630 Hz Concept  

630 Hz is the exponential centre of the audible spectrum.  Most sound systems do not meet the ideal of human perception and have limits to either or both ends of the spectrum (usually the bass or low end). The 630 Hz concept suggests that to improve the quality of a limited bandwidth  the spectral response should be balanced in the following way:
630 * 630  = (approximately) 400,000.  20 * 20,000 = 400,000.  The 630 Hz concept says that the best sounding audio material will have the product of the lower and upper frequency limits = 400,000. (a balanced spectrum)
Thus if the system has poor low frequency response (only goes to 100 Hz) then the upper frequency limit should be lowered to 4,000 Hz.  The idea is that the upper harmonics will not sound out of proportion. The 630 Hz concept is not a steadfast rule - one must always use one's ears of course - but it can provide a useful strategy for the sound designer.