That's a bugger, thanks for pointing that out. Would an aperiodic vent degrade sound quality? I could go a bit bigger but 100 litres (3.5 cu ft) would be seriously big.
I would keep it the same footprint and build it taller. Or swap out the woofer to an HO, if it's not too late.
Don't know much about aperiodic vents, but I would think the negative effect is minor, considering how much bass you'll be generating.
I can do internal dimensions of 45 cm wide, 60 high and 37 deep, for 99.9 litres. I'm planning on 25 mm thick ply so the external dimensions will be +5 cm in each dimension. I will now begin research on subwoofer cloaking devices. This gives Qtc of 0.812 and Fsc of 36.05 Hz before equalisation, so we're fairly low. I've ordered the HF version, so I'd rather do this justice than swap for the HO.
I've read a bit about various different filling materials adding to the apparent size, and further decreasing the Q value of the box, but would the stuffing do any good to prevent oil-canning?
I would build with 18mm ply, 1x1/2" slat/stud bracing glued on end spaced every 6" (think of a stud wall), and line with real sound damping fiberglass, owens corning 703 2" thick or more against the slats (not the wall, with an air gap). Internally coat the walls with rubbery roofing mastic if you feel like it. That should max actual internal volume, while the fiberglass is the best you can do for stuffing.
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Sounds good, thanks! I was looking at bitumen materials to damp any overtones. I actually had a new stud wall put up in front of the brick dividing wall between my house and next door for soundproofing a few years back, and it was similar construction.
I'm so looking forward to this build. I want to get the amp and look at it to work out how to fit it in to the casing with the fins out next. Just got the confirmation the driver has shipped.
Sounds like a good build indeed, I'm looking at doing a compact 15" HO build myself.
The tip on "floating" the fiberglass comes from recent discussions on internal sound damping. The fiberglass is much more effective than polyfill. Having it away from the wall seems to be advised.
The slat/stud bracing is my tip. Many people put huge braces across the entire enclosure, but I personally don't see how that is very efficient. A simple stud across the panel adds substantial stiffness with minimal use of volume. I'm also a fan of using the stiff plywood with a rubber damping.
It might be obvious, but make sure the box is air tight, use good glue and seal the seams well.
Wow, the driver has arrived. That was quick.I got it from Europe Audio in The Netherlands and it's here in three days, packed in three decent boxes one inside the other, so I'll order from them again. I'm off to Paris next week, so no more ordering bits until I know how much I have left at the end of the month!
One thing I was researching was whether the miniDSP would work down below 20 Hz as all the screen shots show this as the lowest level on the basic equalisation screen. There is a post on the miniDSP site about the advanced biquad filters that confirms that the board rolls off at 5 Hz, and so with the right biquad filters I should be okay.
One thing I was researching was whether the miniDSP would work down below 20 Hz as all the screen shots show this as the lowest level on the basic equalisation screen. There is a post on the miniDSP site about the advanced biquad filters that confirms that the board rolls off at 5 Hz, and so with the right biquad filters I should be okay.
Yes, I believe when it was first released the lowest frequencies you could enter to the filters were 20Hz, which somewhat annoyed me, but I bough it nonetheless. But when I wound up trying it out myself, new software was out and you could chose lower frequencies, not sure how low, but surely low enough, can't imagine you'd ever need anything below 10Hz.
Design pictures
Right, I've ordered the miniDSP, but I went for the one in the box, as I've been looking at how to mount it and decided that as it is so small, it can go outside, on the back of the speaker. I was going to put it inside and run the USB extension to the back of the box, but I was wondering if a metal box would be good for shielding purposes, so I thought, may as well buy the one that fits.
I've ordered the BK MF 300 amp in the end, so now that I know roughly what the size of the bits is, here are some pictures of the design. First up, the front with all the sides on. It is 68 cm high, 50 wide and 45 deep, in 18 mm plywood. The driver is not strictly accurate, except for the outer diameter, I just wanted to see how to lay it all out.
Next a shot with the side off showing the volume around the amp as a solid, although the amp does not fill this volume by any means. This is the volume you could box out and just slide the amp in.
A view from the back with the miniDSP in green, and fins of the amp shown. I've just estimated these at 5 cm but again, this is just guessing.
