Different OB designs for woofer + FR - diyAudio
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Old 31st March 2007, 08:35 AM   #1
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Default Different OB designs for woofer + FR

Hello all,

I have a bit of a problem understanding the differences btw the different woofer + FR on OB designs.

What I am looking at is the following:

- Visaton NoBox and its variants by Dick Olsher: Single 15 in woofer (Hawthorne Audio Auggie, Viaston BSG40) + FR (B200, Fostex 208 Sigma, Feastrex D5nf, Lowther DX55 and C55) on a relatively narrow baffle (aprox 60 cm), slightly sloped.

- BD-Design Quasar MkII. Even narrower baffle (46 cm) but much thicker and straight. Two 15 inch woofers (Bert's own BD15 15 inch woofers, Eminence 15 A) + FR (Fostex FE206E, AER MkI).

- Martin J King projects with same dual Eminence 15A + FR (Lowther PMA2A, Fostex FE 167, ) but on super-large baffle (150 cm wide).

My questions:

- What is the difference btw employing one 15 incher vs two in an OB ? I remember reading somewhere (the Linkwitz site maybe ?) that dipole woofers fundamentally need that radiating area i.e. basically one needs two of those.
Note that both Martin's and Dick's projects feature the same 15 incher but on radically different setups

- Very wide baffle, narrow baffle, even narrower and thick baffle ? Any reason for employing these three with same/similar woofers ? Needless to say I take for granted that both Dick, Martin and Bert know what they're doing so it must be a matter of compromises -- in that case, what are those ?

Thanks in advance for shedding some light into this,

Florian
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Old 31st March 2007, 10:39 AM   #2
ttan98 is online now ttan98  Australia
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I am in the same situation as you, however after that I did some research, this is what I found,

Linwitz's woofer is H frame: the woofers are mounted in a dipole position, in the middle of the enclosure, one facing out and the other facing in.

Martin's woofer is U mounted one or two woofers facing out and flat against the baffle, and baffle width size is important, as it determines the frequency response esp. the bass extension.

Both frames are ok for this situation. I think one person has perference over one frame over the other.

go to this site for more details,
http://www.musicanddesign.com/u_frame.html
there is a s/w package at this site to perform the design for you.

there are many more sites.

I am sure M/s M. King and J.K will give you a better explanation than I can
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Old 31st March 2007, 12:52 PM   #3
SunRa is offline SunRa  Romania
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Hi Florian,

The reason for using two woofers on a baffle is mainly related to the power handling and xmax. To go say 35Hz in OB with a woofer you have two posibilities:

1. you use a large (very large) OB and a high Qts driver to push the rollof of the woofer.
2. you try to electronicaly equalise the response using 6db shelving corection and/or linkwitz transform circuit (see Mr. Linkwitz's site for both).

There are problems in both cases, mainly regarding the power handling. To get 35Hz in an OB, using the first method means you should use a woofer with great output to ensure a proper sound pressure. In OB the driver automatically loses 6db because of dipole radiation. After the fc point, the woofer looses another 6db/octave (the frequency point where the driver starts the rollof is dependent of the baffle size). So it's obviouse you need some very unusual woofer to get low frequency response and high SPL in an OB. Even with the bump you get from using high qts drivers it is usualy not enough if you want to get loud. (I think the Augie is a pretty amaising driver in this regard, as Mr. Olsher uses it)

So that's why you end end up using the second method. But to electonicaly equalise the driver you need either woofers with great xmax and medium sensitivity (as Linkwitz uses the 10" peerles drivers) or big pro audio woofers with big spl and large radiating area like Mr. Martin J. King or BD-design.

Having equalised the woofers, means you may have a smaller OB, with the expense of loosing some power handling because the driver gets faster in limitation because of the xmax. Mr. King has a rather large OB with two high qts, high SPL woofers wich require only mild equalisation, rather than bd-design where the OB is smaller and probably you have to work around more. It is obvious for Linkwit's Orion which takes full advantage of the long stroke peerles driver.

As you see it's the need for high sound pressure level that forces you to use two or more large woofers to get low frequency response and get loud.

