Open baffle vs rear drivers driven in phase

If I'm thinking about this right then the idea behind open baffle is that real instruments would radiate backwards as well as forwards and our ears expect to hear those sounds to complete the auditory illusion to our brain. Well wouldn't real instruments radiate in phase sound in all directions, not 180 out of phase going backwards? Would essentially a back to back sealed speaker potentially make an even more believable auditory illusion?
 
Musical instruments tend to radiate in a fashion similar to an open-baffle driver, especially in the case of bass frequencies. Think of a bass drum - just like a large-cone open-baffle bass driver.

You need to consider the listening room, of course. A different environment, in general, compared to the recorded performance. In practice, open-baffle speakers need at least a metre behind them - but that's just about their only limitation. The most realistic bass I've heard is from open-baffle speakers.

At higher frequencies, front and back phase are of course irrelevant (except for the fact that out-of-phase rear radiation results in a desirable dipole).

Open-baffle dipoles excite fewer room modes. They don't suffer from cabinet resonances. What's not to like?
 
All music Instruments radiate sound differently. It is not rational to try to make a loudspeaker that would imitate one instrument. Forget these analogies!

A dipole or omnipole loudspeaker tries to radiate similar frequency response to all directions. This is practically mission impossible, there will always be lobes at some frequencies and some directions.

However a well excecuted speaker of these types will sound different in a small room than any conventional or highly directional speaker. Some people like that and some like this.
 
Musical instruments tend to radiate in a fashion similar to an open-baffle driver, especially in the case of bass frequencies. Think of a bass drum - just like a large-cone open-baffle bass driver.

... Except bass drums come in all sorts of shapes, sizes, and configurations. Some of them are ported, and guess where the sound engineers put the mics ;)

Open-baffle dipoles excite fewer room modes. They don't suffer from cabinet resonances. What's not to like?

The truly spectacular waste of LF output capability. I'd consider a pair of 15"s to be the absolute minimum requirement for OB use, but those same drivers in sealed boxes will rattle the house, even down at 20Hz.

OBs have their uses, but I don't like it when people try to present them as the ultimate solution, when they have their own set of compromises, some of which make them impossible for some situations.

Chris
 
The truly spectacular waste of LF output capability. I'd consider a pair of 15"s to be the absolute minimum requirement for OB use, but those same drivers in sealed boxes will rattle the house, even down at 20Hz.

OBs have their uses, but I don't like it when people try to present them as the ultimate solution, when they have their own set of compromises, some of which make them impossible for some situations.

Chris

Not so much a waste when you consider the quality of the bass. You have to go to great lengths to achieve such quality with conventional boxes (multiple boxes and/or severe modifications to the listening room).

With OB, all you need are multiple/large drivers and room to breathe behind the speakers. Simple as that.
 

Keith Taylor

Member
Paid Member
2007-11-04 2:06 pm
Adelaide
maggiesnmacs, mimicking the directional properties of real voices and instruments is not a realistic goal. Many instruments change their directivity quite markedly from note to note.

I would recommend a good read at Linkwitz Lab - Loudspeaker Design

One needs to make a distinction between open baffle and dipole. Open baffle can include planar drivers such as ESL's and Magnapans. To get true "dipole action" calls for drivers that are acoustically small over the range they are covering. Acoustically small means that the radiating surface needs to be a smaller size than the wavelengths being radiated.

When this condition is met the figure 8 pattern can extend up to several Kilohertz. There is nothing particularly good or bad about the rear radiation; but the constant directivity that dipole action imposes on the speakers is what is valued. Even though dipoles radiate front and rear there is 4.8dB less total energy being propagated into the room for equal on axis loudness compared to an omnidirectional speaker. This means diploes can be listened to at a greater distance than omnis' before room reflections start to intrude.

Keith
 

ashok

Member
2002-06-06 4:43 am
3RS
Regarding extreme loss of bass in a dipole. Many people do use a sealed sub with the OB made to go go as low as possible without much eq in the bass. However I'm not sure if a sealed sub below say 70 Hz with an OB working down to 70 Hz ( or slightly higher !) is good enough as compared to a full OB. Note that I don't expect it to be exactly the same sound but is it a 'good enough' compromise to get down low enough with enough higher spl capability?

