The velocity of {HEAD ~ the progression} is determined by the given distance of the fixtures at {TAIL ~ a given frequency} 
The velocity of { HEAD ~ the progression } is determined by the given distance of the fixtures at { TAIL ~ a given frequency }
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL