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Old 13th May 2010, 01:34 PM   #1
cr0wl3y is offline cr0wl3y  United Kingdom
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Default Fast sub anyone?

Hi there,

I'm using (and loving) a pair of Lowther Acoustas with DX4 drivers. Although I personally actually rather like their bass, friends and family have criticised it as being a tad "polite". So, I am considering trying out a mono-sub in the middle.

So, here come the questions which I'd appreciate some views on:

1. According to various views I have seen on the web, combining lowther drivers with subs can lead to problems of relative speed. In other words, "legend" has it that most subs are not fast enough to match lowther drivers. Can anyone suggest a sub design that would be fast enough, other than a khorn which I can neither afford nor build with my limited wood-working skills (and which, in addition, would not really suit the room - need a centre mono sub ideally)?

2. Failing that, has anyone had experiences with using delays to match the speed of different drivers in a system? How do you do it and how much impact does it have on the sound?

3. I am keen to keep the signal that goes into the Acoustas cross-over free. Do you see any problems in the fact that certain frequencies will thus be coming out of both the sub and the main speakers?

Thanks!
Nikos
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Old 13th May 2010, 02:17 PM   #2
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I am curious to know what you mean by a slow driver. AFAIK that's a misnomer. I am wondering if what you're describing is the bloated sound that many powered woofers offer versus the less boomy sound offered by something like a PA driver.
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Old 13th May 2010, 02:51 PM   #3
cr0wl3y is offline cr0wl3y  United Kingdom
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Quote:
Originally Posted by Cal Weldon View Post
I am curious to know what you mean by a slow driver. AFAIK that's a misnomer. I am wondering if what you're describing is the bloated sound that many powered woofers offer versus the less boomy sound offered by something like a PA driver.
Good question - I am not 100% sure myself to be honest...I have spent much time trying to figure out what this (which is based on comments and views I have seen on various sites and forums in the past during my research) means.

Eventually, I reached the conclusion that it is probably a contrast to the "direct" (again, given lack of a better word) sound that the Lowthers have compared to (the limited number of) other speakers I have tried. Ultimately, I had assumed that a delay between what comes out of the Lowthers and the woofer (albeit one of milliseconds) would occur.
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Old 13th May 2010, 03:03 PM   #4
Glowbug is offline Glowbug  United States
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There's an Aura NS 18" for sale in the classifieds that would be perfect. A simple sealed enclosure is extremely easy to build, transient response and group delay would be fine...

Quote:
Do you see any problems in the fact that certain frequencies will thus be coming out of both the sub and the main speakers?
Yes. You need some sort of external low pass filter on the sub.
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Old 13th May 2010, 04:27 PM   #5
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A DX4 has a ~0.21 Qts, ergo at a glance your sub needs a ~0.2 Qts 'sub' driver in whatever Qtc sealed or ~aperiodic alignment that most closely matches the Acousta's roll off slope to blend well, i.e. freaking huge if you don't use a Linkwitz-Riley Transformed (LRT) sealed cab and even then probably still fairly large since multiple drivers will be required to get efficiency up enough to keep excursion well within Xmax.

I assume though that you're driving them with a high output impedance, so the driver's effective Qts must be calculated before designing the 'sub' system.

Now you know why most sealed, vented sub systems can't keep up with a 'fast' set of mains since folks typically use a single > 0.4 Qts low efficiency driver.

I may be missing something obvious, but ATM I can't visualize how adding a delay will help such a Qt mis-match. Delays are about matching up differing phase responses.

OK, then you'll need to bi-amp and low pass the sub separately wherever it blends the best with the mains.

GM
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Old 14th May 2010, 02:17 AM   #6
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I think you have the right drivers. If the box is sealed all the better but it needs to be a solid box. Then what your really need is something to flatten out the low frequencies. Low Q is great for control, speed, and output but really does require EQ and a amp with solid bass. If the amp is sloppy there is nothing you can do really. High gain amps with lots of feedback usually but not always have better bass. There are a few different ways to EQ, a second order high pass filter with a Q of 1.5-2 or so set to flatten the sub. Frequency of the filter will be near the frequency of the woofer in the the box, Fb. Good luck!
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Old 14th May 2010, 06:14 AM   #7
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Id go for small sealed or open baffle
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Old 14th May 2010, 06:24 AM   #8
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How about an infinite baffle sub, while we're throwing ideas around...

Is there any amplifier in particular you'll definitely be driving this with, or are you thinking of buying one...?
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Old 14th May 2010, 07:16 AM   #9
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also, if you dont want any filtering of your main speakers you will have to keep the sub within 1/4wavelength within passband. If you filter low at say 90hz this is much less of an issue (0.95m 1/4wave)
Note that distortion wont be decreased too much if the main speakers are still outputting sub frequencies. if the Qts is low this is less of an issue however.

Re: 'fast' I think we are not talking about the rise time, that refers to the high frequency cutoff.

Instead we are talking about resonance after the initial impulse due to tuned pipes(ports)also spurious harmonic products,not just the fundamental tuning and perhaps resonating panels,higher up in frequency if the enclosure is large enough.

I cant remember the math for the high cutoff rate and effects on transient response. There was a site around with scans of some Olson acoustics graphs.

Last edited by neddludd; 14th May 2010 at 07:22 AM.
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Old 14th May 2010, 09:48 AM   #10
cr0wl3y is offline cr0wl3y  United Kingdom
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Quote:
Originally Posted by chris661 View Post
How about an infinite baffle sub, while we're throwing ideas around...

Is there any amplifier in particular you'll definitely be driving this with, or are you thinking of buying one...?
I would be looking to buy or build an amp for this and I should mention that I would not want to spend a huge amount of cash on that - £200-£300 max.

Thanks for all the suggestions to everyone btw.
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