Open baffle speaker design

[Jakob1863]

No need to DBT speakers, every one of them has their own personal tone controls built in and your preference is as good as anyone else
Personally I prefer a excellent set of horns.

But you should keep in mind that fooling yourself as a listener to speakers is as easy as fooling yourself when listening to "typical audiophile" tweaks/modifications.

 

[Sal1950]

But you should keep in mind that fooling yourself as a listener to speakers is as easy as fooling yourself when listening to "typical audiophile" tweaks/modifications.

Sure, it's easier to like a speakers sound based on things like it's visual appeal..
Got to keep the curtain down on the spin-o-rama.

 

[Theo]

Yes, comb filtering is a known issue with dipoles, since the back wave hits the wall behind, inverts, and recombines with the front. I do not like it at all as it really wreaks havoc with FR and imaging. It drives me nuts, but it's a short drive. That is why I routinely damp (ro diffuse) the back wave unless they are far enough away from the wall behind that the cancellation is low enough in frequency and delayed enough to not be a problem. That has rarely been the case in the rooms I have had through the years. The same thing occurs with conventional speakers, natch, due to first reflections. Planer dipoles do not radiate much off the sides and top/bottom over the bass region (below they start to look more like a point source) so first reflections are rarely an issue -- except for the back wave.

This and the 1.5kHz limit mentioned above show that we may want to have a different approach on the OB question depending on which part of the spectrum is concerned. I suppose that a tweeter and a hi-medium speaker don't necessarily need to be in a closed box as the distance behind the transducer would usually be significantly higher than the wavelength (unless you stick it against the wall). Having a woofer or a subwoofer in an OB configuration is another story and becomes a real problem (actually, also the case with boxed loudspeakers, maybe to a lesser extent). OTOH, as the room becomes the speaker box at low frequencies, wouldn't an OB carefully positioned in the room be a mean to cancel some room notches using the fact that an inverted impulse reflected on the back wall will travel back towards the listening position with its original polarity?

 

[Jakob1863]

@Cosmik,

But if the loudspeaker had never been invented and you were given the task of designing it, would you stipulate: "For every impulse emitted from the front of the transducer, an equal and opposite impulse shall be radiated from the rear". If so, why?

<snip>

That seems to imply that on a theoretical appoach the best possible solution would be a speaker with discrete seperated chassis where much of the energy is transformed into heat and unwanted "by-vibration" ; as an engineer i don´t buy that ....

Neither do the DSP'ed box speakers. And the box speakers maintain reasonably sensible time and phase response as you move around them - so the room's reverberation is as though stimulated by something much closer to the ideal speaker. It all 'adds up'.

As usual it depends on the definitions used for "reasonably" and "room´s reverberation" as usually if the listener leaves that small sweet spot, time and phase response weren´t that great anymore and even in this sweet spot the reflected sound waves most likely don´t possess all of the DSP´ed properties.
In practice it isn´t as easy to define the best fitting directional behavior of a loudspeaker as room dimensions furnitures and all the other stuff interacts with the sound field radiated from the loudspeakers.

Not a great feature for the music lover who likes realistic volume levels.

It depends on the size and the bandwidth used.
Systems like this don´t suffer from the same restrictions:

Not so small electrostatic approach (hybrid)

And yes, I have listened to them many times - and never been as stunned as I am by DSP'ed box speakers.

Isn´t that a completely different topic?
Just as a reminder; todays reproduction is a very distorted version of real soundfields and our brain tries all the time to construct from the various cues an impression that is compatible to our experience of the real world.
Different humans, different responses to this distorted version.
As stated before, i would be very surprised if within the "wrongness" only one "correct reproduction" would exist that fits to all humans.

Despite all the limitations i do agree, good electrostats do something for me that other types of speakers don´t do. There are some hypothesis why that could be, but up to now no conclusive theory.

 

[rebbiputzmaker]

This and the 1.5kHz limit mentioned above show that we may want to have a different approach on the OB question depending on which part of the spectrum is concerned. I suppose that a tweeter and a hi-medium speaker don't necessarily need to be in a closed box as the distance behind the transducer would usually be significantly higher than the wavelength (unless you stick it against the wall). Having a woofer or a subwoofer in an OB configuration is another story and becomes a real problem (actually, also the case with boxed loudspeakers, maybe to a lesser extent). OTOH, as the room becomes the speaker box at low frequencies, wouldn't an OB carefully positioned in the room be a mean to cancel some room notches using the fact that an inverted impulse reflected on the back wall will travel back towards the listening position with its original polarity?

