Decibels for Dummies

Not All Decibels Are Created Equal!

Broadly speaking, any form of unwelcome sound is noise pollution, whether it is the roar of a jet plane overhead or the sound of a barking dog a block away. The actual loudness of a sound is only one component of the effect it has on human beings. Other factors to consider are the time and place, the duration, the source of the sound, and whether the listener has any control over it. Most people would not be bothered by the sound of a 21-gun salute on a special occasion. On the other hand, the thump-thump of a neighbor’s music at 2 a.m., even if barely audible, could be a major source of stress.

Measuring Sound by Decibel (dB)

The decibel (dB) is a measure of sound intensity; that is, the magnitude of the fluctuations in air pressure caused by sound waves. The decibel scale is logarithmic, not arithmetic. This means that a doubling of sound intensity is not represented as a doubling of the decibel level. In fact, an increase of just 3 dB means twice as much sound, and an increase of 10 dB means ten times as much sound.

A sound pressure level of 0 dB represents the threshold of hearing in the most sensitive frequency range of a young, healthy ear, while the thresholds of tickling or painful sensations in the ear occur at about 120 to 130 dB.

Decibels are usually measured with a filter that emphasizes sounds in certain frequencies. The “A” filter (dBA) is the one most frequently used. The “C” filter (dBC) puts more weight on low-frequency sounds such as the bass in amplified music (and more accurately measures debris blower sound)

The perception of loudness by the human ear is not directly proportional to the decibel level. For example, a sound 10 dB greater than another is not perceived as being ten times as loud but only about three times as loud.

The intensity of noise diminishes with distance. Outdoors, and in the absence of any close reflecting surface, the effective decibel level diminishes at a rate of 6 dB for each doubling  in distance. For example, a sound measuring 100 dB at 25 feet would be 94 dB at 50 feet, 88 dB at 100 feet, and so on.

Similarly,  a “quiet” 65 db blower at 50 feet is 71 db at 25 feet; 77db at 12.5 feet; 83 db at 6.25 feet; and over 100 db right at the blower.

As far back as 1974, the EPA identified “noise levels requisite to protect public health and welfare against hearing loss, annoyance and activity interference…” Read more here.

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