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Old 12th September 2003, 07:44 PM   #1
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Default highest exceptable Qtc or box size

I recently bought some 15" subs, these are the specs:

750 watt maximum power handling capability
18-100 Hz Frequency Response
Fs =21.57 Hz
Qms=9.101
Qes=0.4231
Qts=0.4043
Vas=263.27 liters
Cms=0.2562E-03 M/N
Re=3.79 ohms
BL=16.060 Newtons/Amp
Sd=0.0.85530 sq. mtrs
Mms=0.212463 Kilograms
Hvc=1.250"
Hag=0.500"
Xmax=0.375
Xlin p-p=.750
Ref Efficiency=0.6001%
Predicted Output=89.982 Db/1W/1M

Recommended box size for sealed is 42.5 liters, which gives a Qtc of 1.08, but I think that's if it's in a car and I still think it's too high.

I'm of how big the box will have to be to achieve the box Qtc I want. 500 liter for a Qtc of .5, which is what I want because I'm interested in SQ. But a 500 liter box is way too big. WinISD recommends a .71 Qtc which is 128 liter. Is this a good compremise or is the Qtc too high.

Is there something else I can do to make the box smaller. Such as using an aperiodic vent? Do they make those vents for 15" subs, I would imagine they would have to be very big. Or is this sub not suited for a aperiodic vent.

Any suggestions are welcome.
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Old 12th September 2003, 09:53 PM   #2
Ron E is offline Ron E  United States
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I wouldn't make a sealed box bigger than 100 liters for that woofer. Unibox gives 101.3Liters for a Qtc of 0.707 assuming some damping mat'l on the walls. A 60Liter box with some damping material would sound fine, but I would hardly call a 40-45ish F3 woofer a subwoofer.

How much power will you feed it? If a lot, you should use a higher Qtc for driver protection. This woofer doesn't have a surplus of excursion....

Any technique you try to make the box smaller (stuffing, Aperiodic, etc) actually reduces bass (Fc will rise a bit) because it is just adding losses. It is not "equivalent to" using a larger box in anything but Qtc.

IMO, Qtc=1 sounds fine. Qtc=0.5 is not a magic number as far as sound quality is concerned - you will not hear dramatic differences in response in the 0.5-1.0 range. People who claim to hear dramatic differences in the bass response of a woofer with a certain Qtc have not tried using an adjustable Linkwitz transform to vary Qtc , keeping Fc constant while listening to music.
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Old 12th September 2003, 09:58 PM   #3
Arx is offline Arx  Canada
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A lot of people consider .707 Optimum. It will give you the flattest frequency response down to cutoff.
Some prefer lower q's for better transient response, but even a q of 1 would not sound awful.
I generally aim for around .7 give or take a little on my designs. Keep in mind that a lot of the stuff you'll see on the shelves down at the department stores are using rediculous q's well over 1, so relative to all that stuff, a q of .7 is very low.

I built a box a couple years ago for some 15's and it had a q of .7 at around 500 litres. I built mine to around 450, and it sounds pretty decent. Very very low clean bass. Honestly, I don't think q is all that critical in that region, unless you plan on doing a rediculous amount of work to deal with room effects anyways.

They're 15's. Just get it in the ballpark and it'll rattle your windows.

And I wouldn't worry too much about the excursion. If your q is at all reasonable, and you're not too much into booming like a civic, (which you're obviously not if you want a low q) .3 inches is plenty even if that is p-p
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Old 13th September 2003, 04:15 PM   #4
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Arx and Ron E thanks for your advise,

I'm to hear that I don't need to build a big box for this subwoofer. And then I'm also to hear that an f3 of 40-45 is hardly a subwoofer. I used to have a very similar subwoofer to this one in my car; Kicker Solobaric 15 D series, this one looks exactly like it's predisessor Solobaric C series. Here's what it looks like http://cgi.ebay.com/ws/eBayISAPI.dll...category=18804

I will power it with a 500 watts amp that came out of a Acoustic Research 315HO speaker, it powered the 15" sub. The 315HO is the earlier model of this speaker http://www.acoustic-research.com/ar1_content.asp. Still looks the same and seems to have the same amp. The amp has a 24 or 36 db/slope fix at 100 Hz.

