Methods - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 238

• Various features of a particular noise, generally identified as tonality, impulsive-
ness, modulation and/or low-frequency components, can increase its potential
interference. These features tend to increase the information content of the
particular noise and thus raise its audibility. The audibility of such a noise is
also influenced by the level and characteristics of other ambient noise in the area.
• Non-acoustic factors that can influence the subjective audibility and annoyance
potential of a particular noise include the subjective value of the activity
generating the noise and the effectiveness or otherwise of any attempt to control
the noise.
• Adaptation to the noise, past experience, individual differences, and personality
can influence the response of a particular listener.

Only a few measures that process this information are reported in this topic
because most of the metrics are utilized in industrial and civil engineering and could
be of less interest for our readers.
Considering that this approach focuses on the effect of noise on human life,
however, it is necessary to add some details before the description of the metrics.
The human ear has different sensitivity according to frequency range: the major
sensitivity is between 500 Hz and 8 kHz, with less sensitivity to very low and very
high frequencies. To account for this physiological constraint, the recorded sound
pressure level (SPL) is corrected accordingly with an A-Weighting , C-Weighting ,or
Z-Weighting procedure to compensate for the nonlinear hearing.

A-Weighting is the most common weighting used in noise measurement. As does
the human ear, this method cuts off the lower and higher frequencies that the
average person cannot hear. The dB is represented as dBA or dB(A).
C-Weighting. At the higher levels sounds produce a flat response in the human ear,
and a measurement considering this limitation may be useful for specific
applications. The dB is represented as dBC or dB(C).
Z-Weighting . In this case, the measurement is independent of frequency perception.

9.7.1.1 Equivalent Continuous Noise Level

The equivalent continuous noise level ( L eq, T ) metric stores the amount of sonic
energy produced by the environment for a specific time lag and then produces an
averaged value (Fig. 9.15 ).

Z T

p 2 ðÞ

p 0

t

1
T

L eq, T ¼ 10 log 10 d t

tT

where
T t 1 is the time interval considered

p 2 ( t ) is the squared instant pressure
p 0 is the square of reference pressure
When the L eq is averaged, the frequency is indicated as LA eq ,LC eq .

¼ t 2