Editing Acoustics (section)

== Fundamental concepts ==
=== Wave propagation: pressure levels ===
{{Main|Sound pressure}}

[[Image:Oh No Girl Spectrogram 2.jpg|thumb|[[Spectrogram]] of a young girl saying "oh, no"]]
In fluids such as air and water, sound waves propagate as disturbances in the ambient pressure level.  While this disturbance is usually small, it is still noticeable to the human ear.  The smallest sound that a person can hear, known as the [[Absolute threshold of hearing|threshold of hearing]], is nine orders of magnitude smaller than the ambient pressure.  The [[loudness]] of these disturbances is related to the [[sound pressure level]] (SPL) which is measured on a logarithmic scale in decibels.

=== Wave propagation: frequency ===
{{further|Sound#Frequency}}

Physicists and acoustic engineers tend to discuss sound pressure levels in terms of frequencies, partly because this is how our [[ears]] interpret sound. What we experience as "higher pitched" or "lower pitched" sounds are pressure vibrations having a higher or lower number of cycles per second. In a common technique of acoustic measurement, acoustic signals are sampled in time, and then presented in more meaningful forms such as octave bands or time frequency plots. Both of these popular methods are used to analyze sound and better understand the acoustic phenomenon.

The entire spectrum can be divided into three sections:  audio, ultrasonic, and infrasonic.  The audio range falls between 20 [[Hertz|Hz]] and 20,000&nbsp;Hz.  This range is important because its frequencies can be detected by the human ear.  This range has a number of applications, including speech communication and music.  The ultrasonic range refers to the very high frequencies: 20,000&nbsp;Hz and higher.  This range has shorter wavelengths which allow better resolution in imaging technologies.  Medical applications such as [[Medical ultrasonography|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.

Analytic instruments such as the [[spectrum analyzer]] facilitate visualization and measurement of acoustic signals and their properties.  The [[spectrogram]] produced by such an instrument is a graphical display of the time varying pressure level and frequency profiles which give a specific acoustic signal its defining character.

=== Transduction in acoustics ===
[[Image:3.5 Inch Speaker.jpg|thumb|An inexpensive low fidelity 3.5&nbsp;inch '''driver''', typically found in small radios]]
A [[transducer]] is a device for converting one form of energy into another.  In an electroacoustic context, this means converting sound energy into electrical energy (or vice versa). Electroacoustic transducers include [[loudspeaker]]s, [[microphone]]s, [[particle velocity]] sensors, [[hydrophone]]s and [[sonar]] projectors. These devices convert a sound wave to or from an electric signal. The most widely used transduction principles are [[electromagnetism]], [[electrostatics]] and [[piezoelectricity]].

The transducers in most common loudspeakers (e.g. [[woofer]]s and [[tweeter]]s), are electromagnetic  devices that generate waves using a suspended diaphragm driven by an electromagnetic [[voice coil]], sending off pressure waves. [[Electret microphone]]s and [[condenser microphone]]s employ electrostatics—as the sound wave strikes the microphone's diaphragm, it moves and induces a voltage change. The ultrasonic systems used in medical ultrasonography employ piezoelectric transducers. These are made from special ceramics in which mechanical vibrations and electrical fields are interlinked through a property of the material itself.