The amplitude of a vibration is the maximum displacement of a vibrating object from its equilibrium position. The greater the amplitude, the louder the sound.
The loudness of a sound is also related to the frequency of the vibration. The frequency of a vibration is the number of vibrations per second. The higher the frequency, the higher the pitch of the sound.
The human ear can hear sounds with frequencies between 20 Hz and 20,000 Hz. Sounds with frequencies below 20 Hz are called infrasound, and sounds with frequencies above 20,000 Hz are called ultrasound. Humans cannot hear infrasound or ultrasound, but some animals can.
The decibel (dB) is a unit of measurement used to express the loudness of a sound. The decibel scale is logarithmic, which means that each increase of 10 dB represents a doubling of the loudness of a sound.
The threshold of hearing is the lowest sound that a human can hear. The threshold of hearing is 0 dB. The threshold of pain is the loudest sound that a human can tolerate. The threshold of pain is 120 dB.
Most people find sounds between 30 dB and 60 dB to be comfortable. Sounds above 80 dB can be harmful to hearing, and sounds above 120 dB can cause permanent hearing damage.