Researchers at the National Institute of Standards and Technology (NIST) have designed and built an optical device that could set a new standard for measuring the pressure exerted by sound waves—a measure related to their loudness—and for calibrating microphones and other sound sensors.
Improved calibration of microphones can enhance surveillance of nuclear weapons testing, which employs large arrays of sound detectors. It can also boost monitoring and control of noise that could pose a public health hazard at construction sites or around airports and refine the ability to detect air turbulence and enhance aerodynamic testing of mechanical parts.
The optical method relies on the fact that light moves more slowly through denser materials. For example, because glass is denser than air, light traveling through glass travels at just two-thirds its speed in air.
When sound waves move through the air, they alternately compress and expand the gas, changing its density. The variation in density alters both the speed and wavelength of light traveling through the same parcel of air. (The higher the density, the shorter the wavelength.)
To use this opto-acoustic effect as a way to measure the sound waves, the NIST team injects infrared laser light into an optical cavity, 12 centimeters in length, and capped by mirrors on either end. In the absence of sound waves, the length of the optical cavity determines the resonance wavelength, or equivalently, the resonance frequency, of the infrared laser light in the cavity—the frequency at which the light can bounce back and forth many times, in standing waves between the two mirrors before exiting the cavity.
When sound waves are introduced into the cavity, they cause the resonance frequency of the cavity to shift ever so slightly—about 30 parts per bill ..
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