Introduction
   The purpose of the  Laser Interferometer Gravitational Wave Observatory (LIGO) is to observe gravitational waves of cosmic origin.  The facility houses a laser interferometer, consisting of mirrors suspended at each of the corners of a gigantic L-shaped vacuum system, measuring 4 kilometers (2.5 miles) on each side. Precision laser beams in the interferometer will sense small motions of the mirrors, which are caused by gravitational waves. A significant problem concerning the LIGO project currently is the level of acoustical noise around the Pre-Stabilized Laser.  Due to the high standards posed by the laser setup, small acoustic vibrations such as the noise produced by cooling fans might create a significant disturbance. We will characterize the noise and will propose a solution based on detailed studies of commercially available acoustical solutions.

Abstract
    The purpose of this project is to rank and determine the best methods to eliminate acoustic noise around and within  the Pre-Stabilized Laser (PSL) table.  In order to do this efficiently and not to interfere with present activities at the PSL, we set up a test area, which consist of a fairly accurate copy of the optical table and its cage geometry.  Different noise reducing/reflecting materials will be examined to see which material, arrangement, etc. lowers the amount of noise the best.

Procedure
   We will first decide which noises in the PSL room need to be eliminated and which acoustic materials would be best for dealing with this type noise.  A test area at the X end station has been constructed using ABS pipes, foil faced foam panels and an optical table.  We have set up a sound system that consists of microphones, speakers, a spectrum analyzer, an oscilloscope, and a power supply.  The sound system must be thoroughly tested with the microphones in different orientations/locations to ensure the repeatebility of the measurements. With the test area completed, we are currently in the process of executing various measurements with the available acoustic materials.
   Data taken previously from a noisy and quiet PSL room has been analyzed together with the data taken during E4.  The average spectral shape and uncertainty due to daily variations is shown on Figure 1. Once the PSL and mode cleaner is in operation again, we will measure the frequency noise with and without the proposed acoustic isolation.

Weekly progress and major events

Week 1
We surveyed the PSL room and decided which areas had the most noise that should be eliminated or reduced by adding acoustic materials.  We also took measurements of the PSL table useful for the construction of the test area cage.  We found several acoustic materials we believe will be good noise reducers.  We began ordering samples of those materials for our tests.

Week 2
We recorded long stretches of PSL microphone data covering times with high and low acoustic noise situations. We extracted the spectra from these and the E4 data. These will serve as a reference when we compare the effect of various orientations and materials. We were working on the test setup around the optical table at the X end station.

Week 3
We finished the construction of the test cage around the optical table. We are evaluating the available microphones and the test system choices. We have already started the experiments with the available materials.

Preliminary results:

   We used the microphone mounted above the PSL table to record long stretches of data, covering very different acoustic noise environments. The results of the analysis are shown on Figure 1.  We cover three different situations:

1. All desirable devices are on, their fans, disks, etc. are operating. The acoustic noise level is as high as it gets (red curve).
2. During the E4 run, when most unnecessary devices were turned off and there were no purge air or filter noise in the LVEA (green curve).
3. Quiet baseline. Everything was turned off inside the laser room, but purge air and filters were operating in LVEA (audible effect inside the PSL room!) (blue curve). Note the huge higher order harmonics of the 60Hz!

   The comparison of the E4(green) and Noisy(red) curves indicate that switching off not absolutely necessary devices have very limited effect and it does not decrease the noise level significantly. The comparison of the Quiet(blue) curve to the others show that devices operating in the room significantly increase the noise level in the PSL room (~10-15dB) above 200Hz. The noise levels below 200Hz seem to be independent from the devices in the room and might require extra attention. One should also notice that the size of the higher harmonics of the 60Hz is ~15dB above the general noise level in the quiet situation and the top height of these peaks is independent very close to be equal for all the curves. These peaks seem to be there independently from the state of the noise sources. However it is not clear yet, whether these peaks are due to electronic noise in the microphone or they have acoustic origin.