The disappearance of the traditional amplifier

K_ilpo_p, thanks for the excellent discussion . . . let me see if I can better refine and clarify my thoughts on the matter.

Also omnidirectional source directivity is constant, and in power response terms the driver type has no inherent effect.

The parameter of total, summed power response as I see it is most useful in trying to correlate the perceived timberal balance vs. measured frequency response for NON-constant-directivity systems, and for establishing the optimum placement and room treatment for a given loudspeaker system. It's pretty much irrelevant for the issue of establishing the best directivity characteristics of the driver(s) themselves. Consider that (for a single driver) electronic equalisation is supremely effective in altering the summed power response, but completely ineffective at solving directivity issues.

Altec made their Mantaray horns, using basically two flares and an abrupt joint between them. First expansion created the vertical pattern, opening into a narrow arc-like slit - being in itself a (horizontally) wide radiator- and the second flare controlled the horizontal pattern. The JBL Bi-radial horns use the same principle. In very simple terms a waveguide could be interpreted as the outer flare of a constant directivity horn, if you so wish.

While not all modern constant-directivity compression-driver waveguides use an abrupt change in expansion rate, I like your description, and find it a useful analogy. So I'll attempt to use it to illustrate my basic point in the whole matter - which is that to substitute a pistonic driver (cone or dome) for the compression driver and throat . . . brings out fundamentally different principles of operation in the waveguide as far as the directivity is concerned. Also, the difference between these two approaches is pretty much unrelated to the traditional view of the difference between compression drivers and direct-radiating drivers - which you accurately state as being efficiency, and acoustic impedance.

I do not understand what you actually mean with saying, that "for true constant-directivity performance to be possible, the wave-front propegation has to be constant with frequency."

Fair enough . . . my use of the term "true" implies a value judgement which I did not intend.

Instead, I'll refer to a compression-driver constant-directivity waveguide system (like the big JBL butt-cheek we've been discussing) as being a "wideband constant-directivity" system. In addition to the traditional points stated above (acoustic impedance and efficiency), a compression driver strives to transform the pistonic movement of the diaphragm into a pressure wave - a wave that has a shape that is (ideally) frequency independent. Early-20th-century practice viewed these as plane waves, examples being devices such as slant-plate acoustic lenses, and the driver measurement apparatus, a "plane-wave tube". And although the plane-wave as a useful, precise mathematical model may be completely outdated (I'll again reference Dr. Geddes' work), it is my understanding that in a "wideband constant-directivity system" (my terminology), the ultimate goal is for the driver to illuminate the waveguide in a manner that is constant with frequency. The result is a device where the useable constant-directivity frequency range is limited solely by the practical size of the waveguide, the mechanical performance of the compression driver, and the compression driver/phase plug/throat meeting the goal of frequency-independent waveguide illumination.

This is in (at least conceptual) contrast to the practice of using a pistonic driver to illuminate a waveguide, because the driver/waveguide relationship isn't (and cannot be) frequency-independent. Rather, (please correct me if I'm wrong) the idea is that the waveguide should dominate the directivity at the bottom of the driver's passband, and as the frequency increases, the directivity is decreasingly defined by the waveguide, and increasingly defined by the driver . . . this occurs because a pistonic driver will ALWAYS have an increase in directivity with an increase in frequency. Thus, in order for the driver/waveguide system to have smooth, predictable directivity performance . . . it is obviously of paramount importance that the driver itself have smooth, predictable directivity performance - in exactly the same manner as it should in a non-waveguide direct-radiating system.

My general conclusion is that while a piston-driver/waveguide combination can maybe acheive "a good practical approximation of constant directivity" (your description), its ability to do this will ALWAYS be limited to a much narrower frequency range than is possible with a wideband, compression-driver constant-directivity waveguide. It's also only effective over a specific range of desired radiation angles, which thankfully correspond to reasonably useful ones for studio monitoring. In the end, the directivity characteristics of the driver itself is the tail that wags the dog, and ultimately determines the extent of effectiveness in the waveguide.

As a final note . . . you make reference to the importance of matching the directivity of the bass driver(s) to the waveguide-loaded device(s) (something I very much agree with), and the effects of the crossover slope on the transition-band directivity. I'd be interested on how you view the common (recommended?) practice of turning i.e. the Genelecs sidewise and simply rotating the waveguide, which I feel makes a mess of these issues in both theory and practice.

