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

Facade Sound Insulation as Protection to Outdoor Noise
Chiara SCROSATI1; Fabio SCAMONI1; Michele DEPALMA1; Matteo GHELLERE1
1

Construction Technologies Institute of the National Research Council of Italy, Italy

ABSTRACTIn recent years the acoustic qualification of buildings has gained more and more importance at international
level. One of the central elements for determining the sound quality of houses is the insulation against
external noise. In fact, many issues related to excessive noise in dwellings arise from their inclusion in noisy
environments, or from their original surrounding environment, which has grown from silent to very noisy
over the years. The main strategy to limit the noise inside the buildings is the façade insulation improvement.
Regulatory requirements and classification schemes in Europe present a high degree of diversity. In particular,
the approach to the façade insulation acoustic requirement adopted is very different in the various countries
also because it is closely related to the outdoor environmental noise. The main issues related to the façade
sound insulation are investigated with particular focus to the choice of the descriptors, the evaluation methods
and their reproducibility. The latter aspect is the more difficult to evaluate.
Keywords: Façade Sound Insulation, Outdoor Noise, Acoustic Classification

1. INTRODUCTION
One of the central elements for determining the sound quality of houses is the insulation against
external noise. In fact, many issues related to excessive noise in dwellings arise from their inclusion
in noisy environments (1), or from their original surrounding environment, which has grown fromsilent to very noisy over the years. The main strategy to limit the noise inside the buildings is the
façade insulation improvement (2).
The sound insulation of the façade can be evaluated according to different procedures; in general
the related requirements are divided into two main categories, the one relating to the building elements
performance (global facade or single element of facade), the second relating to the indoor sound level.
An overview of the different parameters and classification schemes in Europe is given in section 2.
The many difference of these parameters, also from the metrological point of view, implies first of all
the understanding of protection degree needed against the outdoor noise. In fact, to guarantee indoor
acoustic comfort, it is essential to understand the relationship between the environmental noise
surrounding the building and the degree of the acoustic insulation needed for the facade. In particular,
it is useful to correlate the external noise determined according to the European Noise Directive (END)(3) with the limit values for the acoustic classification of buildings. This is a well-known issue
considering the ISO work for a new standard on building acoustic classification (4).
Critical aspects of the evaluation methods for the external level (Lden) have been dealt with several
studies; Gaja et al. (5) have studied the problem of the external noise sampling, to determine anequivalent level over 24-h that is representative of the annual equivalent level. Brambilla et al. (6)have dealt with the issue of assessing the accuracy of a short-term sampling to estimate both the hourly
LAeq and the day and night values.
Furthermore, considering that the END admits that individual countries may define the time
periods differently, it is important to establish the correlations between Lden values of the END andthe ones according to the needs of each government.
In Italy, for example, Zambon et al. (7) analyzed the procedures for the conversion of day andnight values to Lden values and the descriptive statistics of the differences between the Lden level ofthe END and the Italian one.
1

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c.scrosati@itc.cnr.it

Finally, the problem of measuring the indoor level (Lden, indoor ) still needs to be extensively studied,especially with reference to the microphone positions.
In this work the preliminary results of a study carried out at the ITC-CNR laboratories on thecorrelation between the sound insulation of the facade, the environmental noise and the level Lden,indoor
are reported,.

2. Acoustic requirements for facades
The facade sound insulation is evaluated in very different ways in European countries. An overview
including façade sound insulation is summarized in Table 1 (8), where regulatory requirements are
split into two main categories, the first is related to the performance of the building (façade soundinsulation), the latter to the indoor sound level. The comparison of different European performancedescriptors and the influence of acoustic and non-acoustic parameters, has been studied by Scamoni
and Scrosati (9) and Masovic et al. (10) (11).
The façade sound insulation descriptors may be divided into two main categories: the former,
including D nT,w, and Rw expressing the performance (sound insulation of façade) directly, the latter,
including the indoor sound level, expressing it indirectly.
Table 1 – Overview of facade sound insulation descriptors of 18 European countries
Requirement’s
Typology
Façaderequirement
Mixed
Internal level
requirement

Measured quantity

N°
countries

Performance of the building
facadeIndoor sound levelD2m,nT,w ,D2m,nT,w + Ctr
Element façade R'w, Rw

11

1

Element facade R'w, Rw and LA,eq day and night
LA,eq day and night
Lden
LpA24h, LpAFmax

6

Note: overview information only. Detailed requirements and conditions are found in the building codes of thedifferent countries.

