The disappearance of the traditional amplifier

I haven't heard the Audiokinesis loudspeakers, but I have spent a little time with Dr. Geddes' "Summa" loudspeaker, on which (as I understand it) the Audiokinesis designs are based. The Summa's horn/waveguide is a true constant-directivity design, and this is immediately evident in their excellent imaging, and very consistent tonal balance. I actually feel that Dr. Geddes' research in this area is some of the most interesting, competent, and relevant work in loudspeaker design in recent years.

But the "waveguide" designs in the Genelec monitors bear very little resemblence to a true constant-directivity spherical or bi-radial "waveguide" horn, in both the theory and the way they behave. This is mainly because they don't use compression drivers - and the waveguides are so short that the directivity characteristics of the driver itself dominate the polar response of the loudspeaker. Genelec's "waveguides" do seem to clean up the directivity performance of their drivers at the more extreme realms of their off-axis response, but they are NOT constant-directivity.

I'm really not trying to slam Genelec in general, and when used in the vertical configuration, their directivity characteristics similar to many well-behaved direct-radiating monitors. I just feel that some of their recommended setup configurations give lackluster performance, and their marketing material seems to imply that they are truly constant-directivity (even though they don't actually make that claim).

I don't know if it's accurate to say Duke's designs are "based" on Geddes' work, but he certainly pays tribute to the man. (I mean he pays vocal respect and acknowledgment, not that he pays tribute in the Cosa Nostra sense.)

Yes, Dr. Geddes obviously understands acoustics very well and it is enlightening to read his material.

But the "waveguide" designs in the Genelec monitors bear very little resemblence to a true constant-directivity spherical or bi-radial "waveguide" horn, in both the theory and the way they behave.

Good Discussion. Yes but....horns are different in that they use a compression chamber. For horns the exact mathematics and expansion design become critical, however, for a normal non-compression driver a simple conical expansion often has teh best performance and is enough to control dispersion such that a smooth power response is achieved.

See this

and see K&H O500C - click on the "measurements" link in thr right hand corner for directivity plot. This is an example of an impressive response that is extremely smooth and wide both on and off axis and illustrates the use of the "waveguide".

Shadorne, great links! Thanks for the interesting reading.

One source of confusion is that there's a lot of ambiguity between in the terms "horn" and "waveguide" (add "lens" and it gets worse) - I tend to use them rather imprecisely as well. But to differ with AeroNET article (if I was to attempt to be precise), I think that the difference lies not in the efficiency or the type of driver used, but rather in the theoritical basis for its shape. A "horn" is usually based (at least loosely) on Webster's horn equations, which were derived in the early 1920s mainly to calculate the load the horn presents to the driver, for the purpose of maximizing efficiency -- this is the origin of the classic exponential shapes. However, there's very little theoritical basis here for understanding the horn's directivity characteristics, which is why horns for the first half of the 20th century used other techniques (multi-cellular construction, or a slant-plate lens) to control directivity without a good understanding of how the contour itself affects this.

A "waveguide" on the other hand is designed with mathematics that are derived from other fields, using techniques designed to accurately predict the directivity based on the waveguide's contours. The specifics of these maths are way over my head, but I think it's accurate to say that waveguide theory isn't limited to lower rates of expansion or lower acoustic gains.

The root of my skepticism with the direct-radiator/short-waveguide configuration (for which I've made the Genelec monitors the poster-boy) is that with my (admittedly VERY rudimentary and imprecise) understanding of both waveguide techniques and Webster's equations . . . all of it assumes a specific wave-front propegation for the horn/waveguide to work as intended. So for true constant-directivity performance to be possible, the wave-front propegation has to be constant with frequency . . . and the conventional direct-radiating cones and domes used in such configurations do NOT acheive this. Rather, they exhibit the classic increase in directivity with increase in frequency, just like all domes and cones.

The author of the AEROnet article does make mention of this, but then goes on to say that for wide-angle waveguides with direct-radiating drivers "that most of the mathematical detail can be side-stepped." Huh???? If you side-step the mathematical detail, then what you have is simply a random curvy recessed cabinet-front, and NOT a waveguide. He then goes on to make some measurements of some purely emperically-derived combinations . . . and while the final results look nice and smooth, this seems to be the obvious result of simply changing the way the waveguide is illuminated, thereby effectively altering its curve in a theoritical sense.

The K&H monitor does indeed have very smooth directivity plots, but the directivity still increases quite steadily with frequency . . . as one expects with direct-radiating domes. I would say that the smoothness of these plots (compared to i.e. a Genelec 3-way) is a testament to the excellent performance of the drivers themselves, and well-implemented crossover design . . . and the cabinet contours help out at the extremes. But they're also endorsing my other pet-peeve -- horizontal placement, for which they give no directivity plots. Suffice it to say it will be worse, and its horizontal polar response will then exhibit some of the inconsistencies found in the vertical directivity plots. Maybe they can clean this up a bit in the DSP, by changing some of the crossover slope characteristics . . .

I've also heard Duke's speakers at length, and they perform with some of the very best and IMO perform well outside their price points. He definitely did his homework.

FWIW in the old days, field coils and a lot of the other in-house coils were made in a very different economic environment. If we are talking about the 1920s, for example, the dollar now buys less than 1/10th of what it did back then. As the buying power of paper money declined, the industry looked for economic solutions- IOW permanent magnets are a **lot** cheaper to build.

I have an old RCA FC speaker from the 30s that is purported to have about 18,000 gauss once energized. The magnet structure on the thing is immense- so large that it is used for mounting the speaker, not the basket. IOW it puts a lot of Lowthers and the like to shame.