Active speakers have been around forever even Altec Lansing made them yrs ago. Good for certain apps,but taking over no way

Kirkus,

Thanks - you make some really interesting points. I'd like to point out that a lot of the discussion depends on what "driver" you have to begin with. Let me explain.

To me the TWO main ideas of the short open waveguide are as described by K_ilpo_p....

1) it allows a driver to have greater sensitivity (better SPL and lower distortion) at the low end of its passband
2) it can narrow the low end of the passband radiation pattern - which enables one to match the higher frequency driver radiation pattern to that of the lower driver - for a smooth transition at crossover. Note that this type waveguide cannot help with the high end of the passband radiation pattern because that narrows anyway as the wavelengths become smaller than diaphragm.

Without digging into physics it seems clear that only a CONE shape will maintain uniform spherical wavefronts therefore if you have a dome driver (such as a dome tweeter or the midrange in the K&H O500) then you pretty much start with a spherical wave and therefore a cone waveguide is the simple answer to control dispersion (no bending of the wave is needed as in the case of a horn which has to be bent from a plane wave in to a spherical wave).

Here is some more interesting reading.

Shadorne, these are two great points, and I agree that they are very significant potential benefits of the "short open waveguide" approach. But they're not constant-directivity (which was my main point), and since as it does indeed very much depend on what "driver" you have to begin with . . . these behave fundamentally very much like a standard direct-radiating driver.

But as far as the cone vs. a dome to "maintain uniform spherical wavefronts" that's the whole problem, neither of them deliver any kind of wavefront that's consistent with frequency. Cones, domes, inverted domes, ring-radiators . . . they can all exhibit profound differences in their application and execution, but they are all of a similar ilk in their inability to deliver a consistent wavefront independent of frequency. The compression driver differs in the fact that it (at least aims to) acheive this goal.

I enjoyed Mr. White's article to which you kindly provided the link, but the main problem is . . .

The theory behind the waveguides to be described is that a dome driver produces what is fair approximation of a spherical wave over its piston range

I simply can't conceive of this as being valid . . . I wish my knowledge of physics and my mathematical skill was sufficient to expound on this further, but I think it reasonable to say that it would be hard to build a consenus on this among those who do have competencies in these areas. Further, his calculations are based on the idea that the dome behaves as a point source . . . which is certainly impossible except perhaps for an extremely narrow range of frequencies.

After all, if a dome behaved as a point source, then simply screwing it into a baffle of appropriate size would produce absolutely perfect directivity characteristics, and we wouldn't need waveguides at all.

But they're not constant-directivity (which was my
main point), and since as it does indeed very much depend on what
"driver" you have to begin with . . . these behave fundamentally
very much like a standard direct-radiating driver.

I agree absolutely. "Constant-directivity" is indeed a term that
applies to compression horns rather than 'short open conical
waveguide". And the constant directivity in a speaker using short
conical waveguides is achieved primarily by limiting the drivers to covering
frequencies with wavelengths larger than the diaphragm diameter (this means
a three way in most cases rather than a more protypical two way
"CD" horn). The waveguide simply narrows the wide dome
dispersion so as to integrate the dome with the driver covering the lower
frequencies below the crossover.

After all, if a dome behaved as a point source, then
simply screwing it into a baffle of appropriate size would produce absolutely
perfect directivity characteristics.

In general this is true - a dome works very well as a point source...this is why
they are so popular as the standard tweeter in the majority of speakers (used
within a limited bandwidth of course as they do start to become directive
somewhere above about 8 to 12 Khz and also suffer from breakup like any
regular cone at even higher frequencies and, of course, they rapidly drop in
SPL output as you go low in frequency and exceed excursion limits - however
there is not much "music" above 12 Khz anyway and they make
awesome cheap tweeters )

Large domes for covering lower frequencies also have a nice dispersion and
sound great but they have proved much less successful than the ubiquitous
dome tweeter - mainly because they are expensive to build properly (you
need a very large voice coil/motor and rocking can be an issue due to lack of
lateral support/alignment for the motion ( so some designs resort to having
two spiders) - all factors that make large domes extremely expensive
compared to a regular cone so few designers use them (awesome but way too
expensive).