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

Acoustic Comfort in Multi-group Conversation Space: a Field
Experiment in an Actual Café with Different BGM Levels
Naoya MARUYAMA 1; Yasuhiro HIRAGURI 2; Keiji KAWAI 3, Mari UEDA 4
1, 3

Kumamoto University, Japan
2

4

Kindai University, Japan

Kanagawa Institute of Technology, Japan

ABSTRACTWe refer to “Multi-group conversation space (MGCS)” as a space where more than one groups share aspace and make conversations within the groups, such as restaurant or café. In this study, we investigatedacoustic comfort in MGCS considering speech intelligibility and speech privacy as major factors byconducting acoustic measurements and a questionnaire survey to the customers for four days in an actual
café. The volume of background music (BGM) during operating hours was changed each day at three levelsof low, middle and high and no BGM. The indoor noise levels were measured at six representative points
with six sound level meters hung from the ceiling. The questionnaire consisted of eight items including easeof making conversation, comfort, and atmosphere (lively/relaxed). The customers were asked to answer
questions at their table after having their meal or drink and, at the same time, the occupancy of neighboringtable and the number of customers in the café were recorded. Results show that the ease of conversation
tended to be evaluated better when the volume setting of the BGM was middle level than other settings. Also
the impression evaluation varied by gender, age, or seat position and occupancy.
Keywords: BGM, acoustic comfort, café, ease of talking

1. INTRODUCTION

There are rooms, such as restaurant or café, where more than one groups share a space and makeconversations within the groups and we refer to such spaces as “multi-group conversation space(MGCS).” It would be not easy to make conversation when a MGCS had little sound absorption
because of a poor speech intelligibility caused by much reverberation, but it would also be not easy
when a MGCS had much sound absorption because the conversation, particularly a business talk or atalk of private matters, could be clearly heard between groups. Thus it should be assumed some
optimal range of acoustic condition for a comfortable conversation in which both speech intelligibility
and speech privacy are adequately balanced and the purpose of this study is to investigate such a rangeof the acoustic condition regarding background sound and reverberation. Rindel (1) proposed a simple
prediction model for the noise level in restaurant due to many people speaking in a room consideringthe Lombard effect and provided the recommended minimum absorption area, such as 17 m2 for “good”
verbal communication. Also Rindel (2) proposed a simple evaluation index of “Acoustical capacity”
for the speech intelligibility in eating establishments. Astolfi et al. (3) made an attempt to investigate
the acoustical quality in restaurants regarding speech privacy and intelligibility by conducting asurvey in four pizzeria-restaurants in which seat density and sound absorption were discussed withsubjective evaluations. Nahid et al. (4) performed a simulation subjecting actual eating establishments
to find a balanced condition of good speech intelligibility within a table and good speech privacy
between tables and the result indicated that tall and absorptive barriers, which might not be suitable
182d8435@st.kumamoto-u.ac.jp
hiraguri@arch.kindai.ac.jp
3kkawai@kumamoto-u.ac.jp
4m-ueda@ic.kanagawa-it.ac.jp
1
2

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in eating establishments in terms of space and interior design, are effective. As well as architectural
aspects, background music (BGM) is an important element of acoustic design in MGCS and this has
been studied considerably such as Novak (5). As a whole, studies are still few on the acoustic quality
in MGCS and more investigation for comfortable acoustic condition in terms of speech intelligibility
and privacy with factors of background sounds and noises and sound absorption should be needed.
Thus, in the present study, a field experiment with questionnaire and acoustic measurement was
conducted in an actual café, varying the level of BGM.

