PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Psychoacoustical assessment of thermal impression of automotive
HVAC noise
Seiji NAKAGAWA1,2,1,4*; Takuya HOTEHAMA4; Masaru KAMIYA5
1

Department of Medical Engineering, Graduate School of Science and Engineering, Chiba University
2

Center for Frontier Medical Engineering, Chiba University
3

4

Med-Tech Link Center, Chiba University Hospital

National Institute of Advanced Industrial Science and Technology (AIST)5

DENSO Corporation

ABSTRACTTwo subjective assessments were performed focusing on the auditory impression of automotive HVAC noise
concerning coolness and warmness. First, paired comparison tests were carried out under various conditionsof room temperature. Five stimuli were synthesized by stretching the spectral envelopes of recorded
automotive HVAC noise to assess the effect of the spectral centroid. Twelve normal-hearing subjects wereasked to rate the auditory impression of the stimuli for each pair on a seven-point scale according to howmuch the latter is warmer (for the winter tests) or cooler (for the summer tests) than the former. Results showthat the spectral centroid significantly affects the auditory impression concerning coolness and warmness; ahigher spectral centroid induces a cooler auditory impression regardless of the room temperature. Second,
effects of HVAC noises on the subjects’ sensation of coolness and warmness were evaluated by using amethod of continuous judgment by category. Room temperature were controlled to increase/decrease linearly,and HVAC noise, having warm/cool auditory impressions, were presented. Subjects had to answer theirsensation of coolness/warmness at regular time intervals. The results showed that HVAC noise had significant
effect on the time change rate of sensation of coolness/warmness.
Keywords: HVAC noise, auditory impression, warmness/coolness

1. INTRODUCTION
Noise is an important factor of the comfort of the interior of the car cabin. In particular, the noise
associated with a heating, ventilation and air conditioning system (HVAC) has gained importancesince recent advances in the reduction of engine noise. Much effort has been devoted to reducing
levels of automotive HVAC noise, however, the reduction of the noise levels beyond a certain level
sometimes faces with other problems (1). For example, there might be the relative rise of thesecondary noise sources. In addition, silence sometimes causes a loss of operation feeling - werecognize the air-conditioning system doesn't work enough in case it is too quiet, even if it works wellactually. Therefore, there is a need for a novel method to ‘design’ noises. The present study focuseson the thermal impression of sound by designing HVAC noise rather than considering the reductionof noise levels.
Previous literatures have investigated auditory and thermal cross-modal interactions (2–5). They
mainly examined the relation between the thermal sensation and noise level, However, the results
obtained are heterogeneous and sometimes contradictory. Meanwhile, from the point of view of thesound design of an HVAC system, few studies have focused on the thermal sensation and frequency
characteristics of noise. Roussarie et al. (6) found a significant interaction between sound and thermal
comfort, in that a specific sound significantly enhanced thermal comfort and that some types of noise
are more suited to air-conditioning systems than others. It was thus concluded that HVAC product*s-nakgawa@chiba-u.jp6781

engineers have to account for both auditory and thermal perceptions to provide the most comfortable
thermal condition at home or in a vehicle.
The passenger in a vehicle usually obtains information about operating conditions such as the
air volume from HVAC noise consciously or unconsciously. It is thus considered that there is thepotential to enhance the effects of cooling and heating employing HVAC sounds that better convey
cool and warm impressions respectively. In the present study, to clarify the frequency characteristicsof the vehicle HVAC sound and auditory impression concerning coolness and warmness, listening
tests were performed using a paired comparison technique under various conditions of room
temperature.

