Measuring the Intensity of Sound – Decibels Explained

The intensity of sound is measured in a unit called the decibel (dB), which describes the relative intensity of a sound based on an algorithmic decibel scale containing values ranging from 0 to 194. Achieving a reduction in decibel levels is a common goal of soundproofing projects, though obtaining an exact measure of the pre-treatment decibel level incurs added cost and is often unnecessary.

Although a zero value on the decibel scale represents the weakest sound audible to humans and sound intensity increases in correspondence with numeric values, the relationship among the values on the decibel scale is not linear but algorithmic. Therefore, the simple assumption that a sound with a 50 dB level is twice as intense as a sound with a 25 dB level would be incorrect. Rather, in a perfect world, each three decibel increment affects a 50% change in sound pressure levels. Thus, a 3 dB drop reduces sound exposure by 50%, while a 6 dB drop reduces exposure by 75%. Though reducing the decibel level produced by a sound source from 80 to 77 may not seem like a major change, it would actually represent a 50% reduction in audible sound.

The intensity of a sound reaching a person’s ear depends not only on the intensity of the sound produced, but also on the person’s distance from the source of the sound. If you were standing one foot away from a loud machine, for instance, you would experience higher decibel level than if you were ten feet away, even though the intensity of the sound produced remains unchanged. This is so because the intensity of sound decreases as sound waves spread out over time and distance, a behavior demonstrated by the Inverse Square Law. The Inverse Square Law is a calculable equation proving that each time the distance between the source of a sound and its recipient doubles, the recipient will experience a 6 dB drop in sound intensity, assuming that no surfaces are present to create reflections that in real world situations would alter these results. Due to the weakening intensity of sound waves with distance, soundproofing and sound reduction treatments in large rooms tend to be more effective than equivalent treatments in smaller areas.

Exposure to high noise levels for extended periods of time can be damaging to the human ear. For this reason, the Occupational Safety & Health Administration standardizes the maximum sound exposure to which employees may be exposed without protection based on the number of hours that they are exposed to the sound source. In effort to keep workplaces free from hazards that can cause employee harm, OSHA requires hearing protection measures to be taken in work environments with decibel levels of 90 and above, with the duration of exposure determining the decibel level at which the standard applies.

To view examples of the decibel levels produced by common commercial, industrial and residential sound sources, view our Decibel Level Comparison Chart.