Contents lists available at ScienceDirect Applied Acoustics journal homepage: www.elsevier.com/locate/apacoust Technical note HVAC noise control using natural materials to improve vehicle interior sound quality Sneha Singh ⁎ , A.R. Mohanty Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India ARTICLE INFO Keywords: Noise control Sound quality Jute felt Natural materials Sound absorption HVAC ABSTRACT Heating, Ventilation and Air Conditioning (HVAC) unit is a major noise source in a vehicle’s interior space. Reducing this noise will improve the sound quality of the vehicle’s interior space and enhance passengers’ experience. However, current noise control techniques are high-cost and hazardous to environment. Therefore, this paper studies the first ever usage of low-cost, biodegradable natural materials for vehicle’s HVAC noise control. Jute felt and waste cotton were found to have higher sound absorption coefficients than other common sound absorbing natural materials, and were chosen as sound absorbers for noise control treatment of a pro- totype HVAC unit. Noise sources in the unit were identified and ranked, and the treatment was applied to these. The treatment significantly reduced the noise level (by 4 dBA) and loudness level (by 7 sones) due to the unit at the reference passenger’s ear location, with negligible cost and weight. Sound quality evaluation by 24 parti- cipants showed that the treatment significantly reduced the annoyance of the vehicle interior soundscape comprising the HVAC noise. Thus, jute felt and waste cotton are low-cost, light-weight, biodegradable and recyclable natural materials with high potential for HVAC noise control. 1. Introduction Sound quality of a vehicle interior space is an important aspect of a vehicle and demands special NVH (Noise, Vibration and Harshness) attention. Reducing noise inside a vehicle improves the sound quality of the vehicle interior space, which in turn leads to a higher passenger comfort, better driving experience, and lesser driver distraction. Moreover, reducing noise and improving vehicle interior sound quality enhances a customer’s perception of the vehicle brand; thereby the vehicle attracts more customers and gets a competitive advantage in the market [1,2]. Research and development over the last two decades have led to quieter and better sounding engines. As a result, secondary sound sources located within a vehicle cabin such as heating, ventilation and air-conditioning (HVAC) system, entertainment systems, and audio driver assist systems have become more perceptible to passengers [3]. Among the secondary sources, the HVAC system is the most dominant noise source in a vehicle’s interior space as it operates throughout as long as the vehicle is running. Additionally, the HVAC and blower fan noise reaches the interior space without any sound isolation and can strongly impact passengers’ comfort. In the hot climate of a tropical country like India, a vehicle’s HVAC system operates continuously at higher blower speeds and is one of the most important interior noise sources. Therefore, HVAC noise control is needed to improve the sound quality of the vehicle interior space, and it is gaining growing attention from researchers and manufacturers [3–8]. 1.1. Conventional methods for HVAC noise control and sound quality enhancement The major noise source in the HVAC unit is the aerodynamic blower noise [9–12]. Therefore, the conventional strategy for reducing HVAC noise includes design changes to the blower and its blades [9,10,13,14]. However, the unit examined in this paper was already designed and produced by the manufacturer for optimum performance; hence only noise control strategies at post-production stage are discussed in this paper. Existing post-production noise control techniques include active noise control [4,11,15], and passive noise control using synthetic sound absorbing materials such as micro-perforates [5,6], fiberglass, glass wool and polypropylene [16]. These approaches achieve up to 6–10 dB noise reduction. However, active noise control approaches involve costly equipment, work well only in low frequencies, and are effective only in specific zones of the vehicle interior space. Passive noise control approaches work best at frequencies above ≈500 Hz, and they apply noise reduction throughout the interior space. However, the synthetic sound proofing materials are costly as their manufacturing requires https://doi.org/10.1016/j.apacoust.2018.05.013 Received 13 February 2018; Received in revised form 7 May 2018; Accepted 9 May 2018 ⁎ Corresponding author. E-mail address: snehasingh.iitkgp@gmail.com (S. Singh). Applied Acoustics 140 (2018) 100–109 0003-682X/ © 2018 Elsevier Ltd. All rights reserved. T