Qtc must be greater than driver's Qts.

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I'm using the online calculators at DIY Audio & Video to try out various drivers and volumes for my sealed cabinet design, and often when I input data it comes up with a dialogue box with "Qtc must be greater than driver's Qts"

I assume this statement is true, and assuming I am trying to achieve a qtc of 0.707 does this mean that any driver I'm looking at with a QTS greater than say 0.8 is going to be a bit rubbish? Well not rubbish, but should I be focusing my efforts on drivers with lower <0.707 qts for example.
 
A driver with Qts=0.8 will indeed never reach Qtc=0.707 in a standard sealed enclosure. A driver's free-air Fs and Qts can only be raised in a sealed enclosure, now convolved into Fc and Qtc.

There may be a special case, correct me if I'm wrong, where a driver with lots of dense stuffing right behind it will have its Qms lowered, by extension lowering Qts below its free-air value, but putting this in a sealed box will still only raise it from there, although Qtc could be lower than free-air Qts. Whether this can be used to good effect is another matter.
 

GM

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Joined 2003
There may be a special case.......

The only way I know how to acoustically over damp a driver's Qts is via a heavily damped TL or compression horn loading with the former the most practical WRT design ease/bulk, so research Rick Schultz's (Exolinear) Alpha TL design routine [basically Fs/Qts'] for high Qts drivers, which also shows that for a given alignment, fiberglass insulation is superior to polyfil, resulting in a smaller net bulk [Vb].

Otherwise one is limited to using a Linkwitz-Riley Transform [LRT] to electronically trade efficiency for bandwidth [BW], lower effective Qtc, which of course yields by far the smallest bulk.

Qts' = Qts + any added series resistance: mh-audio.nl - Home

GM
 
For a closed box, the system Q cannot be lower than the driver Qts. To make it equal to Qts would require an infinitely large box. As you decrease the box size from infinity both Fc>Fs and Qtc>Qts. The equations that govern these the relationships between driver and box parameters can be found about halfway down this page on closed box loudspeakers:
Sealed enclosure explained in detail and calculator - Audio Judgement

I suggest you read it over.
 
The only way I know how to acoustically over damp a driver's Qts is via a heavily damped TL or compression horn loading with the former the most practical WRT design ease/bulk, so research Rick Schultz's (Exolinear) Alpha TL design routine [basically Fs/Qts'] for high Qts drivers, which also shows that for a given alignment, fiberglass insulation is superior to polyfil, resulting in a smaller net bulk [Vb].

Otherwise one is limited to using a Linkwitz-Riley Transform [LRT] to electronically trade efficiency for bandwidth [BW], lower effective Qtc, which of course yields by far the smallest bulk.

Qts' = Qts + any added series resistance: mh-audio.nl - Home

GM

Nope. Any loading of the driver will increase Qts. Adding damping material only lessens the effect somewhat, it cannot reduce the Q below Qts, although with a TL there are other variables in play that determine the response shape.

The only way to make the effective Qts LOWER is to employ an amplifier with a negative output resistance. Likewise, positive series resistance (via a resistor, or an amplifier with positive output impedance) will increase Qts. Your equation says this same thing.

It's much easier to just start with a driver with a lower Qts to begin with.
 
The only way I know how to acoustically over damp a driver's Qts is via a heavily damped TL or compression horn loading with the former the most practical WRT design ease/bulk, so research Rick Schultz's (Exolinear) Alpha TL design routine [basically Fs/Qts'] for high Qts drivers, which also shows that for a given alignment, fiberglass insulation is superior to polyfil, resulting in a smaller net bulk [Vb].

Otherwise one is limited to using a Linkwitz-Riley Transform [LRT] to electronically trade efficiency for bandwidth [BW], lower effective Qtc, which of course yields by far the smallest bulk.

Qts' = Qts + any added series resistance: mh-audio.nl - Home

GM
You lost me at "over"
 
Hmm, if it lowers the roll off slope for a couple of octaves or more via acoustic mass loading before 'tipping over', which it does, then for all intent and purposes it's effectively a lower Qt [technically Qtp].

GM

I reiterate that "a Lower Qt" (your words) is NOT via reduction of Qts.


The only way I know how to acoustically over damp a driver's Qts is via a heavily damped TL or compression horn loading with the former the most practical WRT design ease/bulk, so research Rick Schultz's (Exolinear) Alpha TL design routine [basically Fs/Qts'] for high Qts drivers, which also shows that for a given alignment, fiberglass insulation is superior to polyfil, resulting in a smaller net bulk [Vb].

Otherwise one is limited to using a Linkwitz-Riley Transform [LRT] to electronically trade efficiency for bandwidth [BW], lower effective Qtc, which of course yields by far the smallest bulk.

GM

Also, much of what you state above is not correct.
 
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A driver with Qts=0.8 will indeed never reach Qtc=0.707 in a standard sealed enclosure. A driver's free-air Fs and Qts can only be raised in a sealed enclosure, now convolved into Fc and Qtc.

There may be a special case, correct me if I'm wrong, where a driver with lots of dense stuffing right behind it will have its Qms lowered, by extension lowering Qts below its free-air value, but putting this in a sealed box will still only raise it from there, although Qtc could be lower than free-air Qts. Whether this can be used to good effect is another matter.

Yes
 
I reiterate that "a Lower Qt" (your words) is NOT via reduction of Qts.

You're misreading. GM was referring to acoustic effects & OA system Q, hence noting 'technically Qtp' WRT a very heavily damped pipe.

Also, much of what you state above is not correct.

Which parts, when you read it as intended, rather than assuming it refers to the driver's innate EM damping?

Anyway, back OT for the OP, what GM essentially meant was that if you've a high Q driver, then that is the minimum system Q you can obtain except in very specific circumstances which in most cases are 'you can't realistically go there' short of a Linkwitz transform or equivalent LF EQ, which are more easily achieved, but not always possible.

Note that a higher Q driver isn't necessarily 'rubbish' -it may be very good for the purposes it was inteded for. It does give you far fewer options though.
 
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You're misreading. GM was referring to acoustic effects & OA system Q, hence noting 'technically Qtp' WRT a very heavily damped pipe.



Which parts, when you read it as intended, rather than assuming it refers to the driver's innate EM damping?

Anyway, back OT for the OP, what GM essentially meant was that if you've a high Q driver, then that is the minimum system Q you can obtain except in very specific circumstances which in most cases are 'you can't realistically go there' short of a Linkwitz transform or equivalent LF EQ, which are more easily achieved, but not always possible.

Note that a higher Q driver isn't necessarily 'rubbish' -it may be very good for the purposes it was inteded for. It does give you far fewer options though.

Thanks for clearing that up, I'm looking drivers with QTS of around 0.5 now!
 
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