An apparatus for measuring a parameter of a fluid passing through a pipe includes a spatial array of at least two sensors disposed at different axial locations along the pipe. Each of the sensors provides a signal indicative of unsteady pressure within the pipe at a corresponding axial location of the pipe. A signal processor constructs at least a portion of a k-.omega. plot using the signals and detects at least one ridge in the k-.omega. plot. A slope of the at least one ridge is indicative of the parameter of the fluid. The signal processor determines a quality metric by comparing an accumulated energy (power) of k-.omega. pairs along the at least one ridge with an accumulated energy (power) of k-.omega. pairs along at least one ray extending in the k-.omega. plot. The quality metric is indicative of a quality of the at least one ridge.