Understanding the Decibel
What is a decibel? A decibel (dB) is a unit of measurement that gages the intensity of sound. The units define how loud a noise source is, ranging on a comparative scale from 0-194. A dB reading of “0” indicates the faintest sound the human ear can detect, while a dB reading of “180” would be the equivalent to standing on a rocket pad during launch.
Your average day is filled with sound sources that typically range from 30-100 dB. Daytime hours average 10 dB more sound pressure than night time hours. Conversational voice levels average a 65 dB rating, while OSHA demands hearing protection for factory workers exposed over an 8 hour period to levels stronger than 85 dB. The pain threshold for human ear starts at about 120 dB.
The decibel scale is logarithmic, not linear. This simply means that for every 3 decibels you move up or down the scale from 0-194, you are adding or dropping 50% of your remaining sound pressure levels to your exposure. By dropping 6 decibels, for instance, you first move 3 dB, and then another 3 dB. For each 3 dB you drop, your sound pressure levels will drop another 50% of the remaining sound pressure. The following table will help illustrate the order of magnitude associated with dB.
dB-Drop Survival Rate
1 dB Drop 79% of your noise has survived
3 dB-Drop 50% of your noise has survived
6 dB-Drop 25% of your noise has survived
9 dB-Drop 12.5% of your noise has survived
10 dB-Drop 10% of your noise has survived
20 dB-Drop 1% of your noise has survived
30 dB-Drop .01% of yoru noise has survived
Inverse Square Law
Sound intensity will diminish over time and distance. The further away from the noise source you are, the lower your decibel level readings will become. The Inverse Square Law teaches us that for every doubling of the distance between a sound source and the recipient of the sound, a 6 dB drop would occur if there were no echo (as from a mountain top). This would be the equivalent of cutting your noise pressure levels by 75%. For instance, if you stand 10 feet from a sound source and experience a 90 dB reading, at 20 feet you would experience an 84 dB reading in the absence of echo. Of course, in the real world, sound reflections will blur these results.
These results would be experienced only in an open field, such as the top of a mountain, where no surrounding surfaces would interrupt the path of the sound and reflect it back to alter the results. Because of this law, soundproofing treatments are able to trigger stronger absorption results in larger rooms than smaller rooms. In smaller rooms, where you are positioned closer to the sound source, the more difficult it becomes to protect from original sound as well as distinguish between the original sound and reflected sound. For this reason, slowing reverberation times down in a larger room, such as a gymnasium, produces a more noticeable acoustic effect than in a smaller room, such as an office.
When combining two sound sources together, the question of determining the sum of their decibels is addressed using the following scale.
If two sound sources differ by 0-1 dB: add 3 dB to the higher number
If two sound sources differ by 2-3 dB: add 2 dB to the higher number
If two sound sources differ by 4-9 dB: add 1 dB to the higher number
If two sound sources differ by 10+ dB: add 0 dB to the higher number
This explains how reflective sounds within a room can grow to become stronger than the single original sound source. For example, if you had a forklift emitting an 87 dB reading driving past a punch press with an 89 dB reading, your overall exposure would be 91 dB (not 176 dB). Conversely, if your voice carried a 70 dB reading at a rock concert emitting 120 dB, your exposure would still be 120 dB.
Decibel levels are controlled by applying soundproofing treatments that attenuate either the “source” of the noise or the “path” the noise is traveling. These treatments are briefly reviewed in our Sound Treatments section of this Academy, and specified in more detail in the Applications Guide of this website. “Source” driven treatments will typically trigger 12-18 dB drops, while “path” driven treatments will typically trigger 3-8 dB drops. The most we’ve ever experienced with a “path” driven treatment is a 14 dB drop. For more aggressive industrial soundproofing applications, 30+ dB drops can be achieved only through the use of full industrial acoustic enclosures. Remember that regardless of your soundproofing treatment, a 10 dB reduction will be perceived as being half as loud to the human ear.
Dosimeters are instruments that can accurately measure your decibel level exposure. The question is often posed to NetWell:
“Do we need to know what our dB reading is before our soundproofing treatment?”
The answer is usually “no”, unless your application has been triggered by litigation or potential OSHA fines, where documentation is a must. The need to place an expert onsite to record dB readings is a costly step that you can avoid. Remember our goal with your soundproofing treatment is to give you better control over your noise problem, not to completely eliminate it. By reproducing what we have done for countless others before you, and your understanding of the decibel, you will soon have your decibel levels under control!Back to Sound 101