What's wrong with too low a Qts/Qtc?

Discussion in 'Archived Threads 2001-2004' started by Michael R Price, Jan 8, 2002.

  1. Michael R Price

    Michael R Price Screenwriter

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    I was just wondering... if lower Q subwoofers (like .5-.577) have better quality bass with less group delay, then what's wrong with having something like a Q of .4? I've read of low-Q drivers like the Maelstrom and BPD-03 drivers being turned away from IB use because they have 'too low' a Qts. Wouldn't a low Q provide a slower rolloff and hence deeper in-room bass? Wouldn't this slower rolloff cause a flatter phase response and hence, lower group delay? I'm a bit confused, since I've heard of both Q=.5 and Q=.577 as being 'critically damped.' What does that mean, and why would sound quality decrease below such a Q?

    As a side question, what is the problem with having an overdamped (way low-tuned) vented enclosure? Doesn't that emulate the characteristics of a low-Q sealed box? (In other words, I would think that the low tuning would cause the port output to 'mess up' the quality in the normal bass region less.)

    Ack thats a lot of questions, but this is something I'm pretty confused/curious about. Even though I have little chance of applying the knowledge anytime soon.
     
  2. ThomasW

    ThomasW Cinematographer

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  3. Dustin B

    Dustin B Producer

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    One thing I've wondered Thomas, is why didn't you use Tempests for any of your IB subs?
     
  4. ThomasW

    ThomasW Cinematographer

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  5. Greg Monfort

    Greg Monfort Supporting Actor

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    >Wouldn't a low Q provide a slower rolloff and hence deeper in-room bass?

    ====

    Correct.

    ====

    >Wouldn't this slower rolloff cause a flatter phase response and hence, lower group delay?

    ====

    Correct.

    ====

    >I'm a bit confused, since I've heard of both Q=.5 and Q=.577 as being 'critically damped.'

    ====

    Q = 0.5 is critically damped and 0.577 is max flat delay response.

    ====

    >What does that mean, and why would sound quality decrease below such a Q?

    ====

    Since it's overdamped, the driver can't reproduce all of the transient peak that's within its BW. IOW the LF driver's fundamental/first, etc. harmonics transient output won't be at the necessary amplitude to match the mid/HF output that completes it.

    This becomes a major issue with two-way and ~fullrange speaker systems, though not so much with subs since the room's Q boosts it to some extent.

    ====

    >As a side question, what is the problem with having an overdamped (way low-tuned) vented enclosure?

    ====

    Same as with a sealed design.

    ====

    > Doesn't that emulate the characteristics of a low-Q sealed box? (In other words, I would think that the low tuning would cause the port output to 'mess up' the quality in the normal bass region less.)

    ====

    Correct, except it still has more group delay. If the GD attenuates before it's audible though, then it's a moot point and can actually have lower GD than a sealed in the audible BW, and why I prefer EBS alignments.

    In general, our hearing acuity falls pretty quick with decreasing frequency below ~200Hz, so even mass quantities of GD (which decays with increasing frequency) falls below our 'signal to noise' acuity if the tuning is
     
  6. Michael R Price

    Michael R Price Screenwriter

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    Thanks for the answers, it seems as if there is nothing intrinsically wrong with a very low Q, but that...

    a) many drivers with low Q have other undesirable characteristics such as a high Fs

    b) a low-Q system may start rolling off at very high frequency (maybe 200 Hz) that would cause lean bass in a wide-range system

    That, plus the fact that unless you have an IB the box would usually have to be the size of a truck.

    I wonder if we'll ever see drivers specifically designed for IB/low-Q use... or do such drivers exist?

    Greg - Is there any particular 'rule of thumb' as to what group delay is inaudible... and is it better to have a flat group delay (even amount of delay at many frequencies, like a Q=.577[?]) or a lower overall, but not flat, group delay? I've heard numbers like 20ms at 20Hz... is there some sort of pre-determined number here, or is it an inexact thing?
     
  7. Dustin B

    Dustin B Producer

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    What about the mini IB though :p)
     
  8. ThomasW

    ThomasW Cinematographer

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    Michael

    Please clarify when you say IB are you talking about a relatively small sealed box?
     
  9. Greg Monfort

    Greg Monfort Supporting Actor

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    >a) many drivers with low Q have other undesirable characteristics such as a high Fs

    ====

    This isn't necessarily undesirable, just that you need to match the driver to the application.

    ====

    >b) a low-Q system may start rolling off at very high frequency (maybe 200 Hz) that would cause lean bass in a wide-range system

    ====

    Correct, some low Q pro drivers begin rolling off at ~700Hz.

    ====

    >That, plus the fact that unless you have an IB the box would usually have to be the size of a truck.

    ====

    Yes, you need between 4-10x Vas depending on the driver.

    ====

    >I wonder if we'll ever see drivers specifically designed for IB/low-Q use... or do such drivers exist?

    ====

    They used to be commom decades ago, except for high excursion capability, but with the advent of the materials/manufacturing revolution.....

    ====

    >Greg - Is there any particular 'rule of thumb' as to what group delay is inaudible... and is it better to have a flat group delay (even amount of delay at many frequencies, like a Q=.577[?]) or a lower overall, but not flat, group delay? I've heard numbers like 20ms at 20Hz... is there some sort of pre-determined number here, or is it an inexact thing?

    ====

    This has been debated for some time now and IMO it really boils down to the indiviual and the room it's in. FWIW, if it's 50ms/
     
  10. Jack Gilvey

    Jack Gilvey Producer

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