What is the low frequency sound pressure level from an open baffle speaker with a given effective piston area and excursion?
A14 – The output depends on the path length difference D for sound reaching the listener from the front and the rear of the piston. For a flat circular baffle of diameter d, with the piston at its center, that path length difference is D = d/2, when measured on-axis and at sufficient distance. For the PHOENIX dipole woofer D = 19″ (483 mm) between front and rear cabinet openings. For the PHOENIX main panel, which is folded back and rectangular, the effective path difference is more difficult to determine and approximately D = 12.5″/2 + 3.5″ = 9.75″ (248 mm). See Design Models A. Using these values for D you can calculate the frequency Fequal at which the dipole output is the same as that of a monopole, closed box speaker, with the same volume displacement (see FAQ5). The dipole output has a slope of 18 dB/oct and the monopole increases at 12 dB/oct for constant piston excursion with frequency. Conversely, you can use the MS Excel 4.0 spreadsheet spl_max1.xls to determine the absolute S
