Here are the step by step plans for my Concrete Subwoofer. I have not named this particular design. It has been years since I have built one, so I will do my best to answer any questions. Sorry if there are any mistakes in these directions.
BUILDING A CONCRETE SUBWOOFER
MOLD PREPARATION
1) Tape some wood pieces varying from between 10 and 20 square inches and up to 9 inches long and 4 inches wide on two opposing sides inside a 1' * 1' * 1' cardboard box ( ' means "foot", " means "inches"). Pack the box full of foam shipping peanuts. Glue the top shut.
2) Make 3/4" thick circular spacers with a diameter 1/8" greater than the cut-out size of the 8" woofers. These can either be cut out of wood or fashioned from cardboard concrete pillar molds.
3) Make 3/8" thick plywood circles that are 8 1/8" in diameter. Set a speaker on it and mark the hole locations. Drill 1/8" diameter holes at an outward slanting angle of about 30 degrees at each speaker mount location.
4) Join the pieces made in Steps 2 and 3 above with the smaller diameter centered on the larger. Be sure that the holes in the wider, thinner piece radiate outward from front to rear, front being the wider side.
5) Set the packed cardboard box on a 16" square piece of plywood with minimum thickness of 1/2". 3/4" thickness is best. Set temporary spacers under the box to raise it about 1" for molding the thickness of the concrete on the bottom. Arrange 4 more equally sized and approximately square boards about 15 to 16" long by 14" wide upright on the bottom around the cardboard box so that the volume contained inside is comprised of a 14" wide by 14" wide by 14" high cube. These boards form a spiral around the inner volume. Complete the outer mold with a top board like the bottom one.
Top view of boards in offset (spiral) pattern:
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6) Temporarily join the spiral boards together with several 2" drywall screws so that the inner pieces are held in place by friction as the foam pellets in the cardboard box push out on the speaker hole spacers. Adjust the spacers and make sure everything is centered. Trace around the 8 1/8 circles with a pencil so that their outlines are left on the inside surfaces of the outer mold boards. Use a permanent marker to mark the spiral boards 1 through 4 and also bottom and top. These labels need to go on the outside surfaces so that concrete residue does not conceal them when/if the mold is used to form later enclosures. Draw marker lines from the end of one board past the edge to the end of the adjoining board to use as later alignment references. Mark them so that each of the 4 edges is marked at a different height. Also mark the edges at the bottoms of the four side boards with the bottom board at different locations for each one. Repeat for the top board at the tops of the side boards
7) Drill 1/4" holes in the top corners of the two outer mold boards which do not contain the speaker spacers. They will hold 3" long # 10 drywall screws wrapped with tape which will penetrate all the way though to the cardboard box inside, creating holes in the concrete for mounting the carrying handles 2" from the top corners of the enclosure.
8) Drill a 3/8" hole in one of speaker face mold boards toward one lower corner about one inch from the wide part of the speaker hole spacer. This hole is for the speaker wire to exit the enclosure. Remove the screws holding the spiral boards together.
9) Align each spacer with the circle drawn on the inside of the corresponding mold board. Temporarily join them with two 1 1/2" drywall screws. Drill the 1/8" holes all the way through from the spacers out through the outer mold boards. Also drill 6 more 1/8" grill-mount holes 1/2" from the each spacer, 60 degrees apart, surrounding the spacer. Once the holes are completed, remove the screws, tip the spacers down from the outer mold boards and mark one spacer as "A" and the corresponding mold as "A" very close to it so that the markings line up when the boards are placed flat together. Mark the other spacer and mold board as "B". This step will allow 2 1/2" drywall screws wrapped with a few layers of tape to go all the way through to form holes for speaker mounting once the poured concrete cures.
10) Coat the inner mold surfaces lightly with baby oil, or other clean mineral oil. Otherwise the molds will not come apart when the concrete cures. Do not use any other type of oil. You may wish to wrap the speaker hole spacers with masking tape first to protect them from damage so that they can be reused.
11) Reassemble the spiral boards with the cardboard box and spacers in proper position. Be sure the reference marks line up for all boards. Use 4 screws per side. Do not use the temporary spacers under the cardboard box this time.
12) Now the cardboard box, spacers, and the two prepared outer mold boards must be screwed together. Drill pilot holes for the screws for joining those three items, stopping at the wooden pieces taped inside the cardboard box. About 6 holes should enable enough screws to penetrate well into some of the inner wood pieces to give a good hold.
13) Screw the bottom board on using 3 screws per edge. Make sure the spaces inside the mold are even before driving the screws through the side boards.
14) Now make 8 temporary dividers to enable the concrete to fill only half of the box at a time. Attach wires to them with short drywall screws. Orient the wires to face into the bottom half of the box. The wires will permit the dividers to be pulled out easily when filling the bottom half of the box. Attach the dividers to the sides of the outer mold boards with drywall screws.
15) Insert tape-wrapped screw spacers. There should already be holes for sliding 2" #8 screws into for speaker mounting and speaker grills, 3" #10 for the carrying handles on each side, and a large diameter one for the speaker wire. They should be long enough to penetrate all the way through to the cardboard box where there they should be held in place.
CASTING THE CABINETS
16) Mix water with about 60 pounds of cement/sand concrete mix. The consistency should be like thick oatmeal. Pour the mix into the top half, and beat it in with a small board to remove air pockets. Do not beat so hard as to smash the inner cardboard mold box. Pack the concrete in all the way to the surface, scrape across the top with a straight board to make sure the surface is level with the ends of the side boards and that there are no voids in the concrete. Line up the reference marks for the top and screw it on with 3 screws per side.
