In simple terms, sound is vibration. The complex internal structure of our ears can respond to the waves produced by vibration, whether in the form of bass drum or an acoustic guitar.
When an instrument produces vibrations, it creates oscillating sound waves. Some of these waves are audible to humans, while others are very deep or high-pitched, and fall outside the human range of hearing.
Sound waves consist of areas of high and low pressure called compressions and rarefactions.Humans can only hear sound waves as distinct pitches when the frequency. lies between about 20 Hz and 20 kHz