by Steven D. Johnson
Racine, Wisconsin
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Sound Safety Tips — Noise Abatement
Out of curiosity, I often wonder what the three most valuable non-tool shop accessories might be
for the average woodworker. Hands down, my first vote goes to my beloved cup warmer. Coffee often
gets forgotten when I "zone out" or "zone in" to a task, but I can return to that cup, still warm,
even after fifteen minutes or more. I love that cup warmer, as only a real coffee-lover can. My
second favorite non-tool shop accessory is my iPod dock/speaker combination that allows me to listen
to music without the encumbrance of headphones.
My iPod dock is about two years old. I distinctly remember that the LED volume readout on the
dock was always at "11" when listening comfortably to background music. Lately, I have noticed that
my more comfortable listening level is "13." Doubting that the iPod or dock has somehow lost power,
my conclusion has been that a slight diminishment in hearing has occurred. Yikes!
Certainly age affects hearing. Experts agree that continued exposure to overly loud sounds also
negatively impacts hearing. My misspent youth probably did my ears no favors, having played
(poorly) in a rock band (drums, no less!) and having attended innumerable concerts. As I got older
and my tastes broadened, I became a devotee of the symphony, but that did not necessarily help my
hearing. In a boisterous movement, a 75-piece orchestra can produce sound in the 130 decibel (dB)
range. The average for a rock concert is about 115 - 120 dB.
So, with a long background of live music performances, a hobby with noisy "instruments," and
relentless aging, I am a prime candidate for hearing loss. In the down-to-earth woodworking shop,
sound (noise) is becoming an increasing safety concern (and should have been a concern all
along).
The Science of Sound — A Brief (and simple) Primer
The loudness (power) of a sound is measured in decibels (dB). The dB scale is logarithmic, not
linear. We humans perceive loudness as doubling every 10 dB of sound increase. For example, a 70 dB
sound is perceived as twice as loud as a 60 dB sound. Higher frequency sounds are perceived as
louder than lower frequency sounds, given the same sound pressure or energy. A bass drum beat at 80
dB will not be perceived to be as loud as a middle C note from a saxophone at the same 80 dB.
Another interesting note is that lower frequency sounds travel further than high frequency sounds
and have a low material absorption rate – low frequency sound can literally travel through walls and
its intensity decreases less over distance than does a similarly loud high frequency sound. This
explains why you can hear the bass beat from the kid's car coming toward you before you hear the
rest of the music, if you hear the rest at all. Higher frequency sounds are more readily absorbed,
and thus do not travel as far, or through material, as easily.
It is said that we humans hear between 20 hertz (hz) and 20 kilohertz (khz) frequencies, but age
and a host of other factors can affect that hearing range, particularly in the higher frequencies,
with many of us not hearing well above 14 to 18 khz. Despite the 20 hz to 20 khz range being the
generally accepted hearing "range" for humans, we actually are capable of hearing lower frequencies
(down to 1 hz in some tests) but the sound pressure needs to be so high (in dB) that we rarely
consciously experience sounds below 20 hz.
Sound can also have physiological effects. Giant pipe organs that routinely produce sound in the
so-called subsonic range (less than 20 hz) cause a range of physiological responses from euphoria to
depression, and have been known to create hallucinations. At 7 hz, roughly the same frequency at
which our brain waves emanate, given sufficient pressure, sound can cause our major internal organs
to cease functioning. We know from history that both sides of the conflict experimented with sound
weapons during World War II and that the U.S., Russia, and other countries continue to research the
possible use of sound as weaponry.
Without delving too deeply into the science, in the woodworking shop we need to be concerned with
just a few facts and reference points in order to protect our hearing and mitigate noise: (1)
Exposure to high sound pressure levels can damage our hearing, and exposure risk is time-based,
meaning that longer exposures can have a cumulative impact. (2) Higher frequency sounds are
perceived as "louder" than lower frequency sounds of the same sound pressure. (3) Distance affects
sound levels differently – higher frequency sounds will diminish in perceived loudness more quickly
than will lower frequency sounds. (4) Higher frequency sounds are generally perceived as more
annoying but are more easily absorbed or directed.
There are readers undoubtedly thinking right now, "Just throw on a pair of ear protectors and get
on with it!" But, like many other things, the solution to noise pollution is not always so
simple.
Dangerous Quiet
Hearing protection is great, and highly recommended in most cases, but with a caveat. We must
remember that we all receive auditory safety cues that will be negated to some extent by the use of
hearing protection.
These "auditory cues" are numerous and varied. In my shop, I can tell when a power tool blade
needs to be cleaned of pitch and gunk by the sound it makes in the wood. I can also painfully
remember the two times I experienced kick back, and vividly remember the distinct change in sound as
the wood began to pinch the blade, just before it became airborne. Last year, while taking a walk,
I heard the unmistakable sound (at least to us in Northern climes) of car tires sliding on the ice,
and turned just in time to see an out-of-control car sliding toward me. I jumped and made it to
safety, thanks to being able to hear the car behind me. There are safety trade-offs, but when there
is traffic, I will still risk frostbitten ears rather than diminish my hearing with earmuffs.
Dangerous Noise
If you use those hearing protectors that look like headphones but that include built-in speakers,
be especially careful. I adjusted my headphones to a comfortable (not too loud, not too soft)
level. Carrie Underwood was singing her hit "Last Name" (yes, my musical tastes are eclectic). I
placed the headphones over my sound pressure meter and was astonished to see readings from 82 dB to
88 dB in the chorus. Trading one damaging noise for a more pleasing, but still damaging, sound is
not a very good trade-off. In fact, public health officials are expressing growing concerns over
the long-term effects and the proliferation of ear buds and headphones. Keep the volume low if you
use these units.