2.1 Lden and façade sound insulation
An example on how to correlate the limit values of external noise level with the façade soundinsulation limit values can be found in an ongoing ISO work (4) for a new standard on building
acoustic classification. In this proposal, the sound insulation of buildings against exterior noise isdescribed by the D nT,A,tr (=D2m,nT,w + Ctr ) indicator, with the class limit depending on the noise levelof the relevant outdoor source, itself described by the Lden indicator as defined in ISO 1996-1 (12) anddetermined in accordance with ISO 1996-2 (13).
The formula proposed to validate the correlation between sound insulation of façade and exterior
noise level is expressed by the Equation 1:

D2 m ,nT ,50 = Lden + 3 − Lden ,indoor(1)

where D2m,nT,50 is given by the Equation 2:

D2 m ,nT ,50 = D2 m ,nT + Ctr ,50− 3150

(2)

where D2m,nT is the standardized level difference defined in ISO 16283-3 (14);C tr,50-3150 is the adaptation term as defined in ISO 717-1 (15);Lden, indoor is the indoor day - evening - night - weighted sound pressure level, with the appropriate
weighting for the day, evening and night period, in the frequency range from 50 to 5000 Hz, as defined3491

in the END for the Lden descriptor.
To evaluate the consistency of Equation 1, a study was carried out at the ITC-CNR laboratory, byusing the average values of the standardized sound insulation, evaluated in the extended frequency
range (50-3150 Hz and 50-5000 Hz), within a Round Robin Test (RRT) carried out on the same facade
used for this comparative study (16).

3. EVALUATION METHODS
To address the aspects related to the position of the external microphone in relation to the surfaceof the façade (with the consequent effects of sound reflection) and in relation to the position normally
adopted for standardized level difference measurements of the façade, the level Lden of the external
noise was measured simultaneously in 3 different points (Figure 1). A first measurement position used
for measuring the acoustic insulation of the façade, at 2 m from the wall, 5.45 height m from theground (corresponding to 1.5 m height with respect to the floor of the receiving room) and horizontally,in the center of the facade under examination is defined as (CH3). The second position (CH4) is
located 1 m away on the horizontal center of the façade and 4 m height with respect to the ground,according to the prescriptions of ISO 1996-2 for the Lden measurement. The third point (CH2) isdirectly placed on the surface of the façade corresponding to the second measurement point; themicrophone was placed on a square plastic slab 0.3 cm thick and 20 cm wide, fixed to the wall withscrews.
Referring to the Law in force in Italy (DM 16/03/98 (17)), the indoor level, Lden, indoor (CH1), wasmeasured simultaneously with the external one Lden, at the height of 1.50 m from the floor, at a distance
1.50 m from the window and at least 1.00 m from any reflective surface in the room. The receiving
unfurnished room has a volume about 40 m3 and is located at the first floor of the experimentalbuilding of the ITC, and its façade, made of precast concrete, has an aluminum frame window with a
4/12/4 mm glazing. The measurements were carried out during 25 days, not all consecutive, in January,
February and March 2016, to obtain a reasonably reliable estimate (5) of the Lden value which, bydefinition, should be measured over a period of one year. The considered year is the relevant year as
regards the emission of sound and an average year as regards the meteorological circumstances. If theincident sound is measured, and the sound reflected by the facade is omitted, as a general rule, a 3 dB
correction must be adopted. When the microphone is placed at a certain distance from a reflecting
surface, provided that some given conditions are met, the direct and reflected sound is equally strong
and when the frequency band considered is wide enough, the reflection causes a doubling of the energyof the direct sound field and a 3 dB increase in sound pressure level (ISO 1996-2) can be found. For
both the CH3 and CH4 positions, the perpendicular distance of the microphone from the reflective
surface of the façade allows to obtain a nominally increase of 3dB. The values measured directly on
the surface of the façade correspond to a position in which an increase of 6 dB of sound pressure levelof the incident sound is obtained ("free field" level).

CH3
CH2

CH3
CH2

(a)

CH4

CH4

(b)

Figure 1 – Microphone location: (a) for the measurement of Lden; (b) for the edge effects evaluation.

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Although ISO 1996-2 suggests that, for a particular receiver, the position of the microphone isrelated to that particular receiver, due to the edge effects in the frequency range from 125Hz to 4kHz,a further measurement of 46 h was made in April 2016. In that case, the CH4 and CH2 microphones
were positioned at a distance b equal to 4m from the edge of the façade for the relation b  4d to be
met, where d equal to 1m is the perpendicular distance of the microphone from the surface of thefacade (CH4). The heights and distances of the two microphones from the surface of the facade havebeen kept constant (Figure 1 (b)).
3.1 LdenSome measurements, performed in the absence of precipitation, fog and/or snow and with windspeed not exceeding 5 m/s, were performed for an overall duration of 25 days. The measurements
were not all uninterrupted (about 5 weeks of which 2 including the weekend) and were carried out
over three months from January to March 2016.
Table 2 shows the following values:• the equivalent continuous sound pressure level LAeqT, as defined in the Equation 3, relating
to the entire measurement period;• the Lden,2002 values, relating to the entire measurement period, for the day – evening - night
periods defined in the END;
• the Lden,I values, relating to the entire measurement period, for the day – evening - night
periods defined in the Law in force in Italy (D. Lgs. 19 August 2005 n. 194).
where Lden, 2002 and Lden,I are defined in Equation 4.

1
LAeqT = 10 lg   p A2 (t ) / p02 dt 

T T

(3)

Lday
Levening + 5
Lnight +10de
24 − d − e
1010
Lden = 10 lg  × 10 + × 10+
× 10 10 
24
24
 24


(4)

Where:• d, e, n are the number of hours of the day, of the evening and of the night respectively;• Lday is the A-weighted equivalent continuous sound pressure level when the reference timeinterval is the day (dB);• Levening is the A-weighted equivalent continuous sound pressure level when the referencetime interval is the evening (dB);• Lnight is the A-weighted equivalent continuous sound pressure level when the referencetime interval is the night (dB);
With the following time intervals:• Default values according to END: the day is 12 hours (07.00-19.00), the evening is 4 hours(19.00-23.00) and the night is 8 hours (11.00-07.00);• Values according to the Law in force in Italy: the day is 14 hours (06.00-20.00), the eveningis 2 hours (20.00-22.00) and the night is 8 hours (22.00-06.00);
The Lden values reported in Table 2 did not include the 3dB correction, as they were used to verify
the validity of Equation 1 (as a difference between the two levels), where the term +3 is used to cancel
the effects of this correction.
Table 3 shows the LAeqT values of external microphone positions, to assess the edge effects (Figure
1 (b)). The reference time is 46h, and the effects are negligible, as can be seen from the comparison
with the LAeqT values in Table 2.

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Table 2 – LAeqT, Lden, 2002 (END) and Lden, I (Italian Law) assessed over the entire measurement period of
approximately 25 days. (Lden values without the 3dB correction)
Microphone position

LAeqT , dB

Lden, 2002 , dB

Lden, I , dB

CH1

32.2

31.3

31.2

CH2

61.4

59.6

60.5

CH3

58.3

56.7

57.5

CH4

58.6

56.8

57.6

Table 3 – LAeqT, over the entire measurement period (46h) to evaluate the edge effects.
Microphone position

LAeqT , dB

CH2(b)

60.9

CH3(b=a)58.5

CH3(b)

58.3

3.2 D2m,nT,w comparison
Table 4 shows the average value (average of 8 laboratories, each of which performed 5 repetitionsfor 40 measurements total) and the in situ standard deviation, obtained in the facade RRT (16), for the
façade under study, according to the scheme in Figure 2. The measurements were carried out according
to the prescriptions of ISO 16283-3, in particular, as the room volume is approximately 40 m3 , it waspossible to make a comparison between the default procedure and the so-called "low-frequency
procedure". Considering that the Lden, indoor measurements were carried out in only one point of theroom and not in the corners, it is more appropriate to apply Equation 1 to the results obtained withthe default procedure, as shown in Table 4.

Figure 2 –Sound source position of each laboratory participating in the façade RRT (16)3494

Table 4 – Average values (5 repetitions for each of the 8 laboratories) of the RRT (16) and thecorresponding in situ standard deviation (ssitu). The subscript "01" indicates that the descriptors were
calculated in steps of 0.1 dB, or with rounding to 0.1 dB in the case of indices with the sum of the spectrum
adaptation terms.
Descriptior (SNQ)

Average,dB

ssitu,dB

Dls,2m,nT,w01

40.0

0.7

Dls,2m,nT,w01+Ctr(50-3150)

34.7

1.0

Dls,2m,nT,w01+Ctr(50-5000)

34.7

1.0

As stated above, the measurements were carried out in 25 non-consecutive days (about 5 weeks).
Gaja et al. (5) estimated for a similar period of time, 4 random weeks, a standard deviation equal to0.81 dB. This standard deviation is comparable with the estimate of in situ standard deviation found
for the façade sound insulation (see Table 4).
The value of D 2m,nT,50 was calculated by applying Equation 1 to the values of Table 2, both for the
microphone position corresponding to the one used for the façade sound insulation measurements as
per ISO 16283-3 (CH3) and the position corresponding to the “environmental” position, 4 m high and
1 m far from the façade (CH4). It was found that, considering Lden,2002, D2m,nT,50 (CH3) was equal to25.4 dB and D 2m,nT,50 (CH4) was equal to 25.5 dB.
By comparing these results with the average values of D ls,2m,nT,w01+C tr(50-3150/5000) coming from the
RRT, equal to 34.7 dB, a difference in the estimation of the façade sound insulation equal to 9.3÷9.2
dB was found.
Repeating the same calculation referring to the law in force in Italy (DM 16/03/98), D2m,nT,50 isequal to 26.3 dB (CH3) and to 26.4 dB (CH4). Therefore, in this case, the difference with
D ls,2m,nT,w01+Ctr(50-3150/5000) reduces to 8.4÷8.3dB. By applying the error propagation, this difference
has the expanded uncertainty for the 90% confidence level for the two-sided test equal to 4.4 dB.
The sound field in the receiving room is not perfectly diffuse, in particular below the Schroeder
frequency. Garai et al. (18) showed that for this room the Schroeder cut-off frequency is 400 Hz.
Figure 3 shows this behavior. Above the Schroeder frequency, all the differences taken into accountin this study show the same trend. Below the Schroeder frequency in semi-diffuse field (down to100Hz) and in non-diffuse field (down to 50 Hz) the trends are very different.
At the higher frequencies (4000 and 5000 Hz) the accuracy and precision of the measurements of
D2m,nT depend on the position of the source and its directivity, as already stated in the reference (16).
In a previous facade RRT (19), in which all the laboratories used the same source position, theaccuracy of the results at high frequencies was much higher. This influence must also be analyzed in
the case of an extended source.

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ϰϲ
ϰϰ

Differences / dBϰϮϰϬŵĞĚŝĂ,ϰͲ,ϭ
Mean value CH4-CH3

ϯϴŵĞĚŝĂ,ϯͲ,ϭ
Mean value CH3-CH1

ϯϲ

CH4-CH1
,ϰͲ,ϭ

ϯϰ

CH3-CH1
,ϯͲ,ϭ

ϯϮ

CH2-CH1
,ϮͲ,ϭ

ϯϬ

Mean value CH2-CH1
ŵĞĚŝĂ,ϮͲ,ϭϮϴ

D2m (8 laboratories)ϮŵϴůĂďŽƌĂƚŽƌŝ

Ϯϲ
Ϯϰ
ϮϮ
ϮϬ
ϭϴ
ϭϲ
ϭϰ
ϭϮ

ϱŬ,ǌ

ϰŬ,ǌ

Ϯ͘ϱŬ,ǌϮŬ,ǌϯ͘ϭϱŬ,ǌ΁ϭ͘ϲŬ,ǌ

ϭŬ,ǌ

ϲϯϬ,ǌ

ϱϬϬ,ǌ

ϴϬϬ,ǌ

΀

Frequency / Hzϭ͘ϮϱŬ,ǌ

Ĩ

ϰϬϬ,ǌ

ϯϭϱ,ǌ

ϮϱϬ,ǌ

ϮϬϬ,ǌ

ϭϲϬ,ǌ

ϭϮϱ,ǌ

ϴϬ,ǌ

ϭϬϬ,ǌ

ϲϯ,ǌ

ϱϬ,ǌ

ϭϬ

Figure 3 –Differences comparisons: 8 laboratoriesRRT average D2m(black), with the error bars in terms ofin situ standard deviation; mean value (time average over 25 days of measurement) of the differences
between the measured values CH4-CH1 (blue), CH3-CH1 (red), and CH2-CH1 (green), with the error bars
in terms of standard deviation of the mean; difference for a maximum value of the spectrum with hourly
average CH4-CH1 (purple), CH3-CH1 (blue) and CH2-CH1 (orange).

4. CONCLUSIONS
The results of the research highlighted that the considered correlation between the noise leveldescriptor (Lden outdoor and indoor) and the façade sound insulation descriptor D2m,nT is not reliable.
The value of D 2m, nT, 50 measured directly differs 8÷ 9 dB from the value calculated by applying
this correlation, and using measured values of Lden and Lden, indoor . Although further verifications arenecessary, based on a greater number of experimental data, obtained in different environmental
situations, and such as to allow a more accurate statistical analysis, it does not seem appropriate to
adopt this correlation as a procedure in order to classify the façade sound insulation requirement.

REFERENCES
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