The Basics of Noise-2013/03

During engine run-up the pilot is exposed to signi cant short-term noise levels. Although seldom used, hearing protection is a
good idea, even with limited exposure. Sound levels at the pilot’s position during a flight are typically 75 dBA or less.
There is no handy library or compilation of sound data for RC aircra , but the author compiled a
chart that shows that the typical sound levels for various types of RC aircra , at a reference distance
of 100 feet.
Noise is one of the major issues
threatening RC airfi elds and
many other recreational activities.
What you consider to be a pleasant
sound from a glow engine at 17,000
rpm can be an unbearable racket for the
neighbors who do not share your passion
for RC fl ight. Part of the defi nition of
noise is, “sound that is unwanted or
undesired.”
The sound factor draws many into RC
aircraft fl ying—a big part of the fun is
the engine noise. Some hobbyists avoid
electric-powered airplanes because of
the lack of engine sounds.
I choose to ride a large Honda touring
motorcycle which is no louder than
a typical car. My neighbor down the
street loves his Harley, complete with
loud aftermarket exhaust pipes. For
him, the noise is an integral part of the
experience. The neighborhood is not as
enamored, especially at 7 a.m.
The Basics of Noise
Our eardrums sense tiny fl uctuations
in air pressure, which we interpret as
sound. The decibel scale is used for
expressing sound levels because we do
not perceive loudness in a linear fashion.
We detect loudness in a logarithmic
way, similar to the Richter scale for
earthquakes.
Although an increase of 3 decibels
(dB) doubles the sound energy or
amplitude, it takes a change of 10 dB to
be judged as twice as loud. Each similar
decrease of 10 dB is considered to be
half as loud as the original sound.
Changes of 1 dB are not perceptible.
A 3 dB change can be heard by a critical
listener under ideal conditions. A 5 dB
change is normally the threshold where
a difference is readily noticeable, either
up or down.
A-weighted decibels (dBA) are
commonly used to measure sound levels.
The A-weighted scale deemphasizes low
frequencies to directly compare loudness
from different sounds. Theoretically, a
diesel locomotive measuring 80 dBA
has the same loudness as a cymbal at 80
dBA, although the frequency content is
much different.
Our ears and hearing system are
constructed to put more emphasis on
high-frequency sound than low tones.
It is important to hear a twig snap
(high-frequency) from behind as the
grizzly bear stalks you. Low-frequency
rumble from distant thunder miles
away is less critical. Most of the
information content from speech is in
the middle and higher frequencies, 500
Hz to 2000 Hz.
The time of day, duration, and
variability of sound affects the
annoyance factor. Steady sounds are less
annoying and more easily accepted than
a varying sound. Sounds with pure tones
or major fl uctuations in level are more
noticeable, which is why those sounds
are used for alarms and sirens.
In the case of RC airplanes, the sound
levels change throughout the fl ight.
The variability makes the noise more
noticeable and annoying to the public.
Two common ways to express sound
levels are the equivalent continuous
noise level (Leq) and the momentary
maximum level (Lmax). The Leq
is the summed total of the sound
energy occurring throughout the
event or a time period. Although not
mathematically precise, Leq can be
thought of as the average sound level.
The Lmax is the loudest noise that
occurs, if only for a second or two.
It takes a more sophisticated sound
meter to measure Leq and calculate the
running average. A decent sound meter
of reasonable accuracy will cost at least
$300. An integrating sound meter that
can directly measure Leq typically costs
$1,500 or more.
Noise Limits
There is no magic number to make
everyone happy and avoid annoying
or offending others. There also is not
a defi nite dB value where everyone
agrees that noise becomes a problem.
Because humans are involved, there is
always a certain amount of subjectivity
and variability.
The U.S. Army has noise criteria
for military firing ranges that define
acceptable noise levels when up to 15%
of the nearby population is annoyed.
The Occupational Safety and Health
Administration allows a workplace level
of 90 dBA for an 8-hour shift, without
running an undue risk of hearing damage
throughout a career.
Not all jurisdictions have a noise
ordinance. Even if a city or county has
an ordinance, it may be flawed and
incomplete. I formerly lived in a small
city that has an ordinance adopted in
the 1950s. It lists octave bands (different
component frequencies rather than the
overall dBA value) that have not been
used since the early 1960s.
In my 28 years as a full-time acoustical
engineer, I have never seen a sound
meter that can measure the octave bands
that are listed. If the text of an ordinance
has technical flaws you are usually stuck
with them. A knowledgeable expert
can sometimes make a case for different
interpretations, but the letter of the
law is usually what counts. Often, the
government staffers do not understand
their own noise ordinance.
Many noise ordinances will express
noise limits as either Leq values or Lmax
values. In the case of Leq, the wording
is usually something similar to: “… noise
level shall not exceed Leq 60 dBA for
any given daytime hour.”
Another approach is listing the
maximum permissible level (Lmax)
regardless of duration such as, “... noise
shall not exceed 70 dBA at a residential
property or property line at any time.”
Noise limits are typically 10 dB lower
between 10 p.m. and 7 a.m.
Another approach to noise limits is
ambient plus five dB. In this case, the
average existing background or ambient
noise in the area is measured without
the noise in question. While RC aircraft
are flying, another set of sound levels are
observed. The two levels cannot differ
by more than 5 decibels.
