Several pointers from an old hand: 1) Subharmonic synthesis (SHS) is an effect, NOT a substitute for a subwoofer. Without a sub, a mixer likely can't hear what s\he's doing, SHS or not. 2) A small room can be driven at VLF (<~100Hz), as can a closed headphone. 3) A subwoofer driver is a different animal of driver, with Thiele-Small parameters that, in proper enclosures and within their Xmax, act larger than e.g. a normal 12in woofer. 4) When specifying a sub, mfgrs' published frequency responses such as "+\-3dB to 30Hz" are misleading. It implies 30Hz is down 6dB - 1/4 power - not an honest engineering spec of f3 - half power. Unlike mid & HF, at 30Hz, -6dB is perceived as less than half as loud [Fletcher-Munson, updated in ISO:226 2003]. Use C-weighted measurement, NOT A-weighted, intended for noise measurements at 40SPL or below. Plus per F-M\ISO:226, harmonic distortion products mainly 1 and 1/-1/2 octaves higher will be disproportionately louder, calling for far less than the 5~10% THD of typical subs. More in whitepapers "Subwoofer Camp" and "Stereo Subwoofers" at www.filmaker.com/papers.htm.
