Apparatus (15, 30) and methods for performing acoustical measurements are provided having some and preferably all of the following features: (A) the
system (15, 30) is operated under near-
field conditions; (B) the piezoelement (40) or piezoelements (40, 48) used in the
system are (i) mechanically (41, 49) and electrically (13, 16) damped and (ii) efficiently electrically coupled to the
signal processing components of the
system; (C) each piezoelement (40, 48) used in the system includes an acoustical
transformer (42, 50) for
coupling the element to a gaseous test medium (9); (D)
speed of sound is determined from the
time difference between two detections of an acoustical pulse (81, 82) at a
receiver (40, FIG. 3; 48, FIG. 7); (E) cross-correlation techniques are employed to detect the acoustical pulse at the
receiver; (F) forward and inverse Fourier transforms employing
fast Fourier transform techniques are used to implement the cross-correlation techniques; in such a mathematical manner that the peak of the cross-
correlation function corresponds to the detection of a pulse at the
receiver and (G) stray path signals through the body (31) of the
acoustic sensor (15, 30) are removed from detected signals prior to
signal analysis. Techniques are also provided for performing acoustical measurements on gases whose thermodynamic properties have not been measured and on mixtures of compressible gases. Methods and apparatus (29) for performing
feedback control of a gas of interest in a mixture of that gas and a carrier gas are provided in which the controlled variable is the flow of the carrier gas.