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  • The Boom Unit

    Introduction:
    For years I've owned a Logitech X-503 multimedia system that provides the sound for my main PC. At the time I purchased it, it was around US$70, which was quite a good price, considering that the system consisted of 10" small 1.7" drivers (two in each of the satellites) and a 4" driver in a gnat-sized vented box to provide bass duties. The satellites aren't too bad (nothing that can't be fixed with a bit of EQ), but the bass unit was definitely a study in compromises. The driver has a relatively high Q and the box is tuned to around 80 Hz, so the end result is a big peak around that frequency that makes everything sound "robust", until you notice that a lot of a low notes are actually missing. Interestingly enough, the output from the bass unit's amplifier is not rolled off (significantly) below 80 Hz, which means that the driver does end up getting signal below Fb. The amplifier must incorporate some sort of dynamic EQ though, because even when turned up, there's no sounds of significant distress from the driver when it's fed very low frequencies.

    Anyway, with no very low frequencies to speak off, the "bass unit' had to go, and I decided to replace it with a subwoofer design based around the Dayton DCS205 8" subwoofer driver. For this particular design, I wanted a solution that had an Fb around 32 Hz, as my intent was to use EQ to address the low-frequency performance in-room, and the lowest band on the simple EQ included with the PC was centered at 32 Hz.

    So, I went to work on a new Excel workbook to describe the design, and I eventually came up with an MTL-looking design that was around 26.6 litres net and 45 litres gross, primarily due to the use of 21mm MDF for the build. The corresponding Hornresp sim showed a response that gently slopes down from 100 Hz to just above 30 Hz, just right for what I wanted. The workbook is also able to export a data file in the required format for easy importing into Hornresp, which made the whole simulation process a lot easer. Note that in Hornresp I opted to include in the model the effect of puttling some polyester fiberfill in the first section of the alignment. According to the sim, this would drop the resonance frequency (Fb) a bit, and smooth the response in the passband. Real-world measurements of the built subwoofer confirm that the added filling achieved these goals. Note also that the predicted frequency response in Hornresp is slightly optimistic, as Hornresp does not account for the impact of box losses. The rolloff from 100 Hz to 40 Hz is slightly steeper than predicted, and the "corner" at around 32 Hz is a bit shallower.

    Results
    After building the box, the first thing I did was perform an impedance measurement, which suggested that Fb, which turned out to be around 33 Hz, was pretty close to that predicted by the Hornresp simularion I also performed some linearity tests, to see if subwoofer maintained its frequency response at higher input power levels (which would happen if port compression was low). Hornresp predicted that Xmax would start to be exceeded at 24.3V (the equivalent of just under 150W), and I tested at up to 25.4V (the equivalent of just over 160W). The subwoofer maintained its composure up to about -6dB, and then vent noise started to become audible. Still, it reached around -3dB from the peak input, or basically 80W, before the response at lower frequencies changed noticeably (according to the measured response), As this subwoofer is not going to see anything more than 25W in actual use (it's going to be powered by the Logitech X530's amplifier, after all), it looks like I'm never going to hear that vent noise - a good thing because this subwoofer is going to be located about three feet away from me, right behind my desk. Peak THD is also pretty low at 5.22% at about 20W input, most of which is 2nd order, so a lot less noticeable and objectionable. This suggests that the subwoofer is going to sound "clean" in actual use, and I can confirm that it does.

    Conclusion
    After messing around with tapped horns for awhile, it was nice to return to relatively simple alignment, and this little subwoofer based around an 8" driver delivers all that I need for my multimedia system. If I was building this for another purpose, I might likely opt to flare the port's exit a bit, to reduce vent noise and compression effects at high volumes even further.

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    Brian Steele
    www.diysubwoofers.org

  • #2
    Nice, it's got a pretty cool form factor as well. Veneer covered MDF?
    I like to read about explanations on why various design decisions were made as they relate to specific goals, it helps me learn how things connect.

    It's amazing how much bass you can get out of a measly 25 watts, right?

    Looks good!

    TomZ
    *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
    *Part 2 *Gluing multiple curved laminations of HDF *Cello's Speaker Project Page

    *Building the "Micro-B 2.1 Plate Amplifier -- Part 1 * Part 2 * Part 3 * Part 4 * * Part 5 'Review' * -- Assembly Instructions PDF

    Comment


    • #3
      Originally posted by tomzarbo View Post
      Nice, it's got a pretty cool form factor as well. Veneer covered MDF?
      I like to read about explanations on why various design decisions were made as they relate to specific goals, it helps me learn how things connect.

