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  • Isobaric subwoofer design, many questions!

    I'm trying to understand the basic concept and would like some help in figuring out a few design choices.

    I'm interested in building a ported isobaric subwoofer where two 10" drivers are mounted face-to-face on the front baffle. This means that the port(s) and the back of one of the woofers will be facing towards the listening area. I posted a few pictures to better illustrate this.

    If I use two 4 ohm woofers, can I wire them in series with one wired out of phase? If I use two 8 ohm woofers, can I wire them in parallel with one wired out of phase?

    The other thing that I'm trying to grasp is this: The baffle will probably have to be at least 1" thick so the long throw woofers don't hit each other because they're mounted face to face. Is it necessary to take this volume between the two woofers into account when designing? Because theoretically, what if the baffle is 5" thick, now there would be quite a bit of volume between the woofers. Is it better that this volume is smaller, or bigger?

    And my last question is regarding higher frequency cutoff. Since the back of the woofer is facing into the listening area, is it possible to lets say achieve a higher cutoff frequency of 160Hz for example? Or will it be much lower, 80Hz and bellow?
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  • #2
    Re: Isobaric subwoofer design, many questions!

    Originally posted by burokas View Post
    I'm trying to understand the basic concept and would like some help in figuring out a few design choices.

    I'm interested in building a ported isobaric subwoofer where two 10" drivers are mounted face-to-face on the front baffle. This means that the port(s) and the back of one of the woofers will be facing towards the listening area. I posted a few pictures to better illustrate this.

    If I use two 4 ohm woofers, can I wire them in series with one wired out of phase? If I use two 8 ohm woofers, can I wire them in parallel with one wired out of phase?

    The other thing that I'm trying to grasp is this: The baffle will probably have to be at least 1" thick so the long throw woofers don't hit each other because they're mounted face to face. Is it necessary to take this volume between the two woofers into account when designing? Because theoretically, what if the baffle is 5" thick, now there would be quite a bit of volume between the woofers. Is it better that this volume is smaller, or bigger?

    And my last question is regarding higher frequency cutoff. Since the back of the woofer is facing into the listening area, is it possible to lets say achieve a higher cutoff frequency of 160Hz for example? Or will it be much lower, 80Hz and bellow?
    First, I will answer your questions, then I will give you some advice.

    Yes, you can wire two drivers in series or in parallel, as long as you are reversing the polarity for one of the woofers.

    The distance between the drivers, if it is a subwoofer is not critical, but there is no reason for it to be large. I need to ask this - who would their excursion cause them to hit if they are moving together in the same direction? I need to make sure we are on the same page here.

    80Hz or 160Hz? I see no difference between the front or back of the driver?

    Now, some advice - I really wouldn't recommend this approach. IMO the trade-offs for an Isobaric or compound design out-weigh the benefits. What you gain is half the Vas, so you get the same cut-off in half the enclosure volume. In exchange you give up 6dB in sensitivity. If you parallel two 8 ohms drivers you can pull yourself back to the original sensitivity, but not efficiency. If you go series you lose another 6dB. Not worth it in my mind. Most of the time we can live the extra box volume. Yes, there is a reduction in even order distortion but this is still not a worthy trade in my mind.

    Jeff B.
    Click here for Jeff Bagby's Loudspeaker Design Software

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    • #3
      Re: Isobaric subwoofer design, many questions!

      I pretty much agree with Jeff B, but if you were making a small sub, you could gain some extension. For example, I have an 8" woofer that in a 4 cu-ft box has an F3 of about 20 hz. With an Isobaric, the box could be 2 cu-ft for the same F3. I believe that the same woofer in a 2.5 cu-ft box would have an F3 around 25 to 28 hz. It's been a while since I calculated boxes for this driver, so I'm going from memory.

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      • #4
        Re: Isobaric subwoofer design, many questions!

        Thank you for the replies and for answering all of the questions. I would like to clarify something just one more time. Even though the woofer's rear will be facing the listening area, I will still be able to get a clean 160Hz signal?

        Comment


        • #5
          Re: Isobaric subwoofer design, many questions!

          You may get a clean 160Hz signal, but that is independent of the isobarik design. This design effectively fell by the wayside with drivers with much lower Vas. There really is no benefit for it now. The losses in efficency simply outweigh the gains in power handling, and lower distortion.
          https://www.facebook.com/Mosaic-Audi...7373763888294/

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          • #6
            Re: Isobaric subwoofer design, many questions!

            You can wire the Voice Coils in either series or parallel. In series, just add the impedance. In Parallel, divide by 2. If you wire 2 4ohm drivers in series, the result is 8ohms and half the available amplifier power. If you wire 2 8ohm drivers in parallel, the result is 4ohms, and twice the amplifier power ( assuming the amp can drive a 4ohm load without difficulty )

            As long as your baffle is at least 3/4 to an inch thick, you should not have a problem with the cones as they will be wired opposite polarity and will be moving in or out together. (they won't bump into each other)

            The advantages of an Isobaric design are:
            1. enclosure volume requirements are cut in half.
            2. you can use twice the amplifier power. (+3db)

            The disadvantages of an Isobaric design are:
            1. Twice the cost of drivers.
            2. Lower sensitivity. ( -6db )
            3. Given the same port diameter, Port length is doubled. (uses more interior volume.)

            The cutoff frequency is in the crossover, either active in the amp, or passive is a subwoofer crossover (expensive), but will not be affected by using an Isobaric design.

            Hope this helps clear things up a little.

            Comment


            • #7
              Re: Isobaric subwoofer design, many questions!