2. SPACE OF FIELD EXPERIMENT
2.1 Overview
The experiment was carried out on four days in December 2018 at a café in Kinosaki town, Japan.
The building originally was an old fire station with RC structure and converted into a café with asmall art gallery in September 2018 (Fig. 1). The floor plan, dimensions and interior surface materials
are presented in Fig. 2 and Tables 1 and 2. Although sound absorptive material (wood wool cement
board) was used on the ceiling, the space had a long reverberation time due to the high ceiling andother reflective surfaces, as described later. The café had ten tables to which IDs from A to I wereassigned as target tables and ID X was assigned as the experimenter table. Six sound level meters(NL-42; Rion Co. Ltd.) were hung 1 m from the ceiling as receiving points for noise measurements.
The noise levels at each of the tables (A-I in Fig. 2) was assumed as the level of the nearest receiving
point or the mean value of the nearest two receiving point if the distances to the two point was similar.

Table 1 Room dimensions
Floor

92.3 m2

Volume285.2 m3

Surface319.1 m2

Height

3.3 m (3.0, 3.6 m)

Seats

43

Table 2 Interior surface materials
ceiling

Wood wool cement
board

wall

Beton brut +emulsion paint finish

floor

Beton brut andcarpetFig. 1 Inside picture of café U.

Fig. 2 Floor plan.

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2.2 Background Sounds
Table 3 BGM volume conditions
The main background sounds without
Day 1
Day 2
Day 3
Day 4
conversation in the cafe were BGM and
(Wed)
(Fri)
(Sat)
(Sun)
operating noise of the air-conditioning unit. The
BGM
no
high
mid
low
BGM was played from two loudspeakers(Electro-Voice S40W) at one end of the room,
level
―
±0 dB
-4.8 dB-9.6 dB
which created a difference in sound pressure
Table 4 Estimated sound level at each tablelevels depending on the distance fromloudspeakers. The BGM volume was set at four
BGM only [LAeq, dB]
AC only
ID SLM [L dB]
conditions (none and three levels of volume) and
Aeq,
low
mid
High
changed in each of the four days of the
A 1&2
57.8
58.3
63.1
67.9
experiment as shown in Table 3. In order to set
B
1&2
57.8
58.3
63.1
67.9
the BGM volume conditions, “high” condition
C
2
57.8
59.0
63.8
68.6
was judged by the experimenter at the nearest
seat of the loudspeakers as the upper limit of the
D
3
59.8
53.8
58.6
63.4
subjectively comfortable volume range. Then
E 3&4
59.1
54.1
58.9
63.7
“mid” and “low” conditions were set at 4.8 dB
F
3
59.8
53.8
58.6
63.4
and 9.6 dB lower, respectively, than the “high”
G
4
58.2
54.4
59.2
64.0
condition as 1.6 dB was the step of the volume
H
5
56.5
52.0
56.8
61.6
dial. The BGM playlist was “Carpenters (nameof musician duo)” channel (USEN MPX-1
I
5
56.5
52.0
56.8
61.6
00032) in which several dozens of the songs of※ background noise was about 50 dB
Carpenters were randomly played.
Table 5 RT and intelligibility indices
The air-conditioning noise was derived by theoperation of a large fan coil unit installed on the
Distance [m]T30 [s]STI
𝐷𝐷50
ceiling. The noise was measured without BGM
1.0
0.77 (0.01)
0.83 (0.03)
and customers in the café. The assumed noise
1.0(0.02)
levels attributable only to the air-conditioning
5.0
0.63 (0.02)
0.50 (0.08)
noise and only to the BGM at each table arepresented in Table 4.
The air-conditioning noise levels was rather high at 56–60 dB and higher than the BGM level atmost of the tables with the “low” condition and at the D, E and F tables with the “mid” condition. The
BGM levels varied 52-69 dB, 17 dB difference, depending on the volume conditions and the tablelocation. The 4-7 dB difference in the BGM level on the same day between the tables A-C and D-I
was similar to or greater than the level differences between one step of the BGM condition.
2.3 Acoustic Characteristics
Room impulse responses were measured using a swept-sine signal emitted by an omnidirectional
loudspeaker to calculate reverberation time and intelligibility indices (STI and D 50 ). Five source
points were set at positions of a chair of different tables and two receiving points were set at distancesof 1 m and 5 m from each of the source points (Fig. 2). The two receiving points were assumed torepresent voices within groups and voices coming from distant tables.
Mean values and standard deviations of these indices are shown in Table 5. The reverberation timewas 1.0 s and the estimated average sound absorption coefficient by the Eyring’s formula was 0.14.
The STI of 1 m / 5 m was 0.77 / 0.63, respectively. This indicated good speech intelligibility not only
within a table but also between distant tables, which might cause a lack of speech privacy whenbackground noise was low. Rindel proposed Acoustical Capacity (AC) to estimate the quality of verbal
communication. AC is calculated from the reverberation time (T) and room volume(V): AC = V / 20T,
and in this case was 13. The ratio of the total number (fully occupied) of seats (43) to AC of this café
was 3.0 and the ratio of the condition during the experiment was around 0.6 in average and 2.0 at themost. AC <1 is evaluated “sufficient” and the ambient noise level at AC = 0.6 is estimated as 65 dB
according to Rindel. The 65 dB seemed close to the actual level as shown in Fig. 3.

