Sound Intensity Level

The human ear perceives sound across an enormous range, from 10−12 watt per meter squared at the hearing threshold to 1 watt per meter squared at the pain threshold. This massive range in SI units makes a linear scale impractical. A logarithmic scale compresses this range into manageable numbers, where the hearing threshold is defined as 0 decibels and the pain threshold is 120 decibels.

The reference intensity is 10−12 watt per meter squared, which represents the threshold at which human ears can barely hear a sound at 1000 hertz. This standard is assigned a value of 0 decibels on the intensity level scale. The intensity level is a unitless ratio comparing any sound intensity to this reference standard.

The human eardrum has an area of approximately one square centimeter. At the hearing threshold of 10−12 watt per meter squared, the eardrum processes about 10−16 watt of energy per second. This demonstrates the extraordinary sensitivity of the human ear, which can detect such minute amounts of acoustic energy.

Common sound intensity levels include rustling leaves at about 10 decibels, an average home at 40 decibels, and noisy traffic at 70 decibels. The threshold of pain, where sound can cause severe damage to the eardrum, occurs at 120 decibels. Sounds exceeding this level can damage eardrums within seconds.

The human ear can perceive sound intensities spanning from 10−12 to 1 watt per meter squared, an enormous range that would be difficult to represent linearly. The logarithmic decibel scale allows the ear's sensitivity to be expressed in a compressed format. This enables scientists and engineers to work with intensity and pressure of sound waves more practically.

The decibel scale is named after Alexander Graham Bell, who invented the telephone and made significant contributions to sound technology and communication. The unit honors his pioneering work. One decibel represents one-tenth of a bel, the original logarithmic unit of sound intensity measurement.

At sound intensity levels greater than 120 decibels, the threshold of pain, eardrums can be damaged within seconds. A sound of 1 watt per meter squared, the upper limit of human perception, causes severe pain and poses immediate risk of permanent hearing damage. Prolonged exposure to such intense sound is dangerous to human health.