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

Speech privacy as a harmonizing factor in rating the soundinsulation between dwellings
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Faculty of Electrical Engineering, University of Belgrade, Serbia

ABSTRACT
Recent consulting practice has shown an increase in the number of complaints concerned with insufficientsound isolation between dwellings. In some of them the sound reduction index value was above the value
proposed in legislation, but the neighbours’ conversation was heard too clear. Comprehensive analysis has
shown that in new buildings there is an extremely low ambient noise level, often below 20 dBA. The energy
efficiency obligation in architecture today introduced extremely good façade windows as a rule, and in thesame time home appliances and equipment have become too quiet. Such environment enable clear
recognition of the neighbours’ speech even with enhanced sound insulation. The conclusion is that inevaluation of sound insulation between dwellings it is suitable to include somehow, besides Rw or DnT, theambient noise level as an embedded factor. A parameter called Speech privacy class as a measure of privacy
had been already introduced in the literature. It is defined as the sum of frequency restricted values of D andambient noise level. The paper discusses the possibility of introducing such information regarding privacy inassessment of the sound isolation between dwellings and discuss the methods for dwelling soundscapecontrol.
Keywords: ambient noise, sound insulation, speech privacy

1. INTRODUCTION
Privacy is an essential element of acoustic comfort in apartments. In multi-dwelling buildings thisimplies that speech originating inside apartment being difficult to hear or understand in the adjoining
dwellings. In architectural design of buildings speech privacy is considered by selecting appropriate
values of weighted apparent sound reduction index R’w or sound level difference D nT in all directions
to adjacent apartments. At that early stage of a building development this is the only physical aspectthat can be considered concerned with acoustic comfort.
One can find in literature what degree of speech privacy can be expected with different amount ofsound isolation. As an illustration, one can see the explanations given in an earlier standard proposal
for the acoustic classification of buildings >1@. It was stated there that with apparent sound reduction
index of 54 dB loud speech can be "audible, but hardly understandable" or normal speech can be "justaudible, but not intelligible”.
A number of information in literature shows that speech privacy is a multi-dimensional concept. It
does not depend only on the partitions’ insulation properties, but also on some other factors. Amongthem the most influencing is the ambient noise level at listener’s position. Descriptions taken fromthe cited document reflects „reasonable quiet environment“ with comment that quieter or noisierenvironment can change that. According to the same document a reasonable quiet environment can beunderstood as the circumstance in which requirement Lden,indoor d30 dBA is met. Practically the samerequirement for ambient noise in dwelling is specified in Serbian legislation, and probably in1emijic@etf.rs
dsumarac@etf.rs
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bjelic@etf.rs
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tm@etf.rs22946

legislations of other countries.
However, control of noise level during sound insulation measurements performed in various newbuildings or in retrofitted apartments revel that one can expect the value of ambient noise level inrooms within wide limits. It was stated that the average level of ambient noise in the living rooms issignificantly below 30 dB. In Belgrade the most frequent value of ambient noise level in new or
renovated buildings is below 25 dBA during the day, and the buildings were found were the equivalentnoise level is about 17 dBA during evening. That implies a different status of speech privacy with thesame partitions between dwellings.
Accordingly, the standard data obtained by sound insulation measurement in a building does notshow to apartments users what kind of property they can expect. Sound insulation measurements show
the value of the apparent sound reduction index R' w or sound level difference DnT , but do not indicate
whether this is "reasonable quiet environment", "extremely quiet environment" or "absolute quiet" inliving rooms. The apparent noise conditions depend on many factors: the noise in the ambient wherebuilding is positioned, the quality of windows, the concept and quality of installations in the building,the proposed level of thermal efficiency (which reflects on windows quality), even the workmanship.
In the same part of the city noise condition can be different in different buildings.
Thus the information that can announced to people the expected level of privacy, at least in Serbia,
remains unknown in the evaluation of buildings’ acoustic quality. Only after the building is moved
tenants in some circumstances address the problem of privacy. Then the first issue arises as how toexpress the state of acoustic comfort in a more complex way, beside R' w or DnT , and second is theways in which noise level can be controlled in order to adjust the speech privacy.

2. A SUGGESTIVE CASE STUDY
An example discovered during sound insulation measurement in a residential building is
schematically explained in Figure 1. Testing of sound insulation quality in a building was made on
tenant’s request. The measured value of weighted sound reduction index between two apartments was
52 dB, as indicated in the Figure. The value is prescribed as requirement in actual legislation >2@ which
is applied as a criterion in buildings design. This means that sound insulation between apartments
meets the requirement.
However, it has been established that the speech privacy in the two adjacent rooms is different. In
one direction it was established that the speech from the neighbour could be heard, thus initiated thetenant's urge, whereas in the opposite direction such an occurrence was not noticed and the tenants
had no complaint about privacy. This difference in voice transfer is indicated in the figure. In analysisof the speech detection difference in two directions, that is, on two sides of the same wall, the samegroup of people was used as the source of speech at both sides and the same group of listeners was atboth rooms, thus eliminating the possible influence of individual differences between speakers andlisteners.

