Sound is vibration transmitted in medium (liquid, gas or solid) and can be percepted by human and animals within certain limits by special organs. Main sound characterisitics are frequency and amplitude.
Person can perceive sound in diapason from 16 Hz to 20 000 Hz. Vibration with frequency lower than 16 Hz is infrasound. Sound within limits from 20 000 Hz to 1 GHz is called ultrasound and which is higher than 1 GHz is hypersound.
Dogs can percept vibrations higher than 20 kHz. Some species of butterflies have mechanoreceptors which accept vibrations and are located on thorax or hind body.
Speed of sound is determined by medium. For example, the speed sound in gases depends on temperature. In 20 C air it is about 1230 km/h. In fresh water at 20 C speed sound is approximately 5335 km/h. In steel it can be 21460 km/h. .
Photo credits: , Olyashevchenko.