Geography Reference
In-Depth Information
Fig. 1 Determination of
appropriate size of
visualization cells.
S
is
noise source,
A
the closest
measurement point in the
cell,
B
most distant possible
measurement point in the
cell,
r
cell diameter,
a
distance from noise source
to closest measurement
point,
b
distance from the
noise source to most distant
measurement point.
The
dotted square
and
hexagon
suggest a possible
approximation of ideal
circular-cell shape
suitable. The largest possible size of one cell must be estimated on the basis of
dependence of noise level change with distance and the average speed of a
pedestrian or cycler (volunteer). According to ISO 1996-1:
2003
, to produce reliable
data, it is desirable to have at least three samples per cell from one pedestrian or
cycler pass, preferably five.
If we take into account that the average speed of a pedestrian is about 6 km/h
(ca 1.7 m/s), the distance between the outer boundary of the cell at a standard
sample rate of 1 Hz (
slow
mode) from its center results in about a 5 m cell for
3 samples, or 8.5 m for 5 samples. If the measured noise level at a distance of 15 m
from the source of noise is 72 dB [e.g. current modern passenger car at a speed of
about 90 km/h, according to Finley and Miles (
2007
)], the level measured at a
distance of 20 m from the source (i.e. at the opposite boundary of the cell with a
diameter of 5 m, see Fig.
1
) should be about 70 dB; while with distance of 23.5 m, it
should be about 68 dB. In the first case, difference between these values is within
the accuracy of measurement itself (
5 dB, see section “Accuracy of Noise
Measurement Conducted by Mobile Phone Microphones”) and regular human
perception (
3 dB), in the second case it is just slightly above the threshold of
both accuracy and perception.
If the noise level is 60 dB at a distance of about 3 m from source (vehicle speed
of 30 km/h—a typical noise exposure situation of pedestrians on the sidewalk in a
residential urban area of Europe), noise at 8 m will be about 52 dB. In this case, the
difference between these two extremes in a given cell is about 8 dB. This difference
is already well perceptible by hearing. But the level of 60 dB is also loudness of
normal human walk (see section “Data Aging”) while a noise level of 52 dB is a
typical city noise background value and it is on edge of being distinguishable by a