3. EXPERIMENTAL IMPLEMENTATION
3.1 Questionnaire Survey
A questionnaire survey was administered to elicit the impressions of customers of the acoustic

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Table 7 Customer attributes

Table 6 Items of impression evaluation
Very comfortable―

Q2

Relaxed―LivelyQ3

Easy to carry out
a conversation―

Difficult to carry out
a conversation

Q4

Not at all noisy―

Extremely noisy

Q 5*

Not at all bothered by
nearby conversations―

Extremely bothered by
nearby conversations

Q 6**

Not at all reverberate―

Extremely reverberate

Q 7**

Not difficult to hear
within the group― Extremely difficult to hear
within the group

Q8

Low (BGM volume)

―

Q1

BGM
volume
Day 1

no

Day 2

high

Day 3

mid

Day 4

low

High (BGM volume)

2
4
1
2
4
6
19
19

1
3
4
4
7
7
9
15

Age groupTotal
35– 45– 55–
65–
44
54
64
1
2
0
1
7
0
4
6
3
20
1
3
8
8
25
2
3
8
4
23
1
4
6
2
25
6
10
7
4
41
4
2
10
8
52
8
14
6
10
72

57

50

23

25–
–24
34
M
F
M
F
M
F
M
F

Total

42

51

40

265

** four-point unipolar scale + “difficult

Day 1 (no BGM)Day 3 (BGM : mid)Day 2 (BGM : high)Day 4 (BGM : low)

55

10

55

10

55

10

55

10

55

50

5

50

5

50

5

50

5

50

0

45

0

45

0

45

0

45

8:00

45

Time

Time

Time

Fig. 3 A-weighted sound pressure level and number of customers.

16:00

60

17:00

15

15:00

60

14:00

15

12:00

60

13:00

15

11:00

60

9:00

15

60

10:00

65

17:00

20

16:00

65

15:00

20

13:00

65

14:00

20

11:00

65

12:00

20

9:00

65

10:00

25

17:00

70

16:00

25

15:00

70

13:00

25

14:00

70

11:00

25

12:00

70

10:00

30

25

8:00

75

70

9:00

30

15:00

75

13:00

30

14:00

75

11:00

30

12:00

75

9:00

30

10:00

75

Time

20
15
10
5

Number of cust0mers

Noise Level (dB)* five-point bipolar scale
to judge”

