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

WATER-BASED SOUND MASKING: AN EXPERIMENTAL STUDY IN
AN OPEN-PLAN OFFICE
Valtteri HONGISTO1,2, Annu HAAPAKANGAS1,21

Finnish Institute of Occupational Health, Turku, Finland
2Turku University of Applied Sciences, Turku, Finland

ABSTRACTIn Finland, the new governmental regulations require that Speech Transmission Index shall be less than 0.50
in open-plan offices. It requires both large amount of absorption materials and artificial sound masking dueto low ventilation noise levels. Commercial sound masking systems use often filtered pseudorandom noise
as a sound signal. A laboratory experiment reported that water sound was experienced more pleasant thanpseudorandom noise when performing cognitive tasks. Our purpose was to determine how different waterbased masking sounds are perceived in an office workplace compared to pseudorandom noise. The study wasconducted in an open-plan office, where sound masking based on pseudorandom noise had been used for 2years at 44 dB LAeq. Our experiment included six conditions: original pseudorandom noise (P1a), four
different water-based sounds (N1N4), and return to original pseudorandom noise (P1b). Each condition
lasted for 3 weeks and were presented at 44 dB LAeq. The employees were asked to respond to a questionnaire
after each condition. Eighteen employees responded to all six questionnaires and their responses wereanalyzed. Water-based sounds appeared to be more disadvantageous than pseudorandom noise for almost all
subjective measures. Due to some methodological weaknesses, more research is justified in this field.
Keywords: Sound masking, open-plan offices, room acoustics

1. INTRODUCTION
Haapakangas et al. (1) found in an extensive field study that noise annoyance was larger in suchopen-plan offices where the distraction distance was larger. Distraction distance, rD, is the central
single-number quantity of the standard ISO 3382-3 which describes the methods to objectively
measure the room acoustic quality in open-plan offices. The value of rD indicates the distance from aspeaker, when the Speech Transmission Index, STI, falls below 0.50 (2). STI can obtain valuesbetween 0.0 and 1.0. STI reflects pretty well the subjective speech intelligibility. When STI falls under
0.50, it has been found (3, 4) that the adverse effects of background speech on cognitive performance
begin to reduce. When STI is under 0.20, performance decrement is usually negligible due to speech.
The value of rD can be reduced by proper room acoustic design which aims at smaller signal-tonoise ratio of speech, SNR. The conventional room acoustic methods for reaching lower SNR are:A. the use of sound-absorbing materials in walls, ceiling, and furniture;B. the use of high sound-insulating screens between workstations;C. the use of artificial sound masking system if the background noise is under 40 dB L Aeq .
The separate and mutual effects of methods AC have been studied both in laboratory (5,6) andfield (7). They prove that methods AC shall be used simultaneously to obtain a small SNR.
The most usual and the cheapest way to implement Method C is to install a loudspeaker system tothe ceiling. Typical density is one loudspeaker per 10 m2 . The spectrum should resemble the spectrumof speech to obtain a good balance between speech masking and acoustic satisfaction towards themasking sound itself (8). The recommended level is 4245 dB LAeq . Sound masking is usually
produced using small loudspeakers and pseudorandom noise within 2005000 Hz. The spectrum has
122482

not available
valtteri.hongisto@turkuamk.fi; annu.haapakangas@turkuamk.fi

usually a slope of -5 to -7 dB per octave doubling (REF in Figure 1).
Haapakangas et al. (9) found in a psychological laboratory experiment that water-based soundmasking (a spring water sound, or water stream) led to higher acoustic satisfaction and cognitive
performance than sound masking based on pseudorandom noise, ventilation sound, music with vocals,
or instrumental music. They suggested that the finding should be confirmed with a field study.
Our purpose was to determine how different water-based masking sounds are perceived in an office
workplace compared to pseudorandom noise while the sound level is constant, 4 dB LAeq , and thedifferences in overall spectrum are not drastic.

2.

