Well, the time is approaching that I'll need to think about the bass enclosures for my 3 way project. You can see the top section thread here:
http://www.diyaudio.com/forums/showthread.php?t=139174&page=2
http://img38.imageshack.us/img38/2685/3dspeakersdarkfronts.jpg/
I have some pretty clear ideas on it myself, but I'm sure there are refinements that can be made.
The driver is Hi-Vi M8a and the enclosure will be sealed. With a final volume ~36L, this gives a Q of 0.62, so slightly on the large side. I believe this gives a more relaxed and potentially more dynamic sound than Q ~0.7 or more. Sealed was chosen for future integration with a subwoofer system. It should also work best in this small room up until that point. Does sealed usually need a small hole drilled to account for atmospheric pressure changes and the like? What would be a good size if required?
The woofer plays to 820Hz, 24db crossover. I want the sound to be very neutral, non-resonant; a pretty standard goal here. I plan an enclosure with 18mm ply and 9mm MDF laminated together as this should help minimise panel resonances. I also with to use acoustic foam inside the enclosure as well as a lot of wadding material:
http://www.studiospares.com/Acoustic-Tiles/Acoustic-Control-100mm-Pyramid-Tile-Blue/invt/465036
This is a thick foam, and as far as I can figure, only a thick high density foam will have much influence over the lower frequencies. Are there any potential problems with using such a thick foam though? Will it disturb the frequency response? There would be foam left over to go elsewhere in the cabinet too, not shown. I'd perhaps consider some thinner foam for the sides around the woofer?
Here are my CAD drawings:
I have also included "spokes" between the side panels, alongside the typical holed braces. These will be drilled into the 18mm ply and glued into place. I'm not certain they'd do much though, they only tie the walls together, mabye an "anchor" point is needed? At the bottom are some drilled solid wood battons for strength, I may use more of these throughout.
The front baffle will have the top half of ~30mm hardwood backed to 9mm MDF and the lower half of 9mm MDF with 18mm ply and 12mm MDF again (or similar).
I basically want as much rigidity as I can get with these 27mm walls and without loosing too much internal volume! Any tips or tricks welcome!
http://www.diyaudio.com/forums/showthread.php?t=139174&page=2
http://img38.imageshack.us/img38/2685/3dspeakersdarkfronts.jpg/
An externally hosted image should be here but it was not working when we last tested it.
I have some pretty clear ideas on it myself, but I'm sure there are refinements that can be made.
The driver is Hi-Vi M8a and the enclosure will be sealed. With a final volume ~36L, this gives a Q of 0.62, so slightly on the large side. I believe this gives a more relaxed and potentially more dynamic sound than Q ~0.7 or more. Sealed was chosen for future integration with a subwoofer system. It should also work best in this small room up until that point. Does sealed usually need a small hole drilled to account for atmospheric pressure changes and the like? What would be a good size if required?
The woofer plays to 820Hz, 24db crossover. I want the sound to be very neutral, non-resonant; a pretty standard goal here. I plan an enclosure with 18mm ply and 9mm MDF laminated together as this should help minimise panel resonances. I also with to use acoustic foam inside the enclosure as well as a lot of wadding material:
http://www.studiospares.com/Acoustic-Tiles/Acoustic-Control-100mm-Pyramid-Tile-Blue/invt/465036
This is a thick foam, and as far as I can figure, only a thick high density foam will have much influence over the lower frequencies. Are there any potential problems with using such a thick foam though? Will it disturb the frequency response? There would be foam left over to go elsewhere in the cabinet too, not shown. I'd perhaps consider some thinner foam for the sides around the woofer?
Here are my CAD drawings:
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
I have also included "spokes" between the side panels, alongside the typical holed braces. These will be drilled into the 18mm ply and glued into place. I'm not certain they'd do much though, they only tie the walls together, mabye an "anchor" point is needed? At the bottom are some drilled solid wood battons for strength, I may use more of these throughout.
The front baffle will have the top half of ~30mm hardwood backed to 9mm MDF and the lower half of 9mm MDF with 18mm ply and 12mm MDF again (or similar).
I basically want as much rigidity as I can get with these 27mm walls and without loosing too much internal volume! Any tips or tricks welcome!
Last edited:
The bass section plays up to 820Hz, that's actually a lot of midrange!
Strong cabinet walls etc do help a lot, but I always feel (from my own works and others I've listened to) the shape of the box plays an even bigger role -- especially in the mid to midbass range.
From the design shown above, it looks quite some effort to build (already). So, why not put some 'unparallel walls' elements into the design? I think unparallel walls help a lot on suppressing the box sound. Not less than the structure and proportions.