Of course the discussion can get on a more detailed level. This is my understanding on this problem as I am not an expert though so any observation would help to better understand these designs.

Good Luck
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Old 31st March 2007, 02:07 PM   #4
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SunRa,

Thanks for the input.

There is one thing that doesn't match: Martin J King project is using the Dayton 15" high efficiency woofers that have a measured Qt of about 0.4 and 95.6 dB sensibility. So the Qts are rather small. Moreover, Mr. King states specifically that he looked for drivers with small Qt, and only eventually tested the Eminence 15A with large Qt (1.2) and less sensibility (92.4 dB).


OTOH the BD-Design Quasar BD15 has a lesser Qt of 0.25 and 99.6 dB sensitivity and is working on much narrower baffle. Moreover, as I understand it there is no electronic eq for the drivers (well, apart being driven by their dedicated amp, but let's not exclude that possibility).

What I gather is that with large OBs the driver has to work less in order to get the SPL resulting in faster response / better dynamics ?!?!) whereas in the smaller OB the driver has to work out more to supplement the narrower OB with the associated drawbacks of long excursion (slower response / relatively bloated ?)

Am I getting closer ?

Thanks,

Florian
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Old 31st March 2007, 03:03 PM   #5
SunRa is offline SunRa  Romania
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Hello,

As I recall Mr. King eventualy liked the eminence more than the dayton drivers, but maybe I'm mistaking.

The thing is that using 15" drivers you get away with a smaller amount of equalisation (or none) than using say 10" drivers. Again, the advantage of using high spl woofers with small xmax is maybe related to what you called drawbacks related to long excursion. There were some discusions on this a while ago. Probably this one of the reasons why many say that large woofers sounds better (not just lower) than smaller ones.

It is quite evident that 15" has a low fs at a low xmax, while a 10" driver may have a low fs but woth long stroke. It's because of the large radiating area of the 15", wich has a better impendance with the air.

Quote:
What I gather is that with large OBs the driver has to work less in order to get the SPL resulting in faster response / better dynamics ?!?!) whereas in the smaller OB the driver has to work out more to supplement the narrower OB with the associated drawbacks of long excursion (slower response / relatively bloated ?)
What I believe regarding this, is that using larger drivers, in narrower OB's overcomes the problems (long stroke related, distorsions, power handling and so on) of using smaller drivers in the same conditions. Without eq, a driver in OB will go as low as the baffle allows it and will not be forced in any way. So yes you are right.

Two 15" units in an OB gives you lots of possibilities regarding experimentation. From my point of wiew both designs, (bd-design and Mr's King) are good and probably very musical too.
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Old 31st March 2007, 04:06 PM   #6
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FlorianO,

If you do not get all of the answer that you need here. You might concider posting over at the Audio Circle Open Baffle forum.

There are a few people who post there with a great deal of experience in OB design.

Good Luck!

NW

http://www.audiocircle.com/circles/index.php?board=90.0
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Old 31st March 2007, 04:16 PM   #7
ttan98 is online now ttan98  Australia
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Default FlorianO

FYI

Eminence Alpha 15A has Qts=1.27 and SPL=97db(not 92.4db)

i check the data.
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Old 31st March 2007, 06:31 PM   #8
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Default Re: FlorianO

Quote:
Originally posted by ttan98
FYI

Eminence Alpha 15A has Qts=1.27 and SPL=97db(not 92.4db)

i check the data.
I took the data from the Martin J King measurements, didn't bother with the data sheet. Still.
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Old 31st March 2007, 07:29 PM   #9
Nuuk is offline Nuuk  United Kingdom
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Is there any reason for not going with the Hawthorne coaxes to go with the Augies? They make a very good OM speaker!
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Old 31st March 2007, 07:54 PM   #10
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Nuuk,

That was not my point. If I wanted an already "digested" (i.e. designed, tested and tried) DIY project I could have go for the Silver Iris, or Robert Bastanis Prometheus or Dick Olsher OB2/Visaton NoBox (my fav atm) or BassZilla, or ... (insert fav DIY OB design here).

My point was trying to _understand_ the issues involved when designing a narrow vs wide baffle and using one vs. two woofers.
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