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Old 27th February 2004, 03:19 PM   #1
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Location: Virginia USA
Question Need Feedback on Design Ideas...Please

Greetings

Well I've finally got my home theater setup rolling and now I am back at my DIY sub project in earnest. Here's my latest design incarnation.

2 ft^3 box, Peerless 12 in XLS driver, 15 in Stryke passive radiator, 250-350W plate amp

Rational:
Low Q, low Vas driver + high displacement, super low Fs, super high Q passive radiator = high alpha, good transient response, 21Hz Fb alignment in a small box - no long folded port, no port noise

I noticed that North Creek Audio and BESL (Bamberg) have both recently introduced very similar designs - so hopefully I am on the right track. They both the Peerless XLS PRs, but those units don't have anywhere near the high Q I am looking for.

Please critique. Is this design stupid/ brilliant /none of the above??? Any feedback from experienced people would be highly appreciated - I've only enough knowledge to be dangerous.

thanks
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Old 2nd March 2004, 12:13 AM   #2
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Default Too shy?

What no opinions? Come on... I can take it!

Did find a quote from the Speaker Design Cookbook where V. Dickason talks about passive radiator designs:
"If however the radiating diaphragm is made more compliant than the woofer, even low alpha (high Qts) alignments will be more like their vented counterparts and have acceptable transient response."_

Now I was hoping to achieve more than "acceptable" transient response by using the low Qts Peerless driver and pairing it with the highly compliant (Vas 978 L) Stryke PR. The 15 inch PR also has over 3 times the displacement of XLS driver and a Fs of 4.3 Hz - waaaaay outside the passband. This should help transient performance too, right?

cheers
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Old 2nd March 2004, 12:50 AM   #3
sreten is offline sreten  United Kingdom
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Just a note that the Q of the PR's at their free air resonant
frequencies may not be relevant to the in box PR tuning
frequency - I'd expect effective Q to be higher.

sreten.
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Old 2nd March 2004, 05:01 AM   #4
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Theory is theory, but in practice I've found that speaker drivers with a small VAS have a habit of sounding worse than their high VAS counterparts. I think this is because the ruggedized spiders and high-excursion surrounds introduce more non-linearities if they are extra stiff.

CM
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Old 2nd March 2004, 08:56 AM   #5
michael is offline michael  Australia
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The XLS drivers arent that stiff, i have only encountered the 12" outside of a box but it seems quite springy compared to some of the stiffer car sub woofers i have seen.
You do not state how much SPL you are after, but i think that a single XLS might not be enough, i would look at the adire DPL as it goes very low and is supposed to sound gread, and also look at the sryke AV12 as it is even louder with itd 23mm of xmax compared to the 12.5 of the XLS. The reason i mention these two drivers is for their larger SPL limits over the XLS and also their reputaion, but the primary reason for their mention is that they will work in small boxes like the XLS and still go deep. But if you can tolerate a larger box go something like a Dayton titanic or Adire Tempest.
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Old 2nd March 2004, 09:04 PM   #6
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First, thanks to all for the feedback.

sreten
My understanding with regard to PRs is that the higher the Q the better. Stryke's 15 inch PR loaded to 1400 gr is rated at a Qms of 74.8, which seems nice and high to me. Now how this plays out in the real world when the PR's Fs is 4.3 Hz I don't know. With a resonant frequency so low does the Q even matter?

CM
The Vas of the XLS is 4.92 cubic feet - doesn't seem terribly stiff.

Michael
It's the high damping (Qts .2) of the XLS that I don't think any of the other drivers can match. I think this is largely due to a strong motor rather than a stiff suspension. As for XMax and total SPL, at only 250W Unibox predicts 108dB at 50Hz and 104.5 at 20. And the peak excursion seems to be well within limits. I've never built a sub before but isn't that pretty good performance, especially if I factor in some room gain?

Everyone
The basic concept I was hoping someone could validate or shoot down was this: If a pair a low Qts, low Fs driver with a very high Q, high Vas, super low Fs PR can't I avoid the steep drop off/poor transient response typical of PR subs?

cheers
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Old 3rd March 2004, 02:25 AM   #7
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Quote:
Originally posted by Javaman
...Everyone
The basic concept I was hoping someone could validate or shoot down was this: If a pair a low Qts, low Fs driver with a very high Q, high Vas, super low Fs PR can't I avoid the steep drop off/poor transient response typical of PR subs?

cheers
What I said earlier was so you can avoid something like a 12" woofer with a VAS of under 2 cubic feet (~55L) or thereabouts, but it doesn't necessarily have to be a high value just so it can be a rule of thumb.... Yep, big magnets are good. It's useful to split the Qts into 2 parts:

Qes - the motor's electrical damping, but also relies on the amp having at least a semi-reasonable output resistance for it to be correct.
Qms - the damping from mechanical things like the surround and spider. I don't normally care about this value, but in some cases a low value is used to make up for a high Qes value.
Qts - parallel sum of the above. Try short-circuiting the terminals of a speaker that's lying around (and not plugged in!), and by finger-tapping on the cone you will see and hear the difference between Qms and Qts.

A couple of things to watch out for: an unusually low Qes value with a small magnet, this may mean that the gap between the magnet's poles and the voice coil is very small and requires a tight mechanical tolerance, or that it uses a stronger material such as an NeFeB magnet. Narrow magnetic gaps do not mix well with high power and soft suspensions, you'd be just asking for trouble. Generally what's needed is a good "all-rounder".

I don't know about PR designs, but you could try tuning the PR to a lower frequency by adding extra mass. The problem is the same as with ported woofers: the speaker doesn't have direct control over the port/PR and there is a large mass of air in between, so you can expect delays and overshoot.

CM
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