17.8:
Sound Intensity Level
On average, human ears are sensitive to a sound of intensity 10−12 watt per meter squared at a frequency of 1000 hertz and can perceive a range of sound up to 1 watt per meter squared. However, a sound of this intensity can cause severe pain.
To incorporate the massive range in SI units, the logarithmic scale is used to define the intensity level. On this scale, the intensity of 10−12 watt per meter squared is 0 decibels.
For instance, the rustle of leaves in the forest has an intensity level of about 10 decibels, an average home is about 40 decibels, noisy traffic is 70 decibels, whereas the threshold of pain is 120 decibels.
17.8:
Sound Intensity Level
Humans perceive sound by hearing. The human ear helps sound waves reach the brain, which then interprets the waves and creates the perception of hearing. The loudness of the environment in which a person is located determines whether they can distinguish between different sound sources.
The human ear can perceive an extensive range of sound intensity, necessitating the use of the logarithmic scale to define a physical quantity—the intensity level. It is a ratio of two intensities and hence a unitless quantity. The standard for measuring the intensity level is taken as the intensity at which human ears can barely hear a sound, 10−12 watt per square meter. The human eardrum is only about one square centimeter; hence, at the hearing threshold, it can process 10−16 watt of energy per second, demonstrating that the ear is an incredibly sensitive organ.
The intensity level is expressed in decibels, named after Alexander Graham Bell, who invented the telephone. On this scale, the threshold of pain is 120 decibels, and at a sound intensity greater than this, eardrums get damaged within seconds.
This text is adapted from Openstax, University Physics Volume 1, Section 17.3: Sound Intensity.