I've used the Peerless 12 inch drivers and Eminence 15 inch drivers and they sound great though cone movement is tremendous with moderate SPL , as expected. The sub would alleviate that. Unfortunately I cannot try it out myself for some time to come.
 

Scott L

Member
Paid Member
2008-12-27 12:32 pm
Knoxville, TN
Cancellation

With open baffle operation of bass drivers, there exists a large degree of front-to-back-wave cancellation. This cancellation is not 100%, but still exists in such a large degree that attempting to produce deep bass becomes virtually impossible, and depends to a very large degree on the baffle size.

Open baffles (as most commonly used) are known as finite baffles.

Some of the finest bass quality can be had with an operation known as the

INFINITE BAFFLE

which is in 100% opposition to the finite baffle.
 
OK that all makes sense. I figured my idea didn't make much sense but it was more of a thought experiment and I feel like I understand better now the fundamental theories of OB.

And I agree that OB is just another speaker with compromises. The fact that you need a very large woofer and lots of space to the real wall are real drawbacks that many people can't afford. Also most often it requires a much more complex crossover that must be line level. There are exceptions to this and I think one of the coolest speakers floating around are 2-way hybrid FAST OB bookshelves that don't offer open baffle below 3 or 400 hz but are very compact, can be passively crossed and have the bass depth of a sealed speaker. But of course that comes with its own compromises as well :)
 

Scott L

Member
Paid Member
2008-12-27 12:32 pm
Knoxville, TN
If I'm thinking about this right then the idea behind open baffle is that real instruments would radiate backwards as well as forwards and our ears expect to hear those sounds to complete the auditory illusion to our brain. Well wouldn't real instruments radiate in phase sound in all directions, not 180 out of phase going backwards? Would essentially a back to back sealed speaker potentially make an even more believable auditory illusion?

Let's examine this further. If an additional out-of-phase content is added in to our attempt at reproduction, this may be considered an aberration, or a distortion of the original. Granted, there is no such absolute thing as high-fidelity playback, and many folks enjoy di-poles. For certain though, they should not be exaulted as "superior". With respect to "influencing the room", I feel another aspect to be considered is [that] a mono-pole radiation is pretty much omni-directional at frequencies where the wavelengths are considerably larger than the physicals of the enclosure, and I feel that "omni" is more uniform than a figure 8. Therefor, I am not "buying" that a figure 8 is a superior tactic.
 
Let's examine this further. If an additional out-of-phase content is added in to our attempt at reproduction, this may be considered an aberration, or a distortion of the original. Granted, there is no such absolute thing as high-fidelity playback, and many folks enjoy di-poles. For certain though, they should not be exaulted as "superior". With respect to "influencing the room", I feel another aspect to be considered is [that] a mono-pole radiation is pretty much omni-directional at frequencies where the wavelengths are considerably larger than the physicals of the enclosure, and I feel that "omni" is more uniform than a figure 8. Therefor, I am not "buying" that a figure 8 is a superior tactic.

True but what I've noticed is that transition from 4pi space to 2pi space occurs at a fairly low frequency, in the low hundreds of hertz for most baffles, where baffle step correction is needed. All frequencies above essentially radiate only forwards and if you stand behind a monopole speaker you hear exactly that. Meanwhile the figure 8 of OB can extend to any frequency essentially.
 

Scott L

Member
Paid Member
2008-12-27 12:32 pm
Knoxville, TN
Yes, that is true as well. For conventional loudspeaker enclosures the baffle step is a concern. They way I tackle this, is to make the transition very close to the Shroeder frequency of the room and to use judicious amounts of sound dampening material on the walls closest to, and between, my loudspeakers.
Once that frequency point is set, I get out of the bass device (crossover frequency) and hand off to a highly directional device, such as a horn. The price to be paid (and I don't mind) is to have only one sweet spot for listening.
 
Here are my measurements of a dipole and monopole loudspeaker in same location of speaker and mic in same room. The room is wide, sidewalls are more than 2m (6ft) away and front wall is 80cm (1 1/3ft) - so this is favourable for the monopole.
Not a big difference but they do sound very much different (spl difference exists of course)!
Guess which is dipole?
 

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