For humans, hearing is normally limited to frequencies between about 12 Hz and 20,000 Hz (20 kHz).

Sound with frequencies of less than 20 Hz is known as infrasound. If a sound has a frequency that exceeds 22,000 Hz it is called ultrasound.

Sound is transmitted through gases, plasma, and liquids as longitudinal waves, also called compression waves.

The sound of a whale song can travel a distance of 800km.

In the air sound travels at the speed of about 335 metres(6100 feet) a second.Sound travels about 4 times as fast in water as it does in air.

Sound waves are invisible because the human eye can’t see pressure waves. But an oscilloscope can detect and show the shape of sound waves very accurately.

The accepted units for sound pressure are metric, Newton per one square meter (N/m2).