PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

HVAC noise in car cabin: a preliminary comparison between ICEVsand HEVs
Massimiliano MASULLO 1; Katsuya YAMAUCHI 2; Yumi NAKATANI 3; Luigi MAFFEI 4
14Università degli Studi della Campania, Department of Architecture and Industrial Design, Italy
23Kyushu University, Department of Communication Design Science, Japan

ABSTRACT
Noise in car cabins is a key aspect of the drivers/passengers’ comfort experience. It is produced by theaerodynamic interactions of the vehicle and the rolling, by the engine and all the auxiliary systems installedin the engine compartment. Although cars manufacturers focused their efforts to reduce the noisetransmission inside the cabin, for Internal Combustion Engine vehicles (ICEVs) the mainly noise source isstill the engine noise. Nevertheless, with the advent of hybrid/full electric vehicles (HEVs) in the automotiveindustry, the engine noise has been reduced dramatically, especially at low rpm, leaving emerging the noiseof auxiliary sources. One of them is the air conditioning noise.
Although doesn’t exist a consolidated model to describe HVAC noise perception inside the vehicles, thepaper intends to explore the expected changes of the drivers/passengers’ auditory perception from ICEVs toHEVs. The acoustic and psychoacoustic metrics of different binaural recordings, carried out inside ICEVsand HEVs, are showed and compared.
Keywords: HVAC, Hybrid Electric Vehicle, Noise.

1. INTRODUCTION
With the transition from the Internal Combustion Engines Vehicles (ICEVs) to Electric or HybridElectric Vehicles (EVs/HEVs) the background noise inside car cabins is changing dramatically. It is
no longer dominated by the engine, rather, by the secondary noise sources which emerge when theinternal combustion engine is replaced with the electrical one. Among all the electro-mechanicalcomponents installed in the engine compartment, the heating, ventilation and air conditioning (HVAC)
system is one of the most critical from the acoustic point of view, especially when cars are moving at
low speed or idling at low engine rpm. Due to this transition between ICEVs to EVs/HEVs, in thenext years it is expected that several comfort indexes and sound quality paradigms defined to describe
the car cabin noise of internal combustion, will change. Consequently, new measurements andexperimental studies need to be carried out on this new generation of vehicles, and new soundquality/comfort predictive models should be developed, to understand the consequences of thetechnical changes on the noise perception.
HVAC noise has been investigated since some decades, within working environments, such as theoffices, to define noise indices (1,2) and to measure its effects on cognitive abilities (3). More recently
it became a key element for passenger’s comfort in train compartments and inside automobiles,
leading to the development of several studies on HVAC sound quality (4,5). In the automotive sector,
researches have been focused to represent the perceptual space of the air-conditioning sounds bymeans of listening tests and subjective questionnaires based on Likert scales, and semantic differentialscales, to be administered to a jury (6-10). Only recently the sound quality studies on HVACs havebeen extended to the HEVs (11), as with the CEVAS project which revealed a 3-dimensional
perceptual space, associated mainly to the unpleasantness, sharpness and fluctuation.
This paper describes the first results of a Collaborative Research Project on HVAC sound quality
inside cars cabins, between the Department of Architecture and Industrial Design of the Universitàdegli Studi della Campania “Luigi Vanvitelli” and the Department of Communication Design Science,
1

massimiliano.masullo@unicampania.it
yamauchi@design.kyushu-u.ac.jp
3nakatani.yumi.222@s.kyushu-u.ac.jp
4luigi.maffei@unicampania.it
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Faculty of Design, Kyushu University. In this first phase of the project, some binaural recordings havebeen carried out, in Italy and in Japan, to characterize the HVAC noise inside ICEVs and HEVs’ cabins,
in different functioning conditions. Acoustic and psychoacoustic metrics have been analyzed and thenused to calculate some sound quality indexes that describe the drivers/passengers' perception insidethe cars.

2. MEASUREMENTS SETUP
Recordings have concerned 2 groups of vehicles, ICEVs and HEVs. Each of them was of 4 units.
In Italy were measured 3 ICEVs and 1 HEV, while in Japan 3 HEVs and 1 ICEV. Vehicles' brands
have been omitted in the paper and the vehicles have been labelled as reported in Table 1. HVAC noise
has been measured at the left position inside each vehicle: the driver position in Italy, the passenger
position in Japan. During the measurement, the tested vehicles were on a private road inside theuniversity campus. The background sound equivalent level inside the vehicles was less than 35 dB(A).
Measurements have been carried out by: 1) a 4-channel-measuring system, SQobold and a binaural
headset BHS II, Head Acoustics, positioned on a Mk1 Cortex manikin (in Italy); and 2) a 2-channelhead and torso simulator (HATS) Type 4100, Brüel&Kjær, (in Japan). The position of the rightmicrophone of the HATS/ BHS II was at about 80 cm from the closest central outlet, and at about 65
cm above the seat. During the measurements all the windows and doors were kept closed while all theair outlets valves were fully opened and slightly oriented to the floor to prevent noise induced by air
flow at the microphones (12). Recordings have been made compatible by a comparative measurement
at the Kyushu University (see Fig. 1) and by the development of an equalizer filters.
Table 1 – Recording labelling
Vehicles category

ItalyJapanICEVs

ICEVA, ICEVB, ICEVC

ICEVD

HEVs

HEVD

HEVA. HEVB, HEVC

Figure 1 – Comparative measurements
Each recording lasted 30 s and was carried out at different Functioning Modalities (FMs) and Air
Flow Rates (AFRs).
• FM01: represent the background noise inside the cabin, when the engine is ON.
• OV (Only Ventilation): represents a typical ICEV setup, where the drivers/passengers can use
the fan to ventilate the cabin, keeping the engine and the air conditioning system OFF.
• VE (Ventilation and Engine): it is the same of OV, but with the engine ON. In case of HEVs theconditions are identical.
• AC/EAC (Air Conditioning and Electric driven Air Conditioning): represent two configurations
with the fan and the air conditioning system ON. The differences are: AC) Air Conditioning8043

system is driven by the ICE; EAC) Air Conditioning system is driven in Electric modality.
Except for FM01, for all the previous functioning modalities the air outlet was set in modality
Frontal, with the Air Flow Rate at 4 different levels:• AFR1: airflow rate 25%;
• AFR2: airflow rate 50%;
• AFR3: airflow rate 75%;
• AFR4: airflow rate 100%.
A total of 88 binaural recordings: 52 for ICEVs (4 vehicles x (3 FMs x 4 AFRs + 1 FM01)) + 36
for HEVs (4 vehicles x (2 FMs x 4 AFRs + 1 FM01)) have been carried out.

Figure 2 – Binaural recordings: Up) HEVs; Down) ICEVs

3. SOUND QUALITY MODELS
Several indexes have been proposed to describe and assess the annoyance and the perceived soundquality of cars, trains or other electromechanical machineries. Among them one the most known is thePsychoacoustic Annoyance model (PA) (13) that combines psychoacoustic descriptors to derive theannoyance rating of people for synthetic and technical sounds (car noise, air conditioner noise, circularsaws, drills, etc.). Similarly, other models aim to predict the subjective annoyance to office airconditioners (14) or to washing machines (15). Regarding the transportations means, predictivemodels have been proposed: to estimate: the subjective responses to HVAC noise (12); the acoustic
comfort inside car cabins (16,17) and trains compartments (18); the characteristics that trigger a‘‘pleasant’’ response from listeners to HVAC noise in automobiles (19).
Some of the previous indexes: Psychoacoustic Annoyance (PA); Subjective ratings (ann, rou, sha);
Discomfort (DC), Acoustic Comfort Index (ACI) and Pleasantness, have been used to calculate how
they change through different typologies of vehicles and working conditions.
3.1 Psychoacoustic Annoyance (PA)
According to this index (13), the psychoacoustic elements of annoying sounds depend by theloudness, the tone colour and the temporal structure of sounds. PA is calculated by measuring 4
psychoacoustic metrics: loudness, N [sone]; sharpness, S [acum]; fluctuation strength, F [vacil] androughness, R [asper], with the formula reported below:
𝑃𝐴 = 𝑁% ∙ (1 + +𝑤- . + 𝑤/0 . 1
Where:
w3 = 0.25 ∙ (S − 1,75) ∙ log(N% + 10) for S > 1,75
𝑤/0 = 2.18 ∙ 𝑁% CD.E (0,4 ∙ 𝐹 + 0,6 ∙ 𝑅)

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(1)

3.2 Subjective ratings: ann, rou, sha
These models (12) have proposed the prediction of the subjective response to HVAC sounds using
the semantic differential technique. Three equations related to subjective responses have been selectedamong those obtained as multiple linear regression of several adjective pairs. Subjective responses
have been used for not-annoying/annoying, smooth/rough and dull/sharp scales showed below:
𝑎𝑛𝑛 = −2,5 + 0,24 ∙ 𝑁(2)𝑟𝑜𝑢 = −5,0 + 0,23 ∙ 𝑅(3)𝑠ℎ𝑎 = −3,8 + 3,1 ∙ 𝑅(4)

