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THE 55 dBA NOISE
LEVEL USED BY FERC AS “STANDARD” WAS NEVER INTENDED BY THE EPA TO BE USED AS
SUCH
Noise Quality
The unit of noise measurement is the decibel (dB), which measures
the energy of the noise. Because the human ear is not uniformly sensitive to all
noise frequencies the "A" weighting frequency scale (dBA) was devised
to correspond with the ear’s sensitivity. The A-weighted frequency scale uses specific weighting
of a sound pressure level for the purpose of determining the human response to sound
and the resulting unit of measure is the dBA. Because noise levels can vary over
a given time period, they are further quantified using the Equivalent Sound Level
(Leq) and Day-Night Level (Ldn). The Leq
is an average of the time-varying sound energy for a specified time period. The
Ldn is an average of the time- varying sound energy for one 24-hour period, with
a 10 dB addition to the sound energy for the time period of 22:00 to 07:00 hours.
If the sound energy does not vary with time, the Ldn level will be equal to the
Leq level plus 6.4 dB.
The EPA has identified an Ldn of 55 dBA as being the maximum sound
level that will not adversely affect public health and welfare by interfering with
speech or other activities in outdoor areas, with an adequate margin of safety (EPA,
1971). Commission regulations (18 CFR § 157.206(b)(5)(i)) require that the noise
attributable to any new compressor engine or modification not exceed an Ldn of 55
dBA at the nearest noise sensitive area (NSA) (i.e., schools, hospitals, or residences)
unless such NSAs are established after facility construction. In addition, the Commission
typically requires that the noise attributable to the full load operation of a compressor
station, including the compressor unit addition, should not exceed the previously
existing noise levels produced by the compressor station at any nearby NSA that
are above an Ldn of 55 dBA.
The 55 dBA level
was never meant by EPA to be used as standard
People
who live near gas compressor sites have been complaining for years about the
compressor and other noises coming from these gas compressor stations. In a series of blogs, we will try to explain
to the public how the noise studies done by the gas pipeline owners are biased,
performed by entities that are either owned by the owners of the pipelines or
are doing a lot of business with the pipeline owners (conflict of interest). Furthermore, we will show that none of these “studies”
are peered reviewed, the calibrations of the instruments are never checked by third
parties and these “studies” are basically one-day or so noise measurements that
are very frequently manipulated by the people who do these studies so that the
pipeline owner is found to be in compliance with the noise “standard”. In many cases, one company called Hoover
& Keith, has reported the insect noises as a nuisance; they basically
compared the high frequency noises of the compressors that are very annoying to
the pleasant “noise” of the insects.
This tells you something about these people.
In
all of their submittals to the FERC, these great scientists make the assertions
that the 55 dBA sound pressure is a standard that was developed by EPA in the
1974 document referenced below. This
cannot be further from the truth, as the EPA wrote that the 55 dBA level is not
a standard. Here are some quotes from
the EPA document:
“It
should be born in mind that this Document is published to present information
required by the Noise Control Act, Section 5(a)(2), and that its contents do
not constitute Agency regulations or standards.
Its statistical generalizations should not be applied to a particular
individual. Moreover, States and
localities will approach this information according to their individual needs
and situations.”
“Undue
interference with activity and annoyance will not occur if outdoor levels are
maintained at an energy equivalent of 55 dB and indoor levels at 45 dB.”
“An
Leq(24) of 70 dB is therefore identified for intermittent noise in order to
prevent hearing damage”.
This
basically means that in general, the noise pressure must be maintained at less
than 70 dB over a period of 24 hours to prevent hearing loss. Now, if someone is exposed to great noise
over an 8-hr period during his/her work, then the rest of his noise exposure
must be lowered so that he does not lose his hearing. This situation was also addressed in the EPA
document as follows:
“As
in the industrial situation, eight hours is generally the amount of time spent
in educational facilities. Therefore an
Leq(8) of 75 dB is considered appropriate to protect against hearing loss, so
long as the exposure over the remaining 16 hours is low enough to result in a
negligible contribution to the 24-hour average.”
SOURCE: “Information
on Levels of Environmental Noise Requisite to Protect Public Health and Welfare with an Adequate
Margin of Safety”,
March 1974, US EPA, Office of Noise
Abatement and Control
18 CFR § 157.206(b)(5)(i))
(5)
(i) The noise attributable to any new
compressor station, compression added to an existing station, or any
modification, upgrade or update of an existing station, must not exceed a
day-night level (Ldn) of 55 dBA at any pre-existing noise-sensitive area (such
as schools, hospitals, or residences).
(ii) A compressor facility installed under
this section must be designed to meet the following noise emissions criteria.
For each new compressor station facility, and for each addition or modification
to an existing compression station, the blanket certificate holder must file a
noise survey with the Secretary within 60 days of placing the facility in
service.
(A) If noise emitted from a new
compressor facility operating at full load exceeds an Ldn of 55 dBA at any
noise-sensitive area (NSA), or if an addition or modification to an existing
compressor station operating at full load at or below an Ldn of 55 dBA at NSAs
causes overall noise attributable to the station to exceed an Ldn of 55 dBA at
an NSA, the blanket certificate holder must come into compliance with an Ldn of
55 dBA at NSAs within 1 year of placing the facility in service.
