Patent ID: 8302490

Claim:
Apparatus for measuring the relative speed of movement between a reference and a fluid, said apparatus comprising: an electro-acoustic transducer coupled to said reference in communication with said fluid having means for imparting acoustic energy into said fluid; an acousto-electric transducer coupled to said reference in communication with said fluid and spaced from said electro-acoustic transducer having means for receiving acoustic energy from said fluid; a bandpass filtered amplifying circuit having an input coupled to said acousto-electric transducer and an output coupled to said electro-acoustic transducer, said bandpass filtered amplifying circuit, said electro acoustic transducer, said acousto-electric transducer and said fluid forming a closed loop defining a loop gain and loop passband within which noise occurs, said closed loop regenerating in response to noise within said closed loop which has frequency components within said loop passband producing a regenerative signal having a frequency which is determined by the relative speed of movement between said reference and said fluid; and means for measuring the frequency of signal produced within said regenerative loop when it regenerates to provide a speed-indicative output.