Hello Dr. Geddes. Because of your contributions to this thread, I went to your website and looked at your speakers and also read your paper describing your design philosophy. I have a question. I notice that your speakers are in sealed boxes, not ported. You state, if I am interpreting you correctly, that you use sealed boxes because you assume the presence of subwoofers in the system. Are there any advantages to sealed vs. ported in the sound of the main speakers? Or do you use sealed solely because you don't need low frequency extension from the main speakers, because subwoofers will be used?
I use closed boxes mostly because I don't need LF extension from the mains, but I also feel that closed box offers a little better result in the end. Ports ring and can create turbulent noises, also EQing below port tuning can be problematic, so I just avoid these two things and use sealed boxes.
These days even my subs are all closed (not always that way) - I just EQ them with a MiniDSP to get an end result that is smooth, uniform and goes down to about 20 Hz.
These days even my subs are all closed (not always that way) - I just EQ them with a MiniDSP to get an end result that is smooth, uniform and goes down to about 20 Hz.
That should win the "smartest post of the week" award. (And I don't say that just because my main system is along the same lines of thought.)
But I can't imagine how ferocious the riticism of such sensible thinking would be in the subwoofer forum where folks are pining to create 6th order boxes (mostly for their trucks, I think).
Really was not a serious listening session,so there is not much to tell about the glass room.
I was visiting a small factory which asked me to lunch with another factory owner, while waiting for them to finish up with some work, they invited me to wait in a fully glassed room with a few systems there on display. I was thinking in my mind, what an awful way to listen to music since from prior experience it would sound fatiguing and really noisy even at pretty low levels. But when they started playing, it was nothing like that! The sound was smooth and relaxing, quite nice for background music, we could carry on conversation without any problem.
My prior experience also in a full glass room was at a telecom company. We just played one of my own systems while talking about other things, the sound was quite annoying that I had to keep lowering the volume. Since this was quite a long time ago, I do wonder whether my ears were more sensitive then.
I agree that closed boxes can give you most control of a driver, this is ideal when you use large drivers designed for closed boxes. Most home use are trying to get the most out of smaller systems, and thus a compromise.
Is this just your first impression?
I know you don't consider driver distortion a major problem, so I am curious if you have considered this seriously.
My own tentative expectation is that they would behave differently because the VC moves and there would be eddy currents from the motion that behave differently from the eddy currents due to flux modulation.
I have looked at the AES literature and there is a reasonable amount of analysis of pole piece eddy currents but none of conductive VC, AFAIK.
The most serious work to reduce eddy current distortion in transducers has been published by JBL/Harman.
This is noteworthy when it is their own senior researchers, Toole and Olive, whose work would seem to downplay the importance of distortion.
The transducers are mainly aimed at professional use so not quite the same as DIY home systems but still...
Best wishes
David
It is probably also because it is just simpler to fabricate, a piece of shim is cut and rolled to make the VC former.
I think even non conductive formers are usually done this way.
It may also be that the slot is used to route the VC wire to return, with some coil layouts.
This is not accurate, see Alex Voishvillo's (JBL) paper in the JAES that analyses this.
Best wishes
David
Shorting rings and the like are essential to lowering distortion in a driver. Flux modulation is probably the most audible distortion in a loudspeaker. But we know how to solve it and any driver that I would use has a shorting ring. So my point was never that these things are unimportant, because they are. My point was that shorting the voice coil is the same thing as a shorting ring around the pole piece as far as the flux modulation is concerned. As someone else pointed out, I did neglect the damping effect that the eddy currents would have on the moving coil (my bad.) This damping would not be a good thing because it would simply lower the drivers efficiency across the bandwidth.
So: Shorting rings are good - even essential, but once used, the distortion is no long an issue; shorted voice coils would be bad because they would loose too much efficiency.
Toole and Olive are, like me, of the mind that nonlinear distortion in a loudspeaker is under control and no longer an issue (not that it never was!) Remember that JBL has been using shorting rings for more than half a century, so this is not new territory.
Last edited:
The issue of distortion, topic of this thread, can be considered in another light and one which has been oddly missing from this thread: electrostatic speakers.
