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thoughts on sub design

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  • #16
    Re: thoughts on sub design

    Originally posted by billfitzmaurice View Post
    Series resistors are for burning off power, which is of benefit when level matching mids and tweeters, but of less than zero value with woofers IMO. With the wide variety of woofers available there's just no need to force square pegs into round holes with resistors. If the driver Qts demands a vented box then use a vented box. If you must have a sealed alignment use a driver appropriate for it.
    Bill F. please do not comment on anything speakers/electronics related as you misdirect others whom might be trying to learn something.

    System efficiency is governed by the IRON LAW and no amount of series resistance can change it in the slightest.

    That is once you know Fs and Vas then system efficiency ([email protected]) will be determined by Qe (the higher Qe the lower system efficiency will be).

    Doesn't matter if the driver starts with Qe right where you want it or you need to triple Re by adding external resistors in series with the voice coil to achieve the desired Qe, efficiency will not (cannot) be anything other than what is dictated by the iron law.

    Comment


    • #17
      Re: thoughts on sub design

      Originally posted by daryl View Post
      Bill F. please do not comment on anything speakers/electronics related as you misdirect others whom might be trying to learn something.

      System efficiency is governed by the IRON LAW and no amount of series resistance can change it in the slightest.

      That is once you know Fs and Vas then system efficiency ([email protected]) will be determined by Qe (the higher Qe the lower system efficiency will be).

      Doesn't matter if the driver starts with Qe right where you want it or you need to triple Re to achieve the desired Qe, efficiency will not (cannot) budge.
      Bill does a fairly good job of explaining most things.
      Instead of attacking Bill, why don't you just do a courteous follow up?
      I'm sure you can "clean up" his statements without making them personal attacks.

      Not all of us DIY'ers are interested in all the heavy duty engineering principals, formulas.
      We just want it to work, the first time, every time and to enjoy the fruits of our labors.
      You're taking the fun out of it.
      Loudspeaker building is as much an art as it is science. Balance is needed.;)

      Comment


      • #18
        Subwoofers 101

        The W8-740P you have chosen is an interesting driver.

        This driver has a heavy moving mass which will yield small enclosures for a given bass extension.

        A 12mm Xmax and 220cm^2 Sd gives this driver a p-p displacement of 528cm^3 and means this drivers output will be strong for an 8" driver, but if you are using this driver you are building a small, modest subwoofer and should expect as much (adding more would be great).

        Low bass extension in a small enclosure will of course mean low efficiency but this is inconsequential as you will be able to fully utilize 528cm^3 p-p displacement with moderate power levels even with low efficiency.

        I have entered the specs for this driver into LEAP and modeled the drivers Le characteristic by creating a semi-inductance that is three quarters the way between a resistor and inductor (closer to an inductor).

        I have adjusted the semi-inductance value so that Mms and Le resonate just below 100hz with the driver not in an enclosure like the impedance measurement shown in the manufacturers data sheet.

        With drivers like this Le has a profound influence on the resulting transfer function so a reasonable Le model is important for a good system model.

        A sealed enclosure with an Fs=28hz driver will not yield very much bass extension without an active filter designed to reshape it's response.

        A ported enclosure with a Fs=28hz driver can achieve sufficient extension although in a small enclosure you might wan't to use a a passive radiator at additional cost rather than an ungainly large vent.

        A third-order-sealed system will achieve bass extension below Ftc and an Fs of 28hz is quite usable.

        I will model the third-order-sealed system.

        A 0.5ft^3 net, unstuffed enclosure will result in an Ftc about 45hz and looks like a good place to start.

        Next I will increase Re by 2ohms by adding resistance (two of these http://www.partsexpress.com/pe/showd...rtnumber=017-4 parallel connected) in series with the voice coil which will transform this driver into an 8ohm driver with a Qts of 0.48 and allow alignments with lower reaching bass extension.

        Finally I will add 1000uf of capacitance (two of these http://www.partsexpress.com/pe/showd...number=027-378 parallel connected) in series with the voice coil which will resonate with the compliance of the enclosure and driver suspension at 24hz boosting low frequency output while reducing the peak just above Ftc.

        Here is the resulting transfer function.


        You see the resulting sensitivity/efficiency is 83.25db both [email protected] and [email protected] since the system is 8ohm nominal.

        The transfer function is intentionally falling with lower frequency to yield a flatter inroom response with roomgain and not waste cone excursion by overemphasizing low bass.

        You can expect roughly 3db/oct roomgain below 80hz which means 5db roomgain at 25hz, looking at 25hz on the chart you see the response has fallen to -8db which means an inroom F3 of about 25hz (impressive for 0.5ft^3 sealed).

        Here is the excursion with 100 watts into 8 ohms.


        40hz is a good place to rate the excursion limited power handling for a sub.

        Here you see with 100 watts the subs 12mm Xmax is utilized at 40hz yeilding a rating of 100 watts at 8 ohms and 103.25db output.

        Notice excursion is falling below the passband (a characteristic of third-order-sealed systems).

        Here is the impedance for the system.


        Between 32hz and 80hz impedance is 8 ohms or higher.

        The impedance minimums are [email protected] and [email protected] which is very reasonable for an 8ohm nominal system.

        Obviously you would use plain amplifier with flat frequency response for this system (no bass boost).

        Here is a schematic for the system.


        Three resistors and three capacitors are shown in the diagram arbitrarily.

        For this design you would use only two of each but in the spirit of the diagram above.
        Last edited by daryl; 09-15-2008, 12:38 PM.

        Comment


        • #19
          Re: thoughts on sub design

          Bill does a fairly good job of explaining most things.
          Instead of attacking Bill, why don't you just do a courteous follow up?
          I'm sure you can "clean up" his statements without making them personal attacks.

