The United Space Alliance sought outsideassistance to evaluate and develop noise controlmeasures for NASA’s two CTs at KSC aspart of a modernisation and upgrading program.NASA selected Noise Solutions Inc. (ageneral acoustical contractor) and their subcontractorFaszer Farquharson & Associates ofCalgary, Canada to engage in this project. Thefirst phase of the sound study encompassedtesting and engineering analysis of significantsound sources to measure and record full frequencyspectra and intensity of the variousnoise sources and to determine the potential fornoise reduction. Real-time sound intensitymeasurements and sound pressure level (SPL)measurements of the significant noise sourceswere undertaken on a fully loaded crawler inorder to obtain actual maximum operationalnoise level conditions.
Finding the Noise Source
SPL measurements provide an overall sound level of the combined noise sources at the microphone position – useful when it’s possible to measure one piece of equipment at a time. SPL measurements of the CT’s ventilation fans and radiator fans were made using Brüel & Kjær’s Type 2260 Hand-held Sound Level Meter and Type 4189 Microphone. 1/3-octave sound pressure level spectra were measured and recorded. The system was field calibrated at the start of each series of measurements and re-checked upon completion using a Type 4231 Sound Level Calibrator. Sound intensity measurements, on the other hand, provide an indication of both the sound intensity level and the direction of the sound energy flow. That is, a sound intensity measurement indicates that sound may be flowing out of or into a surface. Sound intensity measurements were taken of the major noise sources that change level with load including the diesel generator sets, the hydraulic systems, and various control room and cab surfaces. These measurements were undertaken during a shuttle rollout using two Type 2260 Sound Intensity Analyzers each equipped with a Type4197 Microphone Pair mounted on a Type2683 Dual Preamplifier. 1/3-octave band frequency sound intensity and sound level spectra were measured and recorded. Again, the system was field calibrated at the start of each series of measurements and re-checked upon completion using a Type 3541 Sound Intensity Calibrator with Type 4228 Pistonphone. Local Brüel & Kjær Agent, Xscala Sound & Vibration, worked closely with Noise Solutions and Faszer Farquharson & Associates to ensure all necessary instrumentation was on hand for these critical measurements.
The sound levels were analysed and sound intensity mapping techniques used to determine the major noise sources on the crawler and the sound level contributions of the various noise sources. Conceptual noise control measures were then determined and presented. This resulted in the removal of the old and installation of new, custom designed and upgraded engine exhaust mufflers, and upgraded engine/pump room ventilation. The first phase was designed to reduce noise levels around the CT both on the ground and on the walk ways, but a secondary benefit of the upgrades is improved air quality in the engine/pump room. Previously, the engine exhaust outlets were located directly under the CT, so operators walking under it to inspect the large trucks were exposed to both noise and diesel smoke. Along with the upgraded mufflers, the exhaust pipes were extended beyond the end of the CT, ending up in front of the radiator fan outlets. This greatly increased the distance between the operator and the exhaust outlet noise and diesel smoke. Utilising the air movement from the radiator fans, the diesel smoke is now blown out from the end of the CT rather than exhausted under the CT. In the past, the ventilation air was drawn in from the sidewalls of the engine/pump room where it would first sweep across the engines before being driven down over the workers and exhausted out through twelve louvered outlets in the floor. The ventilation air is now drawn in from under the CT through twelve filtered and acoustically treated inlet hoods, thus using the coolest air available. The cool air is forced up through the floor grates, and first sweeps across the workers. It then flows across the engines to the ceiling where it is exhausted via fourteen silenced outlets. This change in airflow direction as well as the increased volume of ventilation air allows the engine/pump room to operate with a greatly reduced temperature increase, and allows the doors to remain closed during operation. The closed doors in conjunction with the silenced floor and wall ventilation openings have significantly reduced noise levels around the CT, both on the catwalks and on the ground, enhancing operational conditions for the CT crew.
The second phase of the noise-control plan is to suppress the JEL (jacking, equalization, levelling)hydraulic systems noise. The hydraulic systems are all rigidly mounted on the CT’s super structure and use rigid piping except for the final connections to the hydraulic cylinders. The pump tone generated by the systems was identified as one of the major noise sources in both the control room and the driver cabs, even though the hydraulic lines do not run directly under these areas. The JEL system noise-control measures include mounting the JEL motors and pump skids on rubber isolators, the use of elastomeric pipe-mounting clamps to isolate the hydraulic lines from the CT’s superstructure, installation of flexible hose between the pumps and the rigid hydraulic lines, and installation of in-line hydraulic silencers to reduce the pump tone. The third phase will encompass the installation of acoustical absorption on the walls and ceilings of the engine/pump room, use of sound lock vestibules at the engine/pump room’s doors, and application of an upgraded sound reduction wall between the engine/pump room and the control room. This will reduce the noise levels in the control room, and lower noise levels outside the CT due to a decrease in the reverberant sound level inside the engine/pump room.