DrWho Posted July 15, 2005 Share Posted July 15, 2005 I just thought I would post some plans for a new subwoofer I'll be building that uses the driver from the promedia subs. I've got 4 drivers right now with 2 amp modules too and even a pair of satelites. I think my total cost after shipping put me right around $60 in parts. Right now I have this Sony surround setup connected to my pc and I really hate the sub. The sats are liveable for right now, but will eventually be upgraded to the promedia sats as well. The sub however needs a big upgrade and that's where this plan comes into play... The basic idea is to use the same amps and to keep things simple I wanted to present the same load to the amp (which really peters out at frequencies below 40Hz). I will be building two enclosures, each with two drivers. Each enclosure will have the drivers wired in series, but the second enclosure will be a passive box wired in parallel with the first one. (The reason I'm wiring in series inside the cabinet is so that the amp doesn't blow if somehow the passive side gets disconnected). I will still be installing an amp on the passive cabinet that way I can power the sats in the future (for a 4.2 setup). I plan on installing a switch inside each cabinet so that I can easily switch between passive and active mode. Ok, enough talk...let's see some pics: (Dimensions: 15x12x18 (HxWxD in inches) for a single cabinet. 17" total height with feet (needed for down-firing woofer)) As you can see, it's a front-firing / down-firing design with slightly more volume per driver. The original 2.1 subs are in a .55 cubic foot enclosure tuned to 40Hz. This setup will be .75 cubic feet per driver and tuned to about 30Hz. (so that's 3 cubic foot total volume for the setup). The original PM subs uses an EQ and I don't know the T/S parameters for these drivers so my design comes from modelling a similar driver, adding an EQ to give it the posted specs and then calling this response "0". I then modelled the same driver in a similar cabinet and tried to moderately extend the LF response. I figured the original subs were at least 3dB down at 30Hz, if not 10dB so I allowed for a slight boost above my "0" line. Here is a pic of the modelling: The curves show the response with the EQ enabled...response below 30Hz is probably inaccurate. As you can see, the new design starts gradually deviating at 65Hz and will eventually be 3dB louder at 30Hz. In the end I'm expecting to get about 112dB max acoustic output and far less distortion. But more importantly, the port noise will be vastly reduced. The current amp is more than capable of bottoming out the drivers so by adding 4 drivers, I should be running at 1/4th the excursion. My total cost is going to be around $120 total, but I'll comment on that after it's finished. And yes, I do plan on making the cabinets look nice. When going the veneer route, do I need to veneer the pieces before or after I assemble the cabinet? I've never tried veneering before and I don't know how to make the edges look good. Quote Link to comment Share on other sites More sharing options...
DrWho Posted July 15, 2005 Author Share Posted July 15, 2005 One idea I'm toying with is going with a dual side-firing design instead. I was thinking that side-firing would perhaps eliminate all the non-linear distortions (similar to an isobarik arrangement). The reason I didn't go this route is I felt it would be a lot harder to properly integrate into the system. I like the idea of the speakers firing at me, not to the sides and bouncing around before it gets to me. I'm also toying with the idea of dual down-firing... So ya, any thoughts or opinions any of the 3 topologies or even other ideas would be much appreciated. Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted July 15, 2005 Share Posted July 15, 2005 I'm also toying with the idea of dual down-firing... ---------------- It seems to work for SVS Looks like a fun project. Quote Link to comment Share on other sites More sharing options...
DrWho Posted July 15, 2005 Author Share Posted July 15, 2005 I should probably mention that the promedia sub crosses over at 200Hz...not sure what kind of problems that might pose. Quote Link to comment Share on other sites More sharing options...
InnovaZero Posted July 15, 2005 Share Posted July 15, 2005 ---------------- On 7/15/2005 4:58:50 PM DrWho wrote: I should probably mention that the promedia sub crosses over at 200Hz...not sure what kind of problems that might pose. ---------------- Damn, is the 2.1 crossover different? On a second note, you think this will outdo the Ultra sub, or prob. not? I dunno if it'd get it down low. It should be interested none the less. Looks like fun Quote Link to comment Share on other sites More sharing options...
MD1032 Posted July 15, 2005 Share Posted July 15, 2005 Think this is something I could do to my PM4.1's and have their response improve? Quote Link to comment Share on other sites More sharing options...
DrWho Posted July 15, 2005 Author Share Posted July 15, 2005 ---------------- On 7/15/2005 10:00:32 PM InnovaZero wrote: ---------------- On 7/15/2005 4:58:50 PM DrWho wrote: I should probably mention that the promedia sub crosses over at 200Hz...not sure what kind of problems that might pose. ---------------- Damn, is the 2.1 crossover different? On a second note, you think this will outdo the Ultra sub, or prob. not? I dunno if it'd get it down low. It should be interested none the less. Looks like fun ---------------- Well I don't think the 2.1 even has a lowpass on the sub...200Hz is where my reciever does the crossover at. I do think it should outperform the 5.1 sub since this will have more cone surface area and more cabinet. If I put polyfill in the cabinet, it'll even increase the apparent volume and even lower the tuning a bit...putting it closer to -3dB at 25Hz versus the -5dB of the 5.1. I'll be sure to post results once I get it finished...I just got an RTA so I'll just need a measurement mic and then I can take some real measurements outside. Quote Link to comment Share on other sites More sharing options...
Blackmesa Posted July 18, 2005 Share Posted July 18, 2005 sounds fun, good luck! here are some quick drawnings I did to help ya.. Quote Link to comment Share on other sites More sharing options...
DrWho Posted July 18, 2005 Author Share Posted July 18, 2005 sweet pics...though I don't think my building capablities would come close to matching that clean look though. Perhaps you should redraw with crooked edges Quote Link to comment Share on other sites More sharing options...
DrWho Posted July 18, 2005 Author Share Posted July 18, 2005 Well I started building it today and completed the cabinet (went with the front/down firing design). It's not perfectly square but it's close and I did a ton better on the edges this time I've got a normal 2.1 sub playing right now in front of me so I'm going to build the one side first and then do some frequency response comparisons and I'm still crossing my fingers hoping it'll work out and sound good with the built in EQ. Does it normally take over 5 hours just to build a simple box? Man I'm so exhausted. I need to get me a table saw cuz doing all these cuts with a jigsaw really sucks. Quote Link to comment Share on other sites More sharing options...
rjon17469 Posted July 18, 2005 Share Posted July 18, 2005 ---------------- On 7/15/2005 10:55:29 AM DrWho wrote: The original PM subs uses an EQ and I don't know the T/S parameters for these drivers so my design comes from modelling a similar driver ---------------- Not the best way to build a subwoofer and expect good results. If you want to get good results, you're going to need to take the T/S parameters of the drivers you're using. Taking the T/S parameters of the drivers you're using will answer one more critical question: is this driver suited for a down-firing application? When mounted in a down-firing position, all drivers sag a certain amount. You can determine the amount of sag from the T/S parameters. As a rule of thumb, if the sag exceeds 5% of the driver's one-way linear excursion, it's not suited for a down-firing application. Reece Quote Link to comment Share on other sites More sharing options...
DrWho Posted July 19, 2005 Author Share Posted July 19, 2005 I'm actually not worried about the sag at all...the suspension on these drivers is far too stiff to even come close to Xmax, which was probably intended to make the drivers more rugged (something definetly needed with the typical computer speaker croud). Even my fingers aren't strong enough to push it in all the way. As far as T/S parameters go...the original design concept was simply to run 4 drivers off a single amp (considering they're so readily available on ebay these days). At the same time I figured it would be a good opportunity to redo the ports to get rid of that annoying port noise. So technically, if I stayed with the same volume per driver and the same tuning, then I shouldn't even have to worry about the T/S specs. I opted to slightly increase the volume of the cabinet per driver because it seems like a little extra free extension. I've modelled hundreds of speakers and you always have to slightly increase the volume per driver when you increase the number of drivers in your cabinet in order to maintain the same slope. The reason I modelled another driver was to get a concept of the magnitude that the cabinet volume change would have. I was sneaky in this design though in that I can seal off a portion of the internal volume and I'll be right back to the original volume per driver, but the best part is I won't have to change the ports because the smaller cabinet also raises the tuning back to the original as well Gosh, all this work simply because I have no means to measure the drivers myself. Quote Link to comment Share on other sites More sharing options...
rjon17469 Posted July 19, 2005 Share Posted July 19, 2005 One element of how much it sags is the mechanical damping factor, but the other part of it is the cone weight. The higher the cone weight, the lower the fs of the driver, and therefore the better sub driver it makes. To calculate the percent of sag, you can use this equation: 24,849 / (xmax * fs^2). Remember, there's no qms factor in there, which means the suspension stiffness is not directly related to the amount of sag (however it is related to the driver fs). If you do want to measure T/S parameters, here's a website I like which makes it pretty easy: http://www.bcae1.com/spboxad3.htm Reece Quote Link to comment Share on other sites More sharing options...
Blackmesa Posted July 19, 2005 Share Posted July 19, 2005 the 2.1 drivers are small, I don't he has to worry about sag. Quote Link to comment Share on other sites More sharing options...
MD1032 Posted July 23, 2005 Share Posted July 23, 2005 Same here. And he's right about the surrounds - I've run some crazy bass tests through these and they can't seem to bottom out. Quote Link to comment Share on other sites More sharing options...
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