I tried a few 3D modelling packages to make this, the free Google sketchup to start with, which had no CSG at all, then the Autodesk 123D Beta. Both if these seem determined to insulate the user from the coordinates of the things on screen. I sort of got somewhere with the Autodesk 123D Beta, the free version, but I just want to type in dimensions and rotations, and then see if it has worked out how I planned. I don't really like having to select anchor points off the screen. I never knew quite what context I was working in, and trying to find how to cut a hole in a plate was a bit of a drag. The tutorials look amazing, but it also crashed quite a few times, so I gave up and used COMSOL to draw the plans up. I have this for work, and it is a full Finite Element modelling suite, and I have seen people doing acoustic models with it. I have no experience of that yet, so this is just for the pictures. I have not added bracing yet, but there will be some. I might just see if I can use the acoustic model to work out the best arrangment.
With regard to the amp, I have mounted it vertically to let the fins work efficiently, and I'm just trying to decide if I'll just create an open box facing outwards (but sealed off from the main volume) of the correct dimensions right against the left side at the back so it can slide in, and I can have some allen key headed bolts projecting through the side (countersunk in flush so they don't stick out) to secure it once in place, or even make the side above it open out for easy access The alternative is to mount the amp as a plate amp, creating a seal around the heatsink. It does not look like it is designed for this though, so this may not be easy, or effective. I also worry about microphony of the components, and even shortening their lifetime by fatigue inside the box. The final option would be to build it into a separate metal or wood case and sit it on top, or on the back but that does not seem very elegant, and the cost of the case might be quite high for something 33 cm by 10 by 10 or so.
Not shown but also ordered are a 7812 Regulator, and the two recommended caps to go with it for smoothing, a little bit of circuit board to put them on to and some hole supports to mount them, and phono socket to fit on the back of the box to take the signal through to the amp. I'll be taking the power for the miniDSP from the amp, regulating it and feeding through, as jwmbro suggested. I cannot seem to find details of what the plug on the miniDSP is, or is it just a terminal block that could just accept tinned wires? I might have to get a new shielded audio cable to connect the lfe output on my amp to the miniDSP. Any recommendations gratefully considered. I have also ordered the volume control pot with the DSP so my wife can turn the sub down without connecting a USB cable to it, so that will be mounted on the back too.
Anyway, the final details from the construction point of view will be to round over the edges with a (borrowed) router to about a 12 or 15 mm radius and then paint the case to match my floor, in gloss French grey. I was wondering about making another recess in the back big enough to sit the miniDSP in, but as the fins for the amp have to stick out a bit to get decent cooling, it hardly seems worth it.
So, just waiting for the other parts, but may be next weekend I'll get the wood and start cutting. I have a final question about using non-magnetic bolts for fixing the speaker driveror the amp to the box. Is it necessary?
Right, I've ordered the miniDSP, but I went for the one in the box, as I've been looking at how to mount it and decided that as it is so small, it can go outside, on the back of the speaker. I was going to put it inside and run the USB extension to the back of the box, but I was wondering if a metal box would be good for shielding purposes, so I thought, may as well buy the one that fits.
I've ordered the BK MF 300 amp in the end, so now that I know roughly what the size of the bits is, here are some pictures of the design. First up, the front with all the sides on. It is 68 cm high, 50 wide and 45 deep, in 18 mm plywood. The driver is not strictly accurate, except for the outer diameter, I just wanted to see how to lay it all out.
An externally hosted image should be here but it was not working when we last tested it.
Next a shot with the side off showing the volume around the amp as a solid, although the amp does not fill this volume by any means. This is the volume you could box out and just slide the amp in.
An externally hosted image should be here but it was not working when we last tested it.
A view from the back with the miniDSP in green, and fins of the amp shown. I've just estimated these at 5 cm but again, this is just guessing.
An externally hosted image should be here but it was not working when we last tested it.
