Abstract: The length of a column of a gaseous or liquid substance or of a solid bar is determined by generating a standing wave of known frequency, wavelength and/or wave velocity in the column or in the bar. The wave frequency is varied until at least two consecutive maxima (antinodes), two consecutive minima (nodes) or a minimum following a maximum of the amplitude of the standing wave have been detected. The length of the column of medium at known frequency and wave velocity of the standing wave generated in that column is computed from the equationL=c/[2(f.sub.n -f.sub.n-1)] (1),where L is the length of the column of medium, c is the wave velocity, f.sub.n is the frequency of the nth maximum and f.sub.n-1 the frequency of the (n-1)th maximum. Apparatus for implementing the method comprises a loudspeaker 1 mounted in a resonance chamber 2 with a tubular acoustic exit aperture 3.

Abstract: The length of a column of a gaseous or liquid substance or of a solid bar is determined by generating a standing wave of known frequency, wavelength and/or wave velocity in the column or in the bar. The wave frequency is varied until at least two consecutive maxima (antinodes), two consecutive minima (nodes) or a minimum following a maximum of the amplitude of the standing wave have been detected. The length of the column of medium at known frequency and wave velocity of the standing wave generated in that column is computed from the equationL=c/[2(f.sub.n -f.sub.n-1)] (1),where L is the length of the column of medium, C is the wave velocity, F.sub.n is the frequency of the nth maximum and F.sub.n-1 the frequency of the (n-1)th maximum. Apparatus for implementing the method comprises a loudspeaker 1 mounted in a resonance chamber 2 with a tubular acoustic exit aperture 3.