Vehicles, today, are less noisy. Engines and transmission systems are quieter than ever before.The main source of noise now is the air conditioning unit. Although available technologies provide reasonable soundproofing, manufacturers are hamstrung in the pricing and recyclability of the materials. Active noise control methods to attenuate low-frequency noise tend to be expensive. Passive silencers, made from synthetic materials, work with higher frequencies but are largely non-biodegradable. So there is a need for universal, inexpensive techniques that operate well across a range of frequencies.
In an attempt to tackle the issue, Sneha Singh and A.R. Mohanty from the Department of Mechanical Engineering, IIT Kharagpur analysed the acoustics of Heating Ventilation Air Conditioning units. Blowers in such units, they found, are the noisiest parts, causing discomfort to passengers.
Using a HVAC prototype, made from acrylonitrile butadiene styrene plastic, they tested four operating presets provided by the manufacturer: low, medium-low, medium-high, and high speed. They removed the ducts that circulate air to the rear of the vehicle, to focus on the sound as perceived by passengers in front. The loudness in medium-high and high modes of operations crossed the threshold for hearing damage — exceeding 32 sones. And this noise unequivocally deserved processing.
To identify the noise sources, they placed extremely sensitive microphones at different positions around the unit. Additionally, they placed a microphone at the manufacturer-prescribed front passenger location. The examination of the noise spectrum at different locations showed that the unit did not excite in any of the natural frequencies. So the major noises produced must be aerodynamic in nature.
They tested the unit in recirculation mode where the air from the cabin is processed and recirculated back into the vehicle. The team came up with an ingenious idea to attenuate the sound from the noisiest parts of the AC unit: a jute felt and cotton. Jute felt and cotton have some of the highest sound absorption coefficients, says A R Mohanti. IIT Kharagpur
Besides lining defrost, foot, and cabin vents with 8mm jute felt, the researchers stuffed cavities with cotton. These measures reduced loudness by 7 sones and the sound pressure level by 3.7dB. The sound was more comfortable to hear for longer periods of time.
This research shows how biodegradable, natural materials can be used to significantly attenuate noise produced by AC units. The measurements would change with the inclusion of rear ducts. However, the efficacy of noise reduction should remain the same, says Sneha Singh, IIT Kharagpur.
Appl. Accoustics, 140: 100-109 (2018); DOI: 10.1016/j.apacoust.2018.05.013