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  • My big subwoofer build thread

    Well, I thought I would start a build thread about my next subwoofer project. This is sort of an evolution of my now defunct 2.12.18PR sub, which had pretty good extension to around 13-15Hz (see more details on the 2.12.18PR at the project page of my web site).

    The goals of this subwoofer are:
    frequency extension to around 10Hz
    smooth low end roll off
    minimum passband response: 105dB
    low distortion
    I will be using the following:
    TWO Dayton Audio RSS390HO-4 drivers
    ONE Acoustic Elegance (AE Speakers) 18" PR with 2100g mass
    Behringer EP2500 power amp



    Here are the results of some initial modeling. I used the program "Basta!" to make these plots, and have confirmed the models using Jeff Bagby's Woofer Box and Circuit Designer:

    FULL POWER FREQUENCY RESPONSE AND CONE EXCURSION:

    Red lines = excursion; black line = SPL

    The above graph is the frequency response I can expect at "maximum output", which is around 107dB in the passband. The output at the low end is limited by the PR excursion first, the driver excursion limit around 8Hz second, and lastly by the available amplifier power (see next post for more details on this). I plan to use the EP2500 bridged, and use the drivers in series. I won't need the full power that the EP2500 can deliver, and this should keep the current draw low enough that I can get away with the "quiet fan mod". I plan to install the amplifier inside the sub cabinet and use acoustically treated tunnels to supply air to (and exhaust air from) the amp.

    ACTIVE CROSSOVER NETWORK:
    In order to get the above response, I use the following:
    12Hz Q=1.4 second order HP filter
    5Hz first order HP filter
    85Hz Q=1.4 second order LP filter
    14Hz first order LP filter
    The 12Hz filter provides a boost around the box tuning and a cut at lower frequencies. It's basically creating a 6th order PR system, which limits the cone excursion below tuning (see the nice symmetric cone excursion in the plot above) and flattens the response. The EP2500 has a built in 5Hz first order filter so I included this in my model. The 85 Hz LP filter will be part of the crossover network leading to the main speakers and is adjustable.

    The last filter, the 14Hz LP filter, shapes and flattens the response. When using these drivers in a 240L box, the models predict that the response will fall off at about 6dB/octave below around 40-50 Hz until the box tuning, and then at 24dB/oct below that. I found that, by using the 14Hz first order LP stage, I could flatten out the response so that F3 is between 10Hz and 11Hz. I increase the corner frequency of this stage to 20Hz if I want a more gradual roll off starting at 30 Hz. I hope that this will allow for some room to tune the response to the listening space. In reality, I should use a shelving LP filter but I could not model this accurately. This will only affect the response above 50Hz and will change the Q and type of LP filter I use in the crossover to the main speakers.

    Next post: how I arrived at this design.

    -Charlie
    Charlie's Audio Pages: http://audio.claub.net

  • #2
    Design background and evolution...

    I will use this post to say a few words about how this design came about.

    As I mentioned, I previously built a nice sub using two 12" TC Sounds drivers and a very nice 18" PR from Acoustic Elegance. This had good extension down in to the mid teens, but I ended up taking it apart and selling the drivers. The sub lent a very nice effect to the bottom end of percussion (drum) sounds, although not much else uses the 15Hz-30Hz octave. Now that a couple of years have passed, I though I would revisit the same theme, but try to do it a little better this time.

    The system would be based around the AE Speakers 18" PR. This has an amazingly large Vas of 1840 liters and mass of 2100g. This sets is apart from just about any other PR on the market. A PR's Vas is a critical parameter, because it sets the natural resonance of the driver when mounted in a box. There is a nice overview about PR's at the AE Speakers web site. Since I want the passband to extend down to around 10Hz, it is important to consider where the notch from the passive radiator will occur. Luckily, with this particular PR, the natural (free air) resonance frequency is down around 3-4Hz, which is sufficiently lower in frequency to make it a non-issue. I found that this was not the case with just about every other large PR that I tried to include in the design - the notch would show up around 20Hz, which would be right in the middle of the lowest octave covered by the sub.

    I wanted to use drivers that would result in low distortion. Because the ear is becoming MORE sensitive to sounds as frequency increases (in the low bass region), distortion products can easily become audible. I turned to the Dayton Audio RSS390HO drivers. These incorporate several components in to the motor to reduce distortion. They also have a relatively stiff suspension (small Vas) so that I could use two in a modestly sized enclosure, which ended up being a good match to the PR. These 15" drivers are also very affordable for the level of performance that they deliver, and have a healthy Xmax of 12mm.

