Sensitivity of Human Ear

The human ear can respond to minute pressure variations in the air if they are in the audible frequency range, roughly 20 Hz - 20 kHz.

It is capable of detecting pressure variations of less than one billionth of atmospheric pressure. The threshold of hearing corresponds to air vibrations on the order of a tenth of an atomic diameter. This incredible sensitivity is enhanced by an effective amplification of the sound signal by the outer and middle ear structures. Contributing to the wide dynamic range of human hearing are protective mechanisms that reduce the ear's response to very loud sounds. Sound intensities over this wide range are usually expressed in decibels.

Dynamic Range of Hearing

In addition to its remarkable sensitivity, the human ear is capable of responding to the widest range of stimuli of any of the senses. The practical dynamic range could be said to be from the threshold of hearing to the threshold of pain:

This remarkable dynamic range is enhanced by an effective amplification structure which extends its low end and by a protective mechanism which extends the high end.

Pitch Resolution

The extremely small size of the cochlea and the extremely high resolution of human pitch perception cast doubt on the sufficiency of the place theory to completely account for the human ear's pitch resolution. Some typical data: Cochlea: turns, about 3.2 cm length. Resolves about 1500 separate pitches with 16,000-20,000 hair cells.

This would require a separate detectable pitch for every 0.002 cm, which is physically unreasonable for a simple peaking action on the membrane.

The normal human ear can detect the difference between 440 Hz and 441 Hz. It is hard to believe it could attain such resolution from selective peaking of the membrane vibrations. Some pitch sharpening mechanism must be operating.

The structures of the outer and middle ear contribute to both the remarkable sensitivity and the wide dynamic range of human hearing. They can be considered to be both a pre-amplifier and a limiter for the human hearing process.

The outer ear (pinna) collects more sound energy than the ear canal would receive without it and thus contributes some area amplification. The numbers here are just representative ... not precise data.

Closed tube resonance of the auditory canal enhances 2000-5000 Hz Tympanic membrane (eardrum) has some 15x area of oval window contributing an area amplification. Ossicles (hammer, anvil and stirrup) contribute a lever-type amplification when listening to soft sounds. Outer ear 2x Tympanic membrane 15x Ossicles 3x

The outer and middle ears contribute something like a factor of 100 or about 20 decibels of amplification under optimum conditions.

Audible Sound

Usually "sound" is used to mean sound which can be perceived by the human ear, i.e., "sound" refers to audible sound unless otherwise classified. A reasonably standard definition of audible sound is that it is a pressure wave with frequency between 20 Hz and 20,000 Hz and with an intensity above the standard threshold of hearing. Since the ear is surrounded by air, or perhaps under water, the sound waves are constrained to be longitudinal waves. Normal ranges of sound pressure and sound intensity may also be specified.

Animal sounds cover the entire range of audible sound and beyond. Blue whales produce sounds around 20 Hz and with intensity measured as high as 189 dB (Sirovic).

For an air temperature of 20°C where the sound speed is 344 m/s, the audible sound waves have wavelengths from 0.0172 m (0.68 inches) to 17.2 meters (56.4 feet).