"They're 15's. Just get it in the ballpark and it'll rattle your windows." Does that mean 15's are not for SQ, maybe I should of bought the 12" version of this sub, but nobody replied to my tread, when I asked people about their opinion on the 12 and 15" versions of this sub. But I think my amp is more suited for an 15 now that I think about it.

Arx Xmax p-p is .75"
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Old 14th September 2003, 12:04 AM   #5
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Default Ron E, I downloaded unibox . . .

And with "minimal leaks" and the "walls covered" for damping I got a volume of 147.3 liter to achieve a Qtc of .707. But I was happy to see that F3 was about 35 Hz. Maybe we have different versions of unibox, mine's 4.03. You think my calculations are correct?

I might still build it only about 60 liters, I can't picture it yet. That is, whether I want to go for .707 or just a small box.
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Old 14th September 2003, 12:41 AM   #6
Volenti is offline Volenti  Australia
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I'm personally using 2 JL15-W6 15'' subs in my setup, each in a seperate 2.5cf (75L) sealed chamber, powered by the plate amp in the foreground.

Click the image to open in full size.

I'm happy with their sound using a 12db x-over at around 45hz, now I just need a set of fact mains that go that low (working on that)

What x-over point do you have in mind with yours?
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Old 14th September 2003, 02:32 AM   #7
Ron E is offline Ron E  United States
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I don't use the damping buttons provided by Unibox - I used Ql=20 and Qa=20.

No model using factory specs is going to tell you the full story...don't count your chickens...
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Old 14th September 2003, 07:30 AM   #8
Arx is offline Arx  Canada
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Nah, I'm not saying there's anything wrong with the sound quality of 15's, just that the q is probably not really all that critical, as the peaks are down at a frequency where your ears aren't all that picky. Personally, I like a bit of a peak down there between 30-40 hz. And besides, you'll be getting tons of peaks and dips from standing waves in the room etc.

Another question.. Are you using a Car sub?
If so, keep in mind that they normally roll off at a higher frequency, which partially compensates for cabin gain. In a house it'll happen a little differently, so be prepared to have to mess with it a bit to make it sound right. Might need a little bit of equalization to flatten it out.
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Old 15th September 2003, 12:02 AM   #9
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Volenti,

You should have my main speakers. When sealed (they have ports, but I don't like ports) they have an f3 of 45-50 Hz and of course a roll-off at 12 db/octive. They're 2.5-way, but can be made 2-way just as easy. Pair of woofers and tweeters sell on ebay (from british columbia, CA) for $130 total, sound very nice, tweeters are vifa and woofers are like Viston Al 200, except mag/copper/aluminum alloy cone.

Anyway . . . my amp is 500 watt and has a 36 db/octive roll-off at 100 Hz (it's described more above). I read that you had 2 JL Audio 15's before, but I never imagined they would be in the same enclosure. I would also like to put my subs in the same enclosure, but I heard stero subs have a more realistic sound, also small size is important.

Ron E,

You're right I should not be counting chickens.

Arx,

Yes, I'm using a sub which is designed for car use. It's very similar to the old round Kicker Solobarics (same as Kicker S15C, link to sub is above).

I thought of using a Linkwitz transformer circuit, but I think that it will require me to build a larger box, right?
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Old 15th September 2003, 11:26 PM   #10
Ron E is offline Ron E  United States
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With a transform, you can build whatever box size (neglecting Q) you desire, then change it to whatever Q you desire - the tradeoff is amp headroom, so boosts of more than 10dB (about an octave extension) are pretty much a no-no.
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