Moreover, without any sound isolation, the HVAC and blower fan noise reaches the interior space can impact passengers' comfort especially in the hot climate of a tropical country such as where a vehicle's HVAC system operates continuously at higher blower speeds and is one of the most important interior noise sources. Therefore, there is a real need to improve the sound quality of the vehicle interior space, and it is gaining growing attention from researchers and manufacturers [4; 5; 6; 7; 8]. One of the major noise sources in the HVAC unit is the noise of the aerodynamic blower [9; 10; 11; 12]. Therefore, the common method for reducing the noise of the HVAC includes redesigning the blower and/or its blades [9; 10; 13; 14]. In order to reduce the level of the noise, there are some techniques that can be used for post-production noise control. These techniques include active noise control reported in [11; 15], and passive noise control using synthetic sound absorbing materials such as micro-perforates [6; 7], fiberglass, glass wool and polypropylene [16]. These passive techniques help to reach up to 6-10 dB noise reduction and work best at frequencies above \(\approx\)500 Hz. However, active noise control techniques involve high-cost equipment, and are effective in specific areas of the vehicle interior space. People may still be made to feel uncomfortable by certain aspects of the sound quality, even when the actual sound level of the air conditioners is low [17]. Therefore, both the sound levels and the sound quality of an air conditioner are essential for the user's acoustic comfort. Many researchers have developed methods that can correlate human annoyance with the noise within a building environment to evaluate air conditioning noise. Noise criterion (NC) curves were proposed to evaluate indoor noise, the noise from air conditioning equipment, and other noise factors [18]. The frequency range of NC curves is from 63 to 8000 Hz. Preferred noise criterion (PNC) curves are modified versions of the original NC curves [19]. The PNC curve evaluates lower frequencies than the NC curve, i.e., the PNC analyzes the 31.5 Hz band. In contrast, the NC does not analyze that band. Noise rating (NR) curves are based on similar assumptions to those of the PNC. The NC and PNC curves are used in the USA, while the NR curve is used in Europe. Balanced noise criterion (NCB) curves are a modified version of the NC and PNC curves that consider the balance between low and high frequency noises, such as rumbling and hissing noises [20]. Room criteria (RC) curves were developed and applied differently to the NC and PNC curves [21]. RC curves have a constant slope in the frequency range from 16 to 4000 Hz. RC Mark II curves are revised versions of the original RC curves and have lower values than the corresponding RC curves at 16 Hz [22; 23]. Rom noise criterion (RNC) curves were proposed to allow technical compromises to be reached between the NC, NCB, and RC curves [24; 25]. RNC curves involve evaluations of the temporal variations in low frequency sound. Large numbers of noise indices have been proposed by numerous researchers for evaluating environmental noise. The A-weighted equivalent sound pressure level (SPL), denoted by LAeq, is the most widely used of the proposed indices. This index accounts for both the magnitude of noise and the sensitivity at different frequencies is simple to measure, and correlates well with many psychological responses to noise, such as annoyance [26; 27; 28]. The A-weighted statistical levels (denoted by LA90, LA10, and LA5) are important because they account for the noise's time dependence. LA90 represents the background noise, while LA10 and LA5 represent the noise peaks, and the difference LA10-LA90 represents the noise fluctuations [29]. The noise pollution level (LNP) was defined as the sum of LAeq and the difference LA10-LA90 and was introduced to describe the degree of annoyance that is caused by fluctuating noise [30].