I tried a few 3D modelling packages to make this, the free Google sketchup to start with, which had no CSG at all, then the Autodesk 123D Beta. Both if these seem determined to insulate the user from the coordinates of the things on screen. I sort of got somewhere with the Autodesk 123D Beta, the free version, but I just want to type in dimensions and rotations, and then see if it has worked out how I planned. I don't really like having to select anchor points off the screen. I never knew quite what context I was working in, and trying to find how to cut a hole in a plate was a bit of a drag. The tutorials look amazing, but it also crashed quite a few times, so I gave up and used COMSOL to draw the plans up. I have this for work, and it is a full Finite Element modelling suite, and I have seen people doing acoustic models with it. I have no experience of that yet, so this is just for the pictures. I have not added bracing yet, but there will be some. I might just see if I can use the acoustic model to work out the best arrangment.
With regard to the amp, I have mounted it vertically to let the fins work efficiently, and I'm just trying to decide if I'll just create an open box facing outwards (but sealed off from the main volume) of the correct dimensions right against the left side at the back so it can slide in, and I can have some allen key headed bolts projecting through the side (countersunk in flush so they don't stick out) to secure it once in place, or even make the side above it open out for easy access The alternative is to mount the amp as a plate amp, creating a seal around the heatsink. It does not look like it is designed for this though, so this may not be easy, or effective. I also worry about microphony of the components, and even shortening their lifetime by fatigue inside the box. The final option would be to build it into a separate metal or wood case and sit it on top, or on the back but that does not seem very elegant, and the cost of the case might be quite high for something 33 cm by 10 by 10 or so.
Not shown but also ordered are a 7812 Regulator, and the two recommended caps to go with it for smoothing, a little bit of circuit board to put them on to and some hole supports to mount them, and phono socket to fit on the back of the box to take the signal through to the amp. I'll be taking the power for the miniDSP from the amp, regulating it and feeding through, as jwmbro suggested. I cannot seem to find details of what the plug on the miniDSP is, or is it just a terminal block that could just accept tinned wires? I might have to get a new shielded audio cable to connect the lfe output on my amp to the miniDSP. Any recommendations gratefully considered. I have also ordered the volume control pot with the DSP so my wife can turn the sub down without connecting a USB cable to it, so that will be mounted on the back too.
Anyway, the final details from the construction point of view will be to round over the edges with a (borrowed) router to about a 12 or 15 mm radius and then paint the case to match my floor, in gloss French grey. I was wondering about making another recess in the back big enough to sit the miniDSP in, but as the fins for the amp have to stick out a bit to get decent cooling, it hardly seems worth it.
So, just waiting for the other parts, but may be next weekend I'll get the wood and start cutting. I have a final question about using non-magnetic bolts for fixing the speaker driveror the amp to the box. Is it necessary?
No. With an aluminum frame driver, there would be no difference whatsoever using non-magnetic bolts, and with a stamped steel frame steel bolts simply extend any magnetic leakage from the basket.
Right, I've been fitting in various tasks at weekends, but have got all the panels cut.
I didn't start gluing however, as I wanted to test the electrical side was all okay first, so this weekend I hooked up all the bits, with the minidsp powered off a 12V output from the BK electronics amp. The hissing noise floor was horrendous, like fm radio between stations. To be fair to the amp, the 12V rail is only supposed to power their LED power meter. I tried putting a voltage regulator in line, with caps before and after, but although it was still better, the noise was now more like a cassette tape from 1985. Next up I tried powering the minidsp with the USB, but the noise was about the same. Next up, I tried a wall wart type power supply from my wife's old mobile phone (a Sendo) and the results was almost no noise. There was still a tiny bit of hiss if I held my ear right in the cone, but barely any.
I don't really like the idea of having two mains connections to the sub, so I think I'll try to find a good unit I can site in the same cavity inside the sub I will have the amp and miniDSP in. Something like this maybe?
Power Pax UK Mini Open Frame SMPSU (EUP) 5VDC 2.3A
AC/DC Module PCB Mount 0.5 Watt Bps 0.5 08 50
Anyway, until I've got the electrics all finalised, I can't start gluing, just in case the cavity I planned is no longer big enough. Anyway, if anyone knows a good UK DC supply for the miniDSP that I can fit inside my sub, please let me know!
An externally hosted image should be here but it was not working when we last tested it.