          Not all of us DIY'ers are interested in all the heavy duty engineering principals, formulas.
          We just want it to work, the first time, every time and to enjoy the fruits of our labors.
          You're taking the fun out of it.
          Loudspeaker building is as much an art as it is science. Balance is needed.;)
          Hi AE,

          This type of thing is reserved for Bill F., Jon Risch and the like.

          Theres no excuse, period.
          Last edited by daryl; 09-15-2008, 11:35 AM.

          Comment


          • #20
            Re: thoughts on sub design

            Originally posted by daryl View Post
            Hi AE,

            This type of thing is reserved for Bill F., Jon Risch and the like.

            Theres no excuse, period.
            Jon Risch?
            O crap.
            He is another one of those people who is full of himself.

            Comment


            • #21
              Re: thoughts on sub design

              Another build idea on the cheap would be the $29 shielded DVC 8" in 1 - 1.25 ft^3 tuned to 33ish hz (3"x12.5"ish). You'll only need 70-100 watts to use all of its Xmax. From memory, it's good for 100db+ from 30hz up. I don't have modelling software in the office to see how it compares to the W8. This is basically the Triskasub from the projects section, using a traditional port instead of the passiver radiators.

              This is essentially what I'm building for my parents right now. I got them the Spherex buyout set and will drop the DVC-8 into the "sub", plug one of the ports to drop the tuning frequency, and run it off of an SA240 I have in the parts closet. They're not looking for reference level output and it's a relatively small room, so I expect this to work really well for them.

              -Brent

              Comment


              • #22
                Re: thoughts on sub design

                Originally posted by daryl View Post
                Bill F. please do not comment on anything speakers/electronics related as you misdirect others whom might be trying to learn something.
                I'm familiar with Bill's professional credits. Daryl, what are yours, if any?
                Also, please explain how 2 ohms of resistance in series with a 4 ohm speaker does not burn off 1/3 the amps output in the resistor.

                Comment


                • #23
                  Re: thoughts on sub design

                  Originally posted by Sounder View Post
                  Also, please explain how 2 ohms of resistance in series with a 4 ohm speaker does not burn off 1/3 the amps output in the resistor.
                  It does burn off 1/3 of the power, but when amp watts cost less than a gallon of gas you're free to waste power if you want, if it helps meet the design goals. And you still have enough left over to launch the cone.

                  Bill doesn't like to burn power. I don't like to burn power. But it is just personal preference.

                  Comment


                  • #24
                    Re: thoughts on sub design

                    Originally posted by wg_ski View Post
                    It does burn off 1/3 of the power, but when amp watts cost less than a gallon of gas you're free to waste power if you want, if it helps meet the design goals. And you still have enough left over to launch the cone.

                    Bill doesn't like to burn power. I don't like to burn power. But it is just personal preference.
                    Thank you. the resistor qts change method Daryl has shown us on this website has a lot of applications. Just as Bill F's horns do. It is a shame that people want to be in separate camps over it. I use fullrange drivers as extended midranges. I am sure that Jordan 92 owners using them as full range think I am a fool. Well that is ok they make great midranges. crossed as low as 75hz and as high as 7k.

                    Comment


                    • #25
                      Re: thoughts on sub design

                      Originally posted by Sounder View Post
                      Also, please explain how 2 ohms of resistance in series with a 4 ohm speaker does not burn off 1/3 the amps output in the resistor.
                      It does...but so what? In this case, Darryl is simply proposing a way to make this driver work. Extension below 30Hz in less than a cubic foot is not easy to do, and there are tradeoffs. What's the difference between burning off a third of the power with series resistors and tripling (or more) the power via some transform or boost filter? They are simply ways of arriving at the same result (extended cutoff) via different means.

                      Comment


                      • #26
                        Re: thoughts on sub design

                        Originally posted by wg_ski View Post
                        Bill doesn't like to burn power.
                        As one would expect from a horn guy. (This really is all just part of a very old argument about efficiency that dates back to the beginnings of hi-fi. In pro-sound applications there is no argument: Efficiency wins every time.)

                        Comment


                        • #27
                          Re: thoughts on sub design

                          Originally posted by philiparcario View Post
                          Thank you. the resistor qts change method Daryl has shown us on this website has a lot of applications. Just as Bill F's horns do. It is a shame that people want to be in separate camps over it.
                          Seems to be an epidemic of that (black/white thinking) these days. I'm easy. I use whatever method gets the best results with what I have at hand. If that means a series resistor to stuff a driver into a small sealed box, so be it. Ditto if it means five cubic feet for a 15 inch high sensitivity woofer to get 94-95 db 1w/1m at 50 Hz. One does what one must, yes?

                          Comment


                          • #28
                            Re: thoughts on sub design

                            i am down with that, hey some guy did you read my post on a flexible port idea. I will try to find and bump up the thread. phil

                            Comment


                            • #29
                              Re: thoughts on sub design

                              Originally posted by wg_ski View Post

                              Bill doesn't like to burn power. I don't like to burn power. But it is just personal preference.
                              Burning power is just the half of it. Raising Qts via added resistance does give the potential to lower f3, but not only does it require more power, it also requires a larger box. Since the goal of the OP is as small a cab as possible the added resistance method is at cross purposes. It would be nice if getting substantially better performance from a sub was as simple a matter as adding a resistor, but the no free lunch rule as always applies.
                              www.billfitzmaurice.com
                              www.billfitzmaurice.info/forum

                              Comment


                              • #30
                                Re: thoughts on sub design

                                Originally posted by philiparcario View Post
                                i am down with that
                                Philip, you are da man! Get down with your funky self, wit all dat street talkin! :D
                                Building it big and playing it loud! Because we all know size really does matter, and a little over compensation never hurt anyone. :eek:

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