17) Turn the whole assembly over and remove the bottom. Remove the dividers! Fill with concrete and pack in place by beating with a small board. Again, don't strike the concrete too hard. Scrape across with a straight board to assure levelness. Place the top mold board on the four sides, lining up the reference marks. Fasten it with 12 drywall screws.
HANDLES
18) After the concrete has cured for several days, remove the mold, first removing all the straw-sheathed screws used to keep screw holes open. Cut 12 plywood squares 1 1/2" on each side and 1/2" thick. Glue 8 of them together to make 2 pairs of double thickness handle spacers. Drill 1/16" holes through the middles of all of these spacers, the 4 single thickness ones and the 4 double thickness ones.
19) Make one handle cross bar of 1" by 2" furring strip size lumber of length about 4" longer than the spaces between the handle mount holes. Align it evenly over the holes on one side of the box and poke a drywall screw through each hole to make an indentation on the crossbar. Drill holes through the indentations. Glue one thin spacer on each end of the crossbar, but both on the same side. Glue one double thickness spacer on each end also, but on the opposite side from the thin ones. Make sure that all the holes line up and that everything is square before setting clamps to hold them while the glue sets for a few minutes..
20) Repeat step 19 but on the other side of the enclosure to make the other handle.
21) Cut a furring strip length about as long as a handle. Place a handle on top of it to determine where a screw hole should be. Mark it on one end with an indentation from a screw. Drill a 1/16" pilot hole at the indentation. Later, this hole will catch the screw sticking through when the handle is mounted. Glue a two inch long piece of 2" by 2" behind the hole so that the hole is in the middle. Glue another one at the other end and on the same side as the 1st. Now one mounting board is complete.
22) Repeat step 21 to make the other mounting board.
23) Push a 8" drywall screw through one handle end and into one of the holes on the speaker enclosure. Push a screw in the other side of the handle and into the opposite enclosure hole to hold the handle level. Hold a mounting board inside the box and find one screw point with the shallow hole. Next, when approximately holding the mounting board level, drive the screw in at the pilot hole. Watch your hand! Now drive in the other screw.
24) Repeat step 23 for the handle on the other side
BUILDING A CONCRETE SUBWOOFER
MOLD PREPARATION
1) Tape some wood pieces varying from between 10 and 20 square inches and up to 9 inches long and 4 inches wide on two opposing sides inside a 1' * 1' * 1' cardboard box ( ' means "foot", " means "inches"). Pack the box full of foam shipping peanuts. Glue the top shut.
2) Make 3/4" thick circular spacers with a diameter 1/8" greater than the cut-out size of the 8" woofers. These can either be cut out of wood or fashioned from cardboard concrete pillar molds.
3) Make 3/8" thick plywood circles that are 8 1/8" in diameter. Set a speaker on it and mark the hole locations. Drill 1/8" diameter holes at an outward slanting angle of about 30 degrees at each speaker mount location.
4) Join the pieces made in Steps 2 and 3 above with the smaller diameter centered on the larger. Be sure that the holes in the wider, thinner piece radiate outward from front to rear, front being the wider side.
5) Set the packed cardboard box on a 16" square piece of plywood with minimum thickness of 1/2". 3/4" thickness is best. Set temporary spacers under the box to raise it about 1" for molding the thickness of the concrete on the bottom. Arrange 4 more equally sized and approximately square boards about 15 to 16" long by 14" wide upright on the bottom around the cardboard box so that the volume contained inside is comprised of a 14" wide by 14" wide by 14" high cube. These boards form a spiral around the inner volume. Complete the outer mold with a top board like the bottom one.
Top view of boards in offset (spiral) pattern:
|--
--|
6) Temporarily join the spiral boards together with several 2" drywall screws so that the inner pieces are held in place by friction as the foam pellets in the cardboard box push out on the speaker hole spacers. Adjust the spacers and make sure everything is centered. Trace around the 8 1/8 circles with a pencil so that their outlines are left on the inside surfaces of the outer mold boards. Use a permanent marker to mark the spiral boards 1 through 4 and also bottom and top. These labels need to go on the outside surfaces so that concrete residue does not conceal them when/if the mold is used to form later enclosures. Draw marker lines from the end of one board past the edge to the end of the adjoining board to use as later alignment references. Mark them so that each of the 4 edges is marked at a different height. Also mark the edges at the bottoms of the four side boards with the bottom board at different locations for each one. Repeat for the top board at the tops of the side boards
7) Drill 1/4" holes in the top corners of the two outer mold boards which do not contain the speaker spacers. They will hold 3" long # 10 drywall screws wrapped with tape which will penetrate all the way though to the cardboard box inside, creating holes in the concrete for mounting the carrying handles 2" from the top corners of the enclosure.
8) Drill a 3/8" hole in one of speaker face mold boards toward one lower corner about one inch from the wide part of the speaker hole spacer. This hole is for the speaker wire to exit the enclosure. Remove the screws holding the spiral boards together.
9) Align each spacer with the circle drawn on the inside of the corresponding mold board. Temporarily join them with two 1 1/2" drywall screws. Drill the 1/8" holes all the way through from the spacers out through the outer mold boards. Also drill 6 more 1/8" grill-mount holes 1/2" from the each spacer, 60 degrees apart, surrounding the spacer. Once the holes are completed, remove the screws, tip the spacers down from the outer mold boards and mark one spacer as "A" and the corresponding mold as "A" very close to it so that the markings line up when the boards are placed flat together. Mark the other spacer and mold board as "B". This step will allow 2 1/2" drywall screws wrapped with a few layers of tape to go all the way through to form holes for speaker mounting once the poured concrete cures.
