Question about O B designs

[jeffsg4mac]

Open baffle designs have always interested me, but I have not actually ever heard one. For use in HT with subs what are the advantages and disadvantages? Could I take a design like these Ushers and put them in an open baffle or does the cross-over have to be completely redesigned? I assume it would have to be.

 

[Swerd]

Open baffle designs have always interested me, but I have not actually ever heard one. For use in HT with subs what are the advantages and disadvantages? Could I take a design like these Ushers and put them in an open baffle or does the cross-over have to be completely redesigned? I assume it would have to be.

The short answer is you could not use those crossovers for those Usher drivers if you changed the cabinets to open baffle.

Without reading the details about open baffle designs, such as here, I can quickly say that properly designed open baffle, dipole, speakers interact with room reflections in an audibly different way than monopole speakers in standard cabinets. If I understand correctly, they need careful placement in a room because of the significant amount of sound coming off the rear of the open baffle. Most dipole designs I've seen or heard were used for stereo music only systems. I can't say how well they might work for a HT.

I'd guess that you should hear some, and play with their room placement, before you make up your mind.

 

[GranteedEV]

There's the deal.

OBs/dipoles need a lot of equalization. About the only successful passive OB i've heard of is Jeff Bagby's Archos, and the thing to note there is that the woofer section is a box / monopole and the drivers used are both very high expensive and very high efficiency.

Passive crossovers can attenuate peaks, but in order to "raise" a dip with a passive crossover, you unrealistically need to attenuate the entire rest of the response.

Now here's the deal with a dipole speaker:

http://www.musicanddesign.com/Dipoles_and_open_baffles.html

as you can see from reading that, dipoles require boost in order to produce the same output that a monopole would. If it starts rolling off at 6db/octave below 300hz for example, that means where a driver might be 0db down at 75hz in a sealed box and, it'll already be 12db down at 75hz on an open baffle. 12db of boost to return it to flat - is a LOT and that's one part of why OBs lack amp headroom.

Now compound that with the fact that any driver in free air or a sealed box rolls off at 12db / octave, so your bass that's already acoustically 12db down at the knee, is now going to roll off at 18db/octave below that knee.

So dipole bass is not very efficient. You need world class drivers and world class amplification.

There's also the reality of driver excursion. Even though you can boost the "12db down 2 octaves below rolloff" as I described above, the boost doesn't mean you're returning the driver motion to where it would have been in a box. It actually has to move more to produce the same output, because of how much cancellation is going on. As a test, run a 30hz tone through a woofer in free air, and run that same tone in a box. One will be louder, even if the driver motion is the same.

That's one bright part about OBs. Because there's no box, there's nothing restricting driver motion. The driver should realistically be well damped so bottom end efficiency is improved. However that too may mean a need for EQ. Most driver have free air q anywhere from .3 to .45 - that's just too low and means the driver rolls off way too early. You probably want a q closer to .55 to .65 so there's another need for equalization, unless you're using a high Q driver of course.

 

[jeffsg4mac]

Forget that mess then. Guess that's why you don't see them much.

 

[GranteedEV]

Forget that mess then. Guess that's why you don't see them much.

I don't think they're a mess, they're just difficult and demanding. They have some definite positives. Since the bass only radiates back and forth rather than in all directions, it is less sensitive than the typical box speaker, to room loading (especially in the region from 80hz to 300hz where multiple subwoofers is NOT an option).

Here is AJ in FLA's OB speaker:

http://www.stereophile.com/content/soundfield-audio

I'm sure it has powerful limitless dynamics and is finely suited for HT, especially with your other subs handling the LFE track.

As you can see, it addresses the lack of efficiency, by using high efficiency 12" mid and 18" woofer.

 

[jeffsg4mac]

Did you see these? http://www.jamo.com/na-en/products/r-909-description/Nice looking but I bet the price is nice too.

 

[MJK]

OB speakers can be very simple to design, easier then boxed speakers IMO. It all comes down to the driver selection, if you pick the right drivers there is no need for all of the EQ or active crossovers. The design can be completely passive and have excellent bass down into the 30 to 40 Hz range. You have to think differently and drivers that you would normally consider for a BR or sealed enclosure design will not work well in a simple OB system. Start by looking for a 15" woofer that is 92 to 94 dB/W/m, with an fs in the 30 to 40 Hz range, and a Qts greater than 1.0. Use a 20" wide by 40" tall baffle. Pick a full range driver that is 88 - 90 dB/W/m (or a similar efficiency mid tweeter combination) and design a second order passive crossover in the 200 Hz range to mate the woofer to the other driver. Dead easy, low cost, and high performance without any of the rocket science you see in more complex OB designs that use woofers meant for boxes.

http://www.quarter-wave.com/OBs/OB_Theory.html

http://www.quarter-wave.com/Project08/Project08.html

http://www.quarter-wave.com/Project09/Project09.html

Martin

 

[GranteedEV]

Ahh yes, I forgot about "wide baffle" OBs. MJK is right on that note.

I guess I personally only think of OBs as "minimal or no baffle" because I want a certain "ideal" polar response characteristic from a dipole. But if that's not what you're chasing, then the above should be considered strongly!!

 

[MJK]

Ideal polar response is a design trade-off that people chase, it places requirements on the types of drivers and the OB system design (baffle width, active crossovers with lots of EQ, and several amps making it very complex). I would maintain that a "wide" OB does not sacrifice the polar response significantly and that once you place either OB style in a room there are probably bigger variables to wrestle with then ideal polar response. I understand the ideal dipole response goal and have decided not to make it a design driver in my personal OB speaker systems. Some people like complexity.

Martin

 

[jeffsg4mac]

What about a simple OB design that only goes to down to 60-80hz range to use with my subs? Using high quality mids and tweeters for the upper part. I don't care about it going low because I have the two titanic subs. I am just looking for ideas because I want new speakers. I still would like an array of some sort as well or some maggies. I guess I am just not sure what I want at this point. Just some higher quality sound than what my monitor 7's are capable of putting out. Which is good, but I would like better.

 

[GranteedEV]

Here is a high end wide baffle OB speaker:

OBL11