Identification and Reduction of Rotary Compressor Pure Tone Noise Sources Using Random Noise Excitation

Sources of noise at discrete frequencies in compressors may be gener ated by acoustic energy in the discharge pulse exciting mechanical or aco ustic resonances within the compressor assembly. In this case history, a known noise source was used in the vicinity of the discharge port to excite those resonances in a bench-type test where they could be easily identified . This test could be run in a1 r at a room ambient rather than at the temper ature and refrigerant environment of a full prototype, Modelling clay was used to experimentally modify acoustic cavities since actual compressor operat ion was not required. This allowed rapid evaluation of several different de sign proposals. A final recommendation was made and verified through test ing of a sample of full prototypes. INTRODUCTION Rotary compressors are widely used in the world today in room air co n-ditioning applications. Quiet operation of these units, especially in densely populated areas, has long been a concern and is steadily increasing in importance in the marketplace. As a result, compressor manufacturers are constantly seeking to reduce radiated noise, including development o f methods to quickly evaluate proposed improvements. The method discussed belo w allows rapid screening of design proposals aimed at modification of acoustic space resonances and reduces the quantity of actual compressor prototypes which might otherwise be built for evaluation. Objectiye of Study This study was directed towards the identification and elimination o f a noise source in a rolling piston type compr"essor used in room air co nditioning applications. Figure 1 is a cross-sect1ona1 illustration. A -signif icant portion of radiated noise was concentrated in the 1250 Hz 1/3 octave ba nd, some also in the 1600 Hz band. The radiated spectrum is shown in Figure 2. The preliminary investigation began with the use of accelerometer measure ments on the outer shell to determine the specific source of the radiated soun d. Preliminary Investigatjon It was found that the top surface of the shell, compared to other re gions, was most active in the 1250 Hz 1/3 octave band. Possible excitation sources could have been either internal gas pulsations or vertical vibration of the shell near the top. Accelerometer measurements taken on the sides o f the shell, in a vertical direction, and near the top ruled out the vertic al shell wall as a source at 1250 Hz.