Methods - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 226

attribute of auditory sensation in terms of which sounds can be ordered on a scale
extending from quiet to loud.”
Loudness is a subjective measure of sound strength and must not be confused
with objective measures of sound energy such as the sound pressure, the sound
intensity, or the sound power. The A-weighting method to measure sound power is
an attempt to adjust sound measurement to human acoustic perception. Loudness
depends on either sound pressure or sound duration. Actually, the human hearing
system averages the effect of sound pressure level (SPL) over an interval of
600-1,000 ms. A sound with a constant SPL seems to increase in loudness after a
duration of 20, 50, 100, or 200 ms up to the duration of 1 s; after that, loudness is
stabilized. For sounds persisting longer than 1 s the loudness is fixed after an
averaging of 600-1,000 ms.

9.2.1.16 Acoustic Impedance

Acoustic impedance indicates the part of sound pressure generated by the vibration
of molecules of a specific medium at a given frequency. The acoustic impedance
Z (or sound impedance) is frequency ( f ) dependent and is very useful for describing
the behavior of musical wind instruments, for example.
The acoustic impedance is calculated as Z

¼ P/vS where P ¼ sound pressure,

v particle velocity, and S is the surface through which an acoustic wave of

frequency f propagates. The specific acoustic impedance Z (also called shock
impedance) is the ratio of sound pressure P to particle velocity v at a single
frequency and is expressed in rayls (Rayleighs).

¼

9.2.2 The Decibel

In acoustics, the decibel is an important unit of measure of the sound power.
The unit dB, which is one tenth of a bel, is a logarithmic (to base 10) unit used to
measure a ratio that may be represented by power, sound pressure, voltage, or
intensity.
The equation that describes

the difference in decibels between two

measurements is

10 log X 2
X 1

Δ X dB ðÞ¼

where X is the difference between two quantities expressed in dB. Considering

that the bel unit is transformed for better use as a decibel (1/10 bel), in the
logarithmic formula it is sufficient to multiply the logarithmic ratio by 10. In
acoustics the sound intensity that is measured by microphones is generally
expressed as variation of sound pressure. To do this a microphone registers the

Δ