probably exceed by budget
2. I'm thinking about rounding the vertical front edges, or at least chamfering them. At what radius should they be rounded for it to worth the effort? And, is the difference between rounding and chamfering audible? Chamfering should be way easier, at least so I've been told.
3. Should I flush mount all the drivers or it is only important for the tweeter?
More question will come as I progress, probably
PS: this should be renamed into "the random cab building questions thread", I think many are disappointed with the newbie questions I keep asking
To make your job easier... just point to the picture you like most
http://txraves.org/~magnetik/diy/IMG_0433.jpg
http://www.lalena.com/audio/faq/build/bracing.gif
http://www.speakerbuilding.com/content/1044/05_bracing.jpg
A few more:
http://infocom.cqu.edu.au/Staff/Derek_Jones/Home_Page/Speakers/Images/Image47.jpg
http://home.insightbb.com/~stephenwmoore/Speakers/Previous_Speakers_files/image004.jpg
http://txraves.org/~magnetik/diy/IMG_0433.jpg
http://www.lalena.com/audio/faq/build/bracing.gif
http://www.speakerbuilding.com/content/1044/05_bracing.jpg
A few more:
http://infocom.cqu.edu.au/Staff/Derek_Jones/Home_Page/Speakers/Images/Image47.jpg
http://home.insightbb.com/~stephenwmoore/Speakers/Previous_Speakers_files/image004.jpg
mr_push_pull said:
Depends on the plant it comes from, but overall it is pretty high quality, with a density of ~50lbs/ft^3, MOE of ~527k psi, so it takes ~1.13" (~28.7mm) thickness to match 19mm BB ply.
What about appleply or void free marine grade?
Third of all... Because I adhere to the theory that humans weren't meant primarily for cummunicating through words, I'd like to see some pictures of proper bracing
I do not save links of internal construction pics of simple boxes, but I remember this one from the other day: http://home.comcast.net/~klone-audio/page30AS-15-1.html
1. Is using a layer of 16 mm MDF for baffle reinforcement, coupled with solid MDF bracing worth it? I mean, from the point of view of the difference in audibility. Using two layers of 16 (or 18) mm MDF for all the walls will probably exceed by budget
Bracing effectively with MDF requires too much work, adds too much weight, and takes up too much internal volume (see above link), so if void free ply is not available, I recommend using either hardwood stock, large dowels such as closet rod, or metal angles/flat iron.
That said, your high aspect ratio/small CSA cab's dims are such that MDF should be fine.
This depends on the driver used and baffle width. The wider the baffle, the less rounding over or chamfering edges affects performance. As it shrinks, the round over must become progressively larger in radius for max benefit, until it winds up being a ~hemispherical shape. Chamfering Vs round over follows the same pattern, though a series of flat cuts to approximate a round over works well enough. Obviously, the driver size affects this, as does its polar response, i.e. if it is radiating into half space or less, it is not diffracting off of any sharp corners.
Depends on the design of the components/baffle. Best to either flush them all, or none and damp the entire baffle, or do both by surface mounting all drivers and make a grill cover using the appropriate thickness base with close fitting/chamfered (if required) cutouts, cover it with thick felt, then stretch some acoustically transparent cloth over it. BTW, doing the latter negates the need for rounding over edges. If you choose this route, it is best to do all testing with them installed, otherwise the speaker will not sound quite right once they are installed and vice versa.
Frankly, I do not understand the preoccupation with exposed drivers. When properly done, grill covers acoustically correct many of the cab's acoustic problems. Must be a cost issue that the marketing dept. has put a bogus 'superior' performance spin on.
GM
Thanks GM, you've been most helpful, as always. To be frank, I doubt I'll find quality plywood, so I think I'll stick to MDF for my first DIY project. As my standards/bugdet will increase, I'll move to more "fancy" materials.
Now I gathered enough info to go and buy/cut the MDF with confidence. I don't know if you remember this, but it's you that recommended me the dimensions for these enclosures in an older thread, I played for a few days with MJK's sheets without decent results, and I couldn't improve on yours by 1%! Do you have a magical formula or something? You seem to be the reference on this forum when it comes to optimum enclosure dimensions, at least for transmission lines. The thing that surprised me the most is that I've obtained very good results (if not the best), when simulating this enclosure as a "normal" bass-reflex one in Unibox. So at lest in theory it should be a fail-safe cab.
Now I gathered enough info to go and buy/cut the MDF with confidence. I don't know if you remember this, but it's you that recommended me the dimensions for these enclosures in an older thread, I played for a few days with MJK's sheets without decent results, and I couldn't improve on yours by 1%! Do you have a magical formula or something? You seem to be the reference on this forum when it comes to optimum enclosure dimensions, at least for transmission lines. The thing that surprised me the most is that I've obtained very good results (if not the best), when simulating this enclosure as a "normal" bass-reflex one in Unibox. So at lest in theory it should be a fail-safe cab.