If the background noise is Leq 59
dBA for example, the noise when RC
airplanes are flying cannot exceed Leq
64 dBA. This option is helpful in areas
that are already noisy.
If the stated limit is 60 dBA but the
highway traffic is 64 dBA, you cannot
easily demonstrate compliance with the
lower limit. The ambient adjustment can
be a double-edged sword. If the area is
quiet and the background noise averages
40 dBA, then your noise limit becomes
45 dBA.
Some ordinances do not have any
numerical limits. They simply use vague
language such as, “… shall not create
a nuisance at nearby residences” or
similar phrasing. But what constitutes a
nuisance? One’s sweet sound is another’s
awful racket. A numerical decibel limit
is more understandable and less open to
interpretation, and more defensible in
court should it come to that.
Noise-Control Techniques
Noise control works on a source-pathreceiver
model. Noise can be reduced
at the source (using mufflers, quieter
engines, electric motors), along the path
of propagation (distance, barriers), or at
the receiver (enclosures, better windows,
earplugs).
In the case of RC aircraft, the noise
source is elevated and moving, so an
enclosure or a property-line noise barrier
is typically not an option. Good luck
getting the neighbors to wear earplugs
because you want to fly!
The available options usually fall into
a few categories:
• Limits on aircraft type (electric
powered versus glow powered, or a ban
on turbines).
• Restricted hours for the field
(typically not after 9 p.m. or before 10
a.m.).
• Noise limits on the aircraft
(requiring mufflers or baffles, or a limit
on engine size).
The Leq value shown on the chart
is for approximately 3 to 5 minutes
of a given flight. Lmax is the highest
momentary level that was observed
during a high-speed pass or a takeoff. It
is complicated to extrapolate a 5-minute
Leq out during an entire hour, as is often
required when assessing compliance
with a local noise ordinance.
That calculation is well beyond a
simple explanation. A 5-minute Leq
taken during a flight will be higher than
the same sounds averaged for an hour. If
the limit is Leq 60 for an hour, and you
measure Leq 65 during each of several
5 to 10-minute flights that hour, the
overall average will likely comply.
There were some surprises in this
data. Turbine aircraft were expected to
be louder than glow engines, but the
difference was only 3 to 4 dBA. Some
glow-powered propeller airplanes
were louder than the smaller turbine
airplanes. A level that is 3 decibels
higher is not dramatically louder. Jets
with turbines rated from P-60 to PT-160
were measured.
The electric helicopters were slightly
quieter than the nitro-powered helis
(600 to 800 series, or rotor diameters
of 600mm to 800mm). Most of the
noise comes from the rotor disc during
aggressive maneuvers, not the engine.
The loudest propeller airplane
measured was a 40% Scale, gaspowered
aerobatic model with a 160cc
four-stroke gas engine. On average,
its noise was Leq 70 throughout the
6-minute routine, but the peak was
higher than other airplanes (90 dBA
during a high-speed pass down the
runway). Smaller gas- or glow-powered
aircraft were in the 72 to 86 dBA range
for their momentary maximum levels,
depending on aircraft size and the
power setting.
It is important to note that any sound
data must include a distance. If someone
says, “My airplane makes 92 dBA,” we
need to know at what distance. It makes
a huge difference whether that 92
dBA level occurs at 5 feet (glow engine
during run-up) or at 500 feet (Boeing
747 during takeoff).
Distance is typically beneficial. Sound
dissipates at roughly 6 dB per doubling
of the distance, according to the inversesquare
law. A level of 90 dBA measured
at 100 feet becomes 84 dBA at 200 feet,
78 dBA at 400 feet, and so forth.
Most are surprised to learn that
trees and vegetation do not make a
big difference in the sound levels at
distances. A row of trees around the
field will not lower sound levels at a
nearby home.
A mature forest that is 100 yards
wide will have roughly a 5 dB net
noise reduction value compared to
an open, grassy field. Five decibels
is slightly noticeable, so the benefit
gained by a couple of rows of thinly
spaced trees is negligible. There
can be an out of sight, out of mind
psychological benefit, but nothing that
sound meters can quantify.
Conclusion
The RC hobby can be noisy,
particularly with fuel-powered
aircraft. Most clubs realize that they
need to be good neighbors and go
to reasonable lengths (or sometimes
well beyond reasonable) to protect
the hobby and keep using their
airfields.
If your club or flying field is
threatened by community complaints
or enforcement action, it is worthwhile
to get advice from someone who
is competent in noise assessment.
These experts are usually listed under
acoustical consultants in the telephone
book.
Navigating the maze of noise
regulations, zoning requirements,
permits, and neighborhood opposition
without some expert guidance is
unwise. It’s similar to representing
yourself in court or diagnosing your
own illness.
—Earl Mullins, professional engineer
[email protected]
ABOUT THE AUTHOR
Earl Mullins has been an
acoustical consultant for 28 years,
and has a mechanical engineering
degree. He has studied and
evaluated noise from a wide variety
of sources, including major airports
and a few RC airfields. He has
expertise in writing and interpreting
noise ordinances—addressing a
variety of specific problems and
community complaints. He also
routinely develops noise-control
measures to meet specific criteria.
Earl learned to fly in Alaska and
spent 14 years doing remote-area
flying, including search-and-rescue
missions. He no longer actively flies
full-scale aircraft.
He recently became an AMA
member and is learning to fly
several different electric-powered
trainer airplanes.