2. EXPEIMENTS
2.1

Stimuli
Model sounds synthesized from the recorded noise of an actual vehicle HVAC system were usedas stimuli. The noise generated by a vehicle HVAC was recorded at approximately the ear position forthe driver's seat. The HVAC system were set at the maximum air flow and were operated steadily. The
long-term power spectrum and spectrogram of the recorded HVAC noise are shown in Fig. 1.
To extract the spectral envelope, the linear predictive coding (LPC) spectral envelope (7) was
calculated from the recorded HVAC noise. Five model sounds were synthesized by stretching thespectral envelope by a factor of 1/2, 2/3, 1, 3/2, and 2 (giving models A, B, C, D, and E, respectively).
Here, the number of LPC dimensions for abstraction of the spectral envelope was 150. Generally, thehigher the number of LPC dimensions, the higher fidelity the synthetic sound has relative to theoriginal sound. From the results of preliminary studies, we selected 150 LPC dimensions so as tocreate synthetic sound similar enough to the original sound.
Auditory stimuli with stretching spectral envelopes (i.e., models A, B, C, D and E) were obtained
as follows by (1) generating Gaussian noise with sampling frequencies that were a product of the basesampling frequency of 48 kHz and the factor 1/2, 2/3, 1, 3/2, and 2, respectively; (2) passing thegenerated noise through the LPC filter; and (3) resampling the filtered signals at 48 kHz.
The power spectra of the synthesized HVAC model sounds are shown in Fig. 2. The spectral
centroids of the synthesized stimuli (A, B, C, D, and E) after the sounds passed through an Aweighting filter were 1678, 1896, 2146, 2334, and 2512 Hz, respectively, according to the equation
𝑆𝑝𝑒𝑐𝑡𝑟𝑎𝑙 𝐶𝑒𝑛𝑡𝑟𝑜𝑖𝑑∑𝑁
𝑘 1𝑘𝑓 𝑘∑𝑁
𝑘 1𝑓 𝑘(1)

where f(k) is the amplitude corresponding to bin k in the spectrum.
2.2

Methods and Apparatus
Two listening tests were carried out in a soundproof room, one in the summer (July 15 to 18) and
one in the winter (February 2 to 6). Twelve subjects participated in each of the summer and wintertests; five females and seven males aged between 21 and 24 years (mean age of 21.8 years) in thesummer experiment and four females and eight males aged between 20 and 23 years (mean age of

Figure 1 – (A) Long-term power spectrum and (B) spectrogram of the recorded HVAC noise.

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Figure 2 – Power spectrum of the HVAC model sounds synthesized by stretching the spectral envelope ofthe recorded HVAC noise by a factor of 1/2, 2/3, 1, 3/2, and 2 (referred as models A, B, C, D, and E,
respectively). G refers to the spectral centroids of the auditory stimuli and GA refers to those after sounds
passed an A-weighting filter.
21.9 years) in the winter test. Six subjects participated in both the summer and winter tests. Table I
gives the room-temperature conditions used in the experiments. In each experiment, there were three
sessions with different conditions of the room temperature, and subjects judged the auditory
impression concerning coolness and warmness in the summer and winter tests for the synthesized
HVAC model sounds employing the Scheffe’s paired comparison. For the control of the roomtemperature condition, the air conditioning equipment of the soundproof room was used. In bothsummer and winter tests, the sequence of the change in the room temperature was from warm to coldfor half of the subjects, and from cold to warm for the other half. The ambient dry-bulb temperature(Ta, °C), relative humidity (RH, %) and predicted mean vote (PMV) of the soundproof room andanterior chamber were measured using a portable amenity meter (AM-101, Kyoto Electronics
Manufacturing Co. Ltd., Japan).
Subjects were instructed to wait approximately 10 minutes before the start of the experiment inan anterior chamber where the room temperature was kept constant at about 25 and 22 °C in thesummer and winter tests, respectively. Furthermore, at the end of each session, subjects were
instructed to wait approximately 20 minutes in the anterior chamber. Subjects wore their own clothing
for the experiments, but they were instructed to adjust their clothing so that they were thermally
comfortable in the anterior chamber. Roughly estimated overall clothing insulation values for the test
subjects were 0.5 and 1.1 clo for the summer and winter tests, respectively. During the experiments,
the room temperature was monitored and the temperature change was confirmed to be within 1 °C.
Subjects were first asked to imagine a scenario: “HVAC noise is heard from the front unit when
you are driving a vehicle.” Additionally, subjects were instructed to assume a cooling operation in thesummer experiment and heating operation in the winter experiment. Auditory stimuli were synthesizedwith Matlab (The Mathworks, Inc., MA, USA) and presented diotically through a digital-to-analog
converter (Audiofire12, Echo Digital Audio Corporation, CA, USA) and headphones (HD650,
Sennheiser Electronic GmbH & Co. KG, Germany). The intensity of the stimulation was adjusted to
70 dBA for all auditory stimuli. The duration of each stimulus was 1.0 s. A comparison pair consists
the auditory impression of the stimuli for each pair on a seven-point scale (−3, −2, −1, 0, 1, 2, 3)
according to how much the latter is warmer (for the winter tests) or cooler (for the summer tests) than
the former. Scores of ±3, ±2, ±1 and 0 respectively indicate extreme, moderate, and slight differences6783

