Qts in OpenBaffle - How low can you go? - diyAudio
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Old 7th April 2011, 02:39 AM   #1
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Default Qts in OpenBaffle - How low can you go?

I gather that for bass extension, a fairly high Qts of around .7 is important in an OB. However, I keep seeing recommendations for drivers with rather low Qts, say around .2 to .3 in OB application (Fostex especially).

Assuming a bass-augmented system with an active crossover goal of about 200 Hz (150 would be better) and an OB size of about 20"-24", what kind of Qts value could one get away with, say for a 8" driver? Is there a tool to help determine this that includes the Qts factor as well as cone size?

I'm having a lot of trouble finding many examples around 8", at least 90 dB (SE tube friendly), inexpensive (under $100 each) with a higher Qts and with a good reputation for sound quality. However, if I could get away with a .3 Qts, that opens up the field a lot. If not, it looks like the Betsy is the best option so far but I would like a few more options to choose from if possible.

I'm starting to think that maybe a smaller driver would be okay too, but power handling for a largish room has been a concern of mine. Dispersion seems adequate on a 8" for me so far.
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Old 7th April 2011, 03:44 AM   #2
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well, i think it will roll off at Fs/qts, not including the open baffle dipole roll-off.

And to me, a driver will have a dip at 1129/distance front to back (but it fill in farfield according to stereophile's measurements of the Jamo open baffle), then there will be a peak at half that frequency (front wave 90 degrees out of phase adding to back wave) then response falling till it hits mass corner then vanishes.

I had about 8" of cone area on an open backed box. The shortest distance (front to back) was 38", I liked it, excellent midbass compared to most speakers that had no baffle step. That put the Feq (+3 to 6db) hump around 180hz. It was liveable to me.

Listening to fostex plus 15" on youtube on a narrow baffle, I hear a severe lack of midbass (not much below 400hz).

Largish room (15' x 18') sitting 12' away, I need at least 8" of cone area plus sub. And I'd like to see a full range get down to 200hz for a crossover point.

The betsy is said to be great for the price. You may want to notch that 5-10db spike near 3-4khz though.


Last edited by norman bates; 7th April 2011 at 03:55 AM.
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Old 7th April 2011, 03:51 AM   #3
Bluto is offline Bluto  United States
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What do you expect for SQ?

I see guys using qts of .3 but they're not getting any bass to speak of or are augmenting with subs or bass bins.

To my way of thinking don't expect SQ out of any 8" fr with less than .7 qts and as a stand alone don't expect to get any lower than maybe 150hz without sacrificing mids and highs which is ob's appeal.

Read Gainphile's site. He's done the most with OB.
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Old 7th April 2011, 04:17 AM   #4
CLS is offline CLS  Taiwan
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"System Q" should be a better POV.

Qts is dominated by Qes in most cases, and then Qes interacts with output impedance of amp.

So, any speaker driver can work with OB, as long as the system Q can be properly set.
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Old 7th April 2011, 04:32 AM   #5
ra7 is offline ra7  United States
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If you are going to augment your full range driver with a bass driver, the Qts of the full range driver is almost irrelevant. Fostex, AN, Lowther, just about anything works.

I'm using a single Eminence Deltalite 2515 per side. Qts=0.38. It needs a little bit more EQ, but it literally whopps the Alpha15s ***. Check out troels site for more info on OB with this driver.

To me, low Qts drivers mean powerful magnets and light cones, a combination which usually results in better sound compared to high Qts drivers.
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Old 7th April 2011, 05:58 AM   #6
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Sorry, I should have been more specific about the bass part. I plan on bi-amping a separate 15" woofer with a MiniDSP for XO. I would like to be able to xo at 150-200 Hz to the FR for the rest of the spectrum.

Thanks Norman for the Fs/Qts formula. That's the math I was missing. I can use Edge for the baffle stuff.

