A pressure of the fluid oscillates according to {HEAD ~ a time varying function}, which causes a density of the fluid also to oscillate according to {TAIL ~ the same time varying function} 
A pressure of the fluid oscillates according to { HEAD ~ a time varying function } , which causes a density of the fluid also to oscillate according to { TAIL ~ the same time varying function }
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL