The Laser Gravitational Wave Observatory, or LIGO, is built to observe gravitational waves as they propagate through space. Advanced LIGO is extremely sensitive
to movements of the test mass as small as 10⁻²¹ m/√Hz , which allows many signals other
than gravitational waves to be detected by the system. Pressure created by external sound can alter the measurement by creating Doppler shifts, intensity fluctuations, and scattering in the laser beam. To determine the areas affected by sound external to the vacuum system, we inject acoustic noise in the laser and vacuum equipment area. On a smaller scale, vibrating a horizontal access module or beam splitting chamber with a shaker tests the impact of sound on single chambers. To calculate the scale at which these vibrations impact the signal as well as the effect of other environmental injections, a program that analyzes ambient background noise signals as well as injections with coupling functions to determine the estimated background level of the environmental signal was created. This data analysis program was used to determine the estimated background level of acoustic coupling for each horizontal access module and a ranking of the vacuum chambers most affected by acoustic coupling was developed from this.It was determined that two vacuum chambers had background levels above 10⁻²⁰ m/√Hz. Tests on materials to limit acoustic coupling within a horizontal access module were also conducted and found that using dampening clips within the vacuum chamber was a possible solution to limit acoustic coupling in the 750 Hz range for that chamber.
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