    I played around with several different ideas for the electronics. I knew that I would need some kind of HP filter to reduce subsonic excursion of the PR and the drivers. Using the Basta! software I was able to come up with a design that allows me to tailor the frequency response to be flat with an F3 of slightly more than 10Hz, or to have a more gentle roll off starting at a higher frequency.

    The power requirements for this sub are pretty significant. The RSS drivers are not very efficient, and there is a lot of boost at low frequencies, requiring several hundred watts of power, per driver. Because of the low frequency extension of this sub, it is critical to take in to account any and all low frequency roll off from electronics, amplifiers, and so on. Because of this, I wanted to shy away from plate amps, which I do not trust for flat power to very low frequencies. Instead I decided that I could use the Behringer EP2500, which has been proved to deliver over 1300 watts (bridged) in to an 8 ohms load. This was a perfect match for two RSS390s in series, and provided more peak power than I needed. Because the highest power requirements are in the 10Hz-20Hz octave and because there is not much power density there for typical music, I believe that the actual power demand that the amp will see will be relatively low and it should run cool, even if I replace the fan with a slower, more quiet one.

    So, with the drivers, electronics and alignment more or less figured out. I only need to design the actual physical box to put it all in. The drivers and PR will need about 240 liters. I plan to create a long "chest" type of box that I will locate behind a couch. The PR will be installed in one end of the chest. In my previous sub using this PR, the 2.12.18PR, I put the PR in the middle of a tall enclosure of modest depth and found that the cabinet vibration in response to the PRs movement to be undesirable. The higher mass of the enclosure and the end location of the PR should solve this problem in the new design. The PR is 18" OD, so the cabinet must be at least this wide and tall. The couch is 96 inches long, but the enclosure volume is not so huge that it needs to be so long. I will have plenty of space for the electronics and amplifier.

    Currently I am deciding how to locate the drivers. I would like them to be mounted facing opposite directions, e.g. one the normal way and the other with the magnet outside the box. This will help to cancel and reduce the even order harmonics. To do this, I am considering installing one driver facing down (the floor) and the other facing up, in the bottom of the cabinet. Since the suspension is relatively stiff, cone sag won't be an issue however there will be unequal acceleration of gravity on the cone moving mass when moving out versus moving in, and this will cause distortion, so I am trying to think of other options.

    Once I have decided on a design, I will post more details in this thread, and some build pics along the way.


    -Charlie
    Last edited by charlielaub; 06-13-2010, 06:46 PM.
    Charlie's Audio Pages: http://audio.claub.net

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    • #3
      Re: My big subwoofer build thread

      Subscribing! I've been tossing ideas around for a "sub-sub" of my own based on four RSS390's in a sealed cabinet, I'm interested in seeing how substituting two subs with one PR works for you!

      What does that PR cost?

      Comment


      • #4
        Re: My big subwoofer build thread

        Originally posted by superspeeder View Post
        Subscribing! I've been tossing ideas around for a "sub-sub" of my own based on four RSS390's in a sealed cabinet, I'm interested in seeing how substituting two subs with one PR works for you!

        What does that PR cost?
        Here's is the link to the PR product over at AE Speakers:
        http://aespeakers.com/shop/catalog/p...products_id=39
        NOT CHEAP!

        You could get two of these instead, for instance:
        http://aespeakers.com/shop/catalog/p...products_id=41

        -Charlie
        Charlie's Audio Pages: http://audio.claub.net

        Comment


        • #5
          Re: My big subwoofer build thread

          While the design thinking is great Charlie, i'm not sure the listening room wouldn't be better served by two seperate ported enclosures with flexibility of placement and smoother in room response than the design you propose. I've worked with the RSS315 and 390HF and agree they really are small box champions.....and as such don't really take full advantage of the PR alignment in regards to enclosure size.

          Comment


          • #6
            Re: My big subwoofer build thread

            Originally posted by Mayhem13 View Post
            While the design thinking is great Charlie, i'm not sure the listening room wouldn't be better served by two seperate ported enclosures with flexibility of placement and smoother in room response than the design you propose. I've worked with the RSS315 and 390HF and agree they really are small box champions.....and as such don't really take full advantage of the PR alignment in regards to enclosure size.
            You are right that multiple subs would be better WRT room modes, etc. But since I already have already purchased all the drivers and the amp, I will give it a go. I'm planning to locate the sub immediately behind my listening position, which I am hoping will put me more or less in the near field. I hope that will reduce issues with room modes.

            If it turns out to be stellar, I could always build another one I guess!

            -Charlie
            Charlie's Audio Pages: http://audio.claub.net

            Comment


            • #7
              Re: My big subwoofer build thread

              just a note... in my experience, while the ep2500 is a capable mains amp, it has trouble handling subs--- just doesn't have the real world damping factor. that's in live PA use, it may be ok for HT levels. So when you test it all out, if it doesn't sound right to you, swap out the amp before doing any serious tuning.

              my $.02

              m

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              • #8
                Re: My big subwoofer build thread

                This is the worst build thread I have ever seen!