and no difference. A comparison pair was presented to a subject only once. Each subject judged 20
Table
1 – Room-temperature
conditions
pairs (5C2 × 2) in each session, anda totalof 60 pairs in three
sessions.
Experiment
Summer

WinterCondition

Ta [°C]RH [%]

PMV

hot2

31

62

2.1

hot1

28

61

1.1

even

25

59

0

even

22

62

-0.3

cool1

19

61

-1

cool2

16

59

-1.7

At end of the experiment, the subjects were asked to answer the following simple queries about
the judgments.
(1)
(2)
(3)
(4)

Were you able to judge the auditory stimuli as vehicle HVAC noise?
Was there any difficulty in judging?
Did you apply any conscious criteria in judging? If so, what were the criteria?
Were you aware that the room temperature affected the thermal impression of the auditory
stimuli?

3. RESULTS
The results obtained in the tests were analyzed employing Scheffe’s paired comparison method(Nakaya variation) [8]. Figure 3 shows the subjective scale values of each auditory stimulus on
yardstick scales as the results of the summer and winter tests. Since the scale value obtained in thewinter experiment
was inverted in terms of the sign, the lower scale values indicate a warmer impression and thehigher scale values indicate a cooler impression in both summer and winter tests. For all conditions,
results show that the auditory stimuli were arranged in the order of the spectral centroid: the stimuli
with a higher spectral centroid gave a cooler auditory impression. The differences in the results forstimuli A to E, taken as the largest difference in subjective scale values for all conditions, were larger
for a high room temperature in both summer and winter tests.
The results of an analysis of variance (ANOVA) reveal that the effect of the stimulus (the main
effect) was significant in both summer and winter tests (summer: F(4,12) = 77.6, p < 0.01; winter:
F(4,12) = 277.2, p < 0.01). Interaction effects of the stimuli and the room temperature conditions were
not significant in either the summer or winter test. The combination effect of stimulation was
significant in the winter test (F(6,12) = 3.25, p < 0.05), while it was not significant in the summer test.
Table II gives the subjective scores of the auditory stimuli obtained for the individual subjects inthe summer and winter tests. In the results of the summer test, the effect of the stimulus was significant
for 10 subjects of the 12 subjects, and its trend corresponded with the overall results: the auditory
stimuli with a higher spectral centroid were considered cooler, and those with a lower spectral centroid
were considered warmer. For one of the remaining two subjects, the effect of the stimulus was not
significant. For the other subject, the effect of stimulus was significant but the trend was the oppositeof that for the overall results (in SS9): the auditory stimuli with a higher spectral centroid wereconsidered warmer. Meanwhile, in the winter test, the effect of the stimulus was significant for 11
subjects of the 12 subjects, and its trend corresponded with the overall results. Again, the results forone subject show that the effect of the stimulus was significant but the trend was opposite that of theoverall results (in SW6). This subject was not a participant in the summer test.
In the questionnaire that was conducted after the experiment, all subjects answered that they
could accept all the auditory stimuli as vehicle HVAC noise. In addition, most of the subjects whose
results corresponded with the overall results reported that they consciously judged the stimuli using acriterion: they perceived the auditory stimuli having “high-frequency” sound as cool and those having“low-frequency” sound as warm. Meanwhile, the subjects whose results opposed the overall results
also consciously used a judging criterion: the opposite of the general trend mentioned above. In

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Figure 3 – Subjective scores of each auditory stimulus on yardstick scales as the result of the Scheffe’s
paired comparison of the summer and winter tests.

addition, some subjects made their judgment according to scenes recalled by the auditory stimuli (e.g.,a low-frequency stimulus recalled a fireplace, and a high-frequency stimulus recalled a cold wind).
Furthermore, although the stimulus intensity was set to 70 dBA for all stimuli, there were somesubjects who answered that a difference in stimulus intensity was a judging criterion. Most subjects
answered that they were unaware that the room temperature affected the thermal impression of thestimuli, while some subjects answered that the auditory impressions were enhanced when the room
temperature was lower/higher.

