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What is "wrong" with a sealed Qtc < .7?

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  • What is "wrong" with a sealed Qtc < .7?

    I was reading a write up recently where the author disparaged a sealed Qtc of less than 0.7. He said it was over damped. Is there a downside to being "over damped"?

  • #2
    Re: What is &quot;wrong&quot; with a sealed Qtc &lt; .7?

    I think "over damped" is the technical term for such an alignment.

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    • #3
      Re: What is &quot;wrong&quot; with a sealed Qtc &lt; .7?

      I never liked any speaker I've built that had sealed qtc less than .7

      .7 to 1.0 is what sounds natural to me. less is anemic IMO

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      • #4
        Re: What is &quot;wrong&quot; with a sealed Qtc &lt; .7?

        If the right driver is picked, placing it in an enclosure so it has a Qtc of .5 can match the rate of room gain well. You need a woofer with a low FS, and generally a large woofer and large enclosure are needed too.
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        • #5
          Re: What is &quot;wrong&quot; with a sealed Qtc &lt; .7?

          A Q of .7 is usually an engineering term for a system that is critically damped so frequency response has smoothed transitions. If it is less example would be a .5 over damped systems are lower output but as usual there is a however. As parts age a systems Q can change since the materials are not forever stable question is how long. I am happy that they build bridges with an over-damped goal. More cost but greater safety.

          Numbers above .7 are consider under-damped which if controlled properly can provide for faster responses but also can cause stability issues.

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          • #6
            Re: What is &quot;wrong&quot; with a sealed Qtc &lt; .7?

            I prefer "overdamped". It relies on motor strength more than air spring. Either critically or over or under damped... one must rely on EQ anyway. So... I'd rather have overdamped (more motor control). This is why I like oversized sealed boxes and IB installs. Of course, this assumes good strong motor design.
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            • #7
              Re: What is &quot;wrong&quot; with a sealed Qtc &lt; .7?

              I could be wrong but I recall 0.5 is critically dampled and 0.7 is occasionally called maximally flat. But Qtc alone does not dictate quality. Advent loudspeaker has a Qtc of about 1.0 while AR 3a has Qtc of about 0.7 and AR 9 claims to have a Qtc of 0.5.

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              • #8
                Re: What is &quot;wrong&quot; with a sealed Qtc &lt; .7?

                Originally posted by johngalt47 View Post
                I was reading a write up recently where the author disparaged a sealed Qtc of less than 0.7. He said it was over damped. Is there a downside to being "over damped"?
                The downside is the general perception of less bass. So, if you like your bass, then don't go there. Stay between 0.7 and 1.0. If you like boom box bass go for >>1.0
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                • #9
                  Re: What is &quot;wrong&quot; with a sealed Qtc &lt; .7?

                  Box design is all about tradeoffs. Every box design, whether air suspension, reflex, transmission line, horn, or whatever, is a balancing act of resonances and damping to achieve a useful response. Air suspension designs have the least complex resonance profile and no additional resonant-acoustic-coupling, and thus, are very forgiving of variations in design. Even in complex designs with very non-linear simulated response, the room will still wind up dominating as the primary contributor to non-linearity below ~200hz.

                  Put simply: Don't read "over-damped" as implying a negative. It should just be interpreted as "damped differently."
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                  • #10
                    Re: What is &quot;wrong&quot; with a sealed Qtc &lt; .7?

                    Originally posted by johngalt47 View Post
                    Is there a downside to being "over damped"?
                    Not if the in-room response is still acceptable. In a small room with lots of cabin gain a low Q may be preferable. It depends on not just Q but also f3. As is almost always the case you can't make a judgement based on one factor alone, you must consider the totality that gives the final result.
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                    • #11
                      Re: What is &quot;wrong&quot; with a sealed Qtc &lt; .7?

                      Fingers moving faster than brain thanks for the correction.

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                      • #12
                        Re: What is &quot;wrong&quot; with a sealed Qtc &lt; .7?

                        Originally posted by carlspeak View Post
                        The downside is the general perception of less bass. So, if you like your bass, then don't go there. Stay between 0.7 and 1.0. If you like boom box bass go for >>1.0
                        I think good bass management has to do with matching the room response and room resonances to the loudspeaker, because you will be hearing all of that. Absolute values of Q aren't as important as getting the whole picture to work out. What you would need in a small room will not work as well in a large room, and vice versa.

                        I would not advise Q>>1 however.
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                        • #13
                          Re: What is &quot;wrong&quot; with a sealed Qtc &lt; .7?

                          I have been thinking about that as the driver is being driven and the voice coil temperature goes up that the Qtc also increases. I designing a enclosure for a midrange driver that will be critical damped (Qtc =0.5) but I'm wondering is I should aim for a overdamped Qtc of 0.45 or something.

                          Shawn

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                          • #14
                            Re: What is &quot;wrong&quot; with a sealed Qtc &lt; .7?

                            Originally posted by johngalt47 View Post
                            I was reading a write up recently where the author disparaged a sealed Qtc of less than 0.7. He said it was over damped. Is there a downside to being "over damped"?
                            Not really part of this question, but box volume requirements may increase dramatically for Qtc < .7.

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                            • #15
                              Re: What is &quot;wrong&quot; with a sealed Qtc &lt; .7?

                              Originally posted by weinstro View Post
                              Not really part of this question, but box volume requirements may increase dramatically for Qtc < .7.
                              True, but if you start with a low Q driver it still may end up a reasonable size. A low Q driver is one reason why you might want a low Qtc, to get low frequency extension that you otherwise could not, and for whatever reason you did not want to use a vented box.
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