from pg 3 of this white paper:
http://www.quarter-wave.com/OBs/U_and_H_Frames.pdfThe U and H frame designs used a common cross-sectional area and cavity
depth. The internal cross-sectional area is 16” wide and 16” tall. The depth of the cavity
was defined as 7.5”. The depth was selected to push the first quarter wavelength
resonance above the desired crossover frequency of approximately 200 Hz.
Leffective = 7.5” + 0.6 x reffective
Leffective = 7.5” + 0.6 x 9.0”
Leffective = 12.9” = 0.328 m
f1/4 = c / (4 x Leffective)
f1/4 = 344 m/sec / (4 x 0.328 m)
f1/4 = 262 Hz
Adding additional length to either the U or H frame drops the quarter wavelength
resonant frequency resulting in a peaking SPL response around the crossover
frequency. Efficiency of the bass output would not be increased with this additional
length. The efficiency of a U or H frame could be increased by using a larger crosssectional
area.
The depth of U frame or either side of H frame enclosures should be set
to place the quarter wavelength resonance above the selected low pass crossover
frequency.