3.3 Discomfort
Despite these models (16) are not related to the HVAC noise but to the interior vehicle noise, it ispossible to derive them, for different road surfaces, from the loudness, sharpness, roughness, andarticulation index. In this paper, only the discomfort index for asphalt was considered.
𝐷𝐶T-UVWXY = 6,111 ∙ 𝑁 + 99,572 ∙ 𝑆 − 107,058(5)
3.4 Acoustic Comfort Index, ACI
Like the previous models, also the Acoustic Comfort Index (17), doesn’t describes the HVAC soundquality, but only the comfort inside motor cars in different conditions. ACI uses, subjective andobjective measurement to estimate the expected comfort in different functioning conditions. In thispaper, only the idle condition has been used.
𝐴𝐶𝐼]^X_ = −3,54 ∙ 𝑅 + 20,07 ∙ 𝐹 + 0,022 ∙ 𝐿T + 22,27(6)3.5 PleasantThis model (19) uses the results obtained from objective and subjective sound quality (SQ)
evaluations of HVAC noise to generate a multiple regression model, that is used with the Semantic
Differential Method to determine what characteristics trigger a ‘‘pleasant’’ response from listeners.
𝑃𝑙 = 9,74 + 0,399 ∙ 𝑁 − 4,38 ∙ 𝑆 − 6,43 ∙ 𝑅 − 0,151 ∙ (𝑁 ∙ 𝑆) − 0,0611 ∙ (𝑁 ∙ 𝑅) − 4,00 ∙ (𝑆 ∙ 𝑅) (7)4. ANALYSES AND RESULTS
Representative chunks of 5 seconds have been extracted from each recording and then analyzed by
the software Artemis (Head Acoustics). For each of them, the average values of several
acoustic/psychoacoustic metrics (L, L A, N, S, R, F, AI) and some statistical value (N5 ) were calculated
for each group of vehicles, ICEVs and HEVs.
Due to reflections over the left window, the results showed that the A-weighted sound pressurelevels at the left microphone were always higher than the right one, and that they increase as the AFR
increases. This is more evident among the HEVs due to the low background noise. The increase fromAFRi to AFRi+1 is in average of about +8 dB (Fig. 3 left).
Loudness of HEVs had values lower than ICEVs, in all the functioning modalities, except at theAir Flow Rates 3 and 4 in Only Ventilation modality (Fig. 3 right).

Fig. 3 – Average values of the A-weighted Sound Pressure Level (left) and Loudness (right)

While slight differences were noticed among the measures of the sharpness and roughness of8045

ICEVs and HEVs, the Fluctuation Strength of ICEVs resulted considerably higher than HEVs,
especially, when the internal combustion engine was on and at low air flow rates (AFR1 and AFR2)(Fig. 4 left). At these air flow rates AI was about the 100% and is reduced to 80% at AFR3 and 60%
at AFR4. Lower percentage were obtained for ICEVs (Fig. 4 right).

Fig. 4 – Average values of the Fluctuation Strength (left) and Articulation Index (right)

The previous data were combined and used to calculate the above described psychoacoustic
indexes. PA and Subjective Ann values of HEVs were higher than ICEVs, only for OV at air flow
rates 3 and 4 (Figg. 5). Rou and Sha didn’t show significant changes.

Fig. 5 – Average values of the Psychoacoustic Annoyance (left) and Subjective Not Annoying/Annoying (right).

Considering the Discomfort/Comfort indexes, DC asphalt showed to be in line with the above results
while ACI idle appeared to be in contrast with the other indexes for the functional modality AC/EAC.
Also, the results of the Pleasant were not sufficiently clear.

5. CONCLUSIONS
The measurement and analyses described above represent the preliminary activities of theCollaborative Research Project on the “HVAC sound quality inside cars cabins”. These activities haveallowed to describe, preliminary, some differences existing between ICEVs and HEVs in several
functional modalities and at different air flow rates.
The most important results emerged from the HEVs and ICEVs measurements showed the greatest
differences can be obtained at low air flow rates (AFR1 and AFR2). In these conditions the low noiseof the ventilation system and presence the internal combustion engine in idle, makes the Annoyance(see PA and SA) or the Discomfort (DCasphalt) of HVAC inside the HEVs considerably lower thanICEVs. The differences between the two engines decrease as the air flow rates increase. In Only
Ventilation condition HEVs have the Annoyance (PA, SA) equal or higher (at AFR4). Less clear were
the results obtained for the other indexes.
These preliminary analyses suggest to carry out more measurements, including subjective
evaluation, and analyses to describe and understand the effects of the combination of different
acoustics characteristics on noise perception. This will be developed in the next step of the project.

ACKNOWLEDGEMENTS
Authors want to thank all the colleagues and friends who helped them to carry out the activities ofthe Collaborative Project "HVAC sound quality inside cars cabins" and who offered their time and8046

vehicles for the measurements.

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