might be nice if eventually users could pass in a longer list of frequencies, in case they don't want their bands to have equal spacing or width.  For example, a list of 2-element tuples would be lower/upper for each band; a flat list with length >2 would use the Nth value as both upper bound for channel N-1 and lower bound for channel N.  Maybe that should be an ENH issue rather than a change to implement now though...