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  • What is "isobaric"?

    Is the Dayton RS1202A subwoofer considered an isobaric design?

    My understanding of the characteristics of an isobaric system is:

    - the two drivers share the same airspace, and are mounted cone-cone, cone-to-magnet, or magnet-to-magnet (as in this design).
    - drivers are wired out-of-phase

    Is this what defines an isobaric design, or are there other characteristics?

    I've heard some say "in an isobaric system you only see one driver" ... that is, one is always "hidden" in the cabinet somewhere. Is this a misconception?

  • #2
    Re: What is "isobaric"?

    I'm not sure this is an isobaric design, it makes no mention of the drivers being wired out of phase from each other.

    I'm also a bit skeptical of that enclosure; it looks well built but 1 12" HF series Dayton needs more than 2.1 cu. ft. (3.42 cu. ft. according to win ISD); however, when I plot the response of two drivers in the 2.1 cu. ft. space it provides an F3 in the mid 30's and a rise in response from 50-100 Hz so it could sound OK but I'd be worried about it sounding boomy, especially with room gain.

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    • #3
      Re: What is "isobaric"?

      Is the Dayton RS1202A subwoofer considered an isobaric design?
      Copy says "true acoustic suspension design"

      Compound enclosure employs two woofers working synchronously with a small sealed air volume contained between their membranes. isobaric (meaning constant pressure)
      Include front-to-back ( cone-to-magnet ). It would involve flipping polarity depending on orientation to sync movement.
      "Not a Speaker Designer - Not even on the Internet"
      “Pride is your greatest enemy, humility is your greatest friend.”
      "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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      • #4
        Re: What is "isobaric"?

        This is not isobaric, just a dual 12, same as if you placed both 12's on the same baffle. They're just on either side.

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        • #5
          Re: What is "isobaric"?

          My understanding (and I may be quite wrong, feel free to correct me :D) is that isobaric systems are used to make two drivers play well in an extremely undersized enclosure. Say you have driver X, and it requires 2 cubic ft sealed to play well. You mount it in a 1 cubic ft box, and then mount a second driver X on top of that so the two drivers essentially act as one driver with both cones moving in unison, coupled by the airspace between them. It has something to do with altering the TS parameters, where you would have 2x the moving mass, but also 2x the suspension and 2x the magnetic force, therefore altering the parameters in such a way that you can use 1/2 the box required for one driver. The downside is you gain no SPL and you need 2x the input power. It was a popular alignment back in the 80's for car audio, when you had a lot of drivers that required a refrigerator sized box. It was an easy way to get bigger bass to fit in the trunk. With modern driver technology (LMS ultra and Dayton Ref. HO)it's no longer an issue, and that would be my guess as to why you don't really see it much anymore. So, no the sub you linked to isn't isobaric, it's a simple dual driver sealed sub with the drivers mounted on opposite sides of the cab to minimize vibration. And yes, if it were isobaric, you would only see one driver.
          My modest builds:
          Armadillo TM, A.K.A. Lil' Dillo
          Tarkus/Armadillo build #2
          Armadillo Center Channel
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          • #6
            Re: What is "isobaric"?

            I know of two speaker companies that use isobaric, EgglestonWorks and Linn, in fact linn had a patent on the "back to front" isobaric loading as opposed to "front to front" or "back to back"

            http://egglestonworks.com/products/the-andra-iii/
            http://uhes.co.uk/news/new-linn-majik-isobarik.html
            "A reduction in distortion at bass frequencies. All bass drive units suffer from non-symmetry; the force required to push the cone forward is different to the force required to pull the cone inward. By mounting two bassdrive units in opposite directions (face to face) we can enforce symmetry and ensure a more linear response to the input current."
            David

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            • #7
              Re: What is "isobaric"?

              From a thermodynamic standpoint, "isobaric" means "constant pressure". In speaker design, one driver fires into another driver's enclosure, thus the pressure in the chamber is held constant.

              An isobaric design would be any that has a net zero change in pressure. Two drivers sharing one enclosure wired out of phase would accomplish this. As one driver pushes out, the other is going in. Another way would be to have the drivers in phase, but the magnet of one hanging out of the enclosure.

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              • #8
                Re: What is "isobaric"?

                Think of it as two drivers in series, acoustically.

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                • #9
                  Re: What is "isobaric"?

                  Originally posted by UGP View Post
                  "A reduction in distortion at bass frequencies. All bass drive units suffer from non-symmetry; the force required to push the cone forward is different to the force required to pull the cone inward. By mounting two bassdrive units in opposite directions (face to face) we can enforce symmetry and ensure a more linear response to the input current."
                  That's not isobaric. It's push-pull, which is not the same thing. Isobaric exposes only one cone to the air. From a design standpoint the result is the same as with one driver with half the Vas. The result is the same output and frequency response as one driver, but due to the halved Vas the net cabinet volume, ie., not including that occupied by the second driver and the plenum connecting the two, is halved. It was a useful arrangement when Vas values in excess of 20 cu ft were common, but with today's drivers not so much.
                  www.billfitzmaurice.com
                  www.billfitzmaurice.info/forum

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                  • #10
                    Re: What is "isobaric"?

                    Originally posted by billfitzmaurice View Post
                    That's not isobaric. It's push-pull, which is not the same thing. Isobaric exposes only one cone to the air. From a design standpoint the result is the same as with one driver with half the Vas. The result is the same output and frequency response as one driver, but due to the halved Vas the net cabinet volume, ie., not including that occupied by the second driver and the plenum connecting the two, is halved. It was a useful arrangement when Vas values in excess of 20 cu ft were common, but with today's drivers not so much.
                    this I know, just a misunderstanding, the Linn speaker/patent is as the above post shows "front to back", unless your saying that isn't isobaric? Harry Olson in the early 1950's had the patent on the "back to back" and the "front to front" I was actually just reading about that in one of the audio mag's that on the stand's now, with a article about the linn speakers.(not that I didn't know about ISO's from my car audio days)
                    That quote is from the linn link I posted, which is kind of odd seeing as in the linn configuration this benefit is not seen

                    http://mobile.jlaudio.com/support_pages.php?page_id=153
                    David

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                    • #11
                      Re: What is "isobaric"?

                      Originally posted by UGP View Post
                      this I know, just a misunderstanding, the Linn speaker/patent is as the above post shows "front to back", unless your saying that isn't isobaric?
                      I didn't see a cross-sectional view of the Linn, so it may or may not be isobaric. The quoted text, however, discusses the reduction in THD that you get with drivers in a push-pull alignment. While isobaric can be push-pull, it doesn't have to be, nor does push-pull have to be isobaric. Most aren't.
                      www.billfitzmaurice.com
                      www.billfitzmaurice.info/forum

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                      • #12
                        Re: What is "isobaric"?

                        Originally posted by billfitzmaurice View Post
                        That's not isobaric. It's push-pull, which is not the same thing. Isobaric exposes only one cone to the air. From a design standpoint the result is the same as with one driver with half the Vas. The result is the same output and frequency response as one driver, but due to the halved Vas the net cabinet volume, ie., not including that occupied by the second driver and the plenum connecting the two, is halved. It was a useful arrangement when Vas values in excess of 20 cu ft were common, but with today's drivers not so much.
                        To add to Bill's comments, the isobaric mounting arrangement also results in a some pretty large response dips in the upper range. In the midbass, the compliance of the small coupling volume between drivers forms an acoustic notch filter. The compliance must remain small to keep this out of the passband. There are also tube resonances to deal with.

                        Here's a nice old isobarik analysis from Dick Pierce from the original rec.audio group, investigating the effect on TS params. Olson's old 50's text (not very well known but published around the time of Beranek) had the original analysis.




                        From: [email protected] (Richard D Pierce)
                        Subject: Re: T/S Perams for Isobarik
                        Date: Fri, 17 Dec 1993 15:16:06 GMT

                        > I was thinking about using two speakers in a isobaric configuration
                        >and got thinking what if the T/S peram's wern't the same. For example,
                        >lets take a extreme case,
                        >
                        > Speaker 1 Speaker 2
                        >Fs 30 40 Hz
                        >Vas 5 7 Cu Ft
                        >Sens 100 110 dB
                        >Qts 5 2
                        >R 8 8 ohms
                        >
                        > If you were to bolt these together face to face and have it
                        >behaive like one speaker and wire say both speakers in parallel what
                        >would be the overall Fs, Vas, Sens. and Qts???

                        We could analyze what would happen in the situation where the drivers
                        would be TIGHTLY coupled at low frequencies. This could well be the case
                        assuming the volume between the cones is small compared to the total
                        system volume.

                        To do the analysis, however, let's pick some driver that are "real". The
                        specs you give above are so far from real drivers (especially the one
                        with the > 100% efficiency!) as to make any conclusions highly suspect.
                        The specs you give are utterly inconsistant with themselves. For example,
                        an Fs of 30 Hz, a Vas of 5 ft^3 (141 L) and a Qts of 5.0 leads to a
                        sensitivity of 80 dB, far from your 100 dB. The other example (Fs = 40,
                        Vas = 7 ft^3 (198L), Qts = 2 give sens = 84 dB, way off your suggestion
                        of 110 dB. Try to fit other parameters to the sensitivity figures leads
                        to interesting anomolies like negative cone mass (-3.74 g to be exact)
                        and a near infinite compliance (70,000L). I and, I suspect, no one else,
                        knows how to do that.

