Humans can hear sounds with amplitudes ranging from the threshold of hearing (0 dB SPL) to the threshold of pain (140 dB SPL). The threshold of hearing is the lowest sound pressure level that a healthy person can hear under ideal listening conditions. The threshold of pain is the highest sound pressure level that a person can tolerate without experiencing pain.

The human ear is most sensitive to sounds in the frequency range of 2000 to 5000 Hz. This is the range of frequencies that includes most of the sounds of human speech. The ear is also less sensitive to sounds at very low and very high frequencies.

The amplitude of a sound is measured in decibels (dB). Decibels are a logarithmic unit of measurement, which means that they represent a ratio of two sound pressure levels. A difference of 1 dB is a barely perceptible difference in sound level. A difference of 10 dB is a perceived doubling of sound level.

The following are some examples of common sounds and their approximate amplitudes:

* Whisper: 30 dB SPL

* Normal conversation: 60 dB SPL

* Loud music: 85 dB SPL

* Motorcycle: 95 dB SPL

* Jackhammer: 110 dB SPL

* Jet engine: 140 dB SPL

The National Institute for Occupational Safety and Health (NIOSH) recommends that workers be exposed to no more than 85 dB SPL of noise for 8 hours per day. Exposure to higher levels of noise can cause hearing damage.