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

Relationship between subjective responses and physicalparameters of air-conditioner noises in a car
Yoshiharu SOETA1; Hiroko KAGAWA1; Yoshisada SAKAMOTO2
1

National Institute of Advanced Industrial Science and Technology (AIST), Japan
2

Suzuki Motor Corporation, Japan

ABSTRACTSound pressure levels (SPL) of air-conditioner noise are decreasing due to the developments of noise controltechniques. However, people can still feel uncomfortable by certain aspects of the sound quality, even when
the actual SPL is low. Not only quantity aspects of the noise, such as SPL, but also quality aspects of the noise
can affect the subjective evaluation of air-conditioner noises in a car. Therefore, both the SPL and the soundquality of an air-conditioner noise are important for the user’s acoustic comfort. The aim of this paper is toclarify the characteristics of air-conditioner noises in a car and determine the factor that is most influential on
the subjective responses caused by the noise. Sound quality can be characterized by factors obtained fromautocorrelation function (ACF) and interaural cross correlation (IACF) of a sound. Subjective loudness andannoyance were evaluated using a paired comparison method. Multiple regression analyses were performed
using linear and nonlinear combinations of SPL, the ACF, and IACF factors, and their standard deviations.
The subjective loudness and annoyances were formulated by the ACF factors.
Keywords: Road noise, Air-conditioner noise, Sound quality

1. INTRODUCTION
Many people spend a lot of time in vehicles; thus, the comfort of a car has become important topassengers. Sound environments in cars are becoming quieter because of acoustic insulation andabsorption, such as in the case of a rubberized road surface (1), and the prevalence of low-noiseengines, such as hybrid and electric engines (2). Reduction of noise levels heightens awareness ofsound quality. A bad car interior sound quality can negatively affect passenger psychology andphysiology (3). Therefore, sound quality is becoming increasingly important as a part of vehicle
designs.
Many research papers on booming sound quality in a car have been published (3-8). In addition togeneral interior noise, there are various other types of vehicle noise, such as suspension shock
absorber rattling noise (3) and heating, ventilation and air conditioning noise (9). People use airconditioner most of the time when they drive. Previous studies investigated sound quality of
air-conditioner noise when the car was idling or stopping (9-11). However, there is little study onsound quality of air-conditioner noise when the car was running.
The aim of this study is to determine the factor that is the most dominant in terms of the subjective
assessment caused by air-conditioner noises in running cars. The effects of the road noise andair-conditioner noises were considered in this study.

2. METHODS
2.1 Measurements of noises in running cars
Two cars were chosen for the measurement. Car A was a sedan while car B was a small car. Noisein the running car cabins were recorded by a laptop computer at a sa mpling rate of 48 kHz and asampling resolution of 24 bits via a head and torso simulator (HATS, Type 4128C, Brüel & Kjær) andan AD/DA converter (Fireface UCX, RME). The HATS was located on the passenger seat. The effectof the air conditioner was investigated by setting the air-conditioning mode to off, weak, and strong.
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y.soeta@aist.go.jp

The cars run on a general road at a speed from 0 to 60 kilometers-per-hour and an express way at aspeed from 70 to 90 kilometers-per-hour. All windows were closed during the measurement. For thecomparison, noise in an idling state was recorded. The background noise level was measured
according to the A-weighted equivalent continuous sound pressure level (L Aeq) and is summarized inTable 1.
Table 1 – Background noise level, LAeq [dB], for each setting
Setting