              Originally posted by burokas View Post
              Thank you for the replies and for answering all of the questions. I would like to clarify something just one more time. Even though the woofer's rear will be facing the listening area, I will still be able to get a clean 160Hz signal?
              At 160hz you're getting high enough that the output will be directional. Typically if you're running subs that high they are facing forward in bass bins or integrated in the cabinet.

              You're getting good advice about isobarics. I wouldn't do it unless volume was the only consideration and I was using 4" woofers to build a bose wave radio. You are simply doubling your cost and cutting efficiency for the sake of reduced box volume.

              Comment


              • #8
                Re: Isobaric subwoofer design, many questions!

                Originally posted by MGrant1957 View Post
                You can wire the Voice Coils in either series or parallel. In series, just add the impedance. In Parallel, divide by 2. If you wire 2 4ohm drivers in series, the result is 8ohms and half the available amplifier power. If you wire 2 8ohm drivers in parallel, the result is 4ohms, and twice the amplifier power ( assuming the amp can drive a 4ohm load without difficulty )

                As long as your baffle is at least 3/4 to an inch thick, you should not have a problem with the cones as they will be wired opposite polarity and will be moving in or out together. (they won't bump into each other)

                The advantages of an Isobaric design are:
                1. enclosure volume requirements are cut in half.
                2. you can use twice the amplifier power. (+3db)

                The disadvantages of an Isobaric design are:
                1. Twice the cost of drivers.
                2. Lower sensitivity. ( -6db )
                3. Given the same port diameter, Port length is doubled. (uses more interior volume.)

                The cutoff frequency is in the crossover, either active in the amp, or passive is a subwoofer crossover (expensive), but will not be affected by using an Isobaric design.

                Hope this helps clear things up a little.
                Disadvantage #3 is not correct. Port length is controlled only by cabinet volume and tuning frequency. The number of drivers and whether Isobaric or conventional will have no effect on port length.

                Sensitivity is a little more complicated because it will depend on the final impedance compared to a single driver impedance.
                Click here for Jeff Bagby's Loudspeaker Design Software

                Comment


                • #9
                  Re: Isobaric subwoofer design, many questions!

                  Originally posted by MGrant1957 View Post
                  You can wire the Voice Coils in either series or parallel. In series, just add the impedance. In Parallel, divide by 2. If you wire 2 4ohm drivers in series, the result is 8ohms and half the available amplifier power. If you wire 2 8ohm drivers in parallel, the result is 4ohms, and twice the amplifier power ( assuming the amp can drive a 4ohm load without difficulty )

                  As long as your baffle is at least 3/4 to an inch thick, you should not have a problem with the cones as they will be wired opposite polarity and will be moving in or out together. (they won't bump into each other)

                  The advantages of an Isobaric design are:
                  1. enclosure volume requirements are cut in half.
                  2. you can use twice the amplifier power. (+3db)

                  The disadvantages of an Isobaric design are:
                  1. Twice the cost of drivers.
                  2. Lower sensitivity. ( -6db )
                  3. Given the same port diameter, Port length is doubled. (uses more interior volume.)

                  The cutoff frequency is in the crossover, either active in the amp, or passive is a subwoofer crossover (expensive), but will not be affected by using an Isobaric design.

                  Hope this helps clear things up a little.
                  Advantage #2 is a little misleading as well. The maximum acoustic power that can be delivered to the room is not changed . . . it is excursion (swept area) limited to the same power that can be delivered by just one of the drivers.

                  "Isobaric" loading is a "solution" looking for a problem . . . and when it finds one there is always a better solution available.
                  "It suggests that there is something that is happening in the real system that is not quite captured in the models."

                  Comment


                  • #10
                    Re: Isobaric subwoofer design, many questions!

                    If you don't like the rear of the driver facing the listening area, you can have both drivers facing the same way, but the construction of the enclosure is a little more complicated (just remember to wire the drivers with the same polarity).

                    Statements: "They usually kill the desire to build anything else."

                    Comment


                    • #11
                      Re: Isobaric subwoofer design, many questions!

                      I've built a few isobaric subs, and a few with the back of the sub facing out. It looks cool to see the "guts" of the woofer facing out, I love the look of the drivers magnets, hook up cables, etc.

                      But, you get a lot of noise from a high excursion subwoofer if the back side is facing out. Most have vented pole pieces to cool the magnet/voice coil. That vent chuffs and huffs on low frequency content -- it's audiable, trust me. Also, depending on the driver, you may have the "leads" that connect your terminals to the voice coil, in some cases, they can make noise, "lead slapping" -- as well. There may even be other noises from the suspension, etc.

                      Having the driver face cone-out is the quietest route. If you want to go isobaric and reverse mount the drivers, there's no problem doing so, but just as a test, try running some sweeps and test tones through your intended drivers and listen to the rear of them to see if the "noises" they make additional to the actual tones are tolerable for you. Have fun, and post a few pics of your creation when you decide what route you're going to take.

                      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

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                      • #12
                        Re: Isobaric subwoofer design, many questions!

                        What it comes down to is: What are you trying to achieve with an isobarik? If it's halving of the net cabinet volume (net being the operative word, not inclusive of the volume of the second driver and the plenum) and are willing to pay for two drivers to get it then go for it. But if that's not your intent build a standard alignment.
                        www.billfitzmaurice.com
                        www.billfitzmaurice.info/forum

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                        • #13
                          Very quick question, just want to clear something up about isobaric sensitivity

                          If I were to hook up two 88dB 8 ohm woofers in parallel (non isobaric), I'd increase sensitivity to 94db (4ohm, double the power needed)
                          Hooking up the same woofers in parallel but Isobaric, am I still at 88dB?

                          Comment


                          • #14
                            Yes, you are at the same sensitivity.

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                            • #15
                              Thanks Ani. For some reason I thought it was -3dB not -6dB (compared to standard parallel)

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