MATERIALS AND METHODSGeneral design. We conducted an experiment in an open-plan office. We studied various
subjective responses (dependent variables) among the office employees six times after exposing themto six masking conditions (MCs, independent variable). The exposure time for each MC was threeweeks. The employees were invited to respond to a questionnaire after eight working days from thestart of the exposure. Employees were informed about the research and it’s purpose in advance but
they were not involved with the design of the experiment nor with the selection of masking sounds.
Office. The office as well as the original sound masking (MC P1a) had been used for 2 years. Themasking sound signal was filtered pseudorandom wide-band noise (see P1a in Figure 1). The office(19x50 m, height 2.4 m, 930 m2 ) involved 54 workstations and various kinds of teams (administration,IT, quality, sales, marketing). Partition walls existed only around meeting rooms. The office wasoccupied by 77 employees. Most of the employees had fixed workstations but there were also a lot ofelements of activity-based office (sofa groups, meeting pods, working pods, drop-by workstations) to
be occasionally used by traveling occupants who did not need a permanent workstation.
Masking conditions. The masking conditions (MCs) are described in Figure 1. MCs P1a and P1b
are equivalent. This MC represents the sound masking to which the employees had been exposed fortwo preceding years. MCs N1N4 involved water-based sounds. They were chosen according to alistening test where fifteen people ranked the acoustic satisfaction of each sound according to thesubjective scales of Ref. (8). It was decided to choose four water-based sounds according to two
criteria: participants’ preferences and dissimilarity between the water sounds.
Sound masking was produced using a commercial sound masking system. The system consisted ofloudspeaker grid (appr. 1 loudspeaker per 12 m2 ) installed over the suspended ceiling. The
loudspeakers were not visible to the employees but the they were all aware of the system.
The masking sounds were carefully adjusted by level and spectrum outside working hours beforelaunching a new MC to avoid any special attention to sound masking during working hours. The effectof suspended ceiling and room on the spectrum of masking sound was compensated using one-thirdoctave digital filtering (Adobe Audition 3.0), so that we could obtain the desired spectra in theworkstations. The sound pressure level (SPL) of a new MC was measured in every workstation during
the weekend before launching a new masking condition. A 15-second integration time was used.
Results are shown in Figure 1.
Room acoustics. Suspended ceiling at a height of 2.4 m was covered by 90% with class A soundabsorbing material (ISO 11654). Floor was covered by full-area textile carpet. Windowless walls weremostly covered by illustrated sound-absorbing materials (class A) from floor to ceiling. Trefoil-shaped
workstation groups involved 1.25 m high screens between tables. The level of sound masking (MCP1a) was 43…47 dB LAeq depending on position.
Room acoustic measurements were conducted in MC P1a according to ISO 3382-3. The distraction
distance was very low, rD=5 m. The spatial decay rate of A-weighted speech level was reasonably
high, D2S=6.5 dB. The A-weighted speech level at four meter distance from speaker was very low,
LAS4m=47 dB. The room acoustic conditions were reasonably good compared to typical conditions
measured in e.g. Finnish open-plan offices (2).
Respondents and statistical analyses. The number of respondents to the six questionnaires was
47, 37, 33, 30, 28 and 28. Eighteen employees responded to all questionnaires. Statistical analysis(ANOVA and paired comparison with Benjamini-Hochberg adjustment) was based on these 18employees using repeated measures design. Our primary interest concerned the differences between
MCs P1aN4 (p 1 ). Furthermore, we conducted a secondary analysis between acoustically equivalent
MCs P1a and P1b (p2 ).

2483

50
P1aN1
N2
N3
N4P1bRef
BG

45Lp,Z [dB]

4035

L Aeq

30

Masking Description of soundcondition
P1aPseudorandom noise
N1
Waterfall, spectrum close to P1aN2
River (or stream)
N3
Babbling river
N4
River and occasional birds
P1bAs P1a2520

SD[dB]
1.1
1.1
1.6
1.6
1.31.48000

4000

2000

1000

50025012515M
[dB]
44.5
44.6
43.8
43.0
44.2
43.7

f [Hz]

Figure 1  Descriptions of six masking conditions. Left) The unweighted sound pressure level, LpZ, as afunction of frequency, f. Ref corresponds to the slope of -5 dB per octave increment. BG corresponds thesituation when masking was off (ventilation and computers on). Right) Verbal descriptions of the masking
conditions and the mean (M) and standard deviation (SD) of A-weighted sound pressure level on 54workstations.

3.

RESULTS

Satisfaction with acoustic environment reduced every time when a new water-based masking
condition (MC) was introduced. MCs P1a and P1b were equally rated (Figure 2a).
Perceived negative effects caused by different environmental factors remained quite much the same
between MCs. The exception was noise and lack of speech privacy. Their negative effects increased
every time when a new water-based MC was introduced (Table 1). Perceived negative effects caused
by noise did not return to the level of MC P1a in the end which suggests that noise caused by working
activities were not equal during the MCs P1a and P1b.
Perceived disturbance of work due to different sound sources was larger in water-based MCs (Table2, Figure 2b) than in P1a. P1A and P1b did not differ from each other.
Sound quality assessments of masking sound during different MCs (Table 3) suggested that P1aand P1b were identical, as expected. MC N4 was unfavorable compared to P1a for all sound quality
attributes. The best rating among water-based MCs was given to N1. It differed from P1a only for two
attributes out of nine.
Job satisfaction reduced from P1a to N1 and did not return to the original level in the end. Perceived
possibilities to influence on matters at the workplace were lower in the end (P1b) than in the beginning(P1a). Perceived stress did not change during the experiment.