Air conditioner

Air conditioner

Air conditioner

offweakstrong

Car A

37.2

46.3

60.6

Car B

40.5

49.2

60.3

2.2 Analysis of noises in running cars
The autocorrelation function (ACF) and interaural cross-correlation function (IACF) factors forsound quality evaluation have been proposed (12, 13). To calculate the ACF factors, the normalized
ACF of the signal recorded from microphones, p(t), as a function of the running step, s, is defined by) (W ; s, T )I( W) = I(W; s, T) =,(1)) (0; s, T )) (0; s  W , T )where)(W ; s, T )1 s T ''³ p (t ) p (t  W )dt .
2T s T(2)where, 2T is the integration interval and p’(t) = p(t)*se(t), where se(t) is the ear sensitivity. In this study,p(t) is the signal that was measured using the HATS. se(t) represents the impulse response of an
A-weighted network, including the transfer functions of the human outer and middle ear, forconvenience (12, 13). Normalization of the ACF is performed using the geometric mean of the energy
at s and the energy at s+W; this ensures that the normalized ACF satisfies the condition 0 ≤ I(τ) ≤ 1.
LAeq is determined based on the A-weighted p(t) signal as a function of s. LAeq is then calculatedusing
L Aeq ( s, T ) 10 log )(0; s, T ) .
(3)
This means that the ACF includes LAeq as a factor.
The other ACF factors are calculated from the normalized ACF. W 1 and I1 are defined as the timedelay and the amplitude of the first maximum peak. W1 and I1 are related to the perceived pitch and thepitch strength of the complex sounds, respectively (12, 14). Higher values of W 1 and I1 indicate that thesound has a lower pitch and a stronger pitch, respectively. The other ACF factor, W I(0) , is defined using
the delay time interval at a normalized ACF value of 0.5, and represents the width of the first decay.
W I(0) is equivalent to the spectral centroid (13). Higher values of W I(0) indicate that the sound includes
a higher proportion of low frequency components. From the IACF analysis, the interauralcross-correlation coefficient (IACC), which is defined as the maximum of the IACF, the delay time at
the IACC, WIACC, and width of the IACC, W IACC, were analyzed. We calculated the ACF and IACF
factors for the noises as a function of time. The integration interval was 2T = 0.5 s and the running step
was s = 0.1 s in all calculations. The analyses were conducted using a MATLAB-based program.
2.3 Subjective assessments
Subjective loudness and annoyance caused by noises measured in car cabins was evaluated toclarify the effects of the ACF and IACF factors on loudness and annoyance. Nine participants took
part in the experiments. All participants had normal hearing, no history of neurological disease, andranged in age between 20 and 40 years. Informed consent was obtained from each participant after
the nature of the study had been explained. The study was approved by the ethics committee of theNational Institute of Advanced Industrial Science and Technology (AIST) of Japan.

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Figure 1 – Mean values of (a) LAeq, (b) W1, (c) I1, (d) WI  , (e) IACC, (f) WIACC, and (f) WIACC (r standarddeviations) of noises generated by idling, general road, and express way conditions at air-conditioning
mode of off, weak, and strong.
Twelve stimuli were selected from the measured noise recorded by the HATS in car cabins
running on an express way to focus on the air-conditioner noises in running cars. The stimuli werepresented binaurally through headphones (HD800, Sennheiser). The stimuli were presented
binaurally at the same LAeq as the actual measured stimuli. LAeq was verified using a dummy headmicrophone (KU100, Neumann) and a sound calibrator (Type 4231, B&K).

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Scheffe’s paired comparison tests (15) were performed for all combinations of pairs of stimuli, byinterchanging the order in which the stimuli in each pair were presented in each session and bypresenting the pairs in random order. The duration of the stimuli was 2.0 s. The rise and fall time swere 100 ms, and the silent interval between stimuli was 1.0 s. After the presentation of each pair ofstimuli, the participants were required to compare the two stimuli in each case based on seven gradesby considering the differences between the two stimuli.
The averaged scale values of loudness and annoyance according to each participant werecalculated based on the modified Scheffe’s method (16). To calculate the effects of each objective
factor on participant loudness and annoyance, multiple regression analyses were conducted using alinear combination of the ACF, IACF factors and their standard deviations (SDs) as predictive
variables by stepwise procedures. The analyses were carried out using SPSS statistical analysis
software (SPSS version 22.0, IBM).