Yes, OB bass can become unwieldy and be impracticable. Having the OB driver working from 150-200hz up is a more reasonable compromise as in the BassZilla.

https://www.blackdahlia.com/tipindex/BassZilla/tip_24.htm

 

[Fitzcaraldo215]

This and the 1.5kHz limit mentioned above show that we may want to have a different approach on the OB question depending on which part of the spectrum is concerned. I suppose that a tweeter and a hi-medium speaker don't necessarily need to be in a closed box as the distance behind the transducer would usually be significantly higher than the wavelength (unless you stick it against the wall). Having a woofer or a subwoofer in an OB configuration is another story and becomes a real problem (actually, also the case with boxed loudspeakers, maybe to a lesser extent). OTOH, as the room becomes the speaker box at low frequencies, wouldn't an OB carefully positioned in the room be a mean to cancel some room notches using the fact that an inverted impulse reflected on the back wall will travel back towards the listening position with its original polarity?

I am not really clear on why comb filtering should be a bigger issue with open back dipoles. See this link:

https://audiosciencereview.com/foru...ptual-effects-of-room-reflections.13/#post-15

Yes, there is abundant comb filtering with all speakers except under anechoic conditions, and Amir is pretty calm about their audibility or some special need to compensate for them, except perhaps those due to horizontal rather than vertical surfaces. So, why single out open back dipoles, which may have more from the frontal direction due to reflections from the front wall, but which have fewer from side walls, ceilings and floors?

 

[Fitzcaraldo215]

Yes, OB bass can become unwieldy and be impracticable. Having the OB driver working from 150-200hz up is a more reasonable compromise as in the BassZilla.

https://www.blackdahlia.com/tipindex/BassZilla/tip_24.htm

Agreed, dipole cancellation in the bass is is physics and not easily cured. Hence also dipole hybrids, like Martin Logan, Sanders, etc. using monopole bass drivers.

 

[RayDunzl]

Measurement, 1/6 octave:

Top - dipole, on-axis (stereo)
Middle - conventional on-axis (stereo)
Bottom - conventional off-axis (stereo), about 6 inches

 

[Blumlein 88]

A random question: should dipolar be the polar pattern of choice for speakers reproducing a recording made with figure 8 mikes, and point source be the polar pattern of choice for recordings made with omnis or cardiods?

Maybe. I don't know. Figure 8 recording sounds very good on panels.

 

[RayDunzl]

Here's a little video I made...

Mono Pink Noise is shown, same signal in left and right speakers, both playing, and an increasing delay is added to the right channel, simulating moving the microphone off-axis, showing the progression of comb filtering.

The maximum delay is half a millisecond, simulating 6.7 inches/172mm or so off-axis.

https://www.screencast.com/t/KenwrJj4WGA

 

[Ossidian]

Wow soo much intellectual argument on the merits of OB speakers, how many have truly sat down and listened to a proper pair?
I will not engage in theories or interpret someones theoretical data on the subject of open baffles. Just as I would not assume to tell
someone how to listen to their music or make blanket statements. I like my open baffle speakers and could care less that they accurately
measure 23 hz to 40 khz ; ) I will just log out and enjoy listening to music, carry on. Oh and 96 db is really efficient.

 

[DrTebi]

[...]Having a woofer or a subwoofer in an OB configuration is another story and becomes a real problem (actually, also the case with boxed loudspeakers, maybe to a lesser extent).

Open baffle bass is not a problem, as long as you can provide the necessary EQ curve and amplifier power

OTOH, as the room becomes the speaker box at low frequencies, wouldn't an OB carefully positioned in the room be a mean to cancel some room notches using the fact that an inverted impulse reflected on the back wall will travel back towards the listening position with its original polarity?

Yes, and that's what makes open baffle bass sound so freakin' good even in a less than optimal room acoustics.

Just talking from experience

I have a 4-way active open baffle system. Took a lot of experimenting and tweaking until I got it 99% perfect (to my ears), but now it's just fantastic.

 

[Cosmik]

...an inverted impulse reflected on the back wall will travel back towards the listening position with its original polarity...

I have seen this myth in this forum before. In fact, the pressure wave has to be reflected with the same reversed polarity - which is why open baffle speakers sound very strange if anywhere near a reflective surface behind - it's like accidental QSound processing or deliberately placing a reverse-wired speaker behind the front speaker.

It is also why some people like the open baffle sound (effect). It's like weird, man.

Reflection of a sound wave at a hard wall
At the wall there can be no particle motion, but the pressure can, and will, vary. In order for the particle velocity to be zero at the wall the pressure gradient must be zero. The reflected wave pulse then travels from right to left, with the same speed and amplitude as the incident wave, but this time with the same polarity (the positive pressure pulse is reflected as a positive pulse).

 

[Burning Sounds]

I have seen this myth in this forum before. In fact, the pressure wave has to be reflected with the same reversed polarity - which is why open baffle speakers sound very strange if anywhere near a reflective surface behind - it's like accidental QSound processing or deliberately placing a reverse-wired speaker behind the front speaker.

It is also why some people like the open baffle sound (effect). It's like weird, man.

