There is described a method and device for measurement of the specific density of a gaseous or liquid medium, whereby a pulsed acoustic signal is injected into the measured medium by means of a transmitting transducer, the signal is detected by a receiver transducer, after passing over a measured path, whereupon the acoustic impedance Z is automatically determined in an analytical device by comparison of a curve for the currently measured received signal with reference curves, predetermined for various values of the acoustic impedances stored for each value of the acoustic impedance, whereby the propagation speed c of the acoustic signal in the medium is determined and the density p of the medium is calculated as the quotient from the acoustic impedance Z and the propagation speed c. The method is preferably of application to an ultrasound mass flow measurement device and hence has the advantage that no additional density sensor is required, nevertheless permitting a robust and precise measurement of density.