2484

2+

#

#+

*+4+DisturbanceSatisfaction

15*

0-1

*

#

#

#++

N3

N4

32-2

1P1a

N1 N2 N3 N4Masking condition

P1bP1aN1N2P1bMasking condition

Figure 2  (Left) Mean satisfaction with the office noise and acoustic environment of the office as a wholein six masking conditions. Range: -2 Very dissatisfied; +2 Very satisfied. (Right) The mean disturbance ofwork due to masking sound. Scale: 1 Not at all, 5 Very much. Labels indicate the statistical significancecompared to masking condition P1a according to + p<0.05, * p<0.01, # p<0.001.
Table 1  Perceived negative effects caused by different environmental factors. Mean values. Response
scale: 1 Not at all, 5 Very much. Column p1 describes the significance level of the main effect of masking
conditions P1aN4. Column p2 describes the significance level between masking conditions P1a and P1b.
Label n.s. means a non-significant difference.
Environmental factor
Draught
ColdHot
Stuffy airNoise
Lack of speech privacy
Lighting
Glare or reflections
Dust or dirt
SmellsDisorderCrowdedness
Movements in vision
Openness

P1a
1.9
2.4
2.4
1.5
2.1
2.1
1.4
1.5
1.4
1.2
1.3
1.0
1.7
1.3

N1
1.4
1.7
2.3
1.5
2.9
2.7
1.4
1.1
1.1
1.0
1.3
1.1
1.2
1.2

Masking conditionN2
N3
N4
1.7
2.1
1.8
1.8
2.3
2.1
2.3
2.4
2.0
1.6
1.9
1.5
3.0
3.7
3.5
3.0
3.3
3.0
1.2
1.5
1.5
1.2
1.4
1.4
1.2
1.2
1.1
1.0
1.2
1.2
1.2
1.3
1.2
1.1
1.4
1.3
1.4
1.7
1.4
1.4
1.7
1.6

P1b
1.7
1.8
1.9
1.6
2.8
2.6
1.6
1.8
1.2
1.3
1.5
1.5
1.7
1.5

p1
n.s.
n.s.
n.s.
n.s.
<0.001
0.018
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.
0.046
n.s.

p2
n.s.
0.029
n.s.
n.s.
<0004
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.
0.024
n.s.
n.s.

2485

Table 2  Perceived disturbance of work due to different sound sources. Mean values. Response scale: 1
Not at all, 5 Very much. Column p1 describes the significance level of the main effect of masking
conditions P1aN4. Column p2 describes the significance level between masking conditions P1a and P1b.
Label n.s. means a non-significant difference.
Masking condition

Sound source
Talking and laughing in other desks

P1aN1N2N3

N4

P1bp1

p2

2.1

2.7

2.5

3.3

2.9

2.2

0.017

n.s.

Talking in jointly used spaces

1.5

2.2

2.1

2.9

2.8

1.7

0.002

n.s.

Ventilation and air-conditioning

1.7

1.4

1.8

2.4

2

1.6

0.029

n.s.

Sound masking

1.9

1.9

2.8

3.7

4.1

1.9

<0.001n.s.

Work-related sounds generated by others

1.4

2.1

2.2

2.4

2.5

1.6

0.024

n.s.

Jointly used office equipment

1.2

1.7

1.8

2.1

1.9

1.2

0.013

n.s.

Phones ringing

1.2

1.4

1.31.4

1.5

1.2

n.s.

n.s.

Table 3  The mean values of some attributes describing the quality of sound masking. Scale: -3 Strongly
disagree, +3 Strongly agree. Bolded value means that the masking condition differs significantly from P1a.
Small value is desirable for attributes with an asterisk. Large value is desirable for attributes without anasterisk.
Attribute
Easy to habituate toDistracting*
Pleasant
Stressful*
NaturalAnnoying*
Tiring*Acceptable
Helpful for my work

P1a1.4
-1.2
-0.6
-1.8
-0.9
-1.9
-1.9
0.70.3N10.8
-1.1
-0.7
-1.2
-0.7
-0.9
-1.0
0.0
-0.2

Masking condition
N2N3
N4
-0.2
-1.8
-2.2
1.3
1.3
-0.1
-1.9
-2.3
-1.2
0.1
0.8
-0.9
-2.3
-1.1
-1.8
0.1
1.0
2.3
-0.7
0.2
1.1
-0.7
-1.8
-2.3
-1.2
-1.6
-2.4

P1b1.4
-1.5
0.2
-1.7
-0.5
-1.3
-1.4
1.2
0.6

p1
<0.001<0.001<0.001<0.001
0.012<0.001<0.001<0.001<0.001

p2
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.