      It's amazing how much bass you can get out of a measly 25 watts, right?

      Looks good!

      TomZ
      ​Thanks Tom. The finish is just flat black latex paint handbrushed onto MDF treated with several applications of aliphatic resin (wood glue) thinned with water. Some areas look like they could have used with a bit more sealing, so I might do a light sanding and repainting this week, just one more thing in a list of boo-boos that happened during this build, albeit a minor one compared to some of the others. Off the top of my head:

      ​1. Designed the box around the use of 18mm MDF, only to discover what I had on hand was 21mm MDF. Luckily I found this out before cutting wood! Easily fixed via the optimizing featured that I built into the Excel workbook describing the design.

      ​2. Used a 30cm ruler for measurements. Some of the panels had to be 45cm wide. no problem - 30cm + 15cm = 45cm, right? So, I made the marks, went inside for a drink, came back outside, drew the cut lines where I'd marked the wood, cut the wood, then discovered that I'd made the cutline on the 30cm mark, not the 45cm mark. And of course there was no more wood left that was at least 45 cm wide!

      3. I decided to just cut some 15cm wide lengths, and join them onto the panels that were cut 15cm short. Than I stuck them together with wood glue, and clamped them, making sure that the panels were lined up as closely as possible, and left it overnight to dry.

      4. I went to continue work the next day on the build, only to discover that I'd somehow managed to stick the wrong panels together.

      5. I decided to break the panels apart and confirmed in the process that yes, it's actually true that wood glue is stronger than the wood. I ended up fracturing both panels.

      6. I fixed the broken panels (wood-glue, filler, clamps and sanding) and turned my attention to the doing the rest of the box, gluing the back panel and the internal separator panel into place, and left overnight to dry.

      7. I wake up next morning, check to see what the work I did looks like, and find out that I glued the separator panel in the wrong location (glued it below the guide line, not above). Sigh. BTW, remember what I said about the wood glue being stronger than the wood? More damaged panels and more repair work required. Well, at least this happened on the inside of the enclosure, so less visible...

      8. I finally finish the damned box, and it looks somewhat passable. I sanded it down smooth, gave it a few coats of DIY sealant, then placed it outside to let the sunlight properly dry it out before I commenced painting, and went inside to get a drink. Of course rain immediately started falling. Remember when I said I was using MDF? I managed to grab the box before it got too wet, but still the MDF sucked up some of that water like a sponge, even with several layers of sealant applied to it. Still, it didn't look like any damage was done, so I left it overnight to dry.

      ​9. The next morning, I examined the box and found that there was minor swelling along one edge of the box. I'm not sure if was there before the rainstorm or arrived afterwards, but it needed to be fixed. Another day lost spent sanding, clamping and sealing.

      ​10. One week after doing a build that should have taken one day, the box is finally finished. I screw the driver into it and perform the linearity tests, only to have the subwoofer give a loud pop and suddenly go silent during the -3dB test. Goddam it, did I blow the driver? I took it out and tested it, and it tested fine, phew! I screwed it back into the box, ran the test, and still no sound from the subwoofer. Strange. I took the driver out, tested it again and it tested fine. So this time I screwed it back into the box while it was playing a signal. The signal cut off as I started screwing the screw directly opposite where the terminals for the driver are located. Turns out that when the driver cutout is 21mm deep, that's just enough to squeeze the driver's push terminals in, and the blinking connection cables were falling out. Chamfering the driver cutout where its terminals would be located solved the problem.

      Other than the above, this build was a pretty easy one .

      ​As for how loud it gets with 25W, yes, that was a pleasant surprise. In fact, I can't turn it up - the top of the desk starts to vibrate .
      Brian Steele
      www.diysubwoofers.org

      Comment


      • #4
        One thing to note - the internal panel that looks like a brace is actually serving another important purpose. I modelled this alignment as a MLTL in Hornresp, and I needed to get the response as smooth as possible up to 200 Hz due to the fact that the satellites are pretty small and the x-over point between the subwoofer and satellites was pretty high as a result. The Hornresp sim suggested that with a certain amount of fill at the start of the "line" and with the driver placed a specific distance down it, I should be able to achieve the response I was looking for. The attached images show what Hornresp predicted the response would look like if (1) there was no filling at all, and (2) if the driver was mounted just about 5cm further down (i.e. about the center of the panel). Notice the "glitch" just above 200 Hz? In reality, those peaks would be a lot less peaky because Hornresp sims don't include the impact of box losses, but still I opted to try and achieve as smooth a response in the sim as possible, with the hope that this would also be reflected in the build. And the build does show a nice smooth response up to and beyond 200 Hz.

