Two chains of reasoning to test the report,
each starting from opposite ends of a chain of inference.
Noise measurements do not confirm the report. |
Assume the report is correct,
then infer noise level at a site directly below the ILS approach
Compare this noise to measured noise. |
|
Assume that noise measurements taken below the ILS approach
are correct and derive noise level perceived by the reporter.
Compare this with research results measuring noise
level required to awaken sleepers.
|
| | We
assume that the noise level at the point of closest approach to the
observer's home as the noise level measured at EDH's west ridge. Actual
noise level will be somewhat less because crossing altitude above
ground level is higher. Approximate crossing altitudes are:
West ridge: 3,900 ft msl 2,800 ft agl
East ridge: 4,500 ft msl 3,300 ft agl
"msl" is "mean sea level" altitude; "agl" is "above ground level". |
Noise reported to be perceived by the observer is sufficient to
wake him up.
Therefore the noise level at the observer's eardrums is at least 55 dB. |
|
Noise levels from freighter approaches over El Dorado Hills at a
measurement site directly under the ILS approach path on the western
ridge showed an average of Lmax = 65 dB in 2004 tests, 64 dB for
non-CDA approaches in 2006 tests, and 61.2 dB for CDA approaches in
2006 tests.
Therefore average noise at the east ridge directly under the ILS approach path is at most 65 dB.
|
Measured sound attenuation by earplugs averages about 30 dB. We
assume that the observer's earplugs provide this level of attenuation.
(Research indicates that if actual attenuation differs it is most
likely to be within ± 5 dB of this average.)
Therefore the noise level within the observer's room is at least 85 dB.
|
|
The
point on the east ridge directly under the ILS has a (vertical) minimum
slant distance of about 3,300 feet. Slant distance to the observer's
home is about 7,800 feet. Noise level
for sound propagating in free air drops 6 dB when its distance traveled
doubles.
Noise
level for sound propagating in free air drops 6 dB when its distance
traveled doubles. Therefore noise outside the observer's home is 7 dB
quieter than noise directly under the ILS approach path.
Therefore average maximum noise outside the observer's home is at most 58 dB. |
Noise level from an outside source, measured inside a
structure with windows open, averages about 15 dB less than the
exterior noise level.
Therefore the outside noise level at the observer's home is at least 100 dB. | | Noise level from an outside source, measured inside a
structure with windows open, averages about 15 dB less than the
exterior noise level.
Therefore the noise level inside the observer's room is at most 43 dB. |
At
the point of closest approach to the observer's site freighters on the
ILS glide slope cross the eastern ridge of El Dorado Hills at an
altitude of approximately 4,500 feet msl. Slant distance to a point
directly below the aircraft at this point is about 3,300 feet. Slant
distance to the observer's home is about 7,800 feet.
Noise level
for sound propagating in free air drops 6 dB when its distance traveled
doubles. Therefore noise directly under the ILS approach path is 7 dB
louder than at the observer's home:
Therefore noise level on the ground
directly under the ILS approach is at least 107 dB. | | Measured sound attenuation by earplugs averages about 30 dB. We
assume that the observer's earplugs provide this level of attenuation.
Therefore the average noise level at the observer's eardrums is at most 13 dB. |
|
Summary based on observer's report
Noise at the observer's ear is at least 55 dB.
Noise directly under the ILS approach is at least 107 dB.
Noise alone causes observer to awaken. | | Summary based on noise measurements
Noise at the observer's ear is at most 13 dB.
Noise directly under the ILS approach is at most 65 dB
Noise alone cannot cause observer to awaken. |
How can we account for the 52-dB discrepancy between these two lines of evidence? - The
noise measurements forming the basis cannot be dramatically incorrect.
Measuring equipment is calibrated at least at the beginning and end of
each measuring session. Federal standards tolerate a maximum
calibration error of 0.5 dB.
- There could be
quantitative issues in parts of the science involved in each chain of
inference. The area most susceptible to the state of knowledge not yet
being fully clear is understanding of human sleep states and their
range of sensitivity to noise among a wide range of individuals.
However, this has been investigated long enough that metrics in this
area appear consistent within approximately 10 dB and the chains of
inference above are deliberately biased to award reasonable margins of
error to favor credibility of the observer's report. The actual
discrepancy between the two assumptions could be as high as about 65 dB.
- If
the observer's statement is correct, then at least several hundred
people in El Dorado Hills are exposed to an average flyby maximum noise
level of at least 107 dB. Complaints from several hundred residents
would support this possibility, but that incidence of complaints is not
present. In contrast, many residents living under or near the ILS
approach path are reporting not having a noise problem.
- Coincidence could possibly be a factor, especially if the reporter has any of several types of sleep disorders. Research
involving large scale field studies found examples of individuals'
sleep being disrupted for reasons independent of aircraft noise at a
time when an aircraft operation occurred. This was more frequent in the
British study, but this involved people living closer to airports and a
much higher rate of night operations than occurs with Mather freighter
approaches.
It is not at all
clear how to validate this report by an individual, of being awakened
while using earplugs within a house with open windows, 1.3 miles from
the ILS approach course. |
