Our distance and proximity sensors work with the ultrasound principle. We use ultrasonic frequencies, i.e. frequencies above 20kHz, as these are rare in nature and thus the sensors are not disturbed. Moreover, man hears no such high frequencies. Incidentally, they are also completely harmless, because the sound energy is extremely small. Industrial ultrasonic sensors operate at frequencies of approximately 80…400kHz, depending on what characteristics the sensor shall have. The greater the required measuring distance is, the lower frequencies must be used.
Few creatures also make use of ultrasound for guidance. These are mainly the dolphin (in water) and the bat (in the air, as our sensors). Both have bad organs of sight and send ultrasonic waves up to 200kHz. In addition, also other creatures can hear frequencies above 20 kHz although they probably don’t have a benefit for their orientation:
Man 20kHz, Dog 50kHz, Cat 60kHz, Grasshoper 95 kHz, Shrew 115kHz
The dolphin transmits and receives ultrasonic waves in water in order to detect obstacles and beasts of prey. Since the acoustic impedance of water is about 3000 times higher than that of air, both waves generated by animal as well as by technique reach much farer in water than in air.
For the bat living in air it is more difficult. Its high-frequency sound waves do not extend so far. But it also has the most advanced ultrasonic location system. The high frequency enables it to get a very fine local resolution. The ultrasonic cries of bats are produced in the larynx and emitted through the mouth to the outside. When the emitted waves meet a flying body, e.g. a small prey animal, they are reflected and returned to the ears, which serve as a sound signal receiver. The hearing organs of bats must have an amazingly good sound analysis capability. It is believed that due to only microsecond short differences in time of sound flight between the left and right ear they are able to get a three-dimensional acoustic image. Therefore a bat can orient itself with their ears in the absolutely dark, as we do with our eyes at daylight.
As engineers we need to give us once more defeated by nature.
