REAL subwoofers!
By Stig Erik Tangen
20 Mar 1996
Specs
This is simulated free-field frequency response curves Fig. 1. Frequency response curves. The solid line is without EQ, and the dotted is with 2nd order high-pass filter at 17 Hz with Q=1.25. The cut-off frequency 16 Hz is marked with a dot on the curve. With EQ there is essentially flat response in the sub-woofer frequency range. Some series resistance caused by cable resistance and amplifier output impedance is taken into account in these simulations.
Maximum SPL with 100W @ 8ohm input is simulated in Fig. 2. Max output SPL. The system is capable of at least 108 dB SPL down to 16 Hz in free-field. A listening room is not free field, so higher SPL levels can be produced in a listening room.
The drivers are connected in parallell. This makes the system impedance to be 4 ohms nominal. A conjugate filter to equalize the impedance is recommended. The easiest way is to parallel the whole system with a 10-12 ohm high power resistor (at least a 20 watt). Use of L-C-R chains could give a more linear impedance, but the large component values needed make this a less attractive solution than just a resistor. A conjugate filter will unfortunately lower the overall impedance, but the goal of this arrangement is to reduce the impedance's phase shift. With most amplifiers, large phase shift is worse than a low and flat impedance. The impedance will reach down to 3 ohms with a 10 ohm parallell resistor.
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