Chapter One: An Acoustics Primer
6. What is Amplitude? | page 2
Amplitude is directly related to the acoustic energy and intensity of a sound. Both amplitude and intensity are related to a sound's power. All three of these characteristics have their own related standardized measurements and will be discussed below.
Amplitude is a measurement of the magnitude of displacement (or maximum disturbance) of a medium from its resting state, as diagramed in the peak deviation example below. If you are watching your speaker woofer create a minute disturbance of air molecules by moving in a out a fraction of an inch from its resting state and it suddenly starts moving in and out by several inches, you can assume that the amplitude has increased because the maximum disturbance of the air molecules has increased (and you'd better cover your ears as well!). A perplexing element in many discussions of amplitude is the confusion between mechanical or electronic measurements of amplitude, and amplitude expressed as an acoustic measure of sound pressure. Since this is an acoustics chapter, let's go with the later, particularly since our ears transfer sound received as pressure waves. The amplitude of sound pressure is frequently measured in pascals (Pa).
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One newton is the amount of force it takes to accelerate a 1-kilogram object by one meter per second (m/s). One pascal is equal to the pressure of one newton per square meter ( N/m2). |
The benchmark threshold of hearing, in other words the smallest perceptible amplitude, is approximately 0.00002 N/m2 for a 1 kHz tone in laboratory conditions (this is actually contradicted by loudness curves discussed below). 60 N/m2 is considered by some to be the threshold of pain, but as we will see, this is also subjective and varies greatly by individual and age.
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Discussions of amplitude depend largely on measurements of the oscillations in barometric pressure from one extreme (or peak) to the other. The degree of change above or below and imaginary center value is referred to as the peak amplitude or peak deviation of that waveform. |
| If we tried to calculate the average amplitude of a sine wave, it would unfortunately equal zero, since it rises and falls symmetrically above and below the zero reference. This would not tell us very much about its amplitude, since low-amplitude and high-amplitude sine waves would appear equivalent. A more meaningful reference has been developed to measure the average amplitude of a wave over time, called the root-mean-squared or rms method. You may also see the rms measurement applied to the power output of an amplifier. |  |
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The rms value of a waveform represents a squaring of the amplitude for each point of a waveform and then taking the total mathematical average. |
The function of the squaring is to eliminate negative values, since all the negative values square to positive ones. This is extremely useful information for those using averaging level meters with audio equipment or software.
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Example: The rms of a sine wave with a hypothetical peak-to peak value of –1 to 1 will be 0.707. This can be used to extrapolate that any rms amplitude = 0.707 x peak amplitude. Peak amplitude = 1.414 x rms amplitude. |
When using audio gear or software, it is important to know whether your meter is a peak-reading meter or averaging meter (or neither). While there are many good reasons to keep an eye on a signal’s peak, the rms average is far more akin to the way we hear. Once you have an understanding of dB’s described below, the markings on the meters should make more sense.