4. DISCUSSION
It is obvious that water-based masking conditions (MCs) were rated more negatively than MCs
with pseudorandom noise. However, we cannot suggest that water-based masking sounds should not
be used because of several methodological weaknesses of our study: The employees were used to MC P1a for 2 years. Exposure time for water-based MCs was only 1.5
to 3 weeks before responding to the questionnaire. Unequal exposure time to different MCs hascaused at least some bias to the result.
 Order effect may have biased the results. In laboratory experiments, order effect can be minimized
by using counterbalanced order of experimental conditions for different participants. We could notconsider this in field setting. Future studies should have only one MC instead of four.
 Employees did not have any control over the experiment although it concerned their physicalworkplace. Employees usually appreciate the possibility to control changes in their workenvironment. Satisfaction with changes made in workplace are usually higher if the employees caninfluence the decisions (10).
 The study was conducted in one workplace. The results might be different in another workplace.
 The experiment was very long (18 weeks). The response rate reduced during the experiment. The
results did not represented the opinion of the whole staff.

2486

5. CONCLUSIONS
We conducted an experiment in an office workplace where the employees were exposed topseudorandom masking sound, four water-based masking sounds, and again the original
pseudorandom masking sound. All conditions were presented at the same level, 44 dB. Water-basedmasking conditions were rated more negatively than masking conditions with pseudorandom sound.
However, we cannot suggest that water-based masking sounds should not be used because of several
methodological weaknesses of our study. More research is warranted in this field. Full version of thiswork has been published in Ref. (11).

ACKNOWLEDGEMENTS
The research work was funded by Plantronics Ltd. and Finnish Institute of Occupational Health.
Final analyses and writing of Ref. (11) and this paper was funded by Turku University of AppliedSciences.

REFERENCES
1.

Haapakangas A, Hongisto V, Eerola M, Kuusisto T. Distraction distance and disturbance by noise – Ananalysis of 21 open-plan offices. J Acoust Soc Am 2017;141(1):127-136.
2. Virjonen P, Keränen J, Hongisto V. Determination of acoustical conditions in open-plan offices - Proposal
for new measurement method and target values. Acta Acust Acust 2009;95(2):279-290.
3. Hongisto V. A model predicting the effect of speech of varying intelligibility on work performance. IndoorAir 2005;15:458-468.
4. Haapakangas A, Hongisto V, Liebl A. The Relation between the Intelligibility of Background Speech andCognitive Performance – A Revised Model. Manuscript submitted for publication, 10 April, 2019.
5. Virjonen P, Keränen J, Helenius R, Hakala J, Hongisto V. Speech privacy between neighboringworkstations in an open office - a laboratory study. Acta Acust Acust 2007;93:771-782.
6. Keränen J, Hongisto V. Prediction of the spatial decay of speech in open-plan offices. Appl Acoust2013;74:1315-1325.
7. Keränen, J., Hongisto, V., Hakala, J., Oliva, D. (2011). Speech privacy in an open-plan office withdifferent room acoustic conditions, Forum Acusticum 2011, paper 135, 1897-1900, 27 June - 1 July,Aalborg, Denmark.
8. Hongisto V, Oliva D, Rekola L. Subjective and objective rating of spectrally different pseudorandom
noises – Implications for speech masking design. J Acoust Soc Am 2015;137(3):1344-1355.
9. Haapakangas A, Kankkunen E, Hongisto V, Virjonen P, Oliva D, Keskinen E. Effects of five speechmasking sounds on performance and acoustic satisfaction - implications for open-plan offices. Acta AcustAcust 2011;97(4):641-655.
10. Hongisto V, Haapakangas A, Varjo J, Helenius R, Koskela H. Refurbishment of an open-plan office –environmental and job satisfaction. J Environ Psychol 2016;45:176-191.
11. Hongisto V, Varjo J, Oliva D, Haapakangas A, Benway E. Perception of water-based masking sounds –Long-term experiment in an open-plan office. Front Psychol 2017;8:1117. Open access at:https://doi.org/10.3389/fpsyg.2017.01177.

2487