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Old 11th February 2008, 07:43 AM   #1
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Default Why is this so hard?

I am looking to build a sealed subwoofer for my HT and hifi system. I'm trying to find a decent driver close to .7 Qts. I am so overwhelmed with driver options at .4 that I may club myself in the face with a hammer.

Am I being mocked because I want a driver with a higher Qts? Seriously, I feel like a leper.

I have found some drivers with adequate t/s, but there is usually a snag.

Sound splinter RL-M15 - Unfortunately I can't really find this speaker only its lower Qts counterpart the RL-P15. and that doesn't help me much.

Macrom tf12f - I can't find this speaker either. This driver quest feels like I'm hunting bigfoot.

Soundstream PWS15 - This speaker I can find and I'm excited by the specs, but I am a little uneasy because it has a rubber spider on the cone and a $90 price tag.



The PWS15 looks like the perfect sub on paper - Qts: .710 Vas: 117L Fs: 28Hz Freq Res: 25-500Hz. Would I be stupid to put this $90 car sub into a hi end home system? It just feels a little off. Has anyone used soundstream drivers, esp the PWS? How well does this driver sound? Does anyone know of any other high quality subs around .7 Qts?


. . . or are you scratching your heads trying to figure out why I would possibly want a sub driver with without a .4 Qts.
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Old 11th February 2008, 08:29 AM   #2
MaVo is offline MaVo  Germany
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Yes. Why do you want to have one? If you were building an open Baffle without EQ, i would understand. In a closed box, low qts is better, because you can make the box smaller.
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Old 11th February 2008, 08:32 AM   #3
tINY is offline tINY  United States
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Is there some reason you want to build a huge box?

Seems to me that a Qts of 0.4 and a Fs down around 17Hz could work. But it seems that most drivers out there are designed for ported systems.




-tINY

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Old 11th February 2008, 09:19 AM   #4
MaVo is offline MaVo  Germany
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Quote:
Originally posted by tINY

Seems to me that a Qts of 0.4 and a Fs down around 17Hz could work. But it seems that most drivers out there are designed for ported systems.
every qts value will work, all one must change is the enclosure size. the box q is a function of driver q and enclosure volume. using high q drivers only leads to a big box.
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Old 11th February 2008, 10:29 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
the Qts of the driver will remain the same if the box size is infinitely big.
If you want a finished speaker Q=0.7 (Butterworth roll off) then fitting a driver of Qts=0.7 to an infite baffle will preserve the Q you are aiming at.

Now fit a driver in a sealed box.
A big box will increase the Q slightly, a smaller box will increase the Q a lot. A very small box will produce a Q that is so high it will become a one note boom that will sound horrible.

The result of all that is that a high Qts driver needs no box.
A medium Qts speaker needs a medium size box
A low Qts speaker needs a small box.

But there is a further compromise that needs your attention. As the box volume gets closer to the Vas of the driver the frequency of the box+driver becomes closer to Fs*2. i.e the bass disappears as the box gets smaller.
This can be EQed to flatten the response but it needs lots of power, lots of Xmax, lots of driver power handling and lots of experimentation to get it sounding right in your room.

That's part of the reasoning behind the advice to choose a driver with a Qts of between 0.3 and 0.5 for a sealed box speaker.
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Old 11th February 2008, 10:59 PM   #6
Ron E is offline Ron E  United States
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Why do you want Qts near 0.7? Explain how you arrived at that specification, as it is unusual... Maybe you are needlessly eliminating 95% of all drivers.
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Old 12th February 2008, 06:14 AM   #7
Svante is offline Svante  Sweden
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Default Re: Why is this so hard?

Quote:
Originally posted by Hirsute Zombie
. . . or are you scratching your heads trying to figure out why I would possibly want a sub driver with without a .4 Qts.

Yes.

Or, I might guess: You want the sub to have a "maximally flat" design. This occurs when the system has a Q of 0.7. Could it be that you are unaware of that the system Q (=Qtc) is different from the driver Q (=Qts)?

If you put a driver in a closed box, the resonace frequency will increase, and so will the Q. For example: If you have a Qts=0.35, fs=20 Hz driver and put it in a box that has a size so that fc=40 Hz, then the Q will increase by a factor fc/fs=40/20=2, ie Qtc will be Qts*fc/fs=0.36*40/20=0.7.
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Old 13th February 2008, 06:16 AM   #8
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When I started designing my sub I was looking at a Qtc of .7 but as I put in more and more numbers into WinISD I started leaning towards a higher QTC usually approaching 1.0. This does mean I have to settle a less flat freq response curve because of a 1 dB hump, but it also drops the low cutoff by about 10 Hz.

I am also planning on building a front box for the sub so that it can be converted into a 4th order bandpass that would ideally cover 20-60Hz or 25-70Hz, so the sealed box parameters also need to work well as the rear box for the 4th order. The drivers that produced the results that I was looking for were usually fairly high Qts, around between .6 and .8. One driver that was preset in WinISD that looks real good for for both boxes is the Sound splinter RL-M12 which only has a Qts of 0.601. Unfortunately the front box is 26.06 with a vent length of 63 cm.

Currently the best results that I could generate with a driver that I could find for sale were from the Sound Stream PWS. I'm looking at a sealed box of 124L and a front box of 60L, but I think I'm willing the sacrifice some living space.

I am no expert in sub design, so if there is anything that I should be warned about with my current designs please let me know. I haven't calculated the power response for any of these drivers yet. I hope that doesn't turn the whole project on it's head.
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Old 13th February 2008, 11:47 PM   #9
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I would rather build a transmission line enclosure for HT
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