The disappearance of the traditional amplifier

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).

~~~~One other reason pro systems use amps in speaker cabs is damping factor, which rises enormously compared to using long runs of large copper, allowing for tight control over cones.

Just The Facts
By

Fbi - It also makes possible to eliminate in-series inductor that limits DF much more (to about 100) than cable itself (it has typical of 0.08 ohm).

Kirkus - Magnet structure is often "immense" in underhung motor speakers.

Here are some photos of field coils.

Kirkuk,

Power response predicts the perceived balance for all types of systems, also for those with constant directivity. Another factor affecting that balance is room absorption vs. frequency.

" 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."

First: The Directivity Factor (DF) is the ratio of the intensity of a source in some specified direction (usually along the acoustic axis of the source) to the intensity, at the same point in space, due to an omnidirectional point source with the same acoustic power. Directivity Index (DI) is logarithm of that factor. The whole issue is defined as intensity ratio, and in itself it does not matter what kind of source you have. For constant directivity it is sufficient to have DI which does not, within tolerances, depend on frequency. In practice this can be achieved in many ways, horns and waveguides being good examples of them.

For simple cone or dome radiators, electronic equalization can be used to equalize the pressure response at a certain axis (mostly the main axis) of the driver. However, equalizing response very flat on one axis often means the response is worse on some other axis. This is a common problem with powerful DSP and diffraction ripple. Electronic eq cannot change how the driver does its radiation job, it can affect only the signal you put into it. Equalizing power response without simultaneously affecting on-axis pressure response is not possible.

As said, power response depends on the radiation characteristics of the driver, i.e. how much its radiation is attenuated off axis compared to on-axis radiation. Most important factor affecting this is the radiator effective diameter in relation to wavelength. When the diameter is much smaller than wavelength, the source is omnidirectional. Hence a 1" dome at 3 kHz is practically omnidirectional but at 20 kHz it is not. However, a 1" throat of a compression driver has same diameter and its directivity characteristics then depend on the shape of the wavefront propagating in the throat and later in the horn. Often also CD horns are pretty directive at 20k as well, but I agree with you that with a good 2" compression driver and well designed horn you can get about 4 octave bandwidth with good directivity control, acceptable frequency response and distortion. With a larger diameter driver you can add an extra octave on the low end at the cost of HF performance. This is a nice system if size and price are not too important. Like all drivers, compression drivers have their own set of compromises and it depends on the desired performance what features are regarded valuable. For example: compression drivers suffer from air nonlinearity causing distortion at high levels, but on the other hand, there is no better way of generating high SPLs. Therefore there have been long standing efforts to eliminate the distortion, one possible way is to predistort the signal in a predictable way. This is basically similar to analog tape recording. DSP can do audible improvements in this respect.

You are right in referring to the aim to match directivity of drivers. However, I see no practical problem in rotating the waveguide in 3-way Genelecs as the MF/HF section still remains vertical. Because the LF/MF crossover is somewhere around 400 Hz (i.e. pretty low) there will be no practical difference whether vertical or horizontal, but of course the room reflection pattern will change due to different height of the woofer. The woofer directivity remains as it is and so does the MF, the only changing parameter is their relative position. You see the same off-axis performance between woofer and MF driver either in vertical or horizontal off-axis measurement, i.e. when going sufficiently off axis you start seeing the crossover. You can naturally calculate the first off-axis zero from the driver distance. If you take 400 Hz as crossover, wavelength is 0.85 m and half of that is 0.43 m. The first zero is at angle where the distance difference is half of wavelength.

I think all two-ways should be used vertical, because of the same off-axis dip appearing at crossover. Naturally the same dip exists also when a two-way speaker is vertical but its audibility is small. Naturally if you have brickwall filters there will be no interference, then you might only notice the changing source position.