Uncomfortable

0
AC
on
off

environment and the relation between impressions and acoustic properties. The questionnaire
consisted of 13 items: (i) eight items about impressions of the acoustic environment with four or five
step evaluation scales (Table 6), (ii) individual attributes, (iii) purposes of use, and (iv) an open
question for free comment. As for a question about reverberation, the actual question sentence was"To what level dose sound carry in this café?" considering that respondents might not know or noticereverberation. The questionnaire sheet was also prepared in English and Chinese version for foreign
visitors.
The questionnaire was carried out during the whole opening hours (8 AM–6 PM). The experimenter(2 people) stayed at table X (Fig. 2) all the time and handed customers the questionnaire sheet after acertain time they made conversation in the café, such as when they had finished eating or drinking.
They were not given explanation about the questions and wrote down the answers by themselves. At
the same time, they were answering the questions, we recorded the number of all the customers in thecafé and their seating tables every 10 min. In total, 265 customers (109 male and 156 female)
responded (Table 7) and, among them, there were 12 foreign respondents from China (6 people), Hong
Kong (2 people), Thailand (2 people), Taiwan (2 people), and Malaysia (2 people), respectively.
3.2 Noise Level Measurement
The noise level was measured continuously during the opening hours on 4 days by six sound level
meters (NL-42; Rion Co. Ltd.) hung at 1 m from the ceiling. The L Aeq,10 min and L A95,10 min (energy
average values of the six receiving points), the number of customers, and the turning on/off of airconditioning unit are portrayed in Fig. 3. On Day 1, the fluctuation of L A95 was greater than those on
the other days due to the condition of no BGM. Actually, L A95 was affected greatly by the intermittent
operation of the air-conditioning unit. Such a fluctuation was also seen with “low” BGM condition on
Day 4 but when the BGM condition was “mid” or “high,” the noise level became rather stable because
the BGM level excessed the air-conditioning noise level.
The noise level received by each respondent (L reci : receiving level) was defined as the L Aeq of themeasured noise level of each of the table during the time from when the customer seated the table towhen they finished answering the questions. The L reci were distributed 53-69 dB.

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4. RESULT
4.1 Impression Evaluation
Results for the eight impression questions by the BGM conditions are shown in Fig. 4. The numberof respondents in each BGM condition varied as shown in table.7. In general, negative responses wererare as the field was a comfortable café. Some negative answers were found in Q4, Q7 or Q8 whenthe BGM was “high.” In Q8, the answer greatly changed with BGM conditions but not evaluateduncomfortable in Q1, which indicated the BGM volume conditions were adequate.
Regarding the questions related to conversation, the negative answer to Q3, related to conversation
comfort, was fewer with “low” and “mid” BGM than with “no” or “high” BGM. The negative answer
to Q5, related to speech privacy, was the fewest with “mid” BGM and increased in “high”, “low” and“no” in this order. As for Q7, related to speech intelligibility, there was no negative answer with “no”
BGM and the negative answer increased along with the volume of the BGM. As a whole, these result
implied that the comfortable condition for conversation could have both aspects of speech
intelligibility and privacy.
4.2 Effects of Neighboring Table Occupancy on Impression Evaluation
The result of Q3: “ease of conversation” indicated that the evaluation was better with “low” and“mid” BGM than with “no” or “high,” as shown in Fig. 4. Here this tendency will be further discussedwith regard to the occupancy of neighboring tables. Figs. 7 and 8 show the ratio of negative answers
to Q3, namely, “difficult” and “slightly difficult” by the occupancy of the neighboring table of the
respondent for either BGM conditions or L reci, respectively. Fig. 7 shows the mean plot of the answers
to Q5: “bothered by nearby conversation” and Q7: “difficulty to hear within the group” also by theoccupancy of the neighboring tables and BGM conditions. The occupancy of the table was observed
0%

40%

20%

80%

60%

no

40

52

high

low

1

mid

15 4

29

Very comfortable

Rather comfortable

Neither

Slightly
uncomfortable

35

40%

20%

39
33

mid
23

high

80%

60%
41

22

24

30

0%

100%

high

46

23

Slightly lively

Neither

Slightly difficult

DifficultNon-response

40%

20%

low

6

mid

44

1

high

35

(3)Ease of conversation
80%

60%

44
35

100%

37

19

49

7
15

2
4

52

13

52

0%

40%

20%

low

27

mid

21

high 8Moderately
Difficult to judge

73
74

mid

80%

46

20

49

4 2

52

18 1 5 242

8 4

Not difficult

Slightly difficult

Moderately difficult

Extremely difficult

Difficult to judge(7)Difficulty of hearing
within the group(6)Room reverberation

100%
0%

20%

low

8

19

mid

2

7

high

40%

62

60%

80%

100%

67

52

51

33
Low

3

33

59
52

14 3 327

Extremely

Very
Non-response

37

52 12

Slightly

Extremely

15

Slightly low

Moderate

Slightly highHigh

Non-response(8)Volume of BGM

Fig. 4 Results of evaluation.