LAeq=30dBA

R’w=52dB

LAeq=20dBA

it's heard
it's not heard

Figure 1 – Illustration of an example that was analysed
In order to explain this phenomenon, beside sound insulation, an equivalent ambient noise level wasmeasured in both rooms, as well. The results are also shown in the Figure 1. In a room where it was
not reported to hear the speech from the neighbour, the value of equivalent noise level was about
30 dBA. It is the value stated in the legislation as the maximum noise level during the night in2947

apartment. The value is consequence of the existing windows’ sound insulation properties, as well assome unknown noise sources distributed somewhere in the apartment. In the adjacent apartment,behind the partition, where the speech from the neighbours was recognized, the value of the equivalentnoise level was only about 20 dBA. It was discovered that new façade windows of high value of thesound reduction index were installed, and all home appliances were brand new and very low noise.
This difference between two apartments has made the difference between signal-to-noise ratio in tworooms. Therefore the speech privacy on two sides of common partition is different even though thesound isolation is unique.
Hence, the speech privacy as an important element of the acoustic comfort is not determined onlyby the value of R' w or DnT, but also by the ambient noise around a potential listener. In addition, in
two phases of sound insulation testing – in building design by calculation and by measurement ofsound isolation after buildings is completed – only sound insulation is consider. In order to obtain amore reliable insight in acoustic comfort the analysis must be extended to include information
concerned with ambient noise in the relevant rooms. Only then the expected speech privacy in thebuilding can be evaluated more reliable.
Testing the building acoustic quality today includes only the sound insulation measurement, so
only the parameters in that domain are available for the building evaluation. The fact is that theambient noise is not the subject of measurements in finished buildings, until some problem occurs aswith excessive noise of nearby mechanical equipment. The fact is also that in this moment there is nodata on ambient noise level statistics in dwellings today during period of noiseless activity, like during
the night. Therefore, no relevant conclusions about the expected speech privacy in buildings. The onlyavailable dwelling noise data is concerned with some noisy activities like cooking, family gathering,
listening music, playing musical instrument, etc. >3@.

3. QUANTIFYING THE EFFECT OF “SILENCE” ON SPEECH PRIVACY
For better understanding the influence of “silence”, i.e. ambient noise, on speech privacy it isimportant to discover some method for measuring the event. A possible solution one can find in an
US standard. Document ASTM E2638 introduces the parameter called Speech Privacy Class (SPC)>4@. The standard is based on results published by Bradley et all >5@. SPC is obtaining by measurementin building and represents the degree of speech privacy provided by a closed room for conversations
occurring inside. Its value is a sum of two basic elements that determine the speech communication
between the room and its surrounding: sound isolation provided by the room envelope and backgroundnoise at the receiving point. Value of SPC is expressed as the sum of sound level difference D andambient noise level L amb , both in dB:(1)
SPC = D + LambBoth element on right side in Eq (1) are unweighted average of 1/3 octave band values from 160 Hz
to 5 kHz, 16 values in total. Thus the SPC value is defined as a sum of two factors that both haveinfluence on speech audibility and intelligibility.
The SPC is defined in the standard as the security and confidentiality measure for the speech inmeeting rooms, i.e. for leakage of speech signal from meeting room interior to some points outside it
where the potential listeners may be located >4@. Although developed for such particular purpose, thatconcept has the potential to be applied for privacy assessments in residential buildings, too.
Effort to describe expected level of speech privacy for different values of the SPC parameter one
can find in literature >4,5@. Description specifies probable level of speech audibility and intelligibility.
For example, at values SPC > 85 speech sounds becomes inaudible, and for SPC between 65 and 70
speech sounds is „frequently audible“ and „brief phrases expected to be rarely intelligible“ >5@.
In the part of the world where ISO standardization is preferred, determination of building’s acoustic
quality is based on measurement of the apparent sound reduction index R’ according to ISO 16283-1>6@ and the equivalent ambient noise level LAeq expressed in dBA according to ISO 1996-1 standard>7@. The result of the soundproofing measurement is expressed by the relevant value of the isolating
power R'w calculated according to the standard IS0 717-l >8@. The authors of this paper in an earlier
work proposed a modification in the SPC concept introducing the values obtained by measurement
procedures according to ISO standards applied in Europe >9@. A parameter equivalent to SPC wasintroduced, named Speech privacy index (IPR) and defined as:(2)
IPG = R’w + LA,eq2948

In a previous study by this authors it has been experimentally shown that the IPG estimate withappropriate accuracy the value of SPC, which means that it can also be used in the same way toevaluate the speech privacy between the rooms, but using parameters defined by ISO and routinely
measured in buildings >9@
The importance of ambient noise for speech privacy is also recognized in evaluation of open space
offices acoustic quality. In an ISO standard where all relevant parameters for these specific areas
quality evaluation is defined, the equivalent ambient noise level is also proposed as an importantparameter>10@. In the evaluation procedure described in this standard the effect of noise is observed
indirectly by means of Speech transmission index (STI) decay with distance form speaker. There is adistance from the speaker where the voice immerse into the existing ambient noise and thus the speech
intelligibility practically disappear. It was shown that speech become unintelligible when STI reachthe value 0,2 >10@. That distance is named “privacy distance”. In many circumstances some supporting
systems for controlling the level of ambient noise are used for that purpose.
Although the SPC was originally defined as a parameter that should show the possibility of leaking
the conversations from the business meeting rooms, such of concept can be used to estimate acoustic
comfort in other circumstances, like in residential buildings. Thus IPG, or just equivalent noise level,
can be used for making statement about speech privacy, along with regular parameters used forcharacterization of sound insulation in the building.

4. CONCLUSIONS
Idea for this paper was initiated during several investigations of complaints by tenants in new andrenovated residential buildings addressed to inadequate level of privacy. The problems are identified
as unwarranted audibility and intelligibility of speech coming from the neighbours. The problem
appears despite the fact that measured values of the parameters showing the sound insulation quality
are in line with design’s goals and proposals in legislation. The presentation in this paper discusses
the need for some changes in engineering practice that can relax the problem to some extent, as well
as open some topics for future consideration. Some suggestions are defined based on these authors
experience in attempting to solve somehow the reported problems. In conclusion, three main topic
found important for further consideration are explained below.
1. The basic goal of any sound insulation measurement in buildings is to obtain relevant
information about the acoustic comfort that tenants can expect. However, the speech privacy does notdepend only on sound insulation, but also on the existing ambient noise level. Therefore, it would be
informative to show in the R 'or D nt measurement reports the measured values of the equivalent noiselevel in the source and receiving room, as well. The time interval of the noise measurement can be
discussed. It can be, for example, an interval of 15 minutes with keeping under control possible
accidental acoustical events in the surroundings.
2. Setting the ambient noise level at the design stage by selecting window sound insulation is not
an action with a controllable results. So, it is easy to achieve the condition of an "excessive silent" inthe dwellings, which enables the audibility and intelligibility of speech coming from the neighbours.
It is therefore reasonable to concern the research of soundscape in dwellings, i.e. research of theambient sound character that could adjust the level of masking without disturbance and which would
allow normal activities, even the dream. Then the "excessive silent" in the dwellings would be only
one tenants’ option, not a permanent status.
3. Research in the domain of sound masking in dwellings have to include design of equipment withphysical characteristics appropriate for application in dwellings. Unlike the equipment used in theoffices, in dwelling that have to fit sophisticatedly into the ambient, remaining sufficiently invisible
and without the possibility for people to detect the position of the sound source. Today's audio
technology offers sufficient solutions that just need to be combined in the right way. This means thatthe concept present from a long time ago in offices has to be adapted and used in dwellings.

ACKNOWLEDGEMENTS
This paper is realized as a part of activities supported by Serbian Ministry of Science andEnvironmental Protection, grant TR-36026.

REFERENCES
1. ISO/TC 43/SC 2 N 1218, TU0901 Proposal CS for NWIP 2013-11-í³$FRXVWLFFODVVLILFDWLRQVFKHPH

2949

for dwellings”
2. SRPS U.J6.201 „Akustika u zgradarstvu – 7HKQLþNLXVORYL]DSURMHNWRYDQMHLJUDÿHQMH]JUDGD³
3. 0$GQDÿHYLü 0 0LMLü ' 6XPDUDF 3DYORYLü ' 0DãRYLü sNoise in dwellings generated in normalhome activities – general approachs, Forum Acusticum, Aalburg, 2011, Proceedings, 1335-134
4. ASTM E2638 – 10, „Standard Test Method for Objective Measurement of the Speech Privacy Provided
by a Closed Room“, 2017.
5. J.Bradley, B. Gover, „Speech Privacy Class for Rating the Speech Privacy of Meeting Rooms“, CanadianAcoustics, Vol. 36 No. 3 (2008) 22-23
6. ISO 16283-1Acoustics — Field measurement of sound insulation in buildings and of building elements— Part 1: Airborne sound insulation environmental noise — Part 1: Basic quantities and assessment
procedures
7. ISO 1996-1 Acoustics — Description, measurement and assessment of environmental noise — Part 1:Basic quantities and assessment procedures
8. ISO 717-l Acoustics - Rating of sound insulation in buildings and of building elements - Part 1: Airborne
sound insulation
9. 0%MHOLü 70LOMNRYLü 'âXPDUDF 3DYORYLü 00LMLü „Speech Privacy Class in ISO standardisation’sworld“, paper in preparation for publishing
10.ISO 3382-3 Acoustics — Measurement of room acoustic parameters — Part 3: Open plan offices2950