Also, that will blend nicer to the mid-high section by some degree of coherence of the forms.
ps. I don't mean mimic the shape of mid-high section. You got a lot of options in making unparallel wall: slant front or back, tapered sides (front to rear), or even tapered downward to a tip at the bottom...
Strong cabinet walls etc do help a lot, but I always feel (from my own works and others I've listened to) the shape of the box plays an even bigger role -- especially in the mid to midbass range.
From the design shown above, it looks quite some effort to build (already). So, why not put some 'unparallel walls' elements into the design? I think unparallel walls help a lot on suppressing the box sound. Not less than the structure and proportions.
Also, that will blend nicer to the mid-high section by some degree of coherence of the forms.
ps. I don't mean mimic the shape of mid-high section. You got a lot of options in making unparallel wall: slant front or back, tapered sides (front to rear), or even tapered downward to a tip at the bottom...
Thanks for the replies!
I intend to fill basically all of the enclosure with BAF wadding type material (mabye lamswool near the driver rear if worth the extra expense). This should actually increase the effective volume too, which is good. I can put the remaining thick foam at the bottom if that's a good location for it.
I had thought about sloping walls. I think it'd be simplest to add them inside the box form, realising this does effectively "waste" volume but it'll be easier for me to do. I could do a few different things here. I could add a gentle tapering back wall profile (sloping down to a smaller area at the bottom), which I think is the best as it effectively acts like a closed end, tapering TL, too short for bass but enough to "suck out" some midrange once heavily stuffed.
Alternatively, I can add a sort of triangle form into the bottom section, almost creating two short TLs. I'd ensure they had non-equal ratios. If I really needed the volume I could even drill holes into the bottom of this triangle insert to reclaim internal volume without much negative effect?
As another possibility, simply adding profiled blocks of wood inside to break up the parallel sides. This is easy to do, but I understand they have to be quite large to have much effect. I actually have some suitable ones which are offcuts, big trinagles of pine with wavy edges. Again, I may be able to drill out a lot of the material to reclaim volume?
I intend to fill basically all of the enclosure with BAF wadding type material (mabye lamswool near the driver rear if worth the extra expense). This should actually increase the effective volume too, which is good. I can put the remaining thick foam at the bottom if that's a good location for it.
I had thought about sloping walls. I think it'd be simplest to add them inside the box form, realising this does effectively "waste" volume but it'll be easier for me to do. I could do a few different things here. I could add a gentle tapering back wall profile (sloping down to a smaller area at the bottom), which I think is the best as it effectively acts like a closed end, tapering TL, too short for bass but enough to "suck out" some midrange once heavily stuffed.
Alternatively, I can add a sort of triangle form into the bottom section, almost creating two short TLs. I'd ensure they had non-equal ratios. If I really needed the volume I could even drill holes into the bottom of this triangle insert to reclaim internal volume without much negative effect?
As another possibility, simply adding profiled blocks of wood inside to break up the parallel sides. This is easy to do, but I understand they have to be quite large to have much effect. I actually have some suitable ones which are offcuts, big trinagles of pine with wavy edges. Again, I may be able to drill out a lot of the material to reclaim volume?
Ah, yes.
A rectangular box is much easier to build than an irregular one. Stuffing with closed-cell type of foam materials should work very well in forming "unparallel inner walls". Many kinds of packaging materials would do. And they are very easy to be moved in and out for adjusting volume if needed.
A rectangular box is much easier to build than an irregular one. Stuffing with closed-cell type of foam materials should work very well in forming "unparallel inner walls". Many kinds of packaging materials would do. And they are very easy to be moved in and out for adjusting volume if needed.
I would leave out the 'spokes' as they look like trouble to mount, may ring like musical instrument strings, and can only act as, um, spokes, ie only really have strength in tension and not in compression.
I'm sure you've taken care to space the internal bracing to avoid creating wall section spans of equal dimensions?
I also wouldn't bother gently sloping the walls, as the needed differences in distances are too large to implement without eating up the volume. But perhaps a 'reflector' oriented at 45º right behind the driver in the upper corner could be useful?
I'm sure you've taken care to space the internal bracing to avoid creating wall section spans of equal dimensions?
I also wouldn't bother gently sloping the walls, as the needed differences in distances are too large to implement without eating up the volume. But perhaps a 'reflector' oriented at 45º right behind the driver in the upper corner could be useful?
Thanks for the input again!
This seems like a good solution, but I'm not certain if there are "side effects". Adding in a sloping board inside, but not joining to the bottom of the cabinet, hence retaining nearly the same internal volume. It seems it would form a 1-0 tapering closed end TL of quite short length, but will the box overall still act just like a sealed? Impedance variations shouldn't be a big concern since the driver is actively powered.
Also adds further bracing to the box. The remaining thick foam can be placed at the end of the line or on the side walls (which are still parallel); I still think it's wise to have most of it directly behind the driver though. The cabinet would still be heavily filled with acoustic wadding too.
Any thoughts on this?
I will be spacing the braces unequally. Is there a technique using prime numbers here too?
This seems like a good solution, but I'm not certain if there are "side effects". Adding in a sloping board inside, but not joining to the bottom of the cabinet, hence retaining nearly the same internal volume. It seems it would form a 1-0 tapering closed end TL of quite short length, but will the box overall still act just like a sealed? Impedance variations shouldn't be a big concern since the driver is actively powered.
An externally hosted image should be here but it was not working when we last tested it.
Also adds further bracing to the box. The remaining thick foam can be placed at the end of the line or on the side walls (which are still parallel); I still think it's wise to have most of it directly behind the driver though. The cabinet would still be heavily filled with acoustic wadding too.
Any thoughts on this?
I will be spacing the braces unequally. Is there a technique using prime numbers here too?
Like you said, that internal wall creates a pipe, so don't do that unless you want a TL. What I was suggesting earlier with the 'reflector' was just a smallish piece (that ends maybe as far down as the center of the driver?) in the upper rear corner (where you have the foam) to help stop sound from bouncing straight back out through the cone. This piece would also have a bracing effect.
I originally wanted a TL in fact, but the ideal model turned out very long indeed with a driver with this fs. Making a much shorter one like that drawn is a problem because it'll cause impedance variations up in the high bass ranges I guess? It a definate no-no?
I could easily incorporate the reflector you suggested, my intention was that the thick foam would absorb sound immediately behind the driver and the rest would travel through enough wadding to decrease it's energy suitably. Though mabye the reflector reduces energy reflected from immediately below the foam covered area by redirecting it to the far end of the cabinet? It'll be a squeeze to fit it in with the 100mm foam on it's face as well, but I'll look into it; perhaps profiling the foam where it encroaches on the drivers rear side.
Any other suggestions welcome! I may get back with a model including the reflector to get an idea of how it sits in
I could easily incorporate the reflector you suggested, my intention was that the thick foam would absorb sound immediately behind the driver and the rest would travel through enough wadding to decrease it's energy suitably. Though mabye the reflector reduces energy reflected from immediately below the foam covered area by redirecting it to the far end of the cabinet? It'll be a squeeze to fit it in with the 100mm foam on it's face as well, but I'll look into it; perhaps profiling the foam where it encroaches on the drivers rear side.
Any other suggestions welcome! I may get back with a model including the reflector to get an idea of how it sits in
Space my braces!
Update for you
I've had the stock for a while, all cut at shop (done a pretty good job too), been working a few days but it's only just starting to get interesting
9mm ply laminated to 18mm MDF, the back and side are done. After doing it the resonance I hear when knocking it is shorter and higher pitched than just the 18mm MDF. What you'd expect I guess. The ply is pretty nasty exterior stuff, but it's got 5 plys at only 9mm thick and is suprisingly heavy so it could be worse.
So I'm now looking to start getting the braces in. I need guidance with the spacing between them! With the total height of 852mm, there are 3 braces plus a top and bottom part leaving 762mm. Divided equally the space between each part is 190.5mm. Obviously I want these distances to be non-equal and not related. What do you suggest? I have heard of the ratio 1:1.68 but don't know exactly how to use it
There will also be wooden beams on the unbraced areas and mabye corner blocks too. I may add in the reflector at the top of the cabinet, I have some board at the correct width as offcuts, just need to bevel it.
Update for you
I've had the stock for a while, all cut at shop (done a pretty good job too), been working a few days but it's only just starting to get interesting
9mm ply laminated to 18mm MDF, the back and side are done. After doing it the resonance I hear when knocking it is shorter and higher pitched than just the 18mm MDF. What you'd expect I guess. The ply is pretty nasty exterior stuff, but it's got 5 plys at only 9mm thick and is suprisingly heavy so it could be worse.
An externally hosted image should be here but it was not working when we last tested it.
So I'm now looking to start getting the braces in. I need guidance with the spacing between them! With the total height of 852mm, there are 3 braces plus a top and bottom part leaving 762mm. Divided equally the space between each part is 190.5mm. Obviously I want these distances to be non-equal and not related. What do you suggest? I have heard of the ratio 1:1.68 but don't know exactly how to use it
There will also be wooden beams on the unbraced areas and mabye corner blocks too. I may add in the reflector at the top of the cabinet, I have some board at the correct width as offcuts, just need to bevel it.
