What our ears perceive as sound are small pressure waves around us. The sound pressure level is measured in decibel (dB). 0 dB is the lowest noise we can hear and we start to feel pain in the ear at 120 dB. 

When noise is described in dB (A) (A-weighted), the decibel values of noise at low frequencies (low-pitched) are reduced because the human ear is less sensitive to these, but more sensitive to noise at high frequency (high-pitched).