20 hz with linkwitz transform fail?

5063 Views 16 Replies 2 Participants Last post by upking, Dec 9, 2016

upking

Discussion Starter · Nov 18, 2016

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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fusseli

· #2 · Nov 18, 2016

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.

upking

Discussion Starter · #3 · Nov 18, 2016

fusseli said:
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?

fusseli

· #4 · Nov 18, 2016

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.

upking

Discussion Starter · #5 · Nov 18, 2016

fusseli said:
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?

fusseli

· #6 · Nov 18, 2016

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

upking

Discussion Starter · #7 · Nov 18, 2016 (Edited)

fusseli said:
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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upking

Discussion Starter · #8 · Nov 18, 2016

Vented X Vented

jbl - green - 108L (F3 21hz)
questo - pink - 72L (F3 23hz)

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fusseli

· #9 · Nov 18, 2016

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

upking

Discussion Starter · #10 · Nov 18, 2016

fusseli said:
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:

upking

Discussion Starter · #11 · Nov 25, 2016

After much search for drivers, I found this one for a reasonable price.

Power RMS: 2000
Znom: 4+4 or 2+2
Fs (Hz): 32
Qms: 7,9
Qes: 0,53
Qts: 0,49
Vas (L): 42
BL (Tesla/m): 30
X-max (mm): 24
Sd (cm2): 540
Sens.(dBs/1w/1m): 86

Would it be able to result in a good linkwitz transform? I'm a little worried about fs, but xmax seems to be ok, I could use 2 of those. Cabinet size is also a plus with those, Vas is very low.

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fusseli

· #12 · Dec 2, 2016

Yeah, that is much better!

In 42L it would take 500W with a +4dB low shelf (LT), and lower the f3 from 47 to about 30 Hz, or about 1500W without the LT.

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2

upking

Discussion Starter · #13 · Dec 3, 2016

I'm in doubt between 2 of those I posted or 1 Dayton UM18-22
F3 30hz is too high, I want at least 25hz

fusseli

· #14 · Dec 4, 2016

Two of those is smaller than than an UM18 that needs about 110L, but like my sub, the UM18 would get a lower f3. If I could do it over again I would consider two small subs over one big one.

Reactions: 1

upking

Discussion Starter · #15 · Dec 4, 2016

fusseli said:
Two of those is smaller than than an UM18 that needs about 110L, but like my sub, the UM18 would get a lower f3. If I could do it over again I would consider two small subs over one big one.

The lower F3 I might achieve with the miniDSP. But could I use the UM18 in a 70L box and equalize it to have a lower F3 aswell and good transient response or am I going to have a "loss" in transients anyway?

fusseli

· #16 · Dec 6, 2016

I think it's going to take a lot of amp power (and EQ) to get a smaller sealed sub to go as low. That being said, you never really know until you take an in-room measurement sweep. You might not need much to make it flat to 20 Hz and beyond with room gain. In this case I still suggest the two smaller subs, though.

upking

Discussion Starter · #17 · Dec 9, 2016

I can't really decide if it's best to go with 2x 12" of those

(unboxing video

)

or 2 of those

Power RMS: 2000
Znom: 4+4 or 2+2
Fs (Hz): 32
Qms: 7,9
Qes: 0,53
Qts: 0,49
Vas (L): 42
BL (Tesla/m): 30
X-max (mm): 24
Sd (cm2): 540
Sens.(dBs/1w/1m): 86
(images here, public page: https://www.facebook.com/pg/protechspeakers/photos/?tab=album&album_id=844356098975071)

I gonna use an 85L enclosure, probably not gonna have enough power with an EP4000 to drive the last ones until xmax, but It'll be close.

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