        Click image for larger version

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        Brian Steele
        www.diysubwoofers.org

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        • #5
          Thanks for sharing your thread here. Though I don't know about this speak more. But I used a Logitech Micro-phone 2 years ago. Overall, it was nice brand and I was happy to use it.

          Comment


          • #6
            Originally posted by Brian Steele View Post

            ​Thanks Tom. The finish is just flat black latex paint handbrushed onto MDF treated with several applications of aliphatic resin (wood glue) thinned with water. Some areas look like they could have used with a bit more sealing, so I might do a light sanding and repainting this week, just one more thing in a list of boo-boos that happened during this build, albeit a minor one compared to some of the others. Off the top of my head:

            ​1. Designed the box around the use of 18mm MDF, only to discover what I had on hand was 21mm MDF. Luckily I found this out before cutting wood! Easily fixed via the optimizing featured that I built into the Excel workbook describing the design.

            ​2. Used a 30cm ruler for measurements. Some of the panels had to be 45cm wide. no problem - 30cm + 15cm = 45cm, right? So, I made the marks, went inside for a drink, came back outside, drew the cut lines where I'd marked the wood, cut the wood, then discovered that I'd made the cutline on the 30cm mark, not the 45cm mark. And of course there was no more wood left that was at least 45 cm wide!

            3. I decided to just cut some 15cm wide lengths, and join them onto the panels that were cut 15cm short. Than I stuck them together with wood glue, and clamped them, making sure that the panels were lined up as closely as possible, and left it overnight to dry.

            4. I went to continue work the next day on the build, only to discover that I'd somehow managed to stick the wrong panels together.

            5. I decided to break the panels apart and confirmed in the process that yes, it's actually true that wood glue is stronger than the wood. I ended up fracturing both panels.

            6. I fixed the broken panels (wood-glue, filler, clamps and sanding) and turned my attention to the doing the rest of the box, gluing the back panel and the internal separator panel into place, and left overnight to dry.

            7. I wake up next morning, check to see what the work I did looks like, and find out that I glued the separator panel in the wrong location (glued it below the guide line, not above). Sigh. BTW, remember what I said about the wood glue being stronger than the wood? More damaged panels and more repair work required. Well, at least this happened on the inside of the enclosure, so less visible...

            8. I finally finish the damned box, and it looks somewhat passable. I sanded it down smooth, gave it a few coats of DIY sealant, then placed it outside to let the sunlight properly dry it out before I commenced painting, and went inside to get a drink. Of course rain immediately started falling. Remember when I said I was using MDF? I managed to grab the box before it got too wet, but still the MDF sucked up some of that water like a sponge, even with several layers of sealant applied to it. Still, it didn't look like any damage was done, so I left it overnight to dry.

            ​9. The next morning, I examined the box and found that there was minor swelling along one edge of the box. I'm not sure if was there before the rainstorm or arrived afterwards, but it needed to be fixed. Another day lost spent sanding, clamping and sealing.

            ​10. One week after doing a build that should have taken one day, the box is finally finished. I screw the driver into it and perform the linearity tests, only to have the subwoofer give a loud pop and suddenly go silent during the -3dB test. Goddam it, did I blow the driver? I took it out and tested it, and it tested fine, phew! I screwed it back into the box, ran the test, and still no sound from the subwoofer. Strange. I took the driver out, tested it again and it tested fine. So this time I screwed it back into the box while it was playing a signal. The signal cut off as I started screwing the screw directly opposite where the terminals for the driver are located. Turns out that when the driver cutout is 21mm deep, that's just enough to squeeze the driver's push terminals in, and the blinking connection cables were falling out. Chamfering the driver cutout where its terminals would be located solved the problem.

            Other than the above, this build was a pretty easy one .

            ​As for how loud it gets with 25W, yes, that was a pleasant surprise. In fact, I can't turn it up - the top of the desk starts to vibrate .
            Wow, Brian...

            We've all been there... but it sounds like you were there an awful lot for that project! I'm just teasing, man. Truthfully, you can't tell any of that bad stuff happened by looking at it. It appears to be just a real nice project. I always have a few buggers happen with every build I do... it's just par for the course.

            How you fix them and keep on going is more important as we both know.

            Problem #10 is kind of amazing. Some of us may have just hurled the whole thing across the work shop and gone and had a root beer or something and just forgot about it. Kudos for continuing.

