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When analysing potential sound paths, the cause of the noise, i.e. the excitation source, should always be considered first. At this point, sound proofing measures can be put into practice most effectively and with the least effort and are therefore preferable to secondary sound protection measures. In the case of heat pumps, the excitation sources are the compressor, the ventilators, and possibly the pumps. With regard to the acoustic optimisation of ventilators, reference is made at this point to the comprehensive literature available on this topic; a very good list is provided by the Association of German Engineers VDI 3731, Sheet 2 [VDI 3731] . The following sections deal with the subject of the sound source relating to the heat pump and the refrigerant circulation device connected with it:
The analysis of the sound sources and the sound paths in compact heat pump units shows that it is the primary task of manufacturers to select low-noise ventilators and compressors. Furthermore, considerable improvement can be achieved by coordinated decoupling of structure-borne sound¬ and optimisation of the refrigerant circuit in the heat pump. Direct enclosure of the heat pump, as well as encasing of the whole unit, is suitable for reducing the transmission of airborne sound. Decoupling structure-borne sound is also of importance for the associated ductwork. The first sound absorber should be placed as near the unit as possible. Dimensioning of the sound absorber should not only ensure that the limit values for the A-rated overall sound level are adhered to, but should also analyse exceeding of the auditory threshold, particularly in the low-frequency range. Which limit values should be adhered to here is explained in more detail in the article relating to standards and regulations.
[VDI 3731] VDI 3731 Blatt 2: Emissionskennwerte technischer Schallquellen, Ventilatoren, VDI-Handbuch Lärmminderung, Dezember 1982. (Emission parameters for technical sound sources, ventilators, VDI Handbook Noise Prevention)