Reprinted from the ACJ - November, 1995
There is an old philosophical question that asks if a tree falls in the woods and no one is around to hear it, does it make a sound?
The same query can be modified for our uses. If a screwdriver falls off of the fender of a Mercedes 450 SL during a flush and fill, bounces against the spinning fan, which shoots it, like an arrow, through the radiator core resulting in the mechanic cursing a blue streak, but the customer is not there to hear it, does it make a sound?
The answer to both questions depends on whether we define sound as the generation of energy waves in the 20 to 20,000 Hz range, or the detection of waves in that spectrum. That is the range of frequencies that our ears are capable of translating into sound.
The way our ears work is nothing less than miraculous. The description of the process I will now give will be far less than miraculous.
The outer ear, or "head handles" as they are cited in the literature, act as inverted megaphones capturing enough sound waves to double their intensity as the waves travel down the ear canal. These waves bump into the eardrum, making it move back and forth.
The ear drum is attached to several small bones that move in unison with the vibration. These are the hammer, stirrup and anvil we learned about in third grade science class. This tack room tool chest of bones then conveys the waves to a fluid-filled cornucopia-shaped tube called the cochlea. Hair-like fibers line this tube, and movement of the fluid moves the hairs.
These hair cells convert movement to energy in a nerve cell attached to our brain. Then a miracle happens and we hear sound (this last step was edited for space considerations). Seems to be a bit of a waste of time when the only sound in the room is your spouse snoring at 3 a.m.
But good sounds or harsh noise, our ears work just the same. That is, until disease or physical damage to our ears cause less than stereophonic performance. Infections, colds, wax buildup and other ailments can cause the inner parts of our ears to swell. This swelling can cause reduced movements of the parts and therefore less sound transmission.
If the ailment is temporary, then the loss of hearing is as well. If the ear parts are damaged, hearing may not fully return. Far and away the leading cause of permanent damage to our ear parts is excessive volume. Loud sounds cause large movements of the ear parts, causing them to wear out like hinges on an old door. Like hinges, if they rub and squeak as they move, we hear a ringing effect called tinnitus. If they get sticky and won't move easily, then we hear less than full volume and/or frequency range.
Because loud noises, either one time or continually, are the health hazards of paramount concern, we should take care to protect ourselves and co-workers from excessive noise. OSHA and ACGIH agree. In the repeated noise category, ACGIH considers any sound that results in a two decibel (db) loss of hearing over a 40-year period to be excessive. That occurs if the sound you put up with, day in and day out, is louder than 75 db continuously.
To avoid this, the eight-hour limit for noise exposure in the work place has been set at 85 db by ACGIH and 90 db by OSHA. A limit for short-term noise exposure has also been set. During a 15-minute period, ACGIH sets a 100 db maximum, while OSHA's limit is 115 db.
Both agencies site a single exposure ceiling limit at 140 db for a 0.1 second blast. That would be about the level experienced during the scream emitted by the Mercedes owner watching the screwdriver go through his radiator.
As an actual comparison, here are some decibel levels of sounds we frequently hear. A ticking watch emits only 10 to 20 db. Normal talking is 60 db, while the noise of a big city is about 70 to 80 db. A pneumatic drill delivers 120 to 130 db, and a jet airplane pushes 130 to 150 db.
Experts cite not only the volume, but the repetition of the sound as a physical danger. For example, a dog bark at 85 db is not so bad, but continual barking is considered a hazard. Maybe only because it drives us insane, a different health concern.
Protection is as easy as wearing earplugs. That is the real reason we have pinkie fingers. But since we need our hands to work, I suggest we look at other types of protection.
When selecting earplugs, make sure that they fit fully in your ear and are comfortable. Keep them very clean to avoid infection, or better yet use a new set of disposable plugs each time you need a pair. It is best if the plugs filter out sound in the upper frequency range —10,000 Hz and up — as these frequencies are the most debilitating.
As we get older, we naturally lose the ability to hear the high sounds first. The hardest part of this type of hearing protection is remembering to put them in each time the noise level goes up. Remember, it is the acute exposures over time that will lead to chronic hearing problems. It is also a good idea not to remove any mufflers or sound deadening accessories from shop equipment. You paid for them, so you might as well benefit from them.
Now let's take a new perspective on our initial question. If the screwdriver falls into the fan and proceeds through the radiator core and the customer is not around to hear it, does it constitute billable work? Remember your ear plugs when you explain it to him.
The above article was written by David M. Brown, Chief Engineer of Johnson Manufacturing Company, Inc. and is published by JOHNSON with the expressed approval of the National Automotive Radiator Service Association and the Automotive Cooling Journal. Other reproduction or distribution of this information is forbidden without the written consent of JOHNSON and NARSA/ACJ. All rights reserved.
JOHNSON MANUFACTURING COMPANY
114 Lost Grove Road / PO Box 96 / Princeton, Iowa 52768-0096
Phone 563-289-5123 or Fax 563-289-3825