            TomZ
            *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
            *Part 2 *Gluing multiple curved laminations of HDF *Cello's Speaker Project Page

            *Building the "Micro-B 2.1 Plate Amplifier -- Part 1 * Part 2 * Part 3 * Part 4 * * Part 5 'Review' * -- Assembly Instructions PDF

            Comment


            • #7
              Originally posted by Brian Steele View Post

              ​Thanks Tom. The finish is just flat black latex paint handbrushed onto MDF treated with several applications of aliphatic resin (wood glue) thinned with water. Some areas look like they could have used with a bit more sealing, so I might do a light sanding and repainting this week, just one more thing in a list of boo-boos that happened during this build, albeit a minor one compared to some of the others. Off the top of my head:

              ​1. Designed the box around the use of 18mm MDF, only to discover what I had on hand was 21mm MDF. Luckily I found this out before cutting wood! Easily fixed via the optimizing featured that I built into the Excel workbook describing the design.

              ​2. Used a 30cm ruler for measurements. Some of the panels had to be 45cm wide. no problem - 30cm + 15cm = 45cm, right? So, I made the marks, went inside for a drink, came back outside, drew the cut lines where I'd marked the wood, cut the wood, then discovered that I'd made the cutline on the 30cm mark, not the 45cm mark. And of course there was no more wood left that was at least 45 cm wide!

              3. I decided to just cut some 15cm wide lengths, and join them onto the panels that were cut 15cm short. Than I stuck them together with wood glue, and clamped them, making sure that the panels were lined up as closely as possible, and left it overnight to dry.

              4. I went to continue work the next day on the build, only to discover that I'd somehow managed to stick the wrong panels together.

              5. I decided to break the panels apart and confirmed in the process that yes, it's actually true that wood glue is stronger than the wood. I ended up fracturing both panels.

              6. I fixed the broken panels (wood-glue, filler, clamps and sanding) and turned my attention to the doing the rest of the box, gluing the back panel and the internal separator panel into place, and left overnight to dry.

              7. I wake up next morning, check to see what the work I did looks like, and find out that I glued the separator panel in the wrong location (glued it below the guide line, not above). Sigh. BTW, remember what I said about the wood glue being stronger than the wood? More damaged panels and more repair work required. Well, at least this happened on the inside of the enclosure, so less visible...

              8. I finally finish the damned box, and it looks somewhat passable. I sanded it down smooth, gave it a few coats of DIY sealant, then placed it outside to let the sunlight properly dry it out before I commenced painting, and went inside to get a drink. Of course rain immediately started falling. Remember when I said I was using MDF? I managed to grab the box before it got too wet, but still the MDF sucked up some of that water like a sponge, even with several layers of sealant applied to it. Still, it didn't look like any damage was done, so I left it overnight to dry.

              ​9. The next morning, I examined the box and found that there was minor swelling along one edge of the box. I'm not sure if was there before the rainstorm or arrived afterwards, but it needed to be fixed. Another day lost spent sanding, clamping and sealing.

              ​10. One week after doing a build that should have taken one day, the box is finally finished. I screw the driver into it and perform the linearity tests, only to have the subwoofer give a loud pop and suddenly go silent during the -3dB test. Goddam it, did I blow the driver? I took it out and tested it, and it tested fine, phew! I screwed it back into the box, ran the test, and still no sound from the subwoofer. Strange. I took the driver out, tested it again and it tested fine. So this time I screwed it back into the box while it was playing a signal. The signal cut off as I started screwing the screw directly opposite where the terminals for the driver are located. Turns out that when the driver cutout is 21mm deep, that's just enough to squeeze the driver's push terminals in, and the blinking connection cables were falling out. Chamfering the driver cutout where its terminals would be located solved the problem.

              Other than the above, this build was a pretty easy one .

              ​As for how loud it gets with 25W, yes, that was a pleasant surprise. In fact, I can't turn it up - the top of the desk starts to vibrate .
              They look great! I really like the black finish, slot "port", and recessed baffle.

              I feel your pain with the challenges you listed, sounds like every one of my projects! The difference is yours ended up perfect where mine leave reminders of every error
              Projects:

              transcenD: http://techtalk.parts-express.com/fo...5035-transcend
              Summits: http://techtalk.parts-express.com/sh...75-The-Summits
              References: http://techtalk.parts-express.com/sh...-My-References
              Vintage Style 2-way: http://techtalk.parts-express.com/sh...-vintage-2-way

              Comment

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