Methods - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 220

Every sound can be distinguished according to these different properties.

• Frequency
• Pitch
• Period
• Wavelength
• Sound speed
• Wavenumber
• Amplitude
• Sound Pressure
• Sound pressure level (SPL)
• Sound power
• Sound power level
• Sound intensity
• Sound intensity level
• Sound exposure level
• Loudness
• Acoustic impedance

9.2.1.1 Frequency

Frequency ( f ) is the number of occurrences of a repeating event per unit time and is
the inverse of the period T where f 1/T and is measured in hertz (Hz) or in cycles

per second (CPS). For instance, if we have 50 events that occur in 25 s the
frequency f

¼

2 Hz.
The hertz (Hz) is a measure of the frequency of sounds expressed in cycles per
second for a regular phenomenon (Fig. 9.1 ). This measure can be applied to every
cyclic phenomenon, such as heartbeat (an infant heart has 120 beats per minute,
which equals 2 Hz; an adult heart has 70 beats per minute, equal to approximately
1.17 Hz). The commonest frequency ranges in acoustics are abbreviated as kilo-
hertz (kHz) (10 3 Hz), megahertz (MHz) (10 6 Hz), and gigahertz (GHz) (10 9 Hz.)
Light, as an electromagnetic radiation, has a frequency ranging from tens
(infrared) to thousands (ultraviolet) of terahertz (10 12 Hz). Radiofrequency radia-
tion is usually measured in kilohertz (kHz), megahertz (MHz), or gigahertz (GHz).
Humans perceive sound frequency as pitch (a subjective perception of an
objective frequency), with differences according to age: infants have a full range
of perception from 20 to 20,000 Hz, adult humans can hear sounds between 20 and
16,000 Hz, and in the aged human the high frequencies are dramatically reduced.
We consider infrasound and ultrasound sounds those with frequencies below and
above human audibility. For instance, whales and other marine mammals are
sensitive to infrared frequency from 20 Hz down to 0.001 Hz. This type of energy
covers long distances and passes over obstacles with little dissipation. This fre-
quency range is generally used for monitoring earthquakes and in oil prospecting.
The relationship between frequency and wavelength is represented by the
equation

¼ 50/25 ¼

λ ¼ c / f , where c ¼ sound speed and λ is wavelength.