The Effect of the Dome Shape of a Hermetic Compressor Housing on Sound Radiation

The sound level of air conditioning units is becoming increasingly more important to the OEM unit manufacturer and the end user of the product. Invariably, the compressor is a significant contributor to the overall sound. This paper will discuss the analysis techniques and ultimate results of redesigning a hermetic compressor housing with the goal of lowering the sound. level. The redesign process consisted of initial finite element analysis to predict resonance frequencies, construction of prototypes, and verification by sound testing and experimental modal analysis. The experimental work showed that the first resonance of the dome increased from 1380 to 2370 Hz. The overall sound pressure level of the compressor was reduced by as much as 2.5 dBA. Even though the finite element analysis did not agree with the experimental modal results, it was a valuable tool for visualizing the benefits of an improved housing shape. IHTROOUCTIOH Since the compressor housing is the ultimate radiator of sound, it follows that the shape of the housing is a very important factor in reducing the sound level. The vibration of the housing surface is of course the source of all the· sound heard by the listener. The object in designing a compressor housing would then be"to make it resistant to excitation. By definition, a stiff structure deflects less per unit force. It follows that the surface velocities are also lower, which translates into less intense sound radiating from the surface. Dynamic forces are transmitted through the suspension springs and the discharge shock loop. These forces are also transmitted in the form of pressure fluctuations acting over the inner surface of the housing. In general, the magnitudes of these forces and pressures decrease as the frequency increases. Also, the housing radiates sound most effectively at frequencies at which it is resonating. Thus, in designing a stiff housing, the resonance frequencies are shifted upwards away from the high amplitude forces. This paper is written to illustrate some of the techniques available to the engineer for determining the effect of dome shape and to aid in designing a compressor housing for reduced sound. Results of finite element modeling conducted on a personal cpmputer will be presented. To verify the results of the mathematical model, simplified experimental modal analyses (EMA) were run, concentrating on the top surface. Finally, sound testing of the compressors in the new housing is presented to show the ultimate benefit of the new housing design in comparison with the original design.