Here are some interesting approximate decibel (dB) reference points:
NIOSH (the National Institute for Occupational Safety and Health) gives us guidelines for noise
exposure:
In the down-to-earth shop, my shop vacuum puts out 81 dB and my dust collector emits 77 dB (both
measured at 6 feet and "ear" level), but the dust collector seems much less obnoxious, since its
sound frequency is lower than the screeching, higher-pitched shop vacuum. My table saw puts out a
relatively "noisy" 92 dB with the dust collector running, but the combined noise level jumps to 96
dB when cutting a piece of ¾ inch plywood. You can see by the chart that NIOSH recommends no more
than about 30 minutes exposure at this level.
My jointer idles peacefully at 80 dB until a piece of wood starts across the knives, then the
noise level jumps to 90 dB when jointing a ¾ inch edge and 102 dB when jointing the face of a 3 ½
inch board. Without hearing protection, I should not use my jointer for more than about 8 minutes,
according to the NIOSH data.
The noisiest machine in the shop is the thickness planer, registering 95 dB while idling and 103
dB planing a six-inch wide board. It gets louder with wider stock.
Just to remind myself of my previous poor work habits, I cranked up the music to be able to hear
it above this din (as I often did in the past) and the cumulative ambient noise level reached an
astonishing 105 db. By the time Carrie got through singing "Last Name," I doubt if I could still
remember my own.
As safety goes, exclusive of noise, the thickness planer is a relatively more "safe" machine than
the table saw, and there are not the same necessary auditory safety cues, so I do routinely wear
hearing protection when power planing. Shame on me, but I have not routinely worn hearing
protection at the jointer or the saw, but my research has opened my eyes, and ears. There are other
additional noise abatement steps that I have taken, too.
Down to Earth Workshop Noise Abatement
Remember that sound intensity lessens with distance and higher frequency sounds are more readily
absorbed and thereby mitigated. I would love to locate my dust collector in a separate soundproof
room, but space does not currently permit that. I am, however, building a lightweight self-standing
screen to place around the collector. This will help muffle the sound and redirect it upward.
I use my shop vacuum as a dust collector for some tools and for general shop cleanup. I am
considering replacing the screaming banshee with a Festool unit based solely and exclusively upon
the stated dB ratings, although I am sure all of the other purported benefits of this unit will be
welcome, as well.
When using any power tool, I now turn off the music. I could not really hear it over most
machines anyway, no matter how much I cranked up the volume, and frankly, less distraction when
using a power tool is probably a good thing.
Hearing protection is not an "all or nothing" proposition, and I have found that disposable foam
earplugs will lessen sound intensity while not entirely eliminating the audible cues I get from the
machine/wood interface sound. For example, I can wear the super quiet headphone-style ear
protectors at the planer and foam earplugs at the table saw.
Just like our first defense against dust is at the source, the first defense against noise is a
well-tuned machine. A smooth-running tool with a sharp blade will always make less noise than a
sloppily maintained machine with a dull blade.
Consider shop environment, too. "Soft" surfaces reflect less sound. Think of wood flooring
rather than tile or concrete, walls of sheet rock instead of hardboard, and if you got rid of those
dusty curtains on the window, maybe reconsider. You can always wash them!
Last, but certainly not least, I now carefully consider the use of any power tool before flipping
the switch. With all due respect to the knickers and puffy shirt-wearing "golden age of
woodworking" throwback crowd, if I have 300 board feet of rough-cut lumber to mill, I will turn on
the power tools, thank you very much. But, by the same token, if I have one quick cut to make,
firing up the table saw and dust collector seems overkill, since I can make the cut more quickly
with a hand saw and a bench hook. And I can still hear the music! A smoothing plane and a scraper
will often leave a better surface than a random orbit sander. These tools whisper instead of scream.
With a single board to plane, I will invariably reach for a jack, smoother, or jointer. Remember,
sound exposure is cumulative, so those quiet moments with a hand plane or scraper help make up for
the hours we have all spent with sanders, planers, and jointers.
There is still much more that can be done. Acoustic engineers could probably tell us how to
arrange our shops and acoustically "tune" them to lessen sound reflections. Tool reviewers should
routinely include sound level measurements in the comparative specifications of all power tools so
we can make informed buying decisions. Manufacturers should consider noise to be one of the safety
concerns they address in the design and engineering process. Harley Davidson and Porsche submit
their vehicles to anechoic sound chambers and fine-tune through engineering to achieve their
distinctive tailpipe sound. Toolmakers should "design in" the less offensive lower frequency sounds
and "engineer out" the more offensive higher frequency sounds. "Tunable" hearing protection is
another option. With today's technology, it is now possible to filter out the loudest and most
offensive of sounds and still permit us to hear important safety auditory cues, voices, etc.
Let me know your strategies for noise reduction and any tricks you have employed. Also let me
know if you find any equipment significantly less noise-offensive than any other. Write to your
favorite reviewers and ask them to routinely take sound level measurements of the equipment they
review. Together we can all look forward to hearing the sweet sounds of music long into our old
age.
Additional Reading, Resources & Interesting Stuff
If you want to experiment with the noise levels in your shop, sound level meters can be purchased
at Radio Shack or Amazon.com for under $40 to well into the $100s. If you have an iPhone, you can
download an App from the iTunes App Store that will convert your iPhone into a serviceable sound
meter for a few dollars.
If you would like to read more about sound and its military application (or rumors thereof) here
are two good places to start:
http://crab.wordpress.com/2008/01/14/a-short-history-of-sound-weapons-pt2-infrasound/
http://www.artesonoro.org/sonicweapons/2008/03/21/acoustic-trauma-bioeffects-of-sound-by-alex-davies
/
For a wealth of workplace safety information, visit NIOSH at
http://www.cdc.gov/niosh/
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