The use of frequency convertto regulatethespeed of rotation according tothe character of minefancan reachthe goal of the modulation of air flow and havea verygoodeffect of saving energy.
"Usually, bat sounds are inaudible to humans as they are too high for us to hear but special 'time expansion' ultrasonic detectors convert these sounds to a lower frequency, " Prof Jones, who is also chairwoman of the UK Bat Conservation Trust, explained.