As with any subwoofer system, the low-pass filtering must be such that the sound output is attenuated rapidly above the crossover frequency (80 Hz). Excessive output, distortion products, or noises at higher frequencies increase the risk that listeners will localize the subwoofers. A second issue relates to the fact that in order for these systems to function fully, the bass must be monophonic below the crossover frequency. Most of the bass in common program material is highly correlated or monophonic to begin with and bass-management systems are commonplace, but some have argued that it is necessary to preserve at least two-channel playback down to some very low frequency. Experimental evidence thus far has not been encouraging to supporters of this notion (see [72] and references therein). Audible differences appear to be near or below the threshold of detection, even when experienced listeners are exposed to isolated low-frequency sounds. Another recent investigation concludes that the audible effects benefiting from channel separation relate to frequencies above about 80 Hz [73]. (In their conclusion, the authors identify a “cutoff-frequency boundary between 50 Hz and 63 Hz,” these being the center frequencies of the octave bands of noise used as signals. However, when the upper frequency limits of the bands are taken into account, the numbers change to about 71 and 89 Hz, the average of which is 80 Hz.)
