PURPOSE: To detect the movement of fluid with good accuracy by providing this recorder with two temp. measuring means to respectively measure the temp. when the fluid stagnates and the temp. when the fluid moves and controlling the filter characteristics of a filter means for filtering the electric signals converted by a voice- to-electricity converting means in accordance with the results of the measurements.

CONSTITUTION: Voices are converted by a microphones 1 to electric signals which are amplified by a voice amplifier 2. The outputs of this amplifier are adjusted in level by an ALC circuit 3 and are filtered by the variable filter circuit 4. The frequency characteristics of this circuit 4 are varied by a CPU 5. First and second thermocouples 6, 7 generate the thermoelectromotive forces according to the respective temps. The thermocouple 6 is installed in the outdoor air and the thermocouple 7 is installed in the air stagnating in a wind protector 8. The thermoelectromotive forces of the respective thermocouples 6, 7 are amplified by first and second differential amplifiers 9, 10 and are outputted to the CPU 5. The CPU 5 changes the frequency characteristics of the variable filter circuit 4 from both outputs. The output of the circuit 4 is passed through an NR circuit 11 and through a clip circuit 2 and the output of a frequency modulating circuit 13 is amplified by a recording amplifier 14 and is recorded on a magnetic tape 16 by a recording head 15.