I mostly agree with one disagreement. Shorting rings don't just affect even orders, they can affect both even and odd. We see the biggest effect on the second, but remember that it is the higher ones that have the most audibility.

On the issue of high versus low efficiency there is another factor to what you have - correctly - said about why. To get good controlled high directivity to lower frequencies the speakers and waveguides must be large. Larger units are usually higher efficiency. Not that it has to be this way, but it is the more likely. One cannot get high controlled directivity out of smallish speakers - the typical low efficiency type - unless you use several of them. But arrays of speakers are very hard to do right - if possible at all. I have never found a solution that I liked with multiple drivers especially when the larger high efficiency units suite the task quite well.

As I have said before, to me the better speakers are always higher directivity - they interact with the room less and bring you more into the recording. This is true of Magnapans, and any of the large panel systems. I just returned from the High End show at CES and to me all the small speaker two way designs all sounded the same and not very good, to a great extent due to the very poor room acoustics, which speakers like this tend to interact with very strongly. ALL of the better speakers were on the larger side with some form of higher directivity and constant coverage control - Magnapans, Edgerhorns, Zimbalies and some others whose names I have forgot. The common factor - high directivity.

I think you agree with me on this Wayne, but you did not mention it. Directivity control, smooth response and low diffraction - thats the ticket for me.

It is also true that audio today is a pennies business for the drivers. I can get decent woofers from China for a couple bucks. Tweeters - about a dollar. These end up at Best buy, etc. at the price point of $50 - $100.

But then there is the absurdity of The High End where they will package about $100 worth of Seaes drivers into a $30,000 system, because it has "magic" in it or some other thing.

But the truth is that cabinetry IS very expensive. In my own speakers the cabinets now rule the cost as they far outweigh the driver costs. This was not my intent, but it is the practicality. I have spent nearly a year trying to get the cost out of the enclosures to the point where they are practicle. They are no wood, by the way, they are molded out of fiberglass and polyester and/or urethane backfills. To make the highly rounded corners in wood is prohibitively expensive - been there - done that. And making the waveguide into the front baffle in wood is all but impossible.

My 2 cents

I agree with you about directivity. I didn't mention it because while most high-efficiency speakers are (directional) horns, not all are.

About shorting rings, I'm not sure how they could reduce odd-order harmonics. I think that mechanical suspension movements and reduction of excursion reduce third-order though, so some of the better high-efficiency drivers probably have reduced third-order from improvements in that regard, in addition to reduced even-order distorton from the use of flux control rings.

Wayne

You need to move away from even and odd order and second and third order thinking in terms of distortion.

Our research showed no difference in even versus odd order distortion perception and it also showed that subjects were virtually imune to second and third order nonlinearities.

Even though flux modulation IS a dominately even order phenomina, second order effects CAN be odd order. This is especially true of the higher orders. It is these higher orders that are most audible and hence most important.

I get a lot of calls from loudspeaker manufacturers who say that they have found no correlation in second and third harmonic distortion levels and subjective quality. They want to know if I have a way out of this situation. Of course I do - Hire us! Not always what they wanted to hear.

People do often like 2nd and 3rd order distortion. We did a test in the hearing aid business once with amps that clipped with a soft clip - mostly 3rd order (about 25% THD) and people liked them better than linear amps. But even a small amount of higher order distortion (about .01%) at the waveform crossover and it sounds awful.

So anyone who says that distortion is irrelavent is absolutely wrong, but saying that there are cases where people like distortion is also quite correct. The point is that there is NO correlation between THD (or IMD) and subjective perception - the proof of this is on my web site.

Here's a little thought experiment that might be interesting to perform on actual devices.

Lets assume we have two similar devices, one built with flux control and the other without. They each use the same cone, voice coil and suspension, and the magnets produce the same flux in the gap. The magnet geometry is the same, with the one difference being that one has an effective shorting ring and a little more magnet to make up the difference in strength lost by adding the ring.

These two speakers sound very much the same, especially at low and moderate listening levels. But as power is increased, the diffrences become a little more apparent. The one with flux control sounds more clean. When a bass drum is struck, you hear the impact of the head as well as the resonance of the drum. And it is not as fatiguing to listen to for extended periods at high volume levels. The one without a shorting ring sounds a little less defined, and the resonance of the drums sounds more pronounced than the impact. The drum head impact becomes lost in the drone of the bass.

The shorting ring reduces even-order harmonics. Measurements show a significant ~20dB reduction of 2nd harmonics. I'm not sure what he levels of 4th, 6th, 8th, ...20th, etc. are. But it stands to reason that they are reduced too.

The driver is capable of generaing from approximately 40Hz to 2kHz on axis within a 20dB range. So the fact that it is capable of generating 40Hz-2kHz sound means that it capable of generating harmonics in this range when sent a low frequency fundamental if the motor produces harmonic artifacts.

That leaves us with the question, what are the proportions of the harmonics and how objectionable are they? This begins to look like the tube-transistor debate. In one sense, it doesn't matter since the mechanisms that reduce 2nd harmonics also reduce 20th harmonics. The trick is improving symmetry, and that reduces all the even-orders. But the reason it might be important is that if, in fact, the high-orders are what are really objectionable, then this gives more credibility to the argument that mechanico-acoustic filtering is a good thing to do.

Absorbent material can be used to attenuate high frequencies. Front chambers like are used in horns and bandpass boxes attenuate high frequencies. The folded path of basshorns attenuate high frequencies. Any of these techniques can be used. A low crossover point won't do it, because harmonics are generated by the motor, not transformed by it. But the lower crossover point will reduce intermodulation distorton and is required if the upper frequency response is to be attenuated anyway.

I guess the thing to do is to measure high-frequency harmonics and see.

Mosty very perceptive. But one error, I think. I don't think that this statement: "The shorting ring reduces even-order harmonics." is true since the shorting ring will work on odd orders too.
"Measurements show a significant ~20dB reduction of 2nd harmonics." True but the higher order odd harmonics are also reduced.

"I'm not sure what he levels of 4th, 6th, 8th, ...20th, etc. are. But it stands to reason that they are reduced too. " This is true.

You have to understand that there is a big difference in the "order of the nonlinearity" and the "order of the Harmonic". Don't get the two confused. You can see a description of this difference in my book in the chapter on distortion.

But I certainly agree, and my experince bears this out, that acoustical low pass schemes do result in a cleaner sounding system.
Waveguides and horns tend to have more gain at low frequencies so they naturally decrease the presence of the electro-acoustically generated harmonics.

Mechanical systems are inherently lower order systems when compared to electronics - although I have seen exceptions. This means that loudspeakers tend to not have objectionable nonlinear distortions at lower sound levels but can become quit objectionable at higher sound levels. Electronics tend to be just the opposite. They can have very high orders of nonlinearity and can be quite objectionable at lower levels and sound OK at higher levels.

One should look for low nonlinearity in electronics at low levels and never use the speakers at levels where the become audibly problematic. Almost any loudspeaker - even very tiny speakers used in hearing aids, have insignificant nonlinearities at low output levels. And they all have some point at which they will start to sound bad. Keep below this point and IMO, nonlinear distortion is not an issue.