Chapter One: An Acoustics Primer
6. What is Amplitude? | page 3
Power and Intensity
Power is a measurement of amplitude over time.
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The unit of measurement for power is the watt, named after James Watt. 1 watt = 1 Newton of work or energy transferred per second* |
The power of the original sound source along with distance of measurement from the sound source combine to form the intensity. Intensity can be measured as watts per square meter or w/m2. Intensity can be seen as amplitude over time over an area. As the surface area of the sound sphere expands, the amount of energy generated by the sound source is distributed over an exponentially increasing surface area. The amount of energy in any given square meter of the expanding sphere's surface decreases exponentially by the inverse square law, which states that the energy drops off by 1/distance2. So acoustic energy twice the distance from the source is spread over four times the area and therefore has one-fourth the intensity, or simply put, relative intensity is the reciprocal of the change in distance squared.
The inverse square law is extremely useful to remember in microphone placement, where even small changes in distance can have a significant impact on the resultant signal strength.
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You may recall from your grade-school math that the  , so as the radius of a sound sphere increases arithmetically, its surface area increases geometrically. The intensity of the source signal energy is distributed over the broadening surface area so that the  , where s = the source intensity. |
Look at it this way: if your kid brother gets ahold of a garden hose that has a pressure of 100 lb./sq. feet (power) and a nozzle that sprays in a sphere (ok, it's a weird nozzle), would you rather be hit in the face from a distance of 0 ft. or 20 ft.? Would the intensity to hit your face be 20x less (5 lb./sq. ft.) at 20 ft. or something else (I'm thinking the pressure hitting your face at any given point is more like .02 lb./sq. ft., calculated using the formula above)?
A few more relationships between amplitude, intensity and power: intensity equals the square of the amplitude, so if the amplitude of a sound is doubled, its intensity is quadrupled. Power is also proportional to amplitude squared, therefore power and intensity are proportional to each other.
Putting it all together
| Term | Property | Unit of Measurement |
| amplitude | the energy of a sound wave present or the magnitude of maximum disturbance of the medium (air in our case) during one cycle of a periodic wave, known as peak deviation. | pascals, or newtons per square meter (n/m2) |
| power | Power is the rate at which energy is being produced or used. In acoustics, this translates into amplitude over time. Measured in terms of pressure, it would be the rms of a sound wave at a specific point in space, or sound pressure level (SPL-see next page). | watt (also related is rms and SPL) |
| intensity | The power present over an area, such as the outer surface of an expanding sound sphere. So from the top, intensity is the amplitude over time over an area. | watts per square meter (w/m2) |
*An energy transfer of one watt per second is equivalent to a Joule, a common measurement in electrical power. The power of one Joule per second is a watt. So a Joule is a measurement of energy and a watt is a measurement of power. These terms are frequently, but mistakenly used as synonymous. Is it all clear now?