10) Coat the inner mold surfaces lightly with baby oil, or other clean mineral oil. Otherwise the molds will not come apart when the concrete cures. Do not use any other type of oil. You may wish to wrap the speaker hole spacers with masking tape first to protect them from damage so that they can be reused.
11) Reassemble the spiral boards with the cardboard box and spacers in proper position. Be sure the reference marks line up for all boards. Use 4 screws per side. Do not use the temporary spacers under the cardboard box this time.
12) Now the cardboard box, spacers, and the two prepared outer mold boards must be screwed together. Drill pilot holes for the screws for joining those three items, stopping at the wooden pieces taped inside the cardboard box. About 6 holes should enable enough screws to penetrate well into some of the inner wood pieces to give a good hold.
13) Screw the bottom board on using 3 screws per edge. Make sure the spaces inside the mold are even before driving the screws through the side boards.
14) Now make 8 temporary dividers to enable the concrete to fill only half of the box at a time. Attach wires to them with short drywall screws. Orient the wires to face into the bottom half of the box. The wires will permit the dividers to be pulled out easily when filling the bottom half of the box. Attach the dividers to the sides of the outer mold boards with drywall screws.
15) Insert tape-wrapped screw spacers. There should already be holes for sliding 2" #8 screws into for speaker mounting and speaker grills, 3" #10 for the carrying handles on each side, and a large diameter one for the speaker wire. They should be long enough to penetrate all the way through to the cardboard box where there they should be held in place.
CASTING THE CABINETS
16) Mix water with about 60 pounds of cement/sand concrete mix. The consistency should be like thick oatmeal. Pour the mix into the top half, and beat it in with a small board to remove air pockets. Do not beat so hard as to smash the inner cardboard mold box. Pack the concrete in all the way to the surface, scrape across the top with a straight board to make sure the surface is level with the ends of the side boards and that there are no voids in the concrete. Line up the reference marks for the top and screw it on with 3 screws per side.
17) Turn the whole assembly over and remove the bottom. Remove the dividers! Fill with concrete and pack in place by beating with a small board. Again, don't strike the concrete too hard. Scrape across with a straight board to assure levelness. Place the top mold board on the four sides, lining up the reference marks. Fasten it with 12 drywall screws.
HANDLES
18) After the concrete has cured for several days, remove the mold, first removing all the straw-sheathed screws used to keep screw holes open. Cut 12 plywood squares 1 1/2" on each side and 1/2" thick. Glue 8 of them together to make 2 pairs of double thickness handle spacers. Drill 1/16" holes through the middles of all of these spacers, the 4 single thickness ones and the 4 double thickness ones.
19) Make one handle cross bar of 1" by 2" furring strip size lumber of length about 4" longer than the spaces between the handle mount holes. Align it evenly over the holes on one side of the box and poke a drywall screw through each hole to make an indentation on the crossbar. Drill holes through the indentations. Glue one thin spacer on each end of the crossbar, but both on the same side. Glue one double thickness spacer on each end also, but on the opposite side from the thin ones. Make sure that all the holes line up and that everything is square before setting clamps to hold them while the glue sets for a few minutes..
20) Repeat step 19 but on the other side of the enclosure to make the other handle.
21) Cut a furring strip length about as long as a handle. Place a handle on top of it to determine where a screw hole should be. Mark it on one end with an indentation from a screw. Drill a 1/16" pilot hole at the indentation. Later, this hole will catch the screw sticking through when the handle is mounted. Glue a two inch long piece of 2" by 2" behind the hole so that the hole is in the middle. Glue another one at the other end and on the same side as the 1st. Now one mounting board is complete.
22) Repeat step 21 to make the other mounting board.
23) Push a 8" drywall screw through one handle end and into one of the holes on the speaker enclosure. Push a screw in the other side of the handle and into the opposite enclosure hole to hold the handle level. Hold a mounting board inside the box and find one screw point with the shallow hole. Next, when approximately holding the mounting board level, drive the screw in at the pilot hole. Watch your hand! Now drive in the other screw.
24) Repeat step 23 for the handle on the other side
continued
INSERTING SPEAKER WIRES
25) Feed 2' lengths of 16 AWG wire into the speaker wire exit hole.
26) Remove the polyester stuffing from a new bed pillow and fill the inside of the enclosure completely, packing it in.
27) Drive twigs into the speaker mounting holes and let them stick out a little. Small honey suckle bush branches work well. Form a gasket in the recessed rim on both sides out of butyl rubber caulk. Break off the twigs at the level of the concrete. Place cling wrap over each hole and cut out the hole with a pair of scissors. Place each speaker in the hole and lightly drive # 6 screws into the twigs.
28) After the caulk cures, remove each speaker to connect each speaker wire. Attach each wire. Reinstall each speaker, tightening the screws just snug--not too tightly.
GRILLS (SPEAKER COVERS)
29) Drive twigs into the 12 grill mounting holes.
30) Place grills over the holes and hold them in place with clips. Use #6 1" drywall screws to fasten the clips.
INSERTING SPEAKER WIRES
25) Feed 2' lengths of 16 AWG wire into the speaker wire exit hole.
26) Remove the polyester stuffing from a new bed pillow and fill the inside of the enclosure completely, packing it in.
27) Drive twigs into the speaker mounting holes and let them stick out a little. Small honey suckle bush branches work well. Form a gasket in the recessed rim on both sides out of butyl rubber caulk. Break off the twigs at the level of the concrete. Place cling wrap over each hole and cut out the hole with a pair of scissors. Place each speaker in the hole and lightly drive # 6 screws into the twigs.
