Audible Frequency Range:

We know that sound is always produced by a vibrating body but when we move or hand to and fro in air no sound is heard. Similarly a pendulum swinging in the laboratory does not produce any sound. The reason is that, the vibration of our and or pendulum does produce longitudinal wave (sound wave) but our ear is limited to a range of frequencies from about 20HZ to 20KHZ or 20000HZ. The sound having frequency less than 20 hertz called infrasonic sound which cannot be heard by the Human ear. For example the Earthquakes, simple pendulum and some animal like whales and elephants, produces infrasonic. The sound having frequency of greater than 20000 hertz called ultrasonic sound our ultrasound. These ultrasonic sounds also cannot be heard by the human ear. The human ear cannot hear ultrasonic sound but dogs can hear the ultrasonic sounds up to frequency of 50000 Hertz. This is the reason why dogs are used for the detective work by police. Similarly other animals like Monkeys, deer, cats bat can also hear ultrasonic sound. But bat can hear the ultrasonic sound up to frequency of 120000 hertz as well as bat can also produce these ultrasonic sounds. That is why bat can fly during night without colliding objects by transmitting ultrasound. The ultrasonic sounds cannot be produced by tuning forks but can be produced be special devices.