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