                Are you still building this monster?
                My projects
                https://picasaweb.google.com/112307725038877176664

                Comment


                • #9
                  Re: My big subwoofer build thread

                  Originally posted by ckmoore View Post
                  This is the worst build thread I have ever seen!



                  Are you still building this monster?
                  Haha, OK you got me. Well, I haven't gotten aroudn to building this yet. I have the PR and one 390HO driver, and the Behringer amp, too. So I still need to buy the other 390HO driver and then get started on construction... still in the works... Just give me another year or two, and then it should be done!

                  Charlie
                  Charlie's Audio Pages: http://audio.claub.net

                  Comment


                  • #10
                    Re: My big subwoofer build thread

                    Originally posted by meep View Post
                    just a note... in my experience, while the ep2500 is a capable mains amp, it has trouble handling subs--- just doesn't have the real world damping factor. that's in live PA use, it may be ok for HT levels. So when you test it all out, if it doesn't sound right to you, swap out the amp before doing any serious tuning.

                    my $.02

                    m
                    ON THE SUBJECT OF DAMPING FACTOR: I've talked to an EE that has designed many amplifiers over the years (decades) and currently designs the amplifiers for Genz Benz. I've also talked to one of the EE's who currently designs the pro audio amps for Peavey. When a damping factor spec is posted, it's nothing more than a marketing tool (which obviously works). This spec should be ignored as it is completely useless to the end consumer with no knowledge of amplifier design. The amplifiers damping factor is much more complicated than just a single number on a spec sheet.

                    I've owned this amp (EP2500) in the past and have had great results with it in multiple pro audio sub applications.

                    ChasW has done testing/measurements on this amp, and has proven that it's THE REAL DEAL. (I can't find the link at the moment. I'll add it as soon as I do)

                    Behringer has a bad rap due to it's unethical business practices and terrible reliability issues. When their products actually work, they work well.
                    Being defeated is often a temporary condition. Giving up is what makes it permanent.

                    Scanspeak Revelator R2904/7000's, Scanspeak Revelator 15M/4531K00's, Scanspeak Revelator 22W/8857T00's, Eminence NSW6021's.
                    MiniDSP 4x10HD. ICE Power amplification and an iNuke 3000.

                    Sennheiser HD650's powered by TEAC amplification.

                    Comment


                    • #11
                      Re: My big subwoofer build thread (similar, but no PR)

                      Hi Charlie,
                      About a year ago, I built a sub containing 2x RSS390 HO-4's. Before I built that, I tested the drivers, and actually used a SINGLE unit in the 3 cubic foot test box. The specs I measured were VERY close to the published numbers, with Vas being slightly higher IIRC. The Fc in the test box was 31.8 w/ the corresponding Qtc= .604 Then, I worked out the numbers for a vented box, and quickly discovered the required tuning ducts to be quite large. I had indeed thought about using a PR, but opted to keep it a vented design. The reason I elected a low-boy coffee table style, was to fit it under a shelf that holds my TV; obviously this is for HT use, and I never even thought I needed to go lower than a 25Hz F3. I can't remember where the actual F3 is, but it will play the 25 Hz warble tone of a Stereophile test CD. I used it in my main room 2 channel audio set up for a while, and it did okay. I have yet to move it in to my HT room; kinda waiting to buy an even bigger TV. Who knows maybe an after Christmas sale might just do the trick. Botttom line is, I think these drivers are very, very good. My measurements:
                      Qts .39899 Fs 21 Vas 4.91 cubic feet <--- amazing for a 15 incher
                      p.s. One pic shows it in a verticle orientation, but will be used horizontal.
                      Attached Files

                      Comment


                      • #12
                        Re: My big subwoofer build thread (similar, but no PR)

                        Ah, this project is slowly taking shape, maybe for real this time! OK, so it's been TWO YEARS since I started this thread... just haven't pulled the trigger yet. But I recently got "permission" from the wife to build a large, long coffee table type monstrosity. So here is the latest go-around of the planning:

                        Recap: I have the drivers and amp in hand: TWO RSS390HO and TWO AE PR1821, plus a Behringer EP2500. Modeling looks promising for this, with nearly flat response down to 10 Hz @ 105dB. Did I mention that I have a very large listening space?

                        So, how to construct this behemoth, which will be something like 18+ cuft? Well, it's IKEA to the rescue again. Some of you may know that I use IKEA kitchen cabinet frames for my "baffleXchange" loudspeaker cabinets, and I like this approach a lot. I am not a woodworker by any stretch of the imagination, and IKEA happy provides me with inexpensive yet good looking if faux wood surfaces, to which I can add baffles etc. That same concept is being followed here. I will use a large "high cabinet" frame that measures 88" tall, 24" wide, and 24" deep. This will be used horizontally to form the sides of the enclosure. On top of the table will be a LAGAN countertop, 96" long, 24-5/8" wide, and 1-1/8" thick butcherblock slab. I will add six or eight legs to lift the frame off of the ground, making the top about 30" off of the ground. Too high for a coffee table really, but what the hell. Maybe it will end up being one of those long tables that you put behind the couch, along the wall...

                        The actual subwoofer enclosures will be built in a similar fashion as my previous IKEA subs:
                        Click image for larger version

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                        Essentially, I will built two subenclosures, like two separate subs with driver and PR mounted in opposite sides (PRs to the outside, RSS390's at middle). This will cancel out cabinet vibration. The subenclosures will include a MDF top and bottom - the IKEA cabinet frame is acting only like a decorative shell or the most part and the countertop just rests on top and is not actually functioning as a wall in either subenclosure. Rather than having any drivers on the sides, drivers and PRs will be located in large openings and the sound wave will pass out the bottom. It is pretty simple to add some MDF baffles and braces to the "inside" of the IKEA cabinets and I know of a very strong adhesive to bond these to the frames. The amp will be located on one end, vertically, in a small compartment with some sound dampening to adsorb some of the fan noise.

                        The great thing is that the IKEA parts for this monster cabinet are very inexpensive! The cabinet frame costs $90 and the countertop $60. Add in about $80 in MDF and some glue and fasteners and that's about it. Total materials: $230. Not bad for something this large.

                        Just don't ask me to bring it to any DIY get-togethers without a team of movers to help me!

                        Ask me in another two years how this project is going... :o

                        -Charlie
                        Charlie's Audio Pages: http://audio.claub.net

                        Comment


                        • #13
                          Re: My big subwoofer build thread

                          Originally posted by Ray Tremblay View Post
                          ON THE SUBJECT OF DAMPING FACTOR: I've talked to an EE that has designed many amplifiers over the years (decades) and currently designs the amplifiers for Genz Benz. I've also talked to one of the EE's who currently designs the pro audio amps for Peavey. When a damping factor spec is posted, it's nothing more than a marketing tool (which obviously works). This spec should be ignored as it is completely useless to the end consumer with no knowledge of amplifier design. The amplifiers damping factor is much more complicated than just a single number on a spec sheet.

                          I've owned this amp (EP2500) in the past and have had great results with it in multiple pro audio sub applications.



                          ChasW has done testing/measurements on this amp, and has proven that it's THE REAL DEAL. (I can't find the link at the moment. I'll add it as soon as I do)

                          Behringer has a bad rap due to it's unethical business practices and terrible reliability issues. When their products actually work, they work well.
                          Ray,
                          Thank you for shedding some light on the damping facter. I was chatting about this topic the other day and I held firmly to the notion that stating a single damping factor spec was simply marketing. Damping is all relative to material being played, freq, and power demands. This marketing spec is similar to the contrast ratio if a tv. Some TV manufacturers example of 1,000,000:1 is no different than another companies 640,000:1
                          As for damping factor, I do think that the spec shown does at least have some resemblance to real life damping (for the well respected companies anyway) becasue back in my car audio days, you surely could tell the difference in amplifiers rated as say 500 damping factor, and those closer to 2500-3000. I know the numbers are BS but when you take one manufacturer, who rates products the same, their amp with the higher damping factor using their scale will show a marked improvement.

                          Comment


                          • #14
                            Re: My big subwoofer build thread

                            OK, slight revision to my recent post about the long coffee table concept...

                            It would probably be wiser to create two separate subs, that could be joined later as one unit, but that could also work separately in different parts of the room when/if desired. Going back to the IKEA well, this is a possibility:
                            AKURUM wall-top cabinet frame 36x24"
                            Click image for larger version

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                            As before, these would be used for the sides of the cabinet, and the LAGAN countertop used above. MDF would be used to create an enclosure inside, with all drivers inside the cabinet frame for a clean look.

                            For amplification, I have a pair of 500W plate amps that I can use in place of the EP2500...

                            This would not provide quite the same cabinet internal volume as the large high cabinet frame, but in the end this might be the best solution and the response is nearly the same. I should be able to get about 6.5 cuft of usable space, which still brings the passband response down to near 10Hz at full power.

                            Again, these cabinet frames are not expensive, about $50 each. I could use the same $60 countertop, so the overall cost is about the same...

                            -Charlie
                            Charlie's Audio Pages: http://audio.claub.net

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