28) After the caulk cures, remove each speaker to connect each speaker wire. Attach each wire. Reinstall each speaker, tightening the screws just snug--not too tightly.
GRILLS (SPEAKER COVERS)
29) Drive twigs into the 12 grill mounting holes.
30) Place grills over the holes and hold them in place with clips. Use #6 1" drywall screws to fasten the clips.
this thread made me think, wouldn't be easier to use two sonotubes, one ,say 2o " set inside another,say 22" evenly spaced and pour the concrete inbetween them .cap with MDF on both ends after it cures, mount the driver on one end and the vent(if used)on the other . after all tht is what these tubes are designed for?
It would be a lot easier. You could even set the inside tube a little short length-wise so that one end is also solid concrete. The drawback is that you no longer have a square box to set flatly and solidly on the floor. It also sacrifices the completely 100% solid concrete structure, though with only one side wooden the effect may be very close.
You could do that. Or....
B&W loudspeakers have done a lot of research in to the effects of cabinet material on speaker response, especially regarding the effects of stored energy and panel modes. I'm sure your'e all familiar with young's modulus, which is a measurement of stiffness and is the first thing to look at when assesing a material for the walls of an enclosure. This modulus shows us that concrete is an extremely stiff material when compared to wood or board and, at first, seems the obvious choice when designing an ultra-rigid enclosure but you have to step back and analyse what the point is of all this rigidity. When a driver moves inwards, it pressurises the cabinet and the cabinet will expand minuteley and after, when the diver moves back out, the cabinet contracts again but not instantly. The box stores some of the energy from being pushed out and when it's not being pushed by the driver any more, it releases this energy as sound. The less rigid the box is, the more energy it stores and, consequently, the louder this sound is and the longer it goes on for. The overall effect on the sound is that the bass sounds "boxy". It will sound loose and wallowy with no definition. In contrast, a tight, rigid box will sound more defined. When the bass stops, the sound stops dead with no ringing or over hang. In general, a more rigid box will sound better in every aspect when compared to a less rigid one. The point of B&W's experiments were to ascertain which method of increasing the rigidity gave the best results. The first being to increase the rigidity of the enclosure walls, i.e: making the box out of concrete and the second being to increase the rigidity of the box itself by using reinforcement i.e: building the box out of 18mm mdf and using further panels the same size with holes cut in slightly spaced apart, running up and down the cabinet so as to create a honeycomb effect. They call this Matrix(tm) bracing and use it in most of their expensive speakers such as the N801 monitor. To understand why they chose such a complicated bracing system, you have to look at whats going on with a panel when it's being excited by a driver. A totally un-braced panel will wobble around more or less in sympathy with the driver, acting kind of like a terrible ABR or port. If it was braced with a battern, supported by a another panel like the opposite wall of the box, it would only really stiffen the panel in the region closest to the end of the batten, where it joins with the wood and the energy would avoid the stiff part of the panel and defract around it and the overall loss in stored energy would be minimal. You could brace a box with loads of battens and, allthough it would be physically very strong, because we are dealing with vibration as opossed to just stress, it still wouldn't be very much stiffer than the unbraced version. A batten is only supported at each end and so has nothing stopping it moving side to side and the join has a very small suface area. A panel on the other hand, is supported (preferably glued and screwed) on all four of it's sides and offers much more support but is still capable of lateral wobble and so we use another panel to brace that and another to brace that e.t.c. until we end up with a grid of panels (matrix) inside the box. The end result is a structre whereby every point is supporting and being supported by every other point. It is because crystals such as diamond have a molecular stucture with the same properties as this (every point supporting every other) that makes them so rock 'ard. Anyway, B&W did some comparison tests between the concrete box and the matrix box, to see which one sounded better. The boxes were of the same size with same thickness of material (18mm). Each box was placed face down in to a huge box of sand, so as to absorb all the energy from the front and a microphone was placed above the back panel, to record the sound output from the box itself. The results were incredible. They found to start with that with your average hifi cabinet, up to 20-30 % of the sound you were hearing was being made by the box itself, instead of the driver, but with regards to the comparison test, the matrix box was found to store ten times less energy than the same size concrete box. In other words, it sounded ten times better. The amount of time and money that goes in to build a matrix cab is much more than the concrete box but the matrix has every other advantage. Using concrete is abit like cracking a nut with a sledgehammer. There's much easier and better ways to make wicked subs than to resort to such extreme materials. Using a complete matrix is a bit over the top but the basic priciple of the bracing is the most important thing. If your not going for a full matrix and just using some panels to stiffen the box then avoid symetry when placing the panels. The more panels you use, the more you will have to increase the box size to make up for loss of volume but this is only a small figure. Many manafucturers use things like 22mm birch ply for their bass cabs but I use 18mm mdf, it's stiffer than normal ply but not birch coz birch ply is like bloody titanium and blunts all your tools and gives you cramps and aches and weighs a ton and costs a fotune (but other than that, it's great) with a bracing system like described and my cabs pee all over professional kit when it comes to sound. Another BIG advantage to a properly ridid box is the surprising increase in efficency. When the driver isn't wasting all that electricity on vibrating the box, it uses it to make sound and the difference is shocking. There are many ways you can brace a box but remember that the point is that the smaller the panel, the more rigid it is and by bracing the centre of a panel with the end of another panel, you effectively turn that in to two panels, each halve the size. A batten support does'nt do this. Use panels to reduce the surace area of other panels and arrange them in such a way so that they are all bracing eachother but try to avoid to much obvious symetry.