4. DISCUSSIONS
The results of the present study reveal that the spectral centroid significantly affects the auditory
impression of coolness and warmness; i.e., a higher spectral centroid induces a cooler auditory
impression regardless of the room temperature. This robust response of the auditory impression of
coolness and warmness to vehicle HVAC noise having different spectral centroids is similar to thethermal impressions induced by “warm” and “cool” colors (see (8) for a comprehensive review).
However, it is clear that the effect is the opposite for some people: a higher/lower spectral centroid
induces a warmer/cooler auditory impression. This suggests that the cultural background andexperience are factors.

5. CONCLUSIONS
The results of the study reveal a tendency that the difference in impressions is wider at higher room
temperature, regardless of the cooling or heating operation. This result suggests that the design of thespectral centroid of HVAC noise would be more effective for the cooling operation in summer,
although there is a need for further investigation.

ACKNOWLEDGEMENTS
The authors wish to acknowledge valuable discussions with Dr. Satoshi Inoue, Dr. Masaharu Sakai,
Dr. Kenji Ishida and Mr. Masashi Watanabe of DENSO Corp. and Dr. Yoshiharu Soeta and Dr. Yosuke
Okamoto of AIST.

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Table 2 – Subjective scores of the auditory stimuli obtained for individual subjects
Subject

Summer

Winter

S S1
S S2
S S3
S S4
S S5
S S6
S S7
S S8
S S9
S S10
S S11
S S12
S w1
S w2
S w3
S w4
S w5
S w6
S w7
S w8
S w9
S w10
S w11
S w12

StimuliSignificance level

A

B

C

D

E

-1.00
-0.40
-0.80
-0.57
-0.73
-1.30
-1.47
0.07
1.17
-0.57
-1.10
-0.07
-0.87
-1.43
-0.87
-1.53
-0.33
1.47
-0.83
-1.20
-0.90
-1.13
-1.03
-0.70

-0.70
-0.17
-0.73
-0.67
-0.70
-1.03
-0.80
0.27
0.40
-0.37
-0.70
-0.03
-0.80
-0.70
-0.73
-1.07
-0.47
0.97
-0.70
-0.63
-0.53
-0.40
-0.70
-0.73

-0.23
-0.07
0.10
0.77
0.07
-0.17
0.10
-0.03
-0.03
-0.20
-0.03
-0.60
0.43
0.27
0.23
0.53
-0.10
-0.67
0.43
0.43
0.13
-0.27
0.47
0.07

0.73
0.20
0.63
0.27
0.37
0.97
0.90
0.03
-0.53
0.67
0.53
0.37
0.53
0.80
0.70
0.90
0.23
-0.87
0.57
0.43
0.50
0.53
0.37
0.60

1.20
0.43
0.80
0.20
1.00
1.53
1.27
-0.33
-1.00
0.47
1.30
0.33
0.70
1.07
0.67
1.17
0.67
-0.90
0.53
0.97
0.80
1.27
0.90
0.77

Sα
**
**
**
**
**
**
**

Sα (B)

Sγ

**
**
**
**
*
**
**
**
**
**
**
**
**
**
**
**
**

*
*

**

Yardstick (Y)Y(0.05)
0.60
0.63
0.49
1.01
0.59
0.72
0.69
0.90
0.62
0.82
0.79
0.91
0.80
0.60
0.61
0.44
0.80
0.71
0.16
0.68
0.52
0.76
0.70
0.33

Y(0.01)
0.77
0.81
0.62
1.30
0.76
0.92
0.89
1.16
0.79
1.05
1.01
1.17
1.02
0.77
0.78
0.56
1.03
0.91
0.21
0.87
0.67
0.97
0.90
0.43

*: 5% significance level; **: 1% significance level

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