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  • Woofer Qms

    I have built many ported boxes, but I always thought Qts was the important parameter of the three Q's, and pretty much ignored Qms. Of course I input it to the simulation program (I use LspCAD), but just didn't care what its value is, since Qts is typically close to Qes (yes, I know Qts is a function of Qes and Qms). Recently, I have looked at some PE Reference woofers that have low Qms, such as the RS225P-8A with Qms=1.25, about the lowest I have ever seen. Some other brand of woofers have typically high Qms above 5, with the marketing claim of a "soft low damping rubber surround for improved transient response". Does anyone have a comment about such a marketing claim, and any other audible experience with low vs. high Qms woofers? Thanks in advance.
    "She don't love my speakers anymore..."

  • #2
    If you consider where you most often see rubber surrounds used, on sub woofers where transient response isn't a consideration, the fallacy of that statement becomes clear. To see what Qms really does model some drivers with similar Qes but differing Qms.
    www.billfitzmaurice.com
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    • #3
      Originally posted by billfitzmaurice View Post
      If you consider where you most often see rubber surrounds used, on sub woofers where transient response isn't a consideration, the fallacy of that statement becomes clear. To see what Qms really does model some drivers with similar Qes but differing Qms.
      I've rarely seen rubber surrounds on subs. Lots of foam surrounds though.
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      • #4
        Originally posted by Pete Schumacher View Post

        I've rarely seen rubber surrounds on subs. Lots of foam surrounds though.
        I guess you don't see many car audio subs?

        Though I have seen foam making somewhat of a comeback for car audio subwoofers of the high-excursion variety.
        Brian Steele
        www.diysubwoofers.org

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        • #5
          Originally posted by Pete Schumacher View Post

          I've rarely seen rubber surrounds on subs. Lots of foam surrounds though.
          Rarely rubber? Foam is rare in all drivers today, subwoofers included. Dayton Reference and Ultimax, Peerless XLS, that new Tymphany monster, all rubber. The only foam surround subwoofer I could think of is the Eminence Lab.

          High Qms is not only a result of the surround/suspension, but also the voice coil former. Aluminum formers will lower Qms, fibreglass former will allow a higher Qms.
          I'm not deaf, I'm just not listening!

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          • #6
            Let me phrase the question this way then: what woofers are your favorites for use from 30ish to 300ish Hz (or higher), in terms of bass sound quality, and what are it's Q values?

            Edit: Right now I am using old Focal 7K415, two per side. Factory specs are: Qms= 2.52, Qes=.29, Qts=.26. Floor mounted. The bass is very 'tight' and non-resonant, but requires about +6 dB or more LF shelf for a satisfying level of bass in my room. No doubt largely due to the low Qes and Qts, and use of an oversize box in an effort to get a low f3.
            "She don't love my speakers anymore..."

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            • #7
              Qms is a measure of mechanical loss in the system, which can be good or bad depending on your goals, much like an aperiodic enclosure. I have found myself attracted to high Qms drivers lately, like the Satori and Wavecor drivers. They seem to edge out low Qms drivers in dynamics. Technically speaking a high Qms driver will be slightly more efficient than a low Qms driver for a given box size and bass extension - up to 1dB at the extremes. This may not make much difference, but perhaps it is related to what I have been liking lately.

              Dan
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              • #8
                Thanks for your reply DanP. It has helped me think more about the issue. I know there is an interaction between the 'traveling wave' that starts at the voice coil and moves outward on the cone, and the damping provided by the surround. Part of the energy of this wave will be absorbed by the surround, and part will be reflected back toward the center of the cone. For metal cones that have no or little internal damping, I think a high damping surround is desired, but this results in a higher Rms and lower Qms. For paper or poly cones that have higher internal damping, a lower damping surround can be used, resulting in a higher Qms. But I think perhaps a very low damping surround allows too much of the wave to be reflected, resulting in rough midrange response. The SB woofers I have looked at which tout the low damping surrounds all have some rough response irregularities around 1kHz. I haven't really studied Satori woofers, but I believe they are designed and made by the same company. A classic example of tradeoffs in design, you can improve something with the result of another parameter being degraded.
                "She don't love my speakers anymore..."

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