When something vibrates, it pushes particles. These pushed particles create a longitudinal wave. If the longitudinal wave has the right frequency and enough energy, your ear drum antennas will pick it up and your brain will turn the energy into what we call sound. The higher the amplitude, the more energy the wave has. Intensity of a measure of a wave’s power per unit area, and is measured in Watts per square meter.
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Waves spread out in circular (or spherical for 3-D) patterns, like ripples on the surface of a pond. The ripples close to the source at the center will have greater intensity that the ones further out. The intensity decreases the further you get from the source by the inverse square law.
Human ears can detect very low intensities (on the order of 10-12 W/m2, which corresponds to the threshold of hearing (TOH), which is at 0 dB (decibels). A sound 10 times greater has a level of 10 dB, or 10-11 W/m2. A sound 100 greater is 20 dB (10-10 W/m2). This measurement of intensity is called the decibel scale.
A whisper is around 20 dB, traffic is around 70 dB, and jets taking off are 140 dB. Eardrum damage occurs over 150 dB.
Everyone perceives intensity differently. Have you ever noticed how older people have hearing issues? That’s because two sounds with the same intensity but different frequencies are not necessarily going to be heard at the same loudness.
Click here to go to next lesson on How You Hear.
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