It is commonly observed - and not just by owners of ESL speakers who seem to always be strong enthusiasts who would prefer nothing else - that ESLs introduce a crystal clarity to the sound*. Many of us were just plain knocked over with delight on the first acquaintance, even hearing early systems which were ESL from 1kHz up.
Eh?
B.
*yes, there's the famous test of an "ML" speaker showing low ratings, reported by Toole. I don't think there's enough information about the test to draw reliable conclusions about ESLs.
Why do you think this?
AFAIK, flux modulation should be essentially completely 2nd harmonic, which should be well masked, as I think you, I, and almost everyone, are in accord.
I am particularly interested in this because some of the early JBL tech notes appear to be seriously flawed.
At first I assumed the mistake must be mine but I now believe that, despite their usual credibility, they published some faulty analysis of flux modulation.
I am in accord with their claim that eddy currents produce mainly 3rd, so worse but still lowish order.
What interested me is that the Voishvillo paper is quite recent, despite, as you say, the fact that JBL has decades old, well developed flux modulation and eddy current counter-measures.
The work is after Toole and Olive's so if all is "under control" why do they continue to add further, not cheap, control measures?
Best wishes
David
Having had some association magnetic braking project like seen in amusement parks, I can say it really takes a very strong magnetic field to have a braking force which equalizes at a fixed velocity which increases with reduced magnetic field strength, conductor conductivity and thickness reduction, surface reduction. Additionally, a flat plate in the magnetic field is all it takes, thus even normal conductive material formers with a slit will have said braking force. A closed ring will have a some shorting effect without any additional braking effects than the same former with a slit.
Last edited:
They are noted for their low moving mass. Having built plasma speakers I have my doubts. Is it their size, are they dipole, could we duplicate them using conventional drivers?
You are somewhat right, an aluminium VC is less than perfect, even with a slit, so that is why better speakers use non-conductive VC formers. However, closing the loop would make things terrible. You are more wrong about this than you can even imagine.
I am away from the PC so will come back to Dave's points after having perused the aes library.
Last edited:
To save you a search I should admit that I wrote JAES but actually had in mind a convention paper, it's number 8989.
There's a few others that look relevant too, unfortunately my university has all the Journals but only restricted access to convention papers.
Best wishes
David
I will check, the AES library as well. However, there are different trade offs in material based on the frequency range which is associated with acceleration and stiffness/hardness of material for the VC former.
Because flux modulation happens across the entire bandwidth of the driver, displacement actuated non-linearity acts only at the very lowest frequencies. This means that a woofer will have non-linearity based on the total current regardless of any excursion. This would, in principle, make for a more audible level of non-linearity. If, as you say (I can't confirm) it is predominately 2nd order then yes it should not be too bad. I have not looked into the flux modulation issue in much detail, but I believe that Klippel has shown it to quite audible if not well controlled.
I can't speak for Alex or why he would be interested in this subject, perhaps to find more cost effective ways of doing the same thing. As you say, these solution techniques are not cheap and lowering their cost while maintaining their effectiveness would be a worthwhile endeavor.
PS. Flux modulation or inductance modulation is dependent on two variable, excursion and current. This means that the total non-linearity will have products of the non-linearities of each of the two variables. This kind of thing tends to raise the orders of the non-linearity very fast. That could be a big factor.
Last edited:
Dave, had a look, unfortunately, they present incomplete results. Probably as they should, being financed by industry. For those of you without easy acces to AES publications, this is the gist of it.
The impedance curves of four otherwise identical drivers where investigated with the four following variations: copper pole piece or not, copper ring in top plate or not.
The only two results presented where those of a driver with both, and of a driver without any of the two copper rings.
It leaves the reader guessing what the effect of a copper ring in the top plate is in isolation.
Does anybody know if JBL has ever implemented a copper or aluminium ring in the top plate?
The impedance curves of four otherwise identical drivers where investigated with the four following variations: copper pole piece or not, copper ring in top plate or not.
The only two results presented where those of a driver with both, and of a driver without any of the two copper rings.
It leaves the reader guessing what the effect of a copper ring in the top plate is in isolation.
Does anybody know if JBL has ever implemented a copper or aluminium ring in the top plate?