What is the optimum Qts for the drivers of a dipole woofer?
A34 – The low frequency roll-off of a woofer and its associated group delay are optimal, from what I have observed, when they follow the response of a 2nd order highpass filter with Q = 0.5. When a driver is mounted in a dipole W-frame or H-frame its mechanical resonance frequency Fs decreases to Fd, due to air mass loading, and Qts increases by a similar percentage to Qtd. For example, a driver with very strong motor, Fs = 18 Hz and Qts = 0.2 might have Fd = 16 Hz and Qtd = 0.22 as determined from an impedance measurement of the baffle mounted driver. With Qtd < 0.5 the low frequency behavior of the woofer is characterized in the complex s-plane by real axis poles at -69 Hz and -3.7 Hz and by 3 zeros at the origin. One of these zeros is due to the front-to-back dipole cancellation with its 6 dB/oct low frequency roll-off. The frequency response of this 3rd order acoustic highpass filter must be equalized to obtain a flat response. A suitable target response could be Fd = 20 Hz and Qtb
