Acoustics is the interdisciplinary science that deals
with the study of all mechanical waves in gases,
liquids, and solids including vibration, sound,
ultrasound and infrasound. The word "acoustic" is
derived from the Greek word ἀκουστικός
(akoustikos), meaning "of or for hearing, ready to
hear". The scientific study of the propagation,
absorption, and reflection of sound waves is called
Sound, ultrasound, infrasound
The entire spectrum can be divided into three sections: audio,
ultrasonic, and infrasonic.
Sound is a mechanical wave that is an oscillation of pressure
transmitted through a solid, liquid, or gas, composed of
frequencies within the range of hearing and of a level
sufficiently strong to be heard.
The audio range falls between 20 Hz and 20,000 Hz. The
ultrasonic range refers to the very high frequencies: 20,000 Hz
and higher. This range has shorter wavelengths which allows
better resolution in imaging technologies. Medical applications
such as ultrasonography and elastography rely on the ultrasonic
frequency range.
On the other end of the spectrum, the lowest frequencies are
known as the infrasonic range. These frequencies can be used to
study geological phenomena such as earthquakes.
Physics of sound
The mechanical vibrations that can be interpreted as
sound are able to travel through all forms of matter: gases,
liquids, solids, and plasmas. The matter that supports the
sound is called the medium. Sound cannot travel through
a vacuum. Sound is transmitted through gases, plasma,
and liquids as longitudinal waves, also called compression
waves. Through solids, however, it can be transmitted as
both longitudinal waves and transverse waves.
Longitudinal sound waves are waves of alternating
pressure deviations from the equilibrium pressure, causing
local regions of compression and rarefaction.
Matter in the medium is periodically displaced by a sound
wave, and thus oscillates.
Sound wave properties and characteristics
A musical tone is a steady periodic sound. A simple tone,
or pure tone, has a sinusoidal waveform.
A compound tone is any musical tone that is not
sinusoidal, but is periodic, such that it can be described as
a sum of simple tones with harmonically related
A noise results from non-periodic vibrations (statistically
random vibration)
Objective sound wave characteristics
Frequency, or its inverse, the period
Harmonic Content (Harmonic spectrum)
Speed (velocity vector)
Direction (wave vector)
Harmonic Content (Harmonic spectrum)
Sound Intensity. Sound Pressure
Threshold of Hearing. Threshold of Pain
Subjective sound perception
• Sounds may be generally characterized by
pitch, loudness, and quality.
Equal Loudness Curves
Ear and Hearing
The Outer Ear
The Tympanic Membrane
"Rule of Thumb" for Loudness
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