What does QTC mean?

Discussion in 'Speakers & Subwoofers' started by EdNichols, Jul 22, 2003.

  1. EdNichols

    EdNichols Second Unit

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    I have read an explanation of this in the Newbie section but still don't quite understand. Does this number have to do with cabinet size vs driver size? What number would be best for music vs HT?
     
  2. EdNichols

    EdNichols Second Unit

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    Surely someone knows!!
     
  3. Chris Quinn

    Chris Quinn Screenwriter

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    Here you go. This is lifted off Acoustic Visions glossary page.

    Q - the magnification of resonance factor of any resonant device or circuit. A driver with a high Q is more resonant than one with a low Q.

    Qes - the electrical Q of the driver.

    Qms - the mechanical Q of the driver.

    Qts - the total Q of the driver at Fs. Qts = Qes x Qms/Qes + Qms.

    Qtc - value for the damping provided for a driver in a sealed enclosure. Denotes the enclosures ability to control the driver response at resonance. Qtc = 0.707 is the optimum value for sealed enclosures, providing flattest response and highest SPL for deep bass extension. Enclosures for this value are often rather large. Lower Qtc can give even better transient response, down to a Qtc of 0.577 for the best damping and transients.
     
  4. EdNichols

    EdNichols Second Unit

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    Thanks..... I think.[​IMG] I guess that means since my sub. cabinet is large I should expect a nice flat response.
     
  5. Frank Grimes

    Frank Grimes Second Unit

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    Not necessarily, it really depends on the sub's parameters. Run it's Theille's through a program like WINISD to find out exactly what your frequency response curve will look like. Realise that if your enclosure is larger than necessary for a Qtc= .707 allignment, your F3 (the frequency at which your sub has rolled off 3 decibels) will rise while slope of the sub's rolloff will become shallower. A higher Q than .707 will again have a higher F3, but a steeper rolloff and possibly a significant SPL peak. Many consider .707 the perfect tradeoff of F3 and low frequency output...
     
  6. EdNichols

    EdNichols Second Unit

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    If that is the case then why would the note above say that the a lower QTC than .707 would give a better transient response?
     
  7. Chris Quinn

    Chris Quinn Screenwriter

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    "Thanks..... I think. "

    I can't translate it either, if that makes you feel any better. I recongize all the words but it might as well be a foreign language. I did get is that lower is better.
     
  8. Frank Grimes

    Frank Grimes Second Unit

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    "If that is the case then why would the note above say that the a lower QTC than .707 would give a better transient response?"

    "Transient response" can be a somewhat misleading term. Putting a subwoofer in a smaller box won't make it sound 'tighter', nor will the sub sound sloppier in a larger enclosure. Going lower than a Qtc of .707 can give a more desired frequency response, but it's really a matter of what kind of sound the person is looking for. Honestly I'm not too knowledgeable when it comes to home theater, but in car audio you rarely see people going much lower than .7 due to cabin gain increasing low frequency output. The gain in home theater is going to be less significant, so that could be a reason for people going with low Q allignments. Also, you're much more likely to find 20-30hz frequencies in movies (rarely a factor in car audio) than music, so I guess that would be another reason...

    Here's a visual example so as to help you understand (hopefully) how this works. Driver is ACI SV12 (Qts=.40, sorry about poor image quality, but the only site I can host on sucks)

    [​IMG]

    Green: Qtc= 1.24, 1ft^3 enclosure, -12.5db @25hz
    Yellow: Qtc= .707, 4ft^3 enclosure, -5.45db @25hz
    Red: Qtc= .41, 100ft^3 enclosure, -5.2db @25hz

    Notice that as the drop to .41 Qtc requires a VERY large enclosure. This is due to the fact that the Qtc can never get lower than the Qts of the driver. Such a large enclosure wouldn't be required for a .41 Qtc if a driver with a lower Qts were used (though low Q drivers are more at home in ported enclosures).
     
  9. Jaime B

    Jaime B Stunt Coordinator

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    Ed:

    I'll bet you pesos for dollars that you now wished you'd never asked that question. In fact you have until Friday to read Messrs. Thiele-Small speaker design theories for a pop quiz!

    After the quiz, visit all the audio/video stores in your area and engage the salesmen in a deep Qtc theory discussion to see what their opinions are. Insist on the factory advertised Qts specs on every speaker in the showroom and ask them why the respective designers chose that specific Qtc.

    Boy, are you going to have FUN!

    JaimeB
     
  10. Frank Grimes

    Frank Grimes Second Unit

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    Or ask your dealers to see the BL curve of a particular speaker. If he says they don't publish them, ask for the reason.

    For some reason my pic came up the first time I came back to this page, but now it's not. If it's not showing for you:
    http://www.soundillusions.net/sites/bababooey/freq.JPG
     
  11. Frank Grimes

    Frank Grimes Second Unit

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    OK, I give up. If you want a clear image I can email it to you.
     
  12. EdNichols

    EdNichols Second Unit

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    Thanks for trying Frank. I think I get the idea.

    Jaime,
    I would dare say that if i went to some of the Audio stores I have visited lately they wouldn't know what QTC was either. And yes you are right I can't recall ever seeing the QTC shown in any of the sub specs I have seen.

    I do wonder how the QTC of my sub. which is a VMPS with a 5.3 cubic ft. cabinet and a 12" driver would compare to something like an Earthquake Supernova with a 12" driver and about 1.5 cubic ft. cabinet. I would think that that mine would have a lower QTC and therefore a deeper base extension and better SPL. I know the driver and amp has something to do with it also, but if they were equal.
     
  13. Jaime B

    Jaime B Stunt Coordinator

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    Ed;

    My previous post was full of sarcasm (it will happen to you once you get over 40)! Of COURSE no salesman knows about QTc, only the speaker designers care about this particular design spec. My bone with salesmen (audio/video, among most retail salespeople) is that most know SH!T about what they sell, even less 'bout tech specs, so go ahead and indulge in an intellectual and technical discourse on speaker design. Do not forget to ask if the sonic attributes of Mapingo Wood are superior to those of run-of-the-mill medium density fiberboard!

    JaimeB[​IMG]
     

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