The human ear is not equally sensitive to all frequencies in the audible range. It may perceive sound waves with the same pressure but different frequencies as having different loudness. Moreover, the perception of sound waves depends on the health of an individual's ears, which decays with age. The health of one's ears may also be affected by regular exposure to loud noises.
The pitch of a sound depends on the frequency and the pressure amplitude of the source. Two sounds of the same frequency and pressure amplitude appear to have the same pitch. However, if the pressure amplitudes are different, the louder source seems to be of lower pitch.
Generally, a sound wave is a complex superposition of many sound waves of different frequencies. If the frequencies are multiples of a fundamental frequency, they are called harmonics. Suppose two sources have the same fundamental frequency and amplitude and hence the same amplitude and pitch. In that case, the number of harmonics present in the two sound waves differentiates their quality or timbre. Moreover, the beginning and end of a sound can also determine its quality.
This text is adapted from Openstax, University Physics Volume 1, Section 17.1: Sound Waves.
