A sensor for measuring the concentration of microorganisms using acoustic spectroscopy in the audible frequency range was evaluated. Tests were made on a monoculture of Escherichia coli in pilot plant scale and on active biological sludge from two municipal water purification plants. The evaluation of acoustic spectra resulted in the conclusion that the sensor is useful for online cell density measuring and for the detection of morphological changes in monocultures but not for use on active biological sludge. For this purpose, a data analytical method, i.e., fingerprint analysis of collected data, might be of use.