My 18" subs are bloody heavy (not like concrete though) but are so rigid that if you touch the sides when they are on full whack, you can't feel any vibration. Minimal loss, maximum sound. Remember: PA companies are marketing a product and sound quality is not their primary concern. Don't be temted to copy what they do because you think it's the proper way to do it because you are spoilt with PA. Big amps, big drivers=loud music. With hifi, your'e limited to small boxes and have to get the most possible out of them which is why all the really cool advancements in speaker technology, that actually make things louder and sound better, come from the hifi world. PA just use horns (yuk). If you apply this technology to PA the result is another level of quality when compared to Turbopants or some other macho brand. Dig in to hifi technology and you can build subs that make a mockery of professional kit. The biggest thing that stands out with a really rigid box (with a quality driver of course) is the content. There is so much going on in the world of bass that we don't normally hear from so-called "subs" (subsonic bass means below hearing, sub is not just another word for bass) that it's a whole new world once you start to play tunes through a proper sub. Concrete subs would look really cool though...
B&W loudspeakers have done a lot of research in to the effects of cabinet material on speaker response, especially regarding the effects of stored energy and panel modes. I'm sure your'e all familiar with young's modulus, which is a measurement of stiffness and is the first thing to look at when assesing a material for the walls of an enclosure. This modulus shows us that concrete is an extremely stiff material when compared to wood or board and, at first, seems the obvious choice when designing an ultra-rigid enclosure but you have to step back and analyse what the point is of all this rigidity. When a driver moves inwards, it pressurises the cabinet and the cabinet will expand minuteley and after, when the diver moves back out, the cabinet contracts again but not instantly. The box stores some of the energy from being pushed out and when it's not being pushed by the driver any more, it releases this energy as sound. The less rigid the box is, the more energy it stores and, consequently, the louder this sound is and the longer it goes on for. The overall effect on the sound is that the bass sounds "boxy". It will sound loose and wallowy with no definition. In contrast, a tight, rigid box will sound more defined. When the bass stops, the sound stops dead with no ringing or over hang. In general, a more rigid box will sound better in every aspect when compared to a less rigid one. The point of B&W's experiments were to ascertain which method of increasing the rigidity gave the best results. The first being to increase the rigidity of the enclosure walls, i.e: making the box out of concrete and the second being to increase the rigidity of the box itself by using reinforcement i.e: building the box out of 18mm mdf and using further panels the same size with holes cut in slightly spaced apart, running up and down the cabinet so as to create a honeycomb effect. They call this Matrix(tm) bracing and use it in most of their expensive speakers such as the N801 monitor. To understand why they chose such a complicated bracing system, you have to look at whats going on with a panel when it's being excited by a driver. A totally un-braced panel will wobble around more or less in sympathy with the driver, acting kind of like a terrible ABR or port. If it was braced with a battern, supported by a another panel like the opposite wall of the box, it would only really stiffen the panel in the region closest to the end of the batten, where it joins with the wood and the energy would avoid the stiff part of the panel and defract around it and the overall loss in stored energy would be minimal. You could brace a box with loads of battens and, allthough it would be physically very strong, because we are dealing with vibration as opossed to just stress, it still wouldn't be very much stiffer than the unbraced version. A batten is only supported at each end and so has nothing stopping it moving side to side and the join has a very small suface area. A panel on the other hand, is supported (preferably glued and screwed) on all four of it's sides and offers much more support but is still capable of lateral wobble and so we use another panel to brace that and another to brace that e.t.c. until we end up with a grid of panels (matrix) inside the box. The end result is a structre whereby every point is supporting and being supported by every other point. It is because crystals such as diamond have a molecular stucture with the same properties as this (every point supporting every other) that makes them so rock 'ard. Anyway, B&W did some comparison tests between the concrete box and the matrix box, to see which one sounded better. The boxes were of the same size with same thickness of material (18mm). Each box was placed face down in to a huge box of sand, so as to absorb all the energy from the front and a microphone was placed above the back panel, to record the sound output from the box itself. The results were incredible. They found to start with that with your average hifi cabinet, up to 20-30 % of the sound you were hearing was being made by the box itself, instead of the driver, but with regards to the comparison test, the matrix box was found to store ten times less energy than the same size concrete box. In other words, it sounded ten times better. The amount of time and money that goes in to build a matrix cab is much more than the concrete box but the matrix has every other advantage. Using concrete is abit like cracking a nut with a sledgehammer. There's much easier and better ways to make wicked subs than to resort to such extreme materials. Using a complete matrix is a bit over the top but the basic priciple of the bracing is the most important thing. If your not going for a full matrix and just using some panels to stiffen the box then avoid symetry when placing the panels. The more panels you use, the more you will have to increase the box size to make up for loss of volume but this is only a small figure. Many manafucturers use things like 22mm birch ply for their bass cabs but I use 18mm mdf, it's stiffer than normal ply but not birch coz birch ply is like bloody titanium and blunts all your tools and gives you cramps and aches and weighs a ton and costs a fotune (but other than that, it's great) with a bracing system like described and my cabs pee all over professional kit when it comes to sound. Another BIG advantage to a properly ridid box is the surprising increase in efficency. When the driver isn't wasting all that electricity on vibrating the box, it uses it to make sound and the difference is shocking. There are many ways you can brace a box but remember that the point is that the smaller the panel, the more rigid it is and by bracing the centre of a panel with the end of another panel, you effectively turn that in to two panels, each halve the size. A batten support does'nt do this. Use panels to reduce the surace area of other panels and arrange them in such a way so that they are all bracing eachother but try to avoid to much obvious symetry.
My 18" subs are bloody heavy (not like concrete though) but are so rigid that if you touch the sides when they are on full whack, you can't feel any vibration. Minimal loss, maximum sound. Remember: PA companies are marketing a product and sound quality is not their primary concern. Don't be temted to copy what they do because you think it's the proper way to do it because you are spoilt with PA. Big amps, big drivers=loud music. With hifi, your'e limited to small boxes and have to get the most possible out of them which is why all the really cool advancements in speaker technology, that actually make things louder and sound better, come from the hifi world. PA just use horns (yuk). If you apply this technology to PA the result is another level of quality when compared to Turbopants or some other macho brand. Dig in to hifi technology and you can build subs that make a mockery of professional kit. The biggest thing that stands out with a really rigid box (with a quality driver of course) is the content. There is so much going on in the world of bass that we don't normally hear from so-called "subs" (subsonic bass means below hearing, sub is not just another word for bass) that it's a whole new world once you start to play tunes through a proper sub. Concrete subs would look really cool though...
Thanks Trev, I do really like the look. It is a look that is easier to turn out half well despite the fact that I am not inclined toward precision woodworking.Your ideas sound right to me. In fact, I made a standby box out of wood, and figured on bracing very similar to how you mentioned. But I never used that box because I liked the concrete. I also suspected that the bracing used in it would not be as stiff as concrete because I used #6 drywall screws and pine braces instead of hardwood. One reason I like the concrete is that once the box is made, it has to be stiff. But I also think that a wooden box done well could do better.
One thing B&W test results do not seem to mention is how the concrete compares to a wooden box with 20-30% losses and related high distortion. If the concrete drops the figure to 2% compared to 0.2% for the honeycomb, the audibility difference may be hard to tell.
I did also consider that the concrete though heavy is more compact than comparable wood with similar inner volume. Another advantage is that I do not feel the need to place the boxes on heavy slabs or suspend them or anything like that. However, your thoughts do seem very valid.
Re: You could do that. Or....
18mm seems kind of thin for concrete. Inspired by this thread I began sketching an easy way to make a concrete enclosure. It is 100% concrete by including the speaker mounting surface which has embeded T nuts. Anyway, I figure 50mm thick is more like it. 50mm mdf might be better but concrete does have its aesthetic attractions.
trev said:
18mm seems kind of thin for concrete. Inspired by this thread I began sketching an easy way to make a concrete enclosure. It is 100% concrete by including the speaker mounting surface which has embeded T nuts. Anyway, I figure 50mm thick is more like it. 50mm mdf might be better but concrete does have its aesthetic attractions.
Subs (not the ones that sink)
I just realised that your original post was about a home subwoofer. I misread two 8" drivers as two 18" drivers. With a box as small as that, it will allready be quite rigid. Using my bracing system has the biggest effect on large cabs with large panel areas. What makes a small improvement in hifi makes a huge one in PA. The concrete box was, obviously, much stiffer than a normal hifi box in the tests but at ten times the weight and with the obvious disadvantage of being a difficult material to work with. It takes a lot of energy to excite something with as low a resonance as concrete but onces it rings, it really rings. On a hifi scale, your concrete cabs are a lot less work and money than a matrix box. If you were building something bigger, it would rapidly get to a point where it would be much better to use my system. As for your point about the audibility of the concrete box's losses, you are right that they will already be very low when compared to an average hifi cab but there is a strange psycho-acoustic effect that means that the more you lower the distortion level of a given program material (i.e. a particular track of music) , the more sensitive our ears become to any other imperfections within that material, especially those of us with a lot of experience at listening to hi quality sound, such as engineers and musicians in studio's with top quality monitors or just audiophiles like us. This is why hifi is a never ending quest and ultimately, the perfecr speaker is all in the ear of the beholder. A flat response is just the beginning. There's stereo focus, depth, accuracy and a whole host of other qualities that we aspire to perfect that means that once you get to a certain level of really hi quality, it all becomes subjective as to what makes a speaker better or worse. B&W'w Naulius retails at around 35 grand, took about 15 years to develop and is wideley regarded by many top engineers as being the best hifi speaker in the world but was described by one top musician as sounding "tinny". This speaker was the first to acheive a perfectly flat respnse in listening room tests and really is the business. But the point is that once you get to a high level of quality, every 0.1% percent is important. There are a lot of misunderstood terms in hifi and a lot of audiophiles miss the point about how to make improvements. Spending hundreds of pounds on gold cable will be do nothing if your speakers aren't hi quality to start with. First, you must get to this hi quality level with good speaker design and positioning and then these thing start to make a difference. Let's say, for example, that you get an improvement of 0.2% by using quality speaker cable, then, you get another 0.3% with better signal cable, then another 0.4% by switching to a balanced line input and a few other improvements that, on there own would not make an audible difference, and it all adds up to a few percent total improvement, this would be an audible difference. Other sound systems are always surprised when they see that I do not use any EQ or compression on any other effects on my rig. The signal goes straight from the mixer to the crossover to the amps. My speakers, being PA but designed using hifi priciples, or allready low and flat enough not to need EQing when playing recorded music (live bands are different, not using compression when the guitarist goes in to a loud solo usually results in all your tweeters going pop) and venues usually don't give me any problem I can't deal with by setting the stacks up differently and tweeking the basic 3 band EQ on the mixer but being hi quality means that you really notice if someones plays an old record or a low fi mp3 or there is some other problem and so I have to make as many of these tiny improvements as I can and small things can be audible, such as dirty mains or un unbalanced input upsetting the signal somewhere. So although it may appear on paper that the difference between a concrete box and a matrix is minimal when you take in to account that were dealing with fractions of a percent, when this is coupled with all the other improvements, it does make an important difference. This is why they chose to use matrix instead of concrete, when concrete would have been cheaper and easier. Interstingly, B&W quickly moved on after this to much better composite and polymer materials, specificaly manafactured for them. The Nautilus is made from a mineral loaded resin and a lot of the materials they use have come from places like NASA, because the most important place to make improvemets is in the speaker design to