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Microphones and Acoustics

Section I: Definitions and Terminology
Section II: Microphone Recommendations
Section III: Calibration and Testing
Section IV: Specification Clarifications
Section V: Specialty Microphone Applications
Section VI: Maintenance and Handling

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How does distance affect the sound pressure level?

This depends on the geometry and relative size of the sound source.  A simple source projects sound from a single location and the size of the source is small compared to the wavelength of sound. As a rule of thumb, if the wavelength of sound is greater than 10 times the radius of the source, then this assumption is valid.  In these cases sound propagates in the form of spherical waves with the source at the center.  These waves are radially symmetric where the pressure is constant at a given radius.  The distance from the source corresponds to the increasing radius of spherical waves as they propagate outward.  As distance from the source increases the sound power is spread over greater and greater distances thus reducing the sound pressure level. The decrease in sound pressure, under ideal conditions, is inversely proportional to the distance from the source. This is equivalent to a 6 dB drop each time the distance is doubled. For example, if the sound pressure level at 5 meters is 100 dB (re 20uPa), then at a distance of 10 meters away it will be 94 dB (re 20uPa). If the source is not small compared to the wavelength of sound (either because it is very large or the frequency is very high resulting in shorter wavelengths) the assumption of a simple source is not valid and additional mathematical analysis is required to determine how the pressure changes with distance.

How does distance affect the sound pressure level?

This depends on the geometry and relative size of the sound source.  A simple source projects sound from a single location and the size of the source is small compared to the wavelength of sound. As a rule of thumb, if the wavelength of sound is greater than 10 times the radius of the source, then this assumption is valid.  In these cases sound propagates in the form of spherical waves with the source at the center.  These waves are radially symmetric where the pressure is constant at a given radius.  The distance from the source corresponds to the increasing radius of spherical waves as they propagate outward.  As distance from the source increases the sound power is spread over greater and greater distances thus reducing the sound pressure level. The decrease in sound pressure, under ideal conditions, is inversely proportional to the distance from the source. This is equivalent to a 6 dB drop each time the distance is doubled. For example, if the sound pressure level at 5 meters is 100 dB (re 20uPa), then at a distance of 10 meters away it will be 94 dB (re 20uPa). If the source is not small compared to the wavelength of sound (either because it is very large or the frequency is very high resulting in shorter wavelengths) the assumption of a simple source is not valid and additional mathematical analysis is required to determine how the pressure changes with distance.