[HiFiNutNut]

I understand general OB theory and all the pros and cons of wide baffles and narrow baffles.

Wide baffles allow higher efficiency towards lower frequencies. The problem is that the first dipole null would be fairly low which would be difficult to deal with. Narrow baffles allow shifting the first dipole null higher to the crossover region so that it can be dealt with by the crossover. Of course, the low efficiency with the narrow baffle increases the costs and design complexity. There are also many other pros and cons.

But there is one aspect that is rarely discusssed - the delayed sound from the back waves. There have been numerous discussions on VER (very early reflections, e.g. less than 1ms would be most harmful to the sound, and within 10ms to be best avoided).

But do the back waves have the same effects as VER? I would assume so. Perhaps fortunately, at higher frequencies (e.g. 800Hz and above) the drivers are quite directional so the VER effect is significantly reduced. And perhaps at lower frequencies, our ears don't care much about the VER?

Would it be the cause of OB lacking of imaging comparing to monopole, as some people claim? or would it be the cause of the unique characteristic OB sound we love so much (openness, live-like)?

I have a narrow baffle OB speakers (John K's NaO) so perhaps the VER is less obvious. Most OB speakers discussed in this site are wide baffle OB speakers. How do you compare yours to the narrow baffle OBs? or to monopoles?

Regards,
Bill

[hurdy_gurdyman]

I couldn't tell you a scientific reason, but of the baffles I've owned, the narrow ones all seem to have a more open and exciting sound stage.

Dave

[Rudolf]

Bill,

in my opinion it is rather short sighted to gear baffle width completely to the low frequency demands. It has been shown that a baffle should not be much wider than 2.2 times the driver cone diameter (I consider 3 times the definite upper limit). Of course this does not apply for drivers with some built-in directivity, because they don´t "see" the baffle width. Within these limits VER should not be much of a problem.
Another aspect: Baffle edge "reflections" (which really are diffractions) are somewhat symmetric while early sidewall reflections are non-symmetric. I don´t know of any scientific reseach regarding the difference but would consider symmetric diffractions less harmful than asymmetric ones.

I agree that OB is lacking of imaging comparing to monopoles (to a certain degree). But I hear it only with those close-miked recordings that are hyper-real in itself and don´t reflect normal listening conditions.

[HiFiNutNut]

Given a wide baffle width of 500mm, the signal coming from the back of the panel travels an additional distance of 250mm and arrives at approximately 0.73ms on the horizontal plane at ear level, longer for other planes. For a 250mm baffle, the time is reduced to half.

If we don't use a rear tweeter, this would only happen to the mid-bass drivers. For a 6.5", it would happen only below about 800Hz. Perhaps in that case, the delayed sound does not cause much problem. But still, it may damage the original sound.

I am wondering if this "delayed" sound causes the loss of image, or the backwaves bouncing off the rear walls cause it. Or perhaps, the "delayed" sound gives the OB characteristics, that the sound is more open and live?

In any case, I do suspect that narrow baffle would sound better than wide baffle, if other parameters are the same.


In reality, my NaO speakers sound really superb. I have definitely no complain about its imaging capability although I have not compared it side by side to other high-end monopole speakers. In fact, I prefer the overall sound quality of the NaO over any other speakers I have listened to, including many high-end speakers with insane price tages.

If I have to split the hair and pick something, I would say that I suspect dipole speakers might have higher diffraction magnitude than closed box conterparts. I measured a bump of 3dB on axis at 2kHz, then turned the panel less than 15 degree, and that bump was completely gone. There were some long discussions on whether dipole speakers reduce or increase the diffraction on the diyAudio forum in the Beyond the Ariel thread, but the result was not conclusive.

My new pair of speakers are open back dampped, angled U-frame and I will start working on them next week. I hope the stuffed (with high quality sheep wool) U-frame would get rid of the "boxed" sound so that they sound the "same" as a flat baffle, which should retain the OB sound, while the backwaves are significantly dampped to reduce the "delayed sound", and a large curve at the front baffle of 75mm radius would reduce the diffraction effect to the minimum.

Regards,
Bill