                        Let's pick two 10" drivers with the following characteristics, one being
                        a driver with a polybutadene rubber suspension, the other with a
                        compressed foam suspension. The magnet structures and voice coils are
                        similar:

                        Fs 25 35 Hz
                        Vas 140 80 Liters
                        Qms 3.0 6.0
                        Qes 0.4 0.5
                        Qts 0.35 0.46
                        Re 7.0 7.0 ohms
                        n0 0.53 0.66 %
                        Sens 89 90 dB SPL 1w/1m

                        This leads to the following electromachanical characteristics:

                        Mms 50 45 g
                        Cms 0.81 0.46 mm/N
                        Rms 2.63 1.64 kg/s
                        Bl 11.8 11.8 N/A

                        These are very typical figures for available medium or high quality
                        drivers. Note the close matches in the cone mass, for example. This is
                        not done for convenience in calculation, but because the specific
                        gravities of most cone materials is within about 25% of 1.0, and the
                        thickness of these cones is defined by the molds used and the material
                        source configuration.

                        Given this, the masses will simply add linearly, the compliances will be
                        in "parallel", the losses add, with the resulting electromechanical
                        parameters:

                        Mms 95 g
                        Cms 0.29 mm/N
                        Rms 4.27 kg/s

                        This results in the following Thiele/Small parameters:

                        Fs 30.3 Hz
                        Vas 42.3 Liters
                        Qms 4.24
                        Qes 0.45
                        Qts 0.41
                        Re 3.5 Ohms
                        n0 0.25%
                        Sens 89 dB 1w/1m

                        As expected, the Vas drops, but the other parameters don't change much.

                        > Could this have any desireable affects like maby a lower Q
                        >(smaller peak at resonance)??

                        In general, the final parameters will be constrained by whichever driver
                        is stiffer, whichever is more lossy, and whichever has the greatest mass.
                        By smaller peak at resonance, do you mean response peak? If so, that's
                        determined essentially by the total system losses at resonance (and that
                        IS the definition of Qts). Do you mean peak in impedance? Well, first,
                        who cares? Second, it, too, is a function of the total losses at
                        resonance, specifically, the minimum impedance is obviously constrained
                        by the DC resitance of the voice coil (Re) while the maximum is the sum
                        of Re and the reflected electrical equivalent of the mechanical losses (Res).

                        You'll find that MOST drivers of reasonable quality and similar size will
                        NOT differ by huge amounts in their various electro-mechanical parameters.

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                        • #13
                          Re: What is "isobaric"?

                          Originally posted by billfitzmaurice View Post
                          I didn't see a cross-sectional view of the Linn, so it may or may not be isobaric. The quoted text, however, discusses the reduction in THD that you get with drivers in a push-pull alignment. While isobaric can be push-pull, it doesn't have to be, nor does push-pull have to be isobaric. Most aren't.
                          but it does not discuss, pull-pull or push-push, J.K.
                          here's their old patent i found of interest:
                          http://www.mickandviv.com/isobarik-faq/Patents.htm
                          looks like they actually filled the chamber with a "curtain of absorbent material"
                          but your right they show no cutaway of their "improved" Iso system
                          David

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                          • #14
                            Re: What is "isobaric"?

                            Originally posted by UGP View Post
                            but it does not discuss, pull-pull or push-push, J.K.
                            here's their old patent i found of interest:
                            http://www.mickandviv.com/isobarik-faq/Patents.htm
                            looks like they actually filled the chamber with a "curtain of absorbent material"
                            but your right they show no cutaway of their "improved" Iso system
                            The art of the patent does show isobariks, some push-pull, some push-push.
                            www.billfitzmaurice.com
                            www.billfitzmaurice.info/forum

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                            • #15
                              Re: What is "isobaric"?

                              Originally posted by tculverhouse View Post
                              From a thermodynamic standpoint, "isobaric" means "constant pressure". In speaker design, one driver fires into another driver's enclosure, thus the pressure in the chamber is held constant.

                              An isobaric design would be any that has a net zero change in pressure. Two drivers sharing one enclosure wired out of phase would accomplish this. As one driver pushes out, the other is going in. Another way would be to have the drivers in phase, but the magnet of one hanging out of the enclosure.
                              I'm injecting my comments here because they would seem appropriate to the previous two posts. 1) A so called isobaric system doesn't really reduce distortion all that much. 2) The pressure in the isobaric chamber is not constant. Thus the name is really inappropriate. The pressure in the chamber varies by the same amount that the pressure would vary if a single driver were mounted in a box twice as large. If you think about it you can see why this must be true. If the front driver is to move in the same way a single driver would move if mounted in a box of 2x the volume then the pressure force on the back side of the driver must be the same in both cases.

                              I have an analysis at http://www.musicanddesign.com/Isobaric.html

                              Read it carefully. The isobaric discussion starts about 2/3's way down.
                              John k.... Music and Design NaO dsp Dipole Loudspeakers.

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