20 hz with linkwitz transform fail? - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 17 Old 11-17-16, 06:33 PM Thread Starter
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20 hz with linkwitz transform fail?

So I'm wondering why my project failed kinda failed, I've got an subwoofer with the following specs:

Qes: 0.419
Qms: 4.888
Qts: 0.386
FS: 21.7
Vas: 335L
xmax: 8mm
SD: 855,3 cm² (15 in)

I tried a sealed box with about 140L, got a really nice response curve, simulations predicted -13db @20hz and that's kinda what happened.

The issue is, whenever I turn up the volume while playing 20hz, it seems the THD rises a lot as soon as I get an audible response, probably due to the harmonics being played.

So now comes the question, is that subwoofer weak for 20hz? I've been told that because of the low qts it couldn't produce extended bass with closed enclosures. Is that true or is it because of the "low" xmax?
Would stuffing my box help?

I can change my sub, got those in mind:

first specs:
Imgur: The most awesome images on the Internet (12 in, xmax: 14.5mm, qts: .47, fs: 28hz )

second specs:
Imgur: The most awesome images on the Internet (12 in, xmax 18mm, qts .48 fs: 31hz)
I am a afraid their fs might be too high, the first is half the price of the second, so I could get 2, seems like a better deal, but I would get away with one if I could in order to have a smaller box.

Btw, I made a box with 140L inside it but it came out way too big, I would stick 70 or less, 100L tops. Less is better. Otherwise I'd build an Transmission Line for the 15in that I already have.


Response and THD tests where measured with a Dayton EMM-6, calibrated by Cross Spectrum


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post #2 of 17 Old 11-17-16, 09:32 PM
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Re: 20 hz with linkwitz transform fail?

It might not just be your sub driver, what and how are you doing your linkwitz? It could be clipping in the signal chain, causing lots of THD. Also, that driver would only take about 100W, BEFORE the linkwitz application, when it would maybe take 10W. So the driver being over driven is also a good chance.


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post #3 of 17 Old 11-17-16, 10:19 PM Thread Starter
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Re: 20 hz with linkwitz transform fail?

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fusseli wrote: View Post
It might not just be your sub driver, what and how are you going your linkwitz? It could be clipping in the signal chain, causing lots of THD. Also, that driver would only take about 100W, BEFORE the linkwitz application, when it would maybe take 10W. So the driver being over driven is also a good chance.
The sub driver can take 350 RMS as specified by the manufacturer
Actually I'm simulating an linkwitz transform via a High Shelf filter using a software as my PC is my source ( don't worry, I got decent DACs. )
I'm pretty sure the problem is not in amplification, it could be the amplifier, but I'm not driving it to it's max, the knob is only halfway through, and audio is pretty clean in other frequences so I'm not really considering that.
I'm almost certain that the signal is clear.

The driver may be overdriven but from a xmax perspective, problem is I don't know how much it is excursioning so I can't tell if it's going over xmax.
Is the low qts really an issue or is it all about xmax when going LT?


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post #4 of 17 Old 11-17-16, 10:26 PM
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Re: 20 hz with linkwitz transform fail?

I don't think low Qts is your issue, I think over driving the sub or clipping are the likely issues.

350W is the thermal rating. Take a look at the cone excursion in the attached WinISD file, it's exceeding limits with just 100W, without any linkwitz added.

8mm is not very much xmax, either, especially for expectations of 20 Hz. You won't be hitting very good SPL at 20 Hz with this setup. The one good thing about your driver is the low Fs, you'd probably get more bang for the buck in a larger vented box tuned for 20 Hz but power handling would still be the limiting factor.
Attached Files upking.wpr (1.8 KB) 


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post #5 of 17 Old 11-17-16, 10:34 PM Thread Starter
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Re: 20 hz with linkwitz transform fail?

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fusseli wrote: View Post
I don't think low Qts is your issue, I think over driving the sub or clipping are the likely issues.

350W is the thermal rating. Take a look at the cone excursion in the attached WinISD file, it's exceeding limits with just 100W, without any linkwitz added.

8mm is not very much xmax, either, especially for expectations of 20 Hz. You won't be hitting very good SPL at 20 Hz with this setup. The one good thing about your driver is the low Fs, you'd probably get more bang for the buck in a larger vented box tuned for 20 Hz but power handling would still be the limiting factor.
Yea I see your point. Would lowering the box size do me any good? Probably not as I would have to apply more power in the lower end, right?
If so, which one from the first post would you pick to make a good LT?


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post #6 of 17 Old 11-17-16, 10:36 PM
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Re: 20 hz with linkwitz transform fail?

Go for the higher xmax, the LT filter will push the driver really hard down low.


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post #7 of 17 Old 11-17-16, 10:37 PM Thread Starter
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Re: 20 hz with linkwitz transform fail?

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Go for the higher xmax, the LT filter will push the driver really hard down low.
Even if I can buy 2 drivers of xmax: 14.5 for the price of one 18mm?

one of those of 14.5mm seems to have F3 on a vented box of 21hz, but I'm not really willing to go vented

some simulations

Questo LT - red (18mm)
jbl vented - green (14.5mm)
jbl single closed - blue (14.5mm)
jbl dual closed - yellow (14.5mm)
questo 15 - what I got now - white (8mm)
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20 hz with linkwitz transform fail?-7b762256396849108e66ee646ded0705.png  



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Last edited by upking; 11-17-16 at 10:50 PM.
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post #8 of 17 Old 11-17-16, 10:54 PM Thread Starter
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Re: 20 hz with linkwitz transform fail?

Vented X Vented

jbl - green - 108L (F3 21hz)
questo - pink - 72L (F3 23hz)
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20 hz with linkwitz transform fail?-screenshot_1.png  



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post #9 of 17 Old 11-17-16, 11:11 PM
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Re: 20 hz with linkwitz transform fail?

I don't believe the JBL vented you have, is excursion below xmax?

If you have the space and amp power for two, go with the 14mm


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post #10 of 17 Old 11-17-16, 11:27 PM Thread Starter
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Quote:
fusseli wrote: View Post
I don't believe the JBL vented you have, is excursion below xmax?

If you have the space and amp power for two, go with the 14mm
Yea, I pumped the watts until Xmax for all simulations.
You can play with it, parameters are at the first post. (14mm driver)
2 jbl in a vented box would be too large of a box. But I think I can make it sealed with 2.
But it would be twice the size of the questo.
Too bad that those jbl 1214 are so ugly D:
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