Sound is a vibration that travels as an acoustic wave in a medium (such as a liquid,gas or solid). All vibrations, including sound, have a frequency, which is a measure of how often it vibrates per unit of time. The frequency unit is hertz (Hz). 

Humans with normal hearing can hear sounds between 20Hz~20,000Hz.

Sound waves with frequencies higher than 20,000 Hz is called ultrasound. Ultrasound cannot be heard by humans. Bats rely on ultrasound to echolocate for foraging and navigation. 

Sound waves with frequencies lower than 20 Hz is called infrasound. It is too low for humans to hear. Elephants, whales and other animals use it to communicate. 

How bats can “see” at night?

Bats emit high-frequency sound waves that are above human hearing to produce echolocation through their mouth or nose. The sound waves bounce off objects and return to their ears. So they can determine the shape, size, location, and texture of objects by the direction and the time it takes for the signal to return.

Ultrasound application: body composition assessment

Like the bat, we use a piezoelectric transducer probe to send and receive sound waves. By repeatedly and rapidly applying and removing the voltage, the piezoelectric crystal rapidly expands and relaxes, producing ultrasonic waves. When the returning sound wave strikes the piezoelectric crystal, it is compressed. The crystal then generates a voltage corresponding to the intensity of the ultrasonic waves striking it. Then we can get real-time image of our tissue.

Ultrasound assess body composition by using how different tissues respond to sound. 

Subcutaneous-tissue-sonogram