Posted by Wayne Parham [ 70.234.133.214 ] on March 19, 2009 at 14:24:49:
In Reply to: Waveguides verses Horns posted by BtHarris on March 19, 2009 at 12:31:35:
The term "waveguide" has been applied to horns that provide constant directivity using a flare that doesn't require diffraction for waveshaping. They're a specialized subset. An acoustic waveguide is a horn, but a horn is not necessarily a waveguide.
Examine the H290 horn and the π wood horn. They both have a smoothly radiused transition from throat angle to the asymptote wall angle. There is no diffraction slot or edge anywhere. The mouth of the H290 terminates at an edge, but the relatively high crossover makes this edge less significant. The π wood horn has a larger mouth and rounded mouth termination to reduce its effect even more. The pattern is uniform over a 90°x40° coverage angle and response is smooth. By all standards, these are attractive features and their performance is quantified by measurements.
Historically, waveguides were transmission lines used in RF at very high (microwave) frequencies. They weren't used as transmitters or receivers - those were antenna. Waveguides were ducts.
In the last decade, the term "waveguide" was adopted in audio. It is not a transmission line in this case, it is a specialized kind of horn, one that provides constant directivity.
Some CD horns or "waveguides" provide an axisymmetrical coverage pattern, others an asymmetrcial pattern like 90°x40°. It is much easier to make a round or square (axisymmetrical) CD horn, but I believe asymmetrical horns are much more useful for two important reasons. First, when sound sources are stacked vertically on a baffle, like a woofer and tweeter, they interact with one another and form nulls above and below the forward axis. It is preferable to limit sound outside these nulls, ideally, no sound would be radiated outside them. Second, boundary reflections are undesirable and probably the most offensive reflection is ceiling slap. It is useful to have fairly wide horizontal pattern for room coverage but limited vertical radiation pattern to reduce floor and ceiling reflections.
Conical horns are the simplest horns to make, and they provide constant directivity. They are a good choice for midhorns loaded with cone speakers, because the source is a spherical wave at low frequencies where the horn is used. But they're less attractive for compression drivers because those generate a plane wave at the throat exit orifice. The phase plug is designed to do this. A horn for a compression driver, therefore, must transform the wavefront, to spread it to the desired wavefront shape and angle.
Most early constant directivity horns used diffraction to widen the pattern. A difraction slot was used in the throat, and sometimes an extra diffration edge about 1/3 from the mouth. The slot in the throat widens the pattern up to relatively high frequencies, and the one closer to the mouth was for lower frequencies. The problem is these diffraction features cause discontinuities which create ripples in impedance and response. They also create a secondary virtual source, which makes the horns appear to have a different acoustic center off axis.
Newer horns use a mimimum surface curve, radiused to transition from the throat exit angle to the horn's wall angle. This gentle transition "nudges" the plane wavefront into a curved wavefront as it progresses down the horn. You can see this illustrated in the drawing below:
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