Bluto, I just don't know about the SQ factor. Is your concern that 8" is too large to give a satisfactory SQ with a low Qts? Do you think that a smaller FR would be better in that regard? I know a lot of people do prefer smaller FR drivers in this kind of application and that is one factor I still am struggling with.

CLS, good points. Thanks

ra7, what can I say, your answer just made my life a lot easier. I think I'll go with it! What do you think of Bluto's concern about SQ of a low Qts 8" FR?
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Old 7th April 2011, 08:28 AM   #7
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I was a bit critical to Troels speaker when someone put it in the Ultimate OB gallery - multiway forum. The Deltalite should be much better than the Alpha no doubt but it's limited X-max will not allow something like 100 dB in the lowbass without overloading, in OB that is. Upper bass would not be a problem.

I have substituted my Alpha15s for Acoustic Elegance IB15 with excellent result. See post #467 here: http://www.diyaudio.com/forums/multi...ml#post2433427 and this link: The BlindStone OB . Also AE IB15 is about half price of the Deltalite.


Last edited by skorpion; 7th April 2011 at 08:40 AM.
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Old 7th April 2011, 09:16 AM   #8
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Originally Posted by skorpion;2530408
I have substituted my Alpha15s for Acoustic Elegance IB15 with excellent result. See post #467 here: [URL
http://www.diyaudio.com/forums/multi-way/123512-ultimate-ob-gallery-47.html#post2433427[/URL] and this link: The BlindStone OB . Also AE IB15 is about half price of the Deltalite.

These seem to be discontinued recently. Too bad, they sounded promising. Guess I'll keep my back-ordered Alpha 15a's coming...
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Old 7th April 2011, 04:21 PM   #9
ra7 is offline ra7  United States
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First off, I don't know what SQ is.

Sprinter, you can try the Beta15s, which have a low Qts, and while they may not be built to the same quality as the deltalites, they will give you a taste of what low Qts bass sounds like. The Alpha15s sound thick and slow in comparison.

My favorite combination is the deltalite with AN8, which is a 8" full range driver with a really low Qts. I cross them at about 130 Hz and everything is fine. With regard to SPL and excursion, I doubt you'll ever exceed the maximum specifications. You'll be in trouble with your neighbours before you break the driver.

NP had his Lowthers crossed to the beta15 at 80Hz LP and 120Hz HP. So make what you will of that.
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Old 7th April 2011, 04:30 PM   #10
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I've used the Betsy 8" (qts around .7), Pioneer B20 (qts around .5) and TB 1808 (qts around .4) and they all sounded good on open baffle with Alpha 15's as bass support. There was not any drastic change in settings for the Alpha's. The baffles were about 20" x 20" sitting on top of H-frames. I also used them all on baffles about 20" x 44" to good effect. When i turn off the woofers bass drops drastically. I hope to get around to trying smaller drivers (4" like many tend to use). I also have a pair of ceiling speakers mounted in an open baffle setup that have a qts of 1.6. Subjectively they have the same amount of bass (down to about 100hz) whether they are in a sealed, ported or on an open baffle.

When simulating responses on open baffles 20 x 20 the drop off is similar regardless of qts (around 250hz unless using a driver with a qts of 1.0 or more, which will extend deeper). On 20 x 44 baffles the bass response extends to about 150hz for drivers with qts below 1.0.

Does any of this make sense? lol

>>> The Alpha15s sound thick and slow in comparison.

Hi ra7, SQ is short for Sound Quality. I don't always know the acronyms but i do know that one. Also, i agree the Alpha's can sound thick and slow in comparison to the main driver if crossed too high in frequency. Keeping it below 100hz (or lower) they sound excellent to me. When i move the crossover higher (around 150hz) they begin to contribute negatively to the overall SQ (lol).

If i was using an equalizer i'd look into the higher priced, higher excursion 15" woofers even with lower qts. But for my applications the Alpha's do the job. They go low enough on open baffle and crossed appropriately, i think they are awesome! I don't doubt the better SQ of the more expensive and more robust products out there.
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Last edited by Godzilla; 7th April 2011 at 04:37 PM.
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