3. RESULTS AND DISCUSSION
3.1 Analysis of noises in running cars
Figures 1 (a) to (d) show the ACF factors (L Aeq, W 1, I1 , and W I(0) ) of the noise that was measured in
the car cabins in an idling state, during running on general road and express way. The setting of theair-conditioning mode was off, weak, or strong. LAeq became larger in the running car. The effect of theair-conditioning mode was small in the running car. The W1 value increased with increasing runningspeed, suggesting the effects of engine sounds. The I1 value decreased in the idling state except whenthe air-conditioning mode was off. This indicates the masking effects of the air-conditioner noise on
the idling sounds. The W I(0) value also decreased in the idling state. When the car run on the express
way, the I1 and W I(0) values decreased in comparison to those when the car run on the general road.
This suggests the effect of the road noise.
Figures 1 (e) to (g) show the IACF factors (IACC, WIACC , and W IACC) of the noise measured in thecar cabins. IACC became larger in the running car, suggesting the effects of sound sources other thanthe engine and air conditioner. The WIACC value show the effects of air-conditioner mode clearly in theidling states. However, effects of the air-conditioner noise on the WIACC value was little in the runningcar. The W IACC value decreased in the idling state. The W IACC value measured on the general road wasgreater than that measured on the express way.
3.2 Subjective assessments
The relationships between the averaged scale value of the loudness, annoyance and each factor arepresented in Figures 2 and 3. Loudness was found to increase slightly with increasing LAeq anddecreasing I1 . Annoyance was found to increase slightly with decreasing I1 .
A multiple linear regression analysis was performed with the scale values of loudness for allparticipants as the outcome variable. The final version indicated that I1 , I1 _SD, and LAeq weresignificant factors:(4)
SVloudness | a1*I1 + a2*I1_SD + a3*LAeq + c.
The model was statistically significant (p < 0.01) and the modified determination coefficient was0.76. The standardized partial regression coefficients of the variables a 1, a 2 , and a 3 in Eq. (4) were−1.42, −0.96, and 0.38, respectively. The negative coefficients of I1 , and I1 _SD indicates that theweaker and unvaried pitch cause greater loudness. The negative coefficients of I1 is consistent withthe previous finding of annoyance (10), however, the negative coefficients of I1 _SD is inconsistent
with the previous finding of annoyance (10). The positive coefficient of LAeq indicates that the larger
LAeq causes greater loudness.
A multiple linear regression analysis was performed with the scale values of annoyance for allparticipants as the outcome variable. The final version indicated that LAeq, LAeq _SD, and W I(0) weresignificant factors:(5)
SVannoyance | b1*LAeq + b2*LAeq_SD + b3* WI(0) + c.
The model was statistically significant (p < 0.01) and the modified determination coefficient was
0.56. The standardized partial regression coefficients of the variables b 1, b 2, and b 3 in Eq. (5) were0.50, 0.48, and −0.22, respectively. The positive coefficients of LAeq and LAeq_SD indicates that thelarger and larger variation of LAeq cause greater annoyance. The negative coefficient of WI(0)6777indicates that the lower frequency components of the noises cause greater annoyance. This isconsistent with the previous finding (10, 11).

4. SUMMARY
This study investigated the dominant factors for the subjective assessments caused byair-conditioner noises in running cars. Pitch strength, I1 , the variation of the pitch strength, I1 , andLAeq are dominant factors for subjective loudness. LAeq, the variation of LAeq and spectral centroid,
which corresponds to W I(0) , are dominant factors for subjective annoyance.

ACKNOWLEDGEMENTS
This work was supported by a Grant-in-Aid for Exploratory Research (Grant No. 18H03324) fromthe Japan Society for the Promotion of Science.

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Figure 2 – Relationships between the scale value of loudness and (a) L Aeq, (b) W 1, (c) I1 , (d) W I  , (e)IACC, (f) WIACC, and (f) W IACC. Error bars indicate standard deviations.

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Figure 3 – Relationships between the scale value of annoyance and (a) L Aeq, (b) W 1, (c) I1 , (d) W I  ,(e) IACC, (f) WIACC, and (f) W IACC. Error bars indicate standard deviations.

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