Determination of Sound Quality of Refrigerant Compressors

Residents of quiet neighborhoods are often disturbed by the noise radiated from the air conditioning or heat pump units installed in their neighbors' or their own yard. In many cases, the sound of the compressor used in the unit is considered annoying while similar sound radiated from the fan of the same unit can be perceived as acceptable. The annoying sounds of the compressors described by consumers as "metalic", "fluctuating", "not smooth", or "rattling" are not necessarily of high acoustical intensity. Instead, these descriptors are often related to the quality of the sound. This paper reviews the validity of several sound quality parameters such as loudness, roughness, sharpness, tonality, and fluctuation strength when they are applied to compressor noises. Comparisons of these sound quality parameters to a widely used "tone corrected sound power" method in ARI 270·84 are also addressed. INTRODUCTION The HVAC system has been one of the major noise sources in the residential neighborhood [1]. In a quiet neighborhood, noise radiated from the air·conditioning or heat pump unit is not only heard outdoors but also transmitted into the house through windows and walls. Typical outdoor unitary equipment consists of a fan which radiates a broad band low frequency noise and a compressor that often radiates multiple·tone higher frequency noise. As the significant progresses have been made in recent years by the HVAC system manufacturers in reducing the fan noise by means of either reducing speed of the fan or better design of the fan orifice, compressor noise becomes the dominant source of the outdoor air·conditioning or heat·pump unit. Two distinct types of sound can be identified from the compressor noise. One is the steady state sound and the other is the time varying sound. Among the sound quality parameters examined below, the descriptors of loudness, sharpness, and tonality are applicable to the steady state sound while the roughness and fluctuation strength are related to the time varying sound. MEASURING THE COMPRESSOR AND HVAC SYSTEM NOISE Two widely used measurement standards to quantify refrigerant compressor sound and HVAC system sound in the North American market are the ARI 270·84 [2] and ARI 530·89 [3], published by the Air Conditioning and Refrigeration Institute. Used for refrigerant compressors, ARI 530·89 specifies a method to measure the airborne noise radiating from a compressor under a load. The measurement results are reported as sound power levels in one· third octave bands between 100 Hz and 1 0,000 Hz and as A·weighted sound (power) level. For the outdoor unitary equipment, ARI 270·84 measures the sound power in a very similar manner. But it also attempts to penalize "discrete tones" radiated from the unitary equipment. When a one· third octave band projects above the linear average of the two adjacent bands by 1.5 dB, a "discrete tone" is determined to be residing in this one·third octave band, some amount of penalty is then applied. The penalties for "discrete tones" are often severe; it is not uncommon to see a penalty of 3 to 5 dB added to certain one·third octave bands. In fact, units with an average level of fan noise are often assessed with the penalty of this Standard just because the spectrum shape of their compressors noise is not "smooth" or "flat". The question becomes can the "discrete tone" be heard when these units are running. The sound of compressors is generally composed of very widely spread multiple tones. A typical narrow band spectrum from a compressor is shown in Figure 1. Many tones (harmonic orders) can be found within a single one·third octave frequency band especially in the high frequencies. On the basis of several laboratory