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.