I didn't start gluing however, as I wanted to test the electrical side was all okay first, so this weekend I hooked up all the bits, with the minidsp powered off a 12V output from the BK electronics amp. The hissing noise floor was horrendous, like fm radio between stations. To be fair to the amp, the 12V rail is only supposed to power their LED power meter. I tried putting a voltage regulator in line, with caps before and after, but although it was still better, the noise was now more like a cassette tape from 1985. Next up I tried powering the minidsp with the USB, but the noise was about the same. Next up, I tried a wall wart type power supply from my wife's old mobile phone (a Sendo) and the results was almost no noise. There was still a tiny bit of hiss if I held my ear right in the cone, but barely any.
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 don't really like the idea of having two mains connections to the sub, so I think I'll try to find a good unit I can site in the same cavity inside the sub I will have the amp and miniDSP in. Something like this maybe?
Power Pax UK Mini Open Frame SMPSU (EUP) 5VDC 2.3A
AC/DC Module PCB Mount 0.5 Watt Bps 0.5 08 50
Anyway, until I've got the electrics all finalised, I can't start gluing, just in case the cavity I planned is no longer big enough. Anyway, if anyone knows a good UK DC supply for the miniDSP that I can fit inside my sub, please let me know!
Right, I got one of these little transformers
Miniature 250mA Transformers : Mains Transformers : Maplin Electronics
and a power supply module
K18231A Power Supply : Miscellaneous Kits : Maplin Electronics
The picture for the power module is wrong, it is this one Velleman, Inc. and has a LM317 regulator in.
Anway, running these off another connection to the power entry module, I get about the same noise from the miniDSP as I had running it from a phone charger psu. I'm going to search and see if there is an earthing issue here, as at the moment, nothing is earthed in the sub, the only earth connection comes in via the screen on the signal cable from the main amp which connects to thge minidsp and then to the amp in the sub. I am thinking I ought to ground the heatsink of the amp as this is the area the kids could touch, and if there is a problem, I'd want it earthed safely. The instructions that come with the amp are to have the input cable screen earthed at the pre-amp, and there are no other earth connections, so I assume no other part of the circuit needs an earth anywhere.
The main amp is outputting a signla from 0-0.9V at full volume, so this matches the range of the miniDSP. At moderate to quiet listening, I measured the signals coming from the main amp LFE channel to be at 24 mV RMS for a range of sine waves, which I really think should be big enough to process without getting noisy. I've tried playing with the gain in the main amp and in the miniDSP filters, but the noise is not affected, which makes me think it is in the earth channel. I'm going to read up on this but tomorrow I may try directly earthcing the signal cable screen to the earth connection on the sub power input. I'de be very grateful if anyone else has had any issues like this and sorted them successfully.
On another track, I measured the voltages output from the main amp from 10-100Hz with the crossover set to 50Hz, and it fits about a fifth order filter.
http://www.vellemanusa.com/products/view/?id=350481
Miniature 250mA Transformers : Mains Transformers : Maplin Electronics
and a power supply module
K18231A Power Supply : Miscellaneous Kits : Maplin Electronics
The picture for the power module is wrong, it is this one Velleman, Inc. and has a LM317 regulator in.
Anway, running these off another connection to the power entry module, I get about the same noise from the miniDSP as I had running it from a phone charger psu. I'm going to search and see if there is an earthing issue here, as at the moment, nothing is earthed in the sub, the only earth connection comes in via the screen on the signal cable from the main amp which connects to thge minidsp and then to the amp in the sub. I am thinking I ought to ground the heatsink of the amp as this is the area the kids could touch, and if there is a problem, I'd want it earthed safely. The instructions that come with the amp are to have the input cable screen earthed at the pre-amp, and there are no other earth connections, so I assume no other part of the circuit needs an earth anywhere.
The main amp is outputting a signla from 0-0.9V at full volume, so this matches the range of the miniDSP. At moderate to quiet listening, I measured the signals coming from the main amp LFE channel to be at 24 mV RMS for a range of sine waves, which I really think should be big enough to process without getting noisy. I've tried playing with the gain in the main amp and in the miniDSP filters, but the noise is not affected, which makes me think it is in the earth channel. I'm going to read up on this but tomorrow I may try directly earthcing the signal cable screen to the earth connection on the sub power input. I'de be very grateful if anyone else has had any issues like this and sorted them successfully.
On another track, I measured the voltages output from the main amp from 10-100Hz with the crossover set to 50Hz, and it fits about a fifth order filter.
http://www.vellemanusa.com/products/view/?id=350481
Right, tried earthing the negative rail of the psu and checked the earth of the signal cable. No change, I can still hear the noise from the driver.
Just to check if I understand the gain levels involved, I ran a 40 Hz sine wave signal from the main amp, which is low enough to pass entirely to the sub with the crossover set to 130 Hz, and measured the ac voltage at the input and output of the miniDSP, and also looked at the signal strength in the RMS meter in the software for the miniDSP, with it running connected to my PC. These results are in the file here. I have no idea what these numbers mean, as if I calculate the gain relative to the maximum signal (1800mV I think, from here) for a 537 mV input, I get -10.5 dB, not -16.9 which is what the software shows me.
TH noise I get from the miniDSP is entirely independent of the input, as I can even disconnect the input lead from the main amp and still hear it. If I disconnect the miniDSP from the sub power amp, then there is no noise from the sub driver at all, and if I connect the main amp directly to the sub amp, then there is no noise either.
At this point, I've reached the limits of my investigative powers, and have emailed the miniDSP support team. Fingers crossed!
Just to check if I understand the gain levels involved, I ran a 40 Hz sine wave signal from the main amp, which is low enough to pass entirely to the sub with the crossover set to 130 Hz, and measured the ac voltage at the input and output of the miniDSP, and also looked at the signal strength in the RMS meter in the software for the miniDSP, with it running connected to my PC. These results are in the file here. I have no idea what these numbers mean, as if I calculate the gain relative to the maximum signal (1800mV I think, from here) for a 537 mV input, I get -10.5 dB, not -16.9 which is what the software shows me.
TH noise I get from the miniDSP is entirely independent of the input, as I can even disconnect the input lead from the main amp and still hear it. If I disconnect the miniDSP from the sub power amp, then there is no noise from the sub driver at all, and if I connect the main amp directly to the sub amp, then there is no noise either.
At this point, I've reached the limits of my investigative powers, and have emailed the miniDSP support team. Fingers crossed!
Right, tried powering the minidsp with a battery, with results not actually any better than with my dc power supply.
Just in case my previous post was unclear, the reason I am worried by the signal measurements shown in the software when I play test tones to the minidsp is that they seem to show that there is something wrong with the ranges for the input. If a 537 mV input signal shows up as -16.9 dB, this implies that the reference potential that would give zero dB, the maximum input, would be 3.7 V. I'm using the equation below to work this out, where V is the input signal 0.537 V, Lv is the signal level in dB, and V0 is the calculated reference voltage level. If anyone can see why this might be wrong, I'd be very grateful?
Finally, this is heading off topic a bit, as if I have got a unit that is not working, it's not really illuminating to read about for anyone who wants to read about a sub build, so I'm not going to add more to this until I fix this issue.
Just in case my previous post was unclear, the reason I am worried by the signal measurements shown in the software when I play test tones to the minidsp is that they seem to show that there is something wrong with the ranges for the input. If a 537 mV input signal shows up as -16.9 dB, this implies that the reference potential that would give zero dB, the maximum input, would be 3.7 V. I'm using the equation below to work this out, where V is the input signal 0.537 V, Lv is the signal level in dB, and V0 is the calculated reference voltage level. If anyone can see why this might be wrong, I'd be very grateful?
An externally hosted image should be here but it was not working when we last tested it.
Finally, this is heading off topic a bit, as if I have got a unit that is not working, it's not really illuminating to read about for anyone who wants to read about a sub build, so I'm not going to add more to this until I fix this issue.
I started with that setup, but that was not good at all! I'm seriously thinking of borrowing an oscilloscope and testing this thing properly over the weekend to see if I can see what is up. I had wanted to get this built by now.
Yes, I got to that point back in post #54, over the weekend. I hope they can help, as I just don't feel like completing the build and boxing it all in until I'm sure I have the system working 100%. My design leaves room to get everything in and out for maintenance, but it's just not knowing if I need extra components that stops me from finishing it.
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