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Noise Issues in Parking Area Sweeping

Decibels Decoded

A few decibels make a big difference

by Wendlyn Alter

Noise levels -- you want to keep them down. But reducing noise costs money. How do you decide whether your investment is cost-effective? Suppose an analysis shows that you could take measures to reduce your noise level from 79 to 72 decibels. That doesn't sound like much improvement, does it? In fact, a noise level of 79 decibels (dB) is more than twice as loud as 72 dB, so this investment could reduce your noise level by better than half.

If you are interested in the mathematical details, the secret lies in the fact that the decibel scale is logarithmic, not linear. You may be familiar with logarithmic scales in the terminology of earthquakes: a quake of 6 on the Richter logarithmic scale is 10 times stronger than a quake of Richter 5, and a quake of Richter 7 is 100 times as severe as a 5. Thus the 8.5 quake in Chile was 1000 times stronger than the equivalent Richter 5.5 of the Hiroshima atomic bomb.

Like the Richter scale, the decibel scale (named after Alexander Graham Bell -- which is why the "B" in the abbreviation dB is capitalized) is a "base ten" logarithmic scale. It describes ratios of sound pressure levels. The sound pressure is the level measured per unit area at a particular location relative to the sound source -- in other words, sound pressure is what the complaining neighbor hears, or what the inspecting police officer measures at a legally determined distance from your machine, expressed in decibels:

dB = 20 log (p1/p0)

where dB is the decibel reading, p1 is the sound pressure at that location, and p0 is the threshold of hearing. ("Log" is a function such that if log y = x, then 10 raised to the power of x = y.) For example, when the officer informs you that your equipment is putting out 79 decibels, that means the racket is approximately 10,000 times louder than the threshold of normal hearing in a young, undamaged ear.

In plain English, this equation tells us that each increase of six decibels means the noise level has approximately doubled -- or, conversely, knocking only six decibels off your sound pressure will reduce your noise output by half, no matter where you're starting on the decibel scale. Lowering your current decibel level by 12 reduces your sound output to one fourth its original level, while a 20 decibel improvement means you're only one tenth as loud.

Decibel Levels in the Real World

    Pain	Shotgun blast; jet taking off 100 ft. away
		Firecrackers; severe thunder; pneumatic jackhammer
 Uncomfortably	Rock Music; hockey crowd
    loud	Concorde jet or 707 jet landing 370 ft. away
    Loud	707 jet taking off 1000 ft. away, gas lawn mower 3 ft. away
		Diesel truck 50 ft. away; food blender or garbage disposal 3 ft. away
  Moderately	747 jet taking off 1000 ft. away, vacuum cleaner 10 ft. away
    loud	Commercial area; normal speech 3 ft. away; quiet typewriter
    Quiet	Quiet urban daytime streets, dishwasher in next room
		Quiet urban nightime streets, household refrigerator
    Very	Quiet suburban nighttime streets, library, average home
    quiet	Quiet rural areas or bedroom at night
    Just	Rustling leaves, whisper
  audible	Threshold for acute hearing
Sources: FAA; Bolt, Beranek and Newman Inc.; "Industrial Noise and Hearing Conversation" by J.B. Olishifiski and E.R. Hartford

Wendlyn Alter is a materials engineer and recipient of NASA's "Most Promising Patent of the Year" Award.

...And Here's What You Can Do To Mitigate Noise

Because there are no national noise statutes yet in place, ordinances and enforcement vary widely, and both are often the result of complaints. Also, because malls are high profile community citizens, they are very sensitive to complaints even when no local ordinance has been broken. For these reasons, many contractors prefer to meet personally with anyone who complains about noise from their sweepers.

Our advice is to also let your client know right away when you have had a complaint. Most who are upset enough to stop you or call the police will also call the mall manager. They will appreciate knowing about it first from you, so they can be prepared.

Handling this type of situation well is very much in their own self interest. Although they don't want unhappy neighbors, they also aren't going to want to pay the extra costs that noise and/or time restrictions usually bring, either.

Knocking only six decibels off your sound pressure will reduce your noise output by half.

Steps you can take to reduce sweeper noise complaints:

  • Run your auxiliary engine at the minimum needed to get the job done.
  • Make certain your mufflers and connecting pipes are in good shape.
  • Keep your head flaps in trim and sealing well.
  • Operate in a complaint area without a flashing light (pre-check for approval with your insurance carrier).
  • Add a sound-deadening shroud, if one is made for your model of sweeper.
  • Clean that part of the lot during the day (more often than not, it's the outer perimeter that is the problem area, and usually just a few cars are parked there in the mornings).
  • Finally, or perhaps firstly, make a sincere effort to work out a successful compromise with the person complaining. Try to understand their position, and figure out a solution to their problem, before it results in a petition to city hall.
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