ATH300EX/S is a moderately wide-beamwidth, controlled-diffraction waveguide with an exceptionally smooth high frequency roll-off. It uses a 19mm (0.75") cylindrical throat and is supposed to be used with internal ring adapters for compression drivers.
Fig 1. shows the Directivity Index. This waveguide is especially suited for constructions with semi-open (cardioid) midrange enclosures.
There is also an experimental version available (offered for free). It is possible to make the Directivity Index virtually flat to 20 kHz with a help of a strong diffraction at the throat (Fig.3). The possible downside is that this is also causing a lot stronger resonances due to reflection (see Fig.5, mind the scale). Anyway, it's not always self-evident to predict the actual acoustic outcome with a real compression driver, as these resonances often seem to be surprisingly well-damped in the end (see the Technical Note).
Even though there is a strong wavefront mismatch at the end of the cylindrical duct, it is confined to a small diameter in terms of wavelengths involved, so the radiation pattern is still very smooth (see Fig.4). This design is left as an experiment for those interested in this kind of device. The other dimensions are basically the same as for the ATH300EX/S.