You are welcome! I am surprised that you can not get a wood product that is processed relatively nearby. I guess they can export all they can manufacture for a better price.
Sorry, but my short term memory is just that, a memory, so do not recall your design.
Yes, I use T/S formulas to get the Vb/Fb/vent size for whatever alignment (I normally post a max flat unless something else is spec'd) and why it models well in Unibox. The main advantages of the ML-TL IMO is using some 1/4WL action to reduce the vent size and better damp it. It also allows for more acoustic gain/lower Fb in oversize cabs before Fb peaking occurs, and once it occurs, you can damp it to suit, whereas in a BR all you can do is stuff it until it becomes somewhat aperiodic. Last, but not least, I started using them decades ago to reduce the 'footprint' of the typically big cabs of the day and to get the driver up closer to face/chest height since most folks I was building for either lived in a small apt. or tiny cottage.
No magic, just a couple of formulas I looked up at the Ga. Tech library back in the '60s. One after a tour of the Altec factory in Ca. and the other after a senior mechanical engr. where I worked at the time made a derogatory remark about how my abysmal lack of engineering knowledge on certain subjects would keep me from ever designing a good performing speaker except through trial and error. One calcs the optimum line length based on what's now called the driver's Qts and a 1/4WL of Fb, and the other for calculating the point along the line to mount the driver, and if need be, the vent if other than at the extreme bottom.
Anyway, good luck with them and let us know how they turn out and what, if any, tweaks were required to make them 'sing'.
GM
Sorry, but my short term memory is just that, a memory, so do not recall your design.
Yes, I use T/S formulas to get the Vb/Fb/vent size for whatever alignment (I normally post a max flat unless something else is spec'd) and why it models well in Unibox. The main advantages of the ML-TL IMO is using some 1/4WL action to reduce the vent size and better damp it. It also allows for more acoustic gain/lower Fb in oversize cabs before Fb peaking occurs, and once it occurs, you can damp it to suit, whereas in a BR all you can do is stuff it until it becomes somewhat aperiodic. Last, but not least, I started using them decades ago to reduce the 'footprint' of the typically big cabs of the day and to get the driver up closer to face/chest height since most folks I was building for either lived in a small apt. or tiny cottage.
No magic, just a couple of formulas I looked up at the Ga. Tech library back in the '60s. One after a tour of the Altec factory in Ca. and the other after a senior mechanical engr. where I worked at the time made a derogatory remark about how my abysmal lack of engineering knowledge on certain subjects would keep me from ever designing a good performing speaker except through trial and error. One calcs the optimum line length based on what's now called the driver's Qts and a 1/4WL of Fb, and the other for calculating the point along the line to mount the driver, and if need be, the vent if other than at the extreme bottom.
Anyway, good luck with them and let us know how they turn out and what, if any, tweaks were required to make them 'sing'.
GM
Grrr
Grrr. Bad news.
One of the baffles is ruined. I made the mistake of trusting one of the "craftsmen" at a woodworking shop to cut the holes for the drivers, the BR tube and the connector. The idiot tried to cut the MDF with that, I don't know the name, rectilinear-moving blade electrical saw. The holes look like they are cut with an axe. The errors are up to 5 mm, in one word, one of the baffles is ruined. At least he realized how bad it sucks at cutting MDF and he stopped.
Now, I'm asking myself, thousands of people can cut 1'' MDF in the garage and specialized woodworking shops can't cut 3/4''? How come? They seem to be used only with the crummiest particleboard that a 5 year old kid can break with one hand, and when they see MDF they go like "gee, this stuff is hard!"
Now, I'm asking you: please advise me, what major mistakes should I avoid next? Well, besides the obvious ones, like trusting idiots. What is the proper tool for cutting (chipping out would be more appropriate) MDF with precision? A picture would be nice, since I might run again into translation issues.
Please help save the enclosures
Grrr. Bad news.
One of the baffles is ruined. I made the mistake of trusting one of the "craftsmen" at a woodworking shop to cut the holes for the drivers, the BR tube and the connector. The idiot tried to cut the MDF with that, I don't know the name, rectilinear-moving blade electrical saw. The holes look like they are cut with an axe. The errors are up to 5 mm, in one word, one of the baffles is ruined. At least he realized how bad it sucks at cutting MDF and he stopped.
Now, I'm asking myself, thousands of people can cut 1'' MDF in the garage and specialized woodworking shops can't cut 3/4''? How come? They seem to be used only with the crummiest particleboard that a 5 year old kid can break with one hand, and when they see MDF they go like "gee, this stuff is hard!"
Now, I'm asking you: please advise me, what major mistakes should I avoid next? Well, besides the obvious ones, like trusting idiots. What is the proper tool for cutting (chipping out would be more appropriate) MDF with precision? A picture would be nice, since I might run again into translation issues.
Please help save the enclosures
You'll want to use a router and a circle jig. See http://www.diyaudio.com/forums/showthread.php?s=&threadid=35472&highlight= for a home made jig that should serve well if the shop you hire does not own one.
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