The Comfort Zone
by Maury Tiernan
Geary Pa cific Corporation
No. 13
September, 1997
Page 1 of 3
The Comfort Zone column appears regularly in the Modular Building Institute’s Commercial Modular Construction Magazine
IS IT MUSIC TO YOUR EARS OR JUST NOISE ?
All too often in our modern society sounds annoy us. Many sounds are unpleasant or unwanted. These we call
noise. However, the level of annoyance depends not only on the quality of the sound, but also our attitude
toward it. The sound of the air conditioner on a newly installed portable classroom may be music to the ears of
the Wall Mount® salesman, but when installed with a direct return, the teachers complain of the noise.
Noise measurements are specific quantities, which describe and rate sound qualities. When applied to
planning and design processes, they can improve the quality of our lives in many ways. Sound measurements
can help us improve building acoustics and the quality of speaker systems, leading to greater music enjoyment
in the concert hall and at home. Measurement and analysis of sound are also powerful diagnostic tools in
facilities noise reduction programs anywhere from airports to factories, homes and schools. Noise measurement
is a tool that can help to enrich the quality of learning and many other aspects of our lives.
In researching this subject, I learned that sound is a pressure variation in any medium (air, water, etc.) that the
human ear can detect. The number of pressure variations per second is called the frequency of the sound and is
measured in hertz. The frequency of a sound produces its distinctive tone. Thus the rumble of distant thunder
has a low frequency, while a whistle has a high frequency.
Sound travels through any medium at a specific speed. You probably already know the familiar rule for
determining how far away a thunder storm is: count five seconds per mile from the time you see the lighting
until you hear the thunder. We know this because sound travels through the air at 770 miles per hour.
Sound travels in a straight line from the point of origin, and can bounce (reflect) off surfaces from that point.
Sound in air can be compared to ripples on a pond. Toss a rock in the middle of the pond and ripples spread out
evenly from the center creating circles. Drop the rock near the shore, and the ripples bounce off of the shore
creating another group of ripples that intersect and affect the original group. We’ll get back to this point in a bit.
Right now let’s get back to our discussion of sound measurement.
To measure loudness, (or sound volume,) people generally use the Pascal scale or the dB (decibel) scale. The
most popular of the two is the dB scale. It gives a much better approximation to the human perception of
relative loudness than the Pascal scale. Generally air conditioners, 5 ton or less, generate approximately 75
dB(A) outside. This does not vary widely by type (roof mount, wall mount, pad mount, etc.). I have heard of
some local building codes requiring HVAC units to be no louder than 55 dB(A) outside. This may be desirable,
but unrealistic. It is less than the noise inside of an average business office. Note these commonly encountered
dB(A) examples:
Geary Pacific Corporation, 1908 N. Enterprise Street, Orange, CA 92865-4102. (800) 444-3279, Fax: (714) 279-2940
The Comfort Zone
No. 13
by Maury Tiernan
Geary Pa cific Corporation
September, 1997
Page 2 of 3
Noise Volume Ratings on the dB(A) Scale:
Rustling of trees
Library
Business office
Semi truck
Pneumatic concrete hammer
Jet take off
The threshold of pain for a
human ear
15 dB
35 dB
65 dB
90 dB
105 dB
125 dB
140 dB
When working with noise measurements, we also need to consider where that 55 dB(A) of sound occurs. . . . 5
feet off the ground, 15 feet away, next to the birdbath, or inside an occupant's office ? If you had 55 dB(A) of
sound emanating from your classroom wall mount today, and the school district sets a new classroom nearby,
creating another hard surface for the sound to bounce sound off of, that 55 dB(A) measurement will change.
(Remember how sound waves move like ripples in the water near the edge of a pond. As they bounce back we
hear them again as increased volume.) That increase in dB(A) is something you have no control over, yet it’s
something you may become responsible for.
Not only are there building codes specifying maximum noise ratings outdoors, some school districts want to
specify a low 35 dB(A) inside the classroom. I have personally tested some classrooms and found that the meter
registered 42 dB(A) when the HVAC unit was off, no children were present, the lights were off (ballast noise)
and 6 a.m. for quiet outside conditions. Understanding sound measurements can help you recognize potential
challenges like this one.
More and more, as an industry, we are being asked to address environmental issues like noise management,
indoor air quality, and energy management. Our industry estimators will have to be knowledgeable about all of
these issues to avoid painful charge-backs or litigations down the line. If the customer calls out for NC35 we
know what the wall is constructed of, but what if they specify 35 dB(A) in the room ? Meeting this spec
involves the choice of HVAC unit and return duct method, the wall construction, floor covering, window type,
light ballast type, etc.
Our customers continue to look to us to solve the ever-encroaching environmental problems associated with our
buildings. It all starts with the specifications at the bid stage. The customer's specifier controls the final
product. In some cases that specifier may even be you. I recommend that you give the customer options on
your bid. Consider including these options:
●● A ducted return HVAC system is quieter than a direct return.
●● Some duct system designs are inherently quieter than others.
●● A bar type register is quieter than a stamped face type.
●● Carpet is quieter than tile flooring.
●● Sound-absorbing material on the walls is quieter than standard paneling
●● Double or triple glaze 1/4" glass is quieter than standard.
(The same goes for sound level impact of lighting, etc.)
Geary Pacific Corporation, 1908 N. Enterprise Street, Orange, CA 92865-4102. (800) 444-3279, Fax: (714) 279-2940
The Comfort Zone
by Maury Tiernan
Geary Pa cific Corporation
No. 13
September, 1997
Page 3 of 3
I’d like to toss into this discussion a few other thoughts for consideration in noise reduction outdoors. It may
help to put up sound absorbing screens, plant trees, face the HVAC units away from the residential area, avoid
roof mount HVAC units (a second story noise source) in residential apartment dwelling areas, add or increase
the height of fencing with rough (sound-absorbing) surfaces, etc.
One last note if you have a noise problem; make sure you get a qualified acoustics technician to do the test.
They know how to test background and other environmental noises on the project. A sound recording device
used improperly can produce results you will not like.
As I finish this up, the dog is barking, the kids are yelling, and my neighbor is using a chain saw (I
hope on a tree). I wish you a quieter time the next time we meet in . . . The Comfort Zone.
Geary Pacific Corporation, 1908 N. Enterprise Street, Orange, CA 92865-4102. (800) 444-3279, Fax: (714) 279-2940