(B) If an addition or modification to an
existing compressor station operating at full load above an Ldn of 55 dBA at
NSAs causes overall noise attributable to the station to increase at an NSA,
the blanket certificate holder must act within 1 year of placing the added or
modified facility in service to reduce noise at NSAs to the level that existed
prior to the addition or modification.
(C) If the initial noise survey demonstrates
a need to take action to mitigate noise, within 60 days of completing such
action, the blanket certificate holder must file a subsequent noise survey with
the Secretary demonstrating that each new compressor station facility, and each
addition or modification to an existing compressor station, complies with the
noise level limits.
(iii) Any horizontal directional drilling
or drilling of wells which will occur between 10 p.m. and 7 a.m. local time
must be conducted with the goal of keeping the perceived noise from the
drilling at any pre-existing noise-sensitive area (such as schools, hospitals,
or residences) at or below a night level (Ln) of 55 dBA.
DEFINITIONS
dBA denotes an “A” -weighted sound level. The ear
has differing sensitivities to different frequencies of sound, and the
“A”-weighting scale was developed to correspond to the relative frequency
response of the human ear. The ear is relatively insensitive to low frequency
sounds compared to mid-and high frequencies as is refl ected in the
“A”-weighting corrections shown below. The “A” -weighted sound level has proven
to be a good indicator of community annoyance to noise and is therefore often
used in design criteria and community noise standards.
Leq is the energy sound level. It is the sound
level of a steady-state sound that has the same (”equivalent”) total energy as
the time-varying sound measured over a specifi c period of time. Leq is a
single-valued level that expresses the time-averaged total energy of a varying
or fl uctuating sound level and is an “A”- weighted level.
Ldn is the day-night sound level, which is
calculated from the following equation:
Ldn =
10 Log 1/24 [15x10(Ld/10) +9x10((Ln+10)/10) ]
Where Ld is the Leq for the 15 daytime hours
between 7 A.M. and 10 P.M. and Ln is the Leq for the 9 nighttime hours between
10 P.M. and 7 A.M. This equation adds a 10 dB penalty to the nighttime Leq to
account for the fact that noise is more disturbing at residential locations
during the typically quieter nighttime hours. Most federal agencies including
the Environmental Protection Agency, Housing and Urban Development and Federal
Energy Regulatory Commission use the Ldn measure for their criteria; in
general, an Ldn of 55 dBA is considered acceptable for residential areas.
Acoustic Efficiency or Conversion Efficiency: The ratio of acoustic
power to mechanical input power, expressed as an efficiency of conversion of mechanical to acoustical power.
A-Weighting Scale: The frequency weighting scale for noise measurements which weights the frequencies near the human hearing
response frequency range most heavily.
Cutoff Frequency: The lowest frequency (first acoustic mode) for sound wave propagation across the diameter
of the pipe within the fluid. Cross modes cannot propagate below the fundamental first mode of the cutoff frequency. The cutoff frequency
is defined as the speed of sound in the gas/fluid
within the pipe divided by the product of 2*pipe diameter, f = SOS / (2D). Compressible gas flows tend
to have much “louder” piping systems,
due to the high potential for broadband noise and the relatively good conversion of mechanical to acoustic
energy. This relationship between the cutoff frequency and the ring frequency effectively defines a frequency band of high noise transmission, where the transmission loss through the pipe will be relatively low.
Decibel: A measure of the amount of energy in an acoustic signal, expressed
as a ratio compared to a certain baseline
reference measurement depending
on the use of the A, B, C, or D decibel scale. The mathematical formulation of the dB is as the common
logarithm of the ratio of the measured
sound pressure to that of a signal that is barely audible.
Pure Tone: A sound for which a waveform can be represented by a sine wave at a single frequency.
Radiation Ratio: The amount
of energy radiated in the form of pressure
fluctuations based on the amount of pipe wall acceleration for a given pipe
Ring Frequency: The frequency for which the longitudinal wavelength is equal to the pipe circumference, to define the upper limit for noise transmission within the pipe material. This is the upper limit for transmission of sound through the pipe wall, beyond which transmission is minimal.
The ring frequency is defined as the speed of sound in the pipe wall material divided by the product
of *pipe diameter, , fr = cL / (D).
Sound Intensity: The continuous flow of power carried
by a sound wave through
an area in space, commonly expressed
in Watts per square meter. Sound intensity
is directional associated with the direction
of the sound wave compared
to the plane in space.
Sound Power: The amount of power radiated by a sound source or the energy radiated
per unit time by a sound source,
typically defined
by a spherical
boundary surrounding the sound source.
Sound Power Level (Lw, dB): A decibel
expression of the sound power. The reference
sound power is 10-12 watts.
Sound Pressure: The variation in pressure above and below atmospheric pressure, normally
expressed as root-mean-square (rms) value of the time averaged
fluctuations. This is a direct
measure of the pressure
variation.
Sound Pressure Level (Lp, dB): A decibel expression of the sound pressure.
The reference sound pressure
is 2 x10-5 Pa, which
is the threshold of human hearing
at 1,000 Hz.
Transmission Loss:
The difference between the incident
and transmitted sound power levels for the candidate
noise control option. Similar to Insertion Loss,
but in some cases introduction of the noise control option
actually increases
the sound power incident on itself.
