Amplitude is the magnitude of change in the oscillating variable, with each
oscillation, within an oscillating system. For instance, sound wavesare oscillations in atmospheric pressureand their amplitudes are proportional to the change in pressureduring one oscillation. If a graph of the system is drawn with the oscillating variable as the vertical axis and time as the horizontal axis then the amplitude may be measured as the vertical distance between points on the curve.
Concepts of amplitude
Peak-to-peak amplitude is the measure of the change between peak and trough. Peak-to-peak amplitudes can be measured by meters with appropriate circuitry, or by viewing the waveform on an
oscilloscope. Semi-amplitude in fields such as astronomyis understood to mean half the peak-to-peak amplitude.Tatum, J. B. " [http://orca.phys.uvic.ca/~tatum/celmechs/celm18.pdf Physics - Celestial Mechanics] ." Paragraph 18.2.12. 2007. Retrieved 2008-08-22]
Some scientists [Regents of the
University of California. " [http://cse.ssl.berkeley.edu/light/measure_amp.html#measure4 Universe of Light: What is the Amplitude of a Wave?] " 1996. Retrieved 2008-08-22] use "amplitude" or "peak amplitude" to mean semi-amplitude, namely half the peak to peak amplitude.
Root mean square amplitude
Root mean square(RMS) amplitude is used especially in electrical engineering: the RMS is defined as the square rootof the meanover time of the square of the vertical distance of the graph from the rest state. [Department of Communicative Disorders University of Wisconsin-Madison. " [http://www.comdis.wisc.edu/vcd202/rms.html RMS Amplitude] ". Retrieved 2008-08-22]
Ambiguity of amplitude
The use of peak amplitude is simple and unambiguous for symmetric, periodic waves, like a
sinewave, a square wave, or a triangular wave. For an asymmetric wave (periodic pulses in one direction, for example), the peak amplitude becomes ambiguous because the value obtained is different depending on whether the maximum positive signal is measured relative to the mean, the maximum negative signal is measured relative to the mean, or the maximum positive signal is measured relative the maximum negative signal (the peak-to-peak amplitude) and then divided by two.
For complex waveforms, especially non-repeating signals like noise, the RMS amplitude is usually used because it is unambiguous and because it has physical significance. For example, the average power transmitted by an acoustic or
electromagnetic waveor by an electrical signal is proportional to the square of the RMS amplitude (and not, in general, to the square of the peak amplitude).
When dealing with
alternating currentelectrical power it is universal to specify RMS values of a sinusoidal waveform. It is important to recognize that the peak-to-peak voltage is nearly 3 times the RMS value when assessing safety, specifying components, etc.
"Note 1:" Pulse amplitude is measured with respect to a specified reference and therefore should be modified by qualifiers, such as "average", "instantaneous", "peak", or "root-mean-square."
"Note 2:" Pulse amplitude also applies to the amplitude of
frequency- and phase-modulated waveformenvelopes.
Federal Standard 1037C
Amplitude in the wave equation
In the simple
"A" is the amplitude of the wave.
Units of amplitude
The units of the amplitude depend on the type of wave.
The amplitude of sound waves and audio signals (also referred to as Volume) conventionally refers to the amplitude of the air pressure in the wave, but sometimes the amplitude of the displacement (movements of the air or the diaphragm of a speaker) is described. The
logarithmof the amplitude squared is usually quoted in dB, so a null amplitude corresponds to -∞ dB. Loudnessis related to amplitude and intensityand is one of most salient qualities of a sound, although in general sounds can be recognized independently of amplitude. The square of the amplitude is proportional to the intensity of the wave.
electromagnetic radiation, the amplitude of a photon corresponds to the changes in the electric fieldof the wave. However radio signals may be carried by electromagnetic radiation; the intensity of the radiation ( amplitude modulation) or the frequency of the radiation ( frequency modulation) is oscillated and then the individual oscillations are varied (modulated) to produce the signal.
Waveforms and amplitude
The amplitude may be constant (in which case the wave is a
continuous wave) or may vary with time and/or position. The form of the variation of amplitude is called the envelope of the wave.
If the waveform is a pure
sine wave, the relationships between peak-to-peak, peak, mean, and RMS amplitudes are fixed and known, but this is not true for an arbitrary waveform which may or may not be periodic.
sine wavethe relationship between RMS and peak-to-peak amplitude is::
Waves and their properties:
*Goldvais, A. Goldvais. [http://img2.tapuz.co.il/forums/1_109580628.pdf Exoplanets] . Retrieved 2008-08-22
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