Sound is a form of energy that travels in waves from one location to another. Although we cannot see sound waves, they are analogous to the ripples that spread outward across the surface of a pond from the splash of a tossed pebble. Sound waves originate with the vibration of an object, such as a tuning fork, a guitar string, a vocal cord, or the wheel of a subway car. As the object vibrates, it rhythmically pushes and pulls the air molecules next to it. Each push creates a region of higher density and pressure in the air, and each pull creates a region of lower density and pressure. In this way, a vibrating object generates waves of alternating high and low pressure, which move outward in all directions, carrying with them a sound's energy.

Sounds vary primarily in frequency and intensity. Frequency is a measure of the regularity of passing sound waves. The closer the waves are to each other, the more often they pass, and the higher pitched a sound will be perceived. Intensity is a measure of the amplitude, or height, of a sound wave, and describes its energy. The greater a wave's amplitude, the louder a sound will be perceived. Unlike sound frequency, which remains the same for the life of the wave, sound intensity changes rapidly with distance. With every unit of distance traveled, sound intensity decreases by the square of that distance. This means that sound intensity decreases by four times with every doubling of distance. Thus, distance from a sound's origin is an important factor in evaluating the potential harm that sound might cause.

Humans are capable of detecting an extremely wide range of sound intensities—from the brush of a fingertip across skin, to the explosion of fireworks. Unfortunately, the most intense sounds can cause permanent hearing loss. A decibel meter can be useful in detecting potentially harmful sound intensities so that protections might be put in place to prevent hearing loss. Given the extremely wide range of sound intensities that the human ear can detect, decibel meters must have a correspondingly high range. The decibel scale is logarithmic and, therefore, nonlinear. Each increase of 10 units, or decibels (dB), indicates a 10-fold increase in intensity. In other words, a sound of 80 dB is 10 times as intense as a sound of 70 dB, 100 times as intense as 60 dB, and 1000 times as intense as 50 dB.

Exposure to sounds of 140 dB or more can cause immediate permanent damage. Prolonged exposure to sounds as low as 90 dB can also result in permanent hearing loss. In light of this, and despite astounding advances in medicine in recent decades, doctors suggest that the best cure for hearing loss is prevention.

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