100%
19

Not at all

Moderately

60%

4

59
52

Non-response(5)Bothered by nearby
conversation

80%

33
55

Extremely noisy

Slightly

60%

44

no

Not so noisy

93

19

An appropriate
mixture of both

Slightly noisy

40%

12

Slightly easy

Neither

20%

9 3

20

Easy

14 1

27

12 6

31
68

Not at all noisy

low

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21

51

Not at all

no

high

13

100%

Slightly relaxedno(4)Noisiness
0%

mid

80%

11 4 11 4

Relaxed

15

36

24
19

low

8 6

(2)Relaxed/Lively

44

low

6 10 160%

70

Lively(1)Comfortable
0%

31

40%

20%

no

23

42
64

high

0%

100%

11 4 441

Uncomfortable

no

80%

60%

81

7 2
20

79

mid

40%

20%

no

19

51

low

0%

100%

81

13

Fig. 5 Difficulty of conversation by BGM

volume and neighboring table occupancy.

Fig. 7 Evaluation by BGM level andoccupancy of neighboring table.

Fig. 6 Difficulty of conversation by noiselevel and neighboring table occupancy

throughout the experiment by the experimenter positioned at the table X in Fig.2 and “occupied” wasdefined as the situation that any of the surrounding neighbor tables of a respondent was seated by anyother customer(s).
As seen in Fig.5, the ratio was the lowest with “mid” BGM and followed by “low,” “no,” and “high”
in this order. Regarding the occupancy of the neighboring tables, the ratio in occupied situation was
higher than in unoccupied situation with “mid” and “high” BGM while the ratios were similar with“no” and “low” BGM. Particularly, the ratio was high with “high” BGM at 25 % when occupied, 2.5
times more than when unoccupied. The result with L reci was similar but the ratio in unoccupied
situation was rather higher than that in occupied situation when the L reci was less than 63 dB (Fig. 6).
The situation where the neighboring table was occupied and L reci was low could be assumed that thecustomers of the neighboring table were quiet and this result might come from this kind of situation.
Presumably BGM would mask surrounding sounds and the conversation from other tables might
become less bothering. However, the result of Q5 in Fig.7 was opposite such that the evaluation
relatively became more bothered with “mid” and “high” BGM when the neighboring table occupied.
Also in Q7, the evaluation became the more difficult to hear with the higher level of BGM when theneighboring table was occupied.

5. THE CUSTOMER CLUSTERING

A correlation matrix for items of impression evaluations is shown in Table 8. Seven items otherthan Q8: “feeling about BGM volume” were correlated at r = 0.3–0.5. To classify the items, a factor
analysis was performed for seven items other than Q8, which can naturally be regarded as anindependent item that is simply affected by the BGM level. Factors were extracted with eigenvaluesof 3.41, 0.81, 0.70, etc. Therefore, we made a varimax rotation for the top two factors. The resultant
factor loadings are presented in Table 8. The first factor was interpreted as related to comfortablenessor sound level nearby the respondent, which highly correlated with the Q2: “room atmosphere (lively/

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Table 8 Correlation matrix of questionnaire items and factor loadings
Item

