{TAIL ~ The acoustic signal} generated by {HEAD ~ the absorption} of the third laser beam is proportional to the propane concentration in the flowing gas sample 
{ TAIL ~ The acoustic signal } generated by { HEAD ~ the absorption } of the third laser beam is proportional to the propane concentration in the flowing gas sample
TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH