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

The effect of reverberated speech on working memory: Toward an
optimal balance of calmness and liveliness in libraries
Kazuma SHAMOTO1; Hiroko TERASAWA2; Hiroshi ITSUMURA3
1, 2, 3

University of Tsukuba, Japan
1

Ono Sokki, Japan

ABSTRACT
In order to accommodate both young talkative children and people who want a quiet library environment, weinvestigated the effects of the reverberation of children’s voices on the working memory function of thosearound them. While modern libraries often hold noisy activities, unlike traditional silent libraries, theirarchitecture has also become more open and reverberant. In this study, we tested the working memory
functions of healthy participants under four sound conditions: (1) children’s conversational voices without
reverberation, (2) children’s voices with reverberations from a traditional library environment (short andmuted reverberation), (3) children’s voices with reverberations from a modern library (long and echoic
reverberation), and (4) silence as a control condition. Working memory capacity was measured by Operation
Span Task (OST) and compared across the four sound conditions. Twenty university students participated in
the experiment. As a result, OTC (i.e., Ospan total correct, the score of OST) was found to be best in silent
conditions, followed by modern library conditions, traditional library conditions, and finally non-reverberant
conditions. In addition, OTC was improved in conditions where participants felt comfortable. This suggeststhat choosing a learning space of our own preference can result in a learning space that provides good working
efficiency for each individual user.
Keywords: Sound, Working Memory Capacity, Room Acoustics, Sound Environment, Library

1. INTRODUCTION
Although quiet sound environments have been idealized as a learning space, modern libraries
include active, social, and interactive learning for people of all ages. While libraries today include
noisy learning activities, the architecture of libraries also became more open and reverberant, reducing
traditional quiet spaces. Active learning spaces in modern libraries ideally have a comfortable murmurof visitor’s speaking voices (i.e., the mixture of the conversational sounds and the reverberations) that
accommodates the effective functioning of working memory. In this study, we investigated the effectof children’s conversation sounds under reverberation on working memory, as well as the impressionof the reverberated conversational sounds.
Traditionally, ideal learning spaces were quiet. Both classrooms and libraries were expected to besilent. Quiet spaces improves working memory (1,2), and in Denmark, for example, the recommended
reverberation time in school classroom is as short as 0.6 seconds (3). However, when the type of
intellectual activity changes, a noisy environment may be more suitable than a quiet environment. In
contrast to the research on school classrooms, some studies have shown that a lively soundenvironment, like a café, is richer in creativity (4). Perhaps this is why many people enjoy working
on a laptop in a café.
Today, intellectual activity and learning are expected to be more creative than mere memorization.
Figure 1 shows three levels of intellectual activities: (1) information processing, (2) knowledge
processing, and (3) knowledge creation. The old style of learning was primarily composed of
information processing and knowledge processing, but current learning trends involve knowledge
12

shamoto.kazuma.ru@alumni.tsukuba.ac.jp
terasawa@slis.tsukuba.ac.jp4537

Figure 1 – The relationship between the hierarchical model ofintellectual activity and architectural space (Adapted from (5))
creation. This change in learning styles calls for a change in acoustical environments and architectural
styles in learning spaces. Quiet and focused learning spaces with a short reverberation time wereconsidered to be optimal for traditional learning. However, if you want to actively exchange ideas andlearn creatively, comfortable background noise may be better for encouraging oral communications.
Many modern libraries aim to provide a comfortable space that includes all three levels ofintellectual activity. Increasing the value of ‘library as a place’, and making the library a place where
various intellectual activities can be performed is a new role for modern libraries (6).
As the purpose and design of libraries changed, new issues arose. In our pilot study, librarians
often mentioned that they receive complaints about conversational voices in the children’s area or thelearning commons. We regarded these noise complaints as an indicator that noise from knowledge
creation tasks may disturb the working memory that is required for information and knowledge
processing tasks.
Information and knowledge processing are constructed upon working memory. Working memory,
including short-term memory, often functions in auditory format in our mind (7). External noise
conflicts with the auditory information in our mind, and disturbs the functions of working memory.
In this way, information and knowledge processing tasks are easily disturbed by speech sounds.
Short-term memory is more disturbed by intelligible speech sounds than by meaningless white
noise (1). If we use room reverberation to transform an intelligible speech sound into ambiguous
speech noise that is less intelligible, the transformed speech noise can better facilitate information andknowledge processing than the original intelligible speech sounds. If we can find an adequate degreeof ambiguity for speech noise transformation to facilitate information and knowledge processing, thenwe can find common ground to accommodate all three levels of intellectual activity.

2. PURPOSE
In this study, we tested working memory under four sound conditions: (1) direct (children’s direct
voices without reverberation), (2) traditional (children’s voices with reverberation of a traditional
library), (3) modern (children’s voices with reverberation of a modern library) and (4) silence (control
condition). The hypothesis underlying this experiment is that conversational speech can betransformed to a comfortable murmur by applying an appropriate reverberation; allowing working
memory to function more easily. Reverberation time is closely related to speech intelligibility,
background noise, and the impression of the space. Finding an appropriate reverberation time that
does not spoil the function of working memory can lead to the development of guidelines for acoustic
design in libraries. In this study, we targeted children's conversational sounds that were mostfrequently complained by library visitors.

3. PREPARATION OF STIMULI
The stimuli for this experiment were prepared according to the following procedures: (1) Measure
impulse responses in traditional and modern libraries, (2) record several children’s conversationalsounds, and (3) convolve the conversational sounds with impulse responses.

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3.1 Impulse response measurement
Impulse responses were measured at two libraries: one traditional university library (University ofTsukuba, Kasuga Library), and another modern public library (Yachiyo City Central Library).

Figure 2 – University of Tsukuba, Kasuga
Library (traditional)

Figure 3 – Yachiyo City Central Library(modern)

Kasuga Library is a traditional university library, holding many books in a quiet sound environment(Figure 2). We measured the impulse response in the quiet study area on the second floor wherebookshelf is dense. The floor area is about 467 m2 (5026.75 ft2 ), the height to the ceiling is about 2.7
m (8.86 ft.), and the average reverberation time is 0.3 seconds.
Yachiyo City Central Library is a modern public library opened in 2015 in Yachiyo City, Chiba
Prefecture (Figure 3). Yachiyo Library has an open design on one spacious floor. The innermost areais the quiet reading area, the middle area is the browsing area, and the outermost area is the children’scorner and reference. The floor area is about 1550 m2 (16684 ft2 ), the varied ceiling height ranges
from 2.9 to -9.0 m (9.5-29.5 ft.), and the average reverberation time is 1.0 seconds.
The following equipment was used for impulse response measurement: An SPL meter withrecording function (Ono Sokki LA-3260), an omnidirectional speaker (solid acoustics sa-755
professional), an amplifier (YAMAHA P4050), a laptop (Macbook Pro 13-inch 2012), a soundcard(RME Babyface), and a laser distance meter (BOSCH GLM7000). The measurement was approved at
the library staff meetings at each library.
Table 1 shows the distance from the sound sources, reverberation time, and definition of theimpulse responses we identified. Definition (i.e., degree of speech intelligibility) is calculated asfollows, where p(t) is sound pressure (Pa) in time.
𝐷𝑒𝑓𝑖𝑛𝑖𝑡𝑖𝑜𝑛 =

/

1234

*𝑝(𝑡). 𝑑𝑡

5

/

∫2

∫2 *𝑝(𝑡). 𝑑𝑡

× 100(%)(1)

The reverberation time for Yachiyo Library is longer, and the definition is larger for the Kasuga
Library. In short, the Yachiyo Library has a long and echoic reverberation and the Kasuga Library has
short and muted reverberation. We controlled the distances from the sound source to the soundreceiving point to be as similar as possible, for these two impulse responses.
Table 1 – Characteristics of the impulse responses we used for the stimuli
Reverberation
Library
Distance (m)Distance (ft)Definition
Time (s)Kasuga Library
5.59
18.3
0.3
0.9
(traditional)
Yachiyo Library
4.95
16.2
0.4
0.7
(modern)
3.2 Recording children’s voices
We recorded children’s conversational sounds in order to simulate the noise from active children

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in the libraries. We used a field recorder (R-05 Roland) in PCM formant, with a 44.1 kHz samplingrate. Five families with children ranging in age from two to ten years old volunteered, with priorconsent to the intended research. The children’s voices were recorded in the dining or living room,
which has little reverberation and enabled us to record direct voice sounds. These locations may not
be acoustically ideal as anechoic chamber, but among the areas at home where children can conduct
natural conversations, they are the best choices. In order to record natural conversation, the parentsof each family recorded the children’s voices without the experimenter present. We explained therecording procedure to each of the parents, and instructed them to adhere to the following procedures:(1) Record when the children are gathered together and having fun, such as during tea time or a party,(2) place the recorder where it will pick up the children’s voices clearly, such as in the centre of thetable, and (3) record for about one and a half hours in total to allow for later editing.
3.3 Stimulus sound without reverberation
We edited the conversational sounds recorded in each household, and prepared the stimulus sounds
before reverberation in the libraries according to the following procedures: (1) Delete sections of therecording that contain private information, that are distorted by loud sounds nearby, and any quiet
sections, (2) calibrate the volume of sounds across the households. (3) divide the sounds about every
1-1.5 minutes and randomly concatenate the sounds of each household. (4) a fade-in/fade-out is edited
in for eight seconds at each transition between household audio clips.
We created three stimulus sounds with a duration of 9-9.5 minutes following the above procedure.
In this research, the calibration of volume is based on ITU-R BS. 1770-4 (8).We calculated the LKFS
value of the sounds which represents loudness, and adjusted the amplitudes of the audio files, so that
their LKFS values are at an equal level, differing less than ±0.05% from each other.
3.4 Stimulus sound with reverberation
We defined four reverberation conditions in this experiment: (1) direct (children’s direct voices
without reverberation), (2) traditional (children’s voices with reverberation of Kasuga Library), (3)modern (children’s voices with reverberation of Yachiyo Library) and (4) silence (control condition).
We convolved the stimulus sounds with the impulse responses from Yachiyo and Kasuga Libraries
in order to achieve the desired reverberation. After reverberation, we calibrated the volume across all
the stimuli equally. A total of nine types of stimulus sounds (three conversational sounds and threereverberation) were created, and used in a random order in the experiment, along with the silence
control condition.

4. WORKING MEMORY EXPERIMENT
4.1 Procedure
We tested working memory using the Operation Span Task (OST (9)) under four sound conditions.
The procedure of the OST is shown in Figure 4, and is designed similarly to that of previously
published research (10), except as to the number of presented letters4. First, we presented a calculation
problem, which the participant answered on the next screen. Next, we presented an alphabet andanother calculation problem. After repeating this, the participant had to recall all of the presented
alphabets in the order they were presented. The feedback was then displayed. One session consistedof this trial repeated for 12 times. An experiment consisted of four sessions (i.e., four different soundconditions in a random order but without a counter balance). We set about 5 minutes break between
sessions. Before starting the experiment, we conducted a practice session of three OST loops for about
five minutes to explain the procedure to the participants, and to acquire baseline data to individually
adjust the experiment parameters. An experiment took about 1.5 hours. This experiment was approvedby institutional review board at University of Tsukuba.

4

The number of presented letters is (3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8) in this study, where the original
research employed (3, 3, 3, 4, 4, 4, 5, 5, 5, 6, 6, 6, 7, 7, 7). The number of letters to present in the first loop
is randomly selected from this list.

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Figure 4 – Procedure of Operation Span Task
4.2 Experimental conditions
Twenty-two undergraduate and graduate students (12 males and 10 females), aged 18-25 at
University of Tsukuba and possessing normal hearing abilities participated in the experiment.
In the experiment, a GENELC 8020C loudspeaker was used to present sound, and a laptop
computer (Macbook Pro 13-inch 2015) was used to display the GUI. We used Inquisit 5 to program
the OST. The speaker was placed at the position in front of the participant, about 1 m (3.28 ft) awayfrom the participant's ears and at a height of 0.95 m (3.12 ft) from the floor (the same level as theparticipant’s ears.) We calibrated the sound pressure level of the stimulus sounds by using a SPL meter(Ono Sokki LA-3260) at these speaker and participant positions.
The experiment was conducted in a soundproof room at University of Tsukuba, with the averagebackground noise level at 18.5 dB. The stimuli were presented with average sound pressure levels of
42 dB at the listening point. The participants seated in front of a speaker and conducted the OST whilehearing the stimulus sounds.
4.3 Questionnaire
We asked the participants to answer both pre- and post-experiment questionnaires and submit to
an interview. The pre-experiment questionnaire consisted of questions about the backgrounds of theparticipants (e.g., age, sex, study environment preference, and frequency of library use). In the postexperiment questionnaire, we asked about their impressions of the stimulus sounds and the taskdifficulty of the experiment. In the interview we asked which sound conditions were comfortable oruncomfortable.
4.4 Ospan Total Correct
There are two generally accepted ways to count correct memory recalls, we chose to employ the
Ospan Total Correct (OTC) (10) method for this study. In order to analyse the experiment results, weused OTC score to count the number of letters that were recalled correctly. OTC represents the total
number of correctly recalled alphabets in the entire task. In this study, if the participants performed
perfectly, their OTC would be 66 (the total number of presented letters).

5. RESULTS
An analysis was conducted for all 20 results, excluding two participants whose accuracy rate wasless than 85% under any condition. We used R for the statistical analysis. The result of the OTCs foreach sound condition are shown in Figure 5. For the OTC, the direct condition had a significantly
lower score than the silent condition (𝑝 < 0.01, corrected with Benjamini & Hochberg (BH) method(11)). Also, the difference between the direct and modern conditions, and the difference between the
traditional silent conditions was not quite significant (𝑝 < 0.09 for both cases, also corrected with BH
method). In the pre-experiment questionnaire, the participants were asked ‘Are you easily distracted
by sounds while working?’ However, their resulting ANOVA showed no significant correspondence
between the questionnaire responses and OTC scores.

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Figure 5 – OTC scores for each sound condition, where**: p < 0.01, *: p < 0.05, †: p < 0.1 (multiple comparison corrected with BH method)

Figure 6 – Comfort judgments of sound condition
In the interview, we asked the participants which sound condition was uncomfortable and which
was comfortable. Figure 6 shows the result of the uncomfortable or comfortable sound conditions as
judged by the participants. Most of the participants felt that the direct condition was the most
uncomfortable, and that the silent condition was the most comfortable. There were more ‘comfortable’
answers in the traditional and modern conditions than in the direct condition. However, there werefewer people who answered ‘moderate’ in the modern condition than in the traditional condition. The
difference of comfort judgments between direct and silent conditions is quite evident, while thedifference of comfort between traditional and modern conditions seems to be more individualized.
Figure 7 shows the Ospan Total Correct sorted by comfort judgments (rated by the participants).
As a result of ANOVA, the correspondence between comfort judgments and OTC was significant (𝑝 <Figure 7 – OTC scores sorted by comfort judgments where**: p < 0.01, *: p < 0.05, †: p < 0.1 (post-hoc analysis with Scheffé’s method)4542

0.05). Post-hoc tests showed that the OTC was significantly higher in the uncomfortable condition
than in moderate and comfortable conditions ( 𝑝 < 0.05, corrected with Scheffé’s method (12)).
Moreover, in the OTC, the score tended to be higher in the comfortable condition than in the moderatecondition (𝑝 < 0.01, corrected with Scheffé’s method).

6. DISCUSSION
6.1 Overall results
Previous studies have showed that short-term memory functions less efficiently when listening toconversational speech than in a silent environment (1, 2, 11). Our research showed a similar
outcome— the difference between direct and silent conditions was significant. The modern condition
tended to show better, although not significant, performance than traditional and direct conditions.
Considering the acoustic design of each library, the Kasuga Library had a small room size and was
designed with many sound absorbing materials. Because the density of book volumes was high, direct
sounds were absorbed by the bookshelves, and the reverberation emphasized low-frequency sounds.
On the other hand, Yachiyo Library had a large room size and low bookshelf density, so thereverberation was quite broad-band. Listening to the synthesized stimuli, the conversational sounds
echoed more pleasantly with the long and broad-band reverberation at Yachiyo Library, than with theshort and low-frequency emphasized reverberation at Kasuga Library. We think this probably resulted
in a more ‘comfortable’ rating for Yachiyo Library than for Kasuga Library.
These results contradict the reported disadvantages of long reverberation time in the field of classroom acoustics. For example, Klatte and Hellbrück (14) reported that environments with longreverberation times induced conversation among children. Persson, et al. (15) reported that the stressof teachers increased in an environment with long reverberation. There may be two reasons why
teachers feel stressed by longer reverberation times. One is that students who are talking earn poorer
grades (14), and the other is that the teacher's voice cannot be clearly transmitted in the classroom
because a long reverberation makes the teacher’s speech unintelligible.
We surmised that the effect of long-reverberation time differs in the context of a public library—
there are no teachers or grades in a library, and people enjoy studying autonomously there. Inducing
children’s conversation may bring a public library a lively and positive atmosphere, which can bebeneficial for knowledge creation. We also speculate that while information and knowledge processing
were well considered in previous classroom acoustics research, consideration of knowledge creation
was neglected. For knowledge creation, longer reverberation time is actually more desirable.
Further, public libraries are much more spacious than classrooms that accommodate diverse people
in the community. For example, zoning is a common technique to spatially distribute library users
based on their intended purpose of visiting, such as reading or discussion. By varying ceiling-heights
and sound-absorbent materials, an ideal reverberation time can be optimally assigned for each zone
within the library.
6.2 The relationship between comfort and sound condition
The resulting data of this study shows that task performance was significantly higher in a
comfortable sound environment than in an uncomfortable environment. This suggests that comfort
and task performance are highly correlated. In other words, choosing a sound environment to perform
intellectual activities based on an individual’s own preference can result in the selection of a soundenvironment with optimal work efficiency. However, in the pre-experiment questionnaire, very few
participants selected lively cafés as preferable to quiet libraries. This response was unexpected, since
students often work in cafés and diners, and Kasuga Library is rarely oversubscribed. In future
inquiries, we could better design our questionnaire to garner more realistic responses. In order toinvestigate the relationship between individual sound environment preference and working memory
capacity, it may also be interesting to recruit a group of participants who prefer a lively soundenvironment in a future experiment.

7. CONCLUSION
In this study, we examined how children's conversational sounds in different reverberation
conditions can affect working memory. We discovered that silent conditions increased working
memory capacity more significantly than direct sound conditions (𝑝 < 0.01). The average score for4543

working memory performance under modern library conditions seemed better than that of traditional
library conditions and direct conditions, but not statistically significant. This result was analysed
along with the preference ratings for the sound environments. In the comfortable sound environment,
the working memory capacity increased (𝑝 < 0.05). This suggests that choosing a sound environment
to perform intellectual activities according to one’s own preference may result in selecting a soundenvironment with good work efficiency.

ACKNOWLEDGEMENTS
We appreciate tremendous supports from our collaborators: Dr. Hiroshi Sato (AIST), Dr. Koji
Ishida (Ono Sokki), Prof. Yuzuru Hiraga and Prof. Keiichi Zempo (University of Tsukuba), Prof.
Atsushi Marui (Tokyo University of the Arts), Mr. Toshihito Yagi (Yachiyo City Central Library), Mr.
Hiroo Yanase (Okada Architect and Associates), and the delightful librarians from University ofTsukuba Libraries.

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