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

Item

F1

F2

(2) relaxed

0.83

0.16

(4) noisiness

0.76

0.24

(1) comfort

0.71

0.24

(5) bothered

0.65

0.25

(5) bothered by nearby -0.41 -0.39 -0.40 -0.49
conversations(7) difficulty

0.13

0.86

(6) room reverberant-0.36 -0.35 -0.37 -0.39 0.34

(6) reverberant

0.28

0.66

(7) difficulty to hear
within the group-0.33 -0.30 -0.45 -0.36 0.31

0.34(3) easiness

0.51

0.54

(8) volume of BGM-0.08 -0.13 -0.11 -0.16 0.00

0.30

(1) comfort(2) relaxed / lively

0.52(3) easiness of carryingout a conversation

0.41

0.52

(4) noisiness

0.44

0.59

0.40

Contribution

0.19

36% 24%

Table 9 Number of respondents in each cluster
Cluster
C1
C2
C3
Total

GenderMale
Female
43
42
27
30
38
82
108
154

Location of table
A–C
D–G
H–I
36
36
13
35
17
5
35
68
17
106
121
35

no
5
2
19
26

BGM volume
low
mid
51
14
19
20
52
33
123
66

High
15
16
16
47

Fig. 8 Variation of impression evaluations of each BGM volume and classification of respondents.

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Total
85
57
120
262

relaxed),” Q1: “comfort,” and Q5: “bothered by the nearby conversations.” The second factor wasinterpreted as related to speech intelligibility, showing high correlation with Q7: “difficulty of hearing
within the group” and “room reverberant.”
We sought some patterns of respondents’ evaluation tendencies. A k-means cluster analysis (k=3)
was applied using the two factor scores of each respondent as Euclidean distance. Fig. 8 shows themean values of items of Q1: “comfort”, Q2: “room atmosphere (lively/relaxed),” Q3: “ease of
conversation” and Q7: “difficulty of hearing within the group” by three clusters (C1, C2, and C3).
The cluster profile is shown in Table 9. Typically, C1 had many respondents in a “low” BGM; C2 hadmany respondents close to the loudspeakers with high BGM. C3 consisted of many women and many
respondents far from the loudspeakers.
Regarding the evaluation patterns, C3 evaluated positively overall. The relative changes of
evaluation to the four items along with the BGM volume were similar between C1 and C2, while C2
evaluated more positively than C1 for the three items other than Q3: “ease of conversation.” Regarding
the item of “ease of conversation,” C2 evaluated negatively in the no BGM while C1 and C2 showedsimilar evaluation when the BGM was played. Although this result was not significant because of alimited sample size, this result implied that some customers might not prefer a quiet situation when
making conversation.

6. CONCLUSIONSIn this study, a field experiment was conducted in an actual cafe with varying BGM volume
conditions and evaluations of customers for items particularly addressing conversations in multigroup conversation space (MGCS) were investigated along with the acoustic conditions and customers’
location.
The results are summarized as follows.
1) Evaluation of Q3: “ease of conversation” tended to be a little negative when the BGM volume or
the received noise level was either high or low. This result suggests that an adequate backgroundsound levels might exist for comfortable conversation.
2) Occupancy of nearby tables tended to affect the comfortableness of conversation with regard to
speech privacy and intelligibility.
3) Results through a cluster analysis indicated that there were some customers who did not prefer aquiet situation.
This study is among the first trials of a field experiment in an actual MGCS and the results are notdecisive nor conclusive. The results suggested some optimum conditions for easy and comfortable
conversations in MGCS and further investigations should be accumulated.

ACKNOWLEDGEMENTS
We thank “Café Utsuroi Tsuchiya Annex” for the cooperation with this field experiment.

REFERENCES

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2. Rindel J.H., Acoustical capacity as a means of noise control in eating establishments. Proceedings of
BNAM 2012, Odense, Denmark, 2012.
3. Astolfi A. and Filippi M. Good Acoustical Quality in Restaurants: A Compromise between SpeechIntelligibility and Privacy. Proc. of ICA2004, 2004.
4. Nahid M. and Hodgson M. Prediction of optimal conditions for verbal-communication quality in eating
establishments. Journal of Acoustical Society of America 2011; 129(4): 2005–2014.
5. Novak CC, Lopa JL and Novak RE. Effects of Sound Pressure Levels and Sensitivity to Noise on Mood
and Behavioral Intent in a Controlled Fine Dining Restaurant Environment. Journal of Culinary Science& Technology 2010; 8: 191–218.

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