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

Airborne sound insulation in dwellings – single numbers weighted
from 50-3150 Hz correlated to Swedish questionnaire surveys

Christian SIMMONS1; Fredrik LJUNGGREN2
1
2Luleå University of Technology, 97187 Luleå, Sweden

Simmons akustik & utveckling, Chalmers Teknikpark, 41288 Gothenburg, Sweden

ABSTRACTDuring the work with the draft standard ISO/DIS 19488 for the sound classification of dwellings, several
studies were presented with recommendations on whether 50 or 100 Hz would be the most appropriate
lower frequency limit when evaluating single numbers of airborne sound insulation between dwellings. It
was observed that 100-3150 Hz is the range used in almost all national building regulations, but including
lower frequencies has been considered in some countries and are recommended for higher sound classes insome standards. The Swedish regulations began including evaluation from 50 Hz in 1999, which means
there is a long-term experience from the field to support this discussion. In this paper, several single
numbers based on field measurements are compared to subjective ratings given by the residents in a varietyof building types. The questionnaire surveys were distributed in total 46 building objects with light-weightor heavy walls and floors. Several single number quantities according to ISO 717-1 were correlated against
the ratings given by the residents. The statistical evidence for a 50 Hz limit was found to be small on the
average, but analysing light-weight buildings separately showed an importance of including soundreduction indices from 50 Hz. Currently, we consider measurements from 50 Hz the best choice when goodsound protection against music at low frequencies may occur, since it will also protect from noise at mid
and high frequencies. If protection against disturbing speech is sufficient, measuring from 100 Hz isenough. In 2019-2021, about 15 surveys will be made in buildings with timber joist, CLT- or concrete
floors.
Keywords: Questionnaire survey, low frequency, single numbers

1. INTRODUCTION
The first modern international standard regarding evaluation of airborne sound insulation between
dwellings was launched in 1968 [1]. The evaluation procedure was based upon a reference curve
originating from the sound reduction properties of a 25 cm brick wall [2]. The frequency range was
set to 100–3150 Hz and the single number quantity (SNQ) R’w – the apparent frequency weighted
sound reduction index – was defined. Later on, complementary spectrum adaptation terms ranging
from 50 Hz were introduced in 1996 [3] in order to take sound insulation at low frequencies into
account. The term C50-3150 was adopted 1999 into the Swedish building code (BBR) in which
requirements for R’w + C50-3150 were stipulated. This was a result of extensive complaints fromresidents, whereby Swedish authorities as well as the building industry realized that the legal
regulations must be stricter. Other countries in the Nordic region chose to keep R’w but increased thelimiting value instead.
Up to now, Sweden is still the only country that mandatorily requires sound insulation to beevaluated from 50 Hz and a lot of experience has been acquired over these almost twenty years. A
reason for Sweden being a pioneer in this context is that light-weight constructions are morecommonly used compared to most other markets and that light-weight buildings, especially at thattime, were prone to suffer from poor sound insulation at low frequencies.
1
2

fredrik.ljunggren@ltu.se
info@simmons.se5063

During the last few years, an increased interest in low frequency sound insulation has been
noticed. As an example, the ISO standards regarding airborne and impact sound insulation nowadays
include a specific low frequency measurement procedure to be applied under certain circumstances[4]. Other examples are to be found in the great amount of research activity in the field. Several
studies based upon listening tests have been reported (2014–16) [5–8] but also attempts to optimize
evaluation parameters against subjective perception can be found (2016) [9,10] as well as
questionnaire surveys (2015) [11]. A common discussion in the given studies is whether airborne
sound insulation between dwellings should be evaluated from 50 Hz or 100 Hz.
A broad general conclusion based upon these studies was that frequencies below 100 Hz are of no
great importance when assessing sound insulation. This standpoint was however clearly rejected in
the reflective paper by Rindel (2017) [12] who believes that the results of the studies in fact point in
the opposite direction, i.e. that frequencies 50–80 Hz must be included in the single number
evaluation. Rindel also claimed that the authors severely misinterpreted their own results andpresented wrong conclusions. A key factor in the understanding of this problem is whether it ispreferable to optimize the evaluation process in order to find the parameter that gives the highest
possible correlation for a specific type of sound source (e.g. loud speech). Or, if it is better to apply amore protective perspective, from the resident’s point of view, leading to higher demand for soundinsulation in terms of a broad banded SNQ that covers most sources in dwellings? Related to this, itis well known that various sound sources, as speech and music, put different demands on thefrequency-weighting factor since many types of music contain proportionally more energy at low
frequencies [13]. For the time being, it is obvious that no general consensus within the addressed
question of 50 or 100 Hz exists.
In this paper, the Swedish experience of evaluating airborne sound insulation is given with the
purpose to find out whether SNQ’s ranging from 50 Hz show higher correlation against the residents’
rated annoyance compared to DnT,w ranging from 100 Hz. Also, the potential difference ofcorrelation in between light-weight and heavyweight constructions will be covered. The following
presentation is the compiled results from field measurements and studies of residents’ rating of soundinsulation during more than 30 years. In the next years, about 15 surveys will be made to broaden the
database with new types of structural systems, e.g. light-weight timber joist floors, massive CLT
floors with various linings. A few older buildings with thin concrete floors will also be included.

2. METHOD
Our database contains results from six Swedish research projects amended by one consultancy
project. It includes data from 46 multi-storey residential buildings. These buildings have a variety offloor structures which can be categorized as light-weight (24 almost exclusively wooden based) or
heavy concrete constructions (22). The projects are called Aku20 (awaiting publications), AkuLite(2013) [14], SBUF (2011) [15], Boverket Byggkostnadsforum (2007) [16], Bodlund (1983) [17] andone additional consultancy project (2014). Only a minor part of the building objects [17] were
designed before the building regulations were changed to include the lower third-octave bands 50–
63–80 Hz. Although the objects range over 30 years in time, the vast majority is built in recent years
i.e. they represent modern building technology in Sweden.
The data collected and correlated are: (a) Residents’ subjective rating of annoyance from noise
related to the airborne sound insulation and (b) Vertical sound insulation measurements in terms of
D’nT, 50–3150 Hz. As mentioned above, new surveys are currently being made, with the same
methodology and post-analysis, which means results and conclusions may change when those data
have been included in the database and the analyses have been updated.
2.1 Questionnaire and subjective rating
The residents were asked to rate the perceived sound insulation by means of a questionnaire.
Within all the projects from 2011, the same questionnaire developed within the European cost action
TU0901 [18] was used. This questionnaire contains 16 questions related to sound and vibration
issues in dwellings. For this study, the following two questions are in focus:
Thinking about the 12 last months in your home, how much are you bothered, disturbed or annoyed by thesesources of noise?
(a)

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Neighbours; daily living, e.g. people talking, telephone, radio, TV through the ceilings or floors

(b)Neighbours; music with bass and drums

and the following two questions have been used mainly for comparison:(c) Neighbours; daily living, e.g. people talking, telephone, radio, TV through the walls(d) Neighbours; footstep noise, i.e. you here when they walk on the floor

Question (a) is assumed to be related to sound sources having a frequency spectrum with limited
energy below 100 Hz while the sources related to question (b) should emit significant amount ofenergy in the 50–100 Hz range. The annoyance is rated on a numerical scale ranging from “0” to “10”where “0” means not at all annoying and “10” means extremely annoying. There was also an option
to answer N/A, in case the source of sound did not exist, but very few used this option.
In the remaining two projects (11 of 46) [16,17], other questionnaire templates were used. Thus,
the questions were not formulated in the exact same way as above and another divergence is thenumerical annoyance scale, which was ranged 1–7 in the reversed direction (1=very annoying).
However, the meaning of the analysed questions from these objects are assumed to be close toquestion (a) above and the answers were translated numerically to fit the annoyance scale from 0 to
10, only for the purpose of comparing their averages to the measured SNQ’s. There was noequivalent question on annoyance from music for these objects though.
Besides the acoustic related questions, the questionnaires contained a personal data section wherecomplementary information e.g. sex, age, and number of persons in the household were given. In
general, the latter parameters were approximately evenly distributed among the residents within eachbuilding objects. However, a couple of the objects were dominated either by younger people(students, ≤25 years of age) or by older (≥65 years). Although a number of building objects were newwhen they were selected for the study, questionnaires were distributed at least six months after thebuilding was taken in use, in some cases more than one year.
The response rate was considered satisfying, typically 50–60% with a total amount of about 1400
filled-in questionnaires.
2.2 Sound insulation measurement and evaluation
Since D’nT is the current descriptor referred to in the Swedish building code, it has been usedthroughout the study. However, several objects have reported the measured data as sound reduction
R’, the previously used descriptor, and in such cases DnT was estimated as DnT = R’-1, a relationthat is valid for normal room heights of about 2.5 m (considering the vertical direction). The soundinsulation was typically measured in two to six rooms for each building object. From themeasurements, a variety of SNQ’s has been calculated. The ISO standardized DnT,w, DnT,w,50 and
DnT,w,50tr (abbreviation for DnT,w, D’nT,w + C50-3150 and DnT,w + Ctr,50-3150 respectively)
together with a couple of alternative SNQ’s reported in the literature; The optimized reference
spectrum for living noise DnT,w,50opt (DnT,w + Copt) by Virjonen et. al. [9] and the modified
reference spectrum DnT,w,50mod (DnT,w + Cmod) by Park/Bradley [13]. For the two latter cases,
the respective evaluation parameter was reported to give slightly higher correlation against
subjectively rated annoyance than the other tested frequency spectra. The spectrum related to Copt isgiven in the frequency range 50–5000Hz but is here adjusted to the interval 50–3150 Hz by adding +1 dB to each third-octave band in accordance with the ISO definitions of C50-3150 and C50-5000.
The reference levels of frequency spectra are compared in Fig. 1. Related to C50-3150, Copt andCmod give less importance to the low frequencies while the opposite prevails for Ctr,50-3150. The
reference level L is a part of the definition of the spectrum adaptation terms in accordance with:

One reason to include only vertical, and not horizontal, measurements is the wide variety of floor
plans. In some buildings there were hardly no partitions facing other dwellings but merely elevator
wells, stairwells, bathrooms, storage spaces, etcetera. This is most likely a positive development, but
makes the meaning of the measured sound insulation difficult to interpret. In the vertical direction,
sounds from more or less loud events are expected to transmit to several rooms below. A second
reason not to include horizontal measurements is that the participating residents have judged theannoyance from noise in vertical direction with, in average, about 70% higher score than noise5065

coming from the horizontal direction. In a Norwegian survey [19], the similar relation between
annoyance from vertically and horizontally directed noise was reported – both regarding sounds fromdaily living activities as well as music with bass and drums – despite the mean vertical soundinsulation being about 2 dB higher (DnT,w and DnT,w,50). These facts support the assumption thatthe vertical direction is of superior interest for the tested objects and it should then also make sense
to compare the measured sound insulation with the outcome of question (b) (about music) although
no direction is specified.
The number of measurements performed within each building object were typically four to six,
evenly distributed among living rooms and master bedrooms

2.3 Statistical analysis
In order to reveal the correlation between the SNQ’s and the subjective ratings from the residents,
linear regression analyses were performed. The mean SNQ from each building object has been usedas the input parameter. For the questionnaires, the input parameter is the mean annoyance rating for
one question from all building objects. The regression analyses are assessed with respect to thecoefficient of determination, R2. It has also been studied whether a statistical relation of 95%
confidence level exists between the annoyance and SNQ, i.e. whether the 95% confidence interval of
the regression line’s slope contains the value “0” or not. The analyses are in accordance with theauthors foregoing papers [20,21] where the method, including the use of the mean annoyance, wasfurther justified [20].

3. RESULTS
3.1 Measured sound insulation
The reported values of DnT,w vary from 49 to 66 dB whereas DnT,w,50 range from 47 to 64 dB.
The overall average of DnT,w is 58.9 dB and the standard deviation is 3.8 dB. The overall average of
DnT,w,50 is 56.3 dB and the standard deviation is 3.5 dB. The distribution according to the Swedish
sound classification [22], based upon DnT,w,50, is shown in Fig. 2. Sound class C corresponds to theminimum requirement of the national building code, DnT,w,50 ≥ 52 dB, while sound class B and Ameans successively 4 dB improved sound insulation. 5 of the 46 building objects (11%) do not fulfil
the minimum requirement while 29 objects (63%) manage sound class B or better.
3.2 Rated sound insulation
The mean annoyance of all the building objects is shown in Fig. 3 for the questions regarding
daily living sounds and music. The annoyance from footsteps, an impact sound source, serves as5066

comparison. The mean annoyance from daily living sounds in the vertical direction on the 0–10
numerical scale is 1.6, where the min/max scores are 0,1/4,2 with the standard deviation 0.8. In
horizontal direction, the mean annoyance is 0.9, where the min/max scores are 0.0/3.0 with thestandard deviation 0.7. The annoyance from vertical sounds is about 70% higher than in thehorizontal direction. The mean annoyance from music is 1.8, where the min/max scores are 0.1/ 4.8
with the standard deviation 1,1. Note that the question regarding music disturbance was only
available for 35 out of the 46 objects. All the airborne sound related questions show low annoyance,
in average, indicating that the residents are in general quite satisfied. (As mentioned above, 63% of
the objects have at least 4 dB higher insulation than the minimum requirement by the building
regulations.) As a comparison, the residents rate the noise from footsteps with a score being about
twice as high. This relation in annoyance between impact and airborne sound sources was establishedalready in one of the underlying studies [20] where mean annoyance scores of 5–6 were reported forsome objects. Thus, inadequate impact sound insulation is, for the included objects, a more
noticeable problem whereas their airborne sound insulation seems rather satisfactory.
3.3 SNQ’s vs. annoyance from daily living sounds and music
In Fig. 4, the linear regressions for DnT,w and DnT,w,50 against the rated annoyance of dailyliving sounds in all included objects are presented. The coefficient of determination, R2, is 8% for
DnT,w and 11% for DnT,w,50. The R2 for the alternative evaluation parameters DnT,w,50tr,
DnT,w,50opt and DnT,w,50mod is 7%, 8% and 6% respectively.

Figure 5 shows the linear regressions between DnT,w and DnT,w,50 and the rated annoyance ofmusic in all objects. The coefficient of determination, R2, is 8% for DnT,w and 14% for DnT,w,50.
The R2 for the alternative evaluation parameters DnT,w,50tr, DnT,w,50opt and DnT,w,50mod is 8%,
9% and 6% respectively. A summary of the outcome of the regression analysis is shown in Table 1.
It also contains information whether there is a statistical relation on 95% confidence level
between the annoyance and respective evaluation parameter. It can be seen that a statisticalsignificant relation is only achieved for DnT,w,50, which holds for both kinds of sound source.

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3.4 Light-weight vs. Concrete constructions
It is often assumed that potential low frequency problems concern primarily light-weight
constructions. Table 2 presents the results from similar regression analysis as above, but this time
separated into two groups; 24 light-weightbuildings (20 for the music annoyance) and 22
concrete buildings (15 for the music annoyance).
The averaged sound insulation and annoyanceare similar between the two groups. The mean
DnT,w,50 is 56.6 and 56.1 dB for light-weightand concrete buildings respectively and themean annoyance from daily living sounds is 1.4
and 1.8 and from music 1.6 and 1.9
respectively.
The regression analysis of annoyance from daily living sounds shows the same coefficient ofdetermination, 10%, for both DnT,w and DnT,w,50 within the heavy (concrete) objects. For thelight-weight buildings, R2 = 4% and 12% for DnT,w and DnT,w,50 respectively, and R2 increases to
20% for DnT,w,50tr which further emphasizes the lower frequencies. For the annoyance from themore low frequency source, music, R2 = 0% in the concrete buildings for both DnT,w and DnT,w,50.
Among the light-weight objects, R2 = 15% and R2 = 31% for DnT,w and DnT,w,50 against music
while R2 = 37% using DnT,w,50tr. It should then be beneficial to include the 50 Hz term dealing
with light-weight constructions while the same connection to concrete buildings is not evident here.
The linear regressions for DnT,w,50 against the rated annoyance from music for the concrete andlight-weight objects separately are shown in Fig. 6.

The alternative adaptation terms, DnT,w,50opt and DnT,w,50mod performs relatively well fordaily living sounds in concrete buildings. In light-weight constructions, for both daily living sounds
and music, DnT,w,50tr shows the highest correlation among the evaluated SNQ’s. This further
strengthens the assumption that evaluation including the low frequency range is of essential
importance regarding light-weight building technique.

4. DISCUSSION
In a broad sense, the measured single numbers are not clearly correlated to the average of thequestionnaire survey results. In most cases, the correlation is so weak that the statistic relation is
insignificant. The reasons for this conclusion are important to discuss.

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The overall average sound insulation was found to be 56.3 dB for DnT,w,50 (i.e. DnT,w +C50-3150) which indicates that 70% of the residents should be “not at all annoyed” according to thelarge study carried out in Norwegian dwellings 2002–2015 [23]. The residents in the study of ourpaper are in general not particularly annoyed about noise from airborne sound sources since most of
the building objects have “good” sound insulation. The combination of low annoyance and highsound insulation makes it hard to find strong mathematical connections between them. If the range ofsound insulation had been wider, it would is likely that the correlation against annoyance would begreater. Furthermore, it must be assumed that the perceived annoyance is not a function of the soundinsulation solely but involves other aspects. The authors are convinced that the neighbours’
behaviour as well as the individual preferences are important as well. These human aspects are likely
the dominating factors of annoyance of airborne sounds, among the relatively few residents being
severely annoyed, when living in buildings with high sound insulation.
From the underlying data, it is hard to make any clear statement whether airborne soundinsulation should be evaluated from 50 or 100 Hz. But strictly statistically, no significant relation
against rated annoyance could be found when using 100 Hz as the lower frequency limit. Evaluationfrom 50 Hz with DnT,w,50 does establish a significant relation although the coefficient ofdetermination is low and the differences compared to DnT,w are small. A possible explanation is that
all of the buildings, except 4 of them, were constructed after 1999, i.e. after that the 50 Hz limit
became mandatory in Sweden. Thereby, it can be assumed that the constructions were designed togive better insulation at lower frequencies than was the case before the regulation was introduced.
Earlier, it was in a way possible to design the construction such that structural resonances, thatnaturally occur, could be avoided above 100 Hz and instead be put at slightly lower frequencies withno further action taken. In this respect, heavy concrete constructions are less sensitive since they, due
to their high mass, better can withstand low frequencies. However, any used SNQ should not only be
related to a specific type ─ but to all kinds ─ of construction. Bearing in mind that a requirement
based on a given SNQ should force the building industry to use constructions with sufficient soundinsulation to all types of noise being prevalent more than occasionally in neighbouring dwellings.
Even though no big difference in the correlation was achieved concerning evaluating from 50 or
100Hz, it is important to reflect about whether high correlation or high protection is the mostimportant. The alternative spectrum adaptation terms DnT,w,50opt and DnT,w,50mod certainly do
consider frequencies from 50Hz, but their weighting curves reduce the importance of lower
frequencies compared to DnT,w,50. These alternative terms have been developed to obtain highcorrelation rather than high protection against all kinds of disturbing noise. Any regulation thatneglect lower frequencies do risk the situation where limited protection from specific sound sources
is given as well as sub optimization of the constructional design. Dimensioning must be performed inaccordance with the weakest case in order to be protective for all cases.
In Sweden, the 50 Hz spectrum adaptation terms have been used regularly during almost 20 years.
The fact that relatively few residents in the presented study complain about the airborne soundinsulation could be taken as a justification of the low frequency requirement in the national building
code. An innovative idea is to include even lower frequencies, i.e. below 50 Hz. However, aprimitive investigation including 10 building objects evaluated down to 20 Hz was done in the earlier
presented study [20] but no support to that idea was given (regarding airborne sound).

5. CONCLUSIONSThe extensive data of measurements and questionnaire surveys from 46 objects of apartment
houses suggest that the evaluation of airborne sound insulation from 50 Hz by DnT,w,50 is a slightly
better choice than evaluation from 100 Hz by DnT,w. However, since most of the included buildings
were originally designed to fulfill the requirements of the Swedish building code, based upon
DnT,w,50, the result could possibly have been different if they were designed for a 100 Hz criterion.
Airborne noise generates generally low annoyance rating from the residents in these constructions, afact that confirms the national regulation.
A criterion including frequencies from 50 Hz puts higher demands on the buildings’ construction,or in other words, it rules out constructions with insufficient low frequency airborne sound insulation.
Even though many daily living sounds contain just a small amount of energy below 100 Hz, theresidents will obtain a certain protection also towards low frequency sources, e.g. music, if criteria
from 50 Hz are used. The authors’ understanding is therefore to encourage evaluation from 50 Hz, on
the basis that the disturbances are often due to loud music.

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Note that the given conclusions are based upon Swedish traditions, which mean that tenants areused to live rather independently in their own home as well as staying indoors because of the climate.
In other countries, the conditions can be different in these respects.

6. Acknowledgement
The authors gratefully acknowledge the financial support by the Swedish research council Formas.

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