No-one I know who has dipoles has them positioned anywhere near a reflective surface (ie the front wall) - the rear wave is either diffused (my preference) or partially absorbed. There is no sound effect @Cosmik and it's most certainly not weird (is that a scientific term? )

 

[Cosmik]

No-one I know who has dipoles has them positioned anywhere near a reflective surface (ie the front wall) - the rear wave is either diffused (my preference) or partially absorbed. There is no sound effect @Cosmik and it's most certainly not weird (is that a scientific term? )

If it makes you feel better. But really it's like taking a reverse-wired speaker - or lots of them - and positioning them around the room. Maybe they're not all that loud (several dB down) and maybe their bandwidth is reduced. In an in-room frequency response measurement their effects may not look anything special, but in terms of the Haas Effect and related sound localisation cues they cause confusion, hence the weird sound.

 

[DonH56]

Open baffle or dipoles will react with the wall behind constructively and destructively. Conventional speakers only have a "front" wave to deal with; dipoles add that back wave. The back wave is inverted with respect to the front, and a boundary (wall) sure enough inverts the wave so now it is no longer inverted with the front wave, but that is not the problem. The problem is the round-trip delay from the back of the speaker to the wall and back where the back wave combines with the front wave, and that modulation continues to the listening position. At that point you can have massive comb filter effects where peaks and valleys are created by the front and back waves combining. Easily heard and measured. The fundamental frequency depends upon distance to the back wall and multiples of that frequency extend all the way up the audio band. At low frequencies, where the panel (or drivers in an open-baffle design using conventional drivers) starts to act like a point source, you have more "normal" SBIR and room mode effects. At higher frequencies, wavelengths are short enough and enough other reflective surfaces around that again it is less an issue. But from the lower midrange right through the vocal band you can have those comb filter effects. That is primarily what has driven me to dampen the back wall whenever I have not had a room big enough to get my Magnepans well out into the room (I've only had a room big enough a few times since the late 1970's when I owned my first pair, and my current room does not qualify -- so it is heavily damped).

Diffusion works as well but is usually more costly, plus I had some low-frequency room mode doubling to deal with (where diffusers would be too large), so I went with lots of absorbers.

 

[Burning Sounds]

In an in-room frequency response measurement their effects may not look anything special, but in terms of the Haas Effect and related sound localisation cues they cause confusion, hence the weird sound.

That the Haas effect is involved in the creation of that perceived spaciousness and "large speaker" sound many people hear when listening to a dipole I'm sure is true.

A simple explanation from Wikipedia - Haas found that humans localize sound sources in the direction of the first arriving sound despite the presence of a single reflection from a different direction. A single auditory event is perceived. A reflection arriving later than 1 ms after the direct sound increases the perceived level and spaciousness (more precisely the perceived width of the sound source). A single reflection arriving within 5 to 30 ms can be up to 10 dB louder than the direct sound without being perceived as a secondary auditory event (echo). This time span varies with the reflection level. If the direct sound is coming from the same direction the listener is facing, the reflection's direction has no significant effect on the results.

For more recent in depth research this might be interesting.

Here's Siegfried Linkwitz' hypothesis:

Confusing cues from the room are minimized if the reflections are:

1 - Left-right symmetrical

2 - if they are delayed >6 ms, and

3 - if they are attenuated copies of the direct sound in spectral content

I think a strong case can be made for our ability to sort out different auditory cues and to focus our attention to hear what is of interest at the moment.

Given your belief that we "listen through the room" (which I 100% agree with) I find it strange that you become confused. But we all hear differently, I guess.

The problem is the round-trip delay from the back of the speaker to the wall and back where the back wave combines with the front wave, and that modulation continues to the listening position. At that point you can have massive comb filter effects where peaks and valleys are created by the front and back waves combining. Easily heard and measured.

Diffusion works as well but is usually more costly, plus I had some low-frequency room mode doubling to deal with (where diffusers would be too large), so I went with lots of absorbers.

Absolutely, I have heard and measured both. Room placement is crucial for a dipole (one of its drawbacks) even though they don't excite the room as much as a monopole.

Regarding diffusion, I just use a wall to wall bookcase behind my speakers (I have both Maggies and Linkwitz LX521), nothing very fancy.

 

[watchnerd]

I still remember when I heard these. I haven't heard a domestic rig since that matched their sense of scale:

 

[Deleted member 2944]

I've been listening to various weird-sounding open-baffle dipole speakers as my primary ones for about forty years now.
All speaker systems have trade-offs, but dipole speakers have (inherently) the least number of said trade-offs.

Dave.

 

[watchnerd]

I've been listening to various weird-sounding open-baffle dipole speakers as my primary ones for about forty years now.
All speaker systems have trade-offs, but dipole speakers have (inherently) the least number of said trade-offs.

Dave.

Yeah, I dunno about that...

I owned 3 different Martin Logan electrostats for over 10 years. I've since gone back to unipole dynamic speakers.

Dipoles are pretty impractical in certain scenarios, such as near field listening or DAW work.

Also, you have a "circle of confusion" issue in that pretty much zero recording studios use dipoles, so using a dipole is putting you farther away from what the recording engineers heard and mixed for.