"You can expect a rigid flat piston radiator to generate a 90° pattern (-6dB) at the frequency where diameter equals wavelength."

Just out of interest, the effect described by Wayne in his statement above can be readily demonstrated in Hornresp.

A circular piston with an area of 929.41 cm^2 has a diameter of one wavelength at 1000 hertz. To simulate in Hornresp, set Ang = 2.0 x Pi (for infinite baffle), S1 = 929.41, S2 = 929.41 and L12 (Exp) = 0.01. Calculate the results, then run the Directivity Beam Width tool from the SPL Response chart, and Sample at 1000 hertz. The -6dB beam width is calculated to be 89.5006 degrees (which rounds to 90 degrees, as indicated by Wayne).

Kind regards,

David