begin with, as opposed to trying to make up for imperfections afterwards with EQing and such. Basicaly: it's up to you how far you want to take speaker design improvements and what is an acceptable quality for you. For B&W (and any real hifi company that truly wants to build the best), you cannot take it far enough and the difference between matrix and concrete is a huge one. We don't have the R&D facilities or the budget to take it as seriously as they do and so we have to look at every aspect of the design procedure to see where we can make a realistic difference within our means. My single 18" matrix subs took me two months to build and use 67 seperate peices of wood because I don't have laser spectrometers and other really expensive R&D equipment and the like but I do have time and lots of wood. Anyway, sorry, I can waffle for hours on this subject so I better cut it short. To summarise: a matrix box the same size as your concrete one would be much more rigid but are you fussy enough to think this is an improvement you want or is it too much hassle? For me personally, I dream of having the resources to build speakers that are out of this world and I have designs that I know would kick *** but I'd need the resources of a large speaker company to build them and so if there is some improvement I can make by just using what Iv'e got (basic tools, basic materials) then I'm going to do it, even if it is a nightmare to build and takes me ages because once they are built, your'e stuck with them and if they aren't good enough, the only thing you can do is start again and build something better.
I just realised that your original post was about a home subwoofer. I misread two 8" drivers as two 18" drivers. With a box as small as that, it will allready be quite rigid. Using my bracing system has the biggest effect on large cabs with large panel areas. What makes a small improvement in hifi makes a huge one in PA. The concrete box was, obviously, much stiffer than a normal hifi box in the tests but at ten times the weight and with the obvious disadvantage of being a difficult material to work with. It takes a lot of energy to excite something with as low a resonance as concrete but onces it rings, it really rings. On a hifi scale, your concrete cabs are a lot less work and money than a matrix box. If you were building something bigger, it would rapidly get to a point where it would be much better to use my system. As for your point about the audibility of the concrete box's losses, you are right that they will already be very low when compared to an average hifi cab but there is a strange psycho-acoustic effect that means that the more you lower the distortion level of a given program material (i.e. a particular track of music) , the more sensitive our ears become to any other imperfections within that material, especially those of us with a lot of experience at listening to hi quality sound, such as engineers and musicians in studio's with top quality monitors or just audiophiles like us. This is why hifi is a never ending quest and ultimately, the perfecr speaker is all in the ear of the beholder. A flat response is just the beginning. There's stereo focus, depth, accuracy and a whole host of other qualities that we aspire to perfect that means that once you get to a certain level of really hi quality, it all becomes subjective as to what makes a speaker better or worse. B&W'w Naulius retails at around 35 grand, took about 15 years to develop and is wideley regarded by many top engineers as being the best hifi speaker in the world but was described by one top musician as sounding "tinny". This speaker was the first to acheive a perfectly flat respnse in listening room tests and really is the business. But the point is that once you get to a high level of quality, every 0.1% percent is important. There are a lot of misunderstood terms in hifi and a lot of audiophiles miss the point about how to make improvements. Spending hundreds of pounds on gold cable will be do nothing if your speakers aren't hi quality to start with. First, you must get to this hi quality level with good speaker design and positioning and then these thing start to make a difference. Let's say, for example, that you get an improvement of 0.2% by using quality speaker cable, then, you get another 0.3% with better signal cable, then another 0.4% by switching to a balanced line input and a few other improvements that, on there own would not make an audible difference, and it all adds up to a few percent total improvement, this would be an audible difference. Other sound systems are always surprised when they see that I do not use any EQ or compression on any other effects on my rig. The signal goes straight from the mixer to the crossover to the amps. My speakers, being PA but designed using hifi priciples, or allready low and flat enough not to need EQing when playing recorded music (live bands are different, not using compression when the guitarist goes in to a loud solo usually results in all your tweeters going pop) and venues usually don't give me any problem I can't deal with by setting the stacks up differently and tweeking the basic 3 band EQ on the mixer but being hi quality means that you really notice if someones plays an old record or a low fi mp3 or there is some other problem and so I have to make as many of these tiny improvements as I can and small things can be audible, such as dirty mains or un unbalanced input upsetting the signal somewhere. So although it may appear on paper that the difference between a concrete box and a matrix is minimal when you take in to account that were dealing with fractions of a percent, when this is coupled with all the other improvements, it does make an important difference. This is why they chose to use matrix instead of concrete, when concrete would have been cheaper and easier. Interstingly, B&W quickly moved on after this to much better composite and polymer materials, specificaly manafactured for them. The Nautilus is made from a mineral loaded resin and a lot of the materials they use have come from places like NASA, because the most important place to make improvemets is in the speaker design to begin with, as opposed to trying to make up for imperfections afterwards with EQing and such. Basicaly: it's up to you how far you want to take speaker design improvements and what is an acceptable quality for you. For B&W (and any real hifi company that truly wants to build the best), you cannot take it far enough and the difference between matrix and concrete is a huge one. We don't have the R&D facilities or the budget to take it as seriously as they do and so we have to look at every aspect of the design procedure to see where we can make a realistic difference within our means. My single 18" matrix subs took me two months to build and use 67 seperate peices of wood because I don't have laser spectrometers and other really expensive R&D equipment and the like but I do have time and lots of wood. Anyway, sorry, I can waffle for hours on this subject so I better cut it short. To summarise: a matrix box the same size as your concrete one would be much more rigid but are you fussy enough to think this is an improvement you want or is it too much hassle? For me personally, I dream of having the resources to build speakers that are out of this world and I have designs that I know would kick *** but I'd need the resources of a large speaker company to build them and so if there is some improvement I can make by just using what Iv'e got (basic tools, basic materials) then I'm going to do it, even if it is a nightmare to build and takes me ages because once they are built, your'e stuck with them and if they aren't good enough, the only thing you can do is start again and build something better.