Thanks!
I won't be putting in the internal baffle as it'll create a sort of TL which might have a negative effect. May do the angled board behind the driver though
I figure I shouldn't put any brace at 190mm spacing as it's the same as the width at the back. I reckon I'll put that first one in at 180mm or so and figure from there.
I won't be putting in the internal baffle as it'll create a sort of TL which might have a negative effect. May do the angled board behind the driver though
I figure I shouldn't put any brace at 190mm spacing as it's the same as the width at the back. I reckon I'll put that first one in at 180mm or so and figure from there.
Good work Dr.EM! 
Nice project
If I may suggest a much better stuffing material for bass cabs (I have found) is rockwool insulation. It kills standing waves. I have found BAF to be completely ineffective for stuffing.
Also, vertical holey braces (rather than the shelf type) would provide better overall bracing and simplify construction slightly.
I too would avoid the spokes.
Nice project If I may suggest a much better stuffing material for bass cabs (I have found) is rockwool insulation. It kills standing waves. I have found BAF to be completely ineffective for stuffing.
Also, vertical holey braces (rather than the shelf type) would provide better overall bracing and simplify construction slightly.
I too would avoid the spokes.
Thanks MJL21193!
I've got some spacings that should work. They arn't mathmatically related to each other or the dimensions of width and depth so should all resonate differently. I actually tried clamping in the braces at these positions and it seemed to be the case. First brace from bottom is at 210mm, the ones in the middle are closer spaced and a larger spacing again at the top for the driver; the reflector should aid rigidity here.
I'll look into alternative wadding! The rockwool is fibreglass housing insulation isn't it? I guess it's quite solid and inflexible? I planned a nearly complete fill either way, mabye a combination will work well.
I have my shelf braces all cut (not drilled yet) so am really stuck with them. I see what you mean though, vertical braces will leave narrow but long areas unsupported which probably do have a harder time trying to resonante. Here's what I have though, I had planned this for a while. Photo before clamping on the brace (which is drying now):
12x32mm wooden battons. A knock test shows an improvement with these added already so it should be good. Spokes arn't going in.
I've got some spacings that should work. They arn't mathmatically related to each other or the dimensions of width and depth so should all resonate differently. I actually tried clamping in the braces at these positions and it seemed to be the case. First brace from bottom is at 210mm, the ones in the middle are closer spaced and a larger spacing again at the top for the driver; the reflector should aid rigidity here.
I'll look into alternative wadding! The rockwool is fibreglass housing insulation isn't it? I guess it's quite solid and inflexible? I planned a nearly complete fill either way, mabye a combination will work well.
I have my shelf braces all cut (not drilled yet) so am really stuck with them. I see what you mean though, vertical braces will leave narrow but long areas unsupported which probably do have a harder time trying to resonante. Here's what I have though, I had planned this for a while. Photo before clamping on the brace (which is drying now):
An externally hosted image should be here but it was not working when we last tested it.
12x32mm wooden battons. A knock test shows an improvement with these added already so it should be good. Spokes arn't going in.
Bit of an update and question. Installing the last brace, here it is just resting in place:
Nearest to you in this picture^^
and from the other end to show overall build:
Before I fit this final brace (then onto top, deflector and other side!) I wonder if it's worth rounding over the bigger holes radius, as it'll be right below the driver and could cause reflection issues? I'd rather not as it'll make a mess and add delays but if it's truly going to make an improvement I'll hold up until I can do it
An externally hosted image should be here but it was not working when we last tested it.
Nearest to you in this picture^^
and from the other end to show overall build:
An externally hosted image should be here but it was not working when we last tested it.
Before I fit this final brace (then onto top, deflector and other side!) I wonder if it's worth rounding over the bigger holes radius, as it'll be right below the driver and could cause reflection issues? I'd rather not as it'll make a mess and add delays but if it's truly going to make an improvement I'll hold up until I can do it
Ah, so that's the main reason for it? I guess if you were adjusting the acoustic materials inside and reaching through braces to get at them. I intend to apply the fill before closing the front, it won't be possible to reach the bottom through only the driver hole, in fact the middle brace may be impossible to reach through too with the biggest hole only 64mm
I guess I'll fit it in place as it is then, then to work on the deflector. I may try and do that from solid wood if I have enough
I guess I'll fit it in place as it is then, then to work on the deflector. I may try and do that from solid wood if I have enough
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