Methods - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 229

Table 9.2 Sound pressure vevel and sound pressure of the most common anthropogenic noises
and level of hearing risk
Sound pressure level (dB) Sound pressure (mPa) Examples

140 200.000.000 Jet engine Harmful condition

130 63.245.555 Jackhammer

120 20.000.000 Airplane

Pain threshold
110

6.324.555 Disco Critical condition

100 2.000.000 Industrial machinery

90 632.455 Heavy truck

80 200.000 Intense traffic Safety condition

70 63.245 Vacuum cleaner

60 20.000 Offices

50 6.324 Low-volume music

40 2.000 Library

30 632 Steps on leaves

20 200 Home at night

10 63 Clock tick-tock

0 20 Hearing threshold

9.3 Digital Representation of Sound

To manage a sound on a digital computer, it must be transformed from an analog
format to a digital format (A/D), which is done by a hardware device called an
analog-to-digital converter or digitizer.
The digitizer usually samples the instantaneous voltage amplitude of a continu-
ously varying analog input signal that results from a microphone output where the
time-varying voltage is proportional to the sound pressure. The digital representa-
tion of a signal produced by a digitizer is represented by a sequence of numeric
values, evenly distributed, that represents the amplitude of the original waveform
(Fig. 9.6 ).
Two parameters are involved in the A/D process: the sampling rate or sampling
frequency (SR) and the number of bits used to represent each amplitude (sample
size or bit depth). Commercial digital audio applications usually utilize sample
rates of 44.1 kHz. The sampling rate at which a signal is stored in a magnetic device
(e.g., AIFF or WAVE format) is saved in the file.
The sampling rate that must be selected to obtain a reasonable digital represen-
tation must be more than twice the highest frequency present in the signal to avoid
producing the phenomenon of aliasing. Aliasing is a phantom frequency that is not
present in the original signal but is the result of an inadequate sample rate (Fig. 9.7 ).
To prevent this effect the signal can be passed to a low-pass filter (anti-aliasing
filter) that removes, if present, every frequency higher than the Nyquist frequency.
The Nyquist frequency is the highest frequency of a digital signal without aliasing
and is equal to half the frequency at which an analog signal is digitized