Differences between OBs and planars

[River757]

I've listened to, but never owned, Magneplanars, Apogees, Bertagni Electroacoustics and Martin-Logans, but the closest I've been to listening to an OB speaker system is a pair of Infinity RS IIs --> three open-backed 5" midbass drivers plus a rear-facing EMIT tweeter, in addition to dual ten inch woofers and a front-facing EMIT.

Since I've been considering building a small OB system, I was wondering if anyone had any opinions concerning the sonic differences and similarities between the two systems. Also, why aren't there more commercially-available OB speaker systems (besides for example Jamo's "R" series), since it seems like:

1) the lack of a full enclosure would be an advantage as far as manufacturing and especially shipping costs (shipping is one of those "hidden" costs but is still a very significant portion of a speaker's street price).

2) building an OB would be much less expensive vs. a planar with its rather delicate and specialized radiating surfaces.

Thanks!

[head_unit]

1) True (compared to a box speaker I think is what you meant), but but but...it might actually be harder to package and ship due to the odd shape. And while somewhat less material could be used for the enclosure, it would probably be somewhat MORE expensive. Why? Because the raw material (if veneered/vinyled MDF) is not really that expensive, and there are huge machines set up to churn out rectangular cabinets all day long. The price of things is most affected by volume; any thing unusual thus tends to cost more.

2) An OB is simply not a planar. It is just a speaker with no back, or at least no back over the midrange drivers. The "better sound" of the planar is not primarily due to the open back. That's just a necessary evil to avoid the huge expense of huge cabinets. (To my recollection only the Beveridge system had enclosed planars). The sound of the planar is primarily due to the diaphragm being driven over a greater portion of the radiating area, with magnetic systems having characteristics very different from cone transducer magnetics. Plus some planars avoid the evils of crossovers, or at least within most of the audio range.

[River757]

The "better sound" of the planar is not primarily due to the open back. That's just a necessary evil to avoid the huge expense of huge cabinets.

I thought the primary attraction of a planar was its dipolar nature and ability to create an "airy" soundfield. And the cone vs. diaphragm driving system aspect, while important too, was secondary.

Btw I'm not trying to start a war between OB fans and planar fans, I'm just trying to accumulate more info on this subject for my own benefit.

[DougL]

To me, OB and plainer speakers have similar sounds.
I attribute that to the dipole radiation and lack of a box.
I have friends that had Quads, Maggie's and I have an OB.

To me, it was no accident that the Dalquest DQ10 had an OB midrange.

Doug

[whgeiger]

Originally Posted by River757

I've listened to, but never owned, Magneplanars, Apogees, Bertagni Electroacoustics and Martin-Logans, but the closest I've been to listening to an OB speaker system is a pair of Infinity RS IIs --> three open-backed 5" midbass drivers plus a rear-facing EMIT tweeter, in addition to dual ten inch woofers and a front-facing EMIT.

Since I've been considering building a small OB system, I was wondering if anyone had any opinions concerning the sonic differences and similarities between the two systems. Also, why aren't there more commercially-available OB speaker systems (besides for example Jamo's "R" series), since it seems like:

1) the lack of a full enclosure would be an advantage as far as manufacturing and especially shipping costs (shipping is one of those "hidden" costs but is still a very significant portion of a speaker's street price).

2) building an OB would be much less expensive vs. a planar with its rather delicate and specialized radiating surfaces.

Thanks!

As the radiating surface of the driver(s) increases, the further behind the loudspeaker the acoustic image of the source recedes.

At lower frequencies, the demands on the driver(s) is increased beyond that of those mounted in a CB enclosure. So, it is not clear that OB loudspeaker would be cheaper to build even if produced and shipped in large quantities.

Regards,
WHG

[a.wayne]

Originally Posted by River757

I thought the primary attraction of a planar was its dipolar nature and ability to create an "airy" soundfield. And the cone vs. diaphragm driving system aspect, while important too, was secondary.

Btw I'm not trying to start a war between OB fans and planar fans, I'm just trying to accumulate more info on this subject for my own benefit.

Correct , along with a massive drive area .....

[River757]

To me, it was no accident that the Dalquest DQ10 had an OB midrange

You know, I've seen pics of that speaker many times, but always figured the felt "blanket" lying across the back of the mid driver was used in a cosmetic manner to cover up a closed-back driver (the 10's use of a plastic piezo[!!] supertweeter prompted that theory). You learn something new every day!

Originally Posted by whgeiger

As the radiating surface of the driver(s) increases, the further behind the loudspeaker the acoustic image of the source recedes.

As Ed McMahon used to say, I did not know that. Thanks.

At lower frequencies, the demands on the driver(s) is increased beyond that of those mounted in a CB enclosure. So, it is not clear that OB loudspeaker would be cheaper to build even if produced and shipped in large quantities.

A good use for a small subwoofer then and just allow the OB to reproduce the frequencies that would allow the price to stay competitive?

[head_unit]

Originally Posted by River757

I thought the primary attraction of a planar was its dipolar nature and ability to create an "airy" soundfield. And the cone vs. diaphragm driving system aspect, while important too, was secondary...

Well, as a speaker engineer I guess I have a certain bias in how I look at things. Really planars have several things going on at once, which are not really separable.

I do believe the dipole effect is a kind of accident of design. I could be wrong, but I'm pretty sure the idea of driving a large surface came first, to:
- Have a very large surface enabling low excursion at each frequency (bass excluded).
- The diaphragm driven over a greater % of the surface, which seems to give better sound.
The magnetics also have a very different field leading to a very different excursion versus distortion relation, and the large rectangular diaphragms have quite different breakup modes compared to cones, both factors leading to different sound versus cone speakers.
In the case of the Bertagnis, in-ceiling or in-wall mounts are not dipoles, and dipole was not a big deal to those guys. I worked with them, that's how I know :-).

Then with such a big diaphragm, it would have been wildly expensive to enclose it-so they made a dipole instead. The dipole then happened to create this "open" sound due to a very different cancellation in the room vs. monopole.

Then some folks said "hey, let's just not put a baffle behind our midrange drivers and get a similar "airy" effect. Others decided to try reflectors and extra tweeters and 360 degree radiating surfaces like the Ohm Walsh as different approaches for "airiness"...
Vive la différence!

[John_E_Janowitz]

The biggest advantage to a ribbon, planar, electrostatic, etc over a standard VC driven driver is the lack of the VC itself. In a VC driven driver where you have a coil of wire, you have inductance to deal with. This inductance leads to issues with flux modulation and the non-linearities in the impedance curve which has a significant effect on the upper end of the response curve due to excursion. Lowering the effects of inductance in a VC driven driver has some significant benefits. This can be accomplished by a properly designed shorting ring. Per our white paper:

AE Speakers --- Superb Quality, Unforgettable Performance, Definitely.


"The copper effectively short circuits the inductance of the VC to an incredibly low level. This further lowers the influence of the inductance variance on the high frequency response of the driver. Also it forces a flatter phase curve for the driver. The main difference between electrostatics, ribbons, and VC driven drivers has always been the inductance. Here we can finally get a woofer to mate with these other drivers as closely as possible."

John

[mige0]

Originally Posted by John_E_Janowitz

The biggest advantage to a ribbon, planar, electrostatic, etc over a standard VC driven driver is the lack of the VC itself.

Allow me question that - and that white paper too - the tenor is more of your sales strategy than anything else


Michael