A while ago I also thought about making a concerete speaker box. You could use that potting foam, for the internal structure to make the internal volume whatever odd non symetrical (and non parallel) shape you want. The stuff im talking about is the stuff they use to make fake flowers stick up. You just need a knife to shape it and stuff. Then when your done you just get a spoon or something and scoop all of the foam out of the inside of the concerete enclosure
Well, Trev, we seem not to be too different in our thinking. I can tend to write long paragraphs.I agree that once the a certain level of performance is reached, small changes become quite noticeable. Because the cabinet is small, It is greatly affected by equalization. I actually took a low-cost 7-band auto-sound type graphic EQ open and tacked in appropriate capacitors to alter the slider center frequencies to 27, 45, 60, 75, 90, 105, and 125 Hz. I don't like to use the concrete sub without it in the signal path. I also use a passive 12 db/octave low-pass filter which kicks in at around 150Hz in the signal path as well.
once they are built, you're stuck with them and if they aren't good enough, the only thing you can do is start again and build something better.
Been there too.
Chris8sirhC,
molded concrete does lend itself to non-flat construction if so desired.
Bill,
I went through such possibilities as cabinets which used concrete building blocks fastened together so as to be disassemblable. One problem with the building blocks is their non-flat surfaces, intended for stability during building construction, cause many gaps that need to be solidly plugged. They tend to be wider at the ends where small bumps are often placed as well. Also, there is still the problem of joining them rigidly--not to imply you were even thinking along these lines in the first place, though.
Enough with the chitter chatter.... I think a CLD technique with concrete would be golden! What I would do is this...
Build a small box, or some sort of shape, out of 3/4 inch MDF. Cut the hole for the speaker and then make a ring out of 3/4 inch mdf with the inner diameter being the same as the speaker cutout and the outer diameter being two inches greater than the inner so that the ring would be an inch wide. Then this would be attached to line up with the speaker cutout hole.
Next a larger box would be built that would fit around the smaller one allowing 3/4" of seperation around the entire box. this would be spaced using the ring. Finally a panel can be reomoved and concrete can be used to fill the 3/4 inch gap between the two peices of wood.
The outer box can be removed if desired, but it can also be used as CLD(constrained layer dampening)
I think I'm going to make two small boxes of this design for my extremises
Build a small box, or some sort of shape, out of 3/4 inch MDF. Cut the hole for the speaker and then make a ring out of 3/4 inch mdf with the inner diameter being the same as the speaker cutout and the outer diameter being two inches greater than the inner so that the ring would be an inch wide. Then this would be attached to line up with the speaker cutout hole.
Next a larger box would be built that would fit around the smaller one allowing 3/4" of seperation around the entire box. this would be spaced using the ring. Finally a panel can be reomoved and concrete can be used to fill the 3/4 inch gap between the two peices of wood.
The outer box can be removed if desired, but it can also be used as CLD(constrained layer dampening)
I think I'm going to make two small boxes of this design for my extremises
I once made a stereo pair from fibre glass - resonance hell, but good for beautiful curvy shapes!
I then lined the inside with concrete by simply pouring in some concrete to an inch thick per side, letting it set, rotating speaker onto another side, and doing the same x6.
Before pouring...
~I added a steel reinforcement frame to the front panel.
Very important - a good temporary "wall" is needed around the driver cutouts to stop concrete caving in. I used "stick-on wax" and fibre glassed it. If i did it again i would try to find a better way. Thin plastic maybe?
~Also drilled holes for the driver bolts, and fibre glassed nuts to the inside.
Using this method, it is possible to make a texture to the inside surface to break-up internal waves. And fun getting hands messy!
Very good results. Great sound. No resonance. Very heavy(needed recessed handles!)
I don't recommend using fibre glass as it takes too much work, and i think some of the fibre glass separated from the concrete but im sure a wooden enclosure could be lined like this.
I then lined the inside with concrete by simply pouring in some concrete to an inch thick per side, letting it set, rotating speaker onto another side, and doing the same x6.
Before pouring...
~I added a steel reinforcement frame to the front panel.
Very important - a good temporary "wall" is needed around the driver cutouts to stop concrete caving in. I used "stick-on wax" and fibre glassed it. If i did it again i would try to find a better way. Thin plastic maybe?
~Also drilled holes for the driver bolts, and fibre glassed nuts to the inside.
Using this method, it is possible to make a texture to the inside surface to break-up internal waves. And fun getting hands messy!
Very good results. Great sound. No resonance. Very heavy(needed recessed handles!)
I don't recommend using fibre glass as it takes too much work, and i think some of the fibre glass separated from the concrete but im sure a wooden enclosure could be lined like this.
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