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

Subjective and objective assessments of noise barriers in terms ofthe loudness levelMarcin Piotr NOWAK1; Piotr KOKOWSKI2
1

AECOM Polska Ltd.; Institute of Acoustics, Adam Mickiewicz University, Poznan, Poland
2

Institute of Acoustics, Adam Mickiewicz University; Akustix Ltd., PolandABSTRACT
The common measure of noise reduction caused by acoustic barrier is its insertion loss, IL. It is expressed inA-weighted decibels, ILA, as a difference between A-weighted time-averaged sound pressure levels, LA,calculated or measured without and with the presence of acoustic barrier. By definition, insertion loss ILA
does not depend on the absolute value of the A-weighted sound pressure level. Thus, for example in case ofroad traffic noise, the ILA is insensitive to the change of factors influencing sound level, like vehicle speed,
fleet composition, distance to the road and so on. On the other hand, it is widely known that for someenvironmental noises LA poorly correlates with subjective noise assessment. A better correlation can beobserved for more sophisticated noise indexes, like Zwicker loudness, expressed in sones. The insertion loss
in sones, ILN, is to be determined as the ratio between the loudness without (N) and with the barrier (Nb). The
results of both, numerical calculations as well as psychoacoustic experiments, will be presented to show therelation between ILA and ILN and their correlation with the subjective assessments of barrier insertion loss asa function of different traffic noise levels.
Keywords: Acoustic barrier, Insertion loss, Loudness

1. INTRODUCTIONAs so far road traffic noise measures are mostly based on the A weighted time-averaged soundpressure level. For many years a lot of effort is made [ex. 1, 2 from early 90’s] to introduce other, moresophisticated objective measures of noise assessment. Some measures are based on subjectiveassessment. For example, in [2] listeners were asked to assess different categories of vehicles (heavyvehicles, delivery vans and passenger-cars) in varying road situations (accelerating, braking, driving
uphill and going downhill). It has been shown [2, 3] that the best correlation bet ween objective noise
measures and subjective annoyance assessment of these noises was obtained for A weighted soundpressure level, LA, and loudness, N.
On the other hand, it is known that any noise reduction measure expressed in A-decibels, including
the insertion loss of the acoustic barrier does not depend on the sound pressure level [4]. It means thatthe insertion loss for a given location of the immission point (observer) will be the same in case of aheavy vehicle and a passenger car pass-bys or for the same vehicle moving at different speeds and soon.
If we adopt loudness to assess barrier insertion loss its value won’t be relative any longer since it
depends on the sound level. In this paper a loudness ratio is proposed as a measure of the barrier’sinsertion loss. The aim of this study was to verify whether the insertion loss expressed in terms ofloudness ratio, IL N , gives better correlation with the subjective assessment of noise compared to theinsertion loss defined by the difference of A-weighted sound levels, IL A. It was made by comparison ofboth measures, IL A and IL N , with the results of psychoacoustic experiment. Both objective measures
were calculated from recorded signals, in the presence of and without the acoustic barrier. In
psychoacoustic part of experiment subjects were asked to judge the loudness of the above mentioned
signals. The stimuli were presented to the listeners for seven noise situations, distinguished by soundlevel values.
1
2

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marcin.nowak@aecom.com
pikoko@amu.edu.pl2. MEASURES OF THE INSERTION LOSS OF THE ACOUSTIC BARRIERInsertion loss defined as a difference of the A-weighted sound levels with- and without the barrier,ILA, comes from ISO 10847:1997‫ܮܫ‬஺ ൌ ‫ܮ‬௣஺ െ ‫ܮ‬௣஺௕ ሾ݀‫ܣܤ‬ሿ.
(1)
Regardless of other factors influencing sound propagation in the presence of barrier the above
formula is equal to the effectiveness of a given acoustic barrier.
Proposed measure of the insertion loss is based on the standardized definition of loudness and the(2)ଶସ஻௔௥௞ ᇱ
method of its calculation (ISO 532B). Loudness, N, is defined asܰ ൌ ‫׬‬଴ܰ ሺ‫ݖ‬ሻ݀‫ݖ‬ሾܵ‫݁݊݋‬ሿ,where N’ is “specific loudness” with a dimension of sone/Bark. The total loudness N is then theintegral of specific loudness over all critical-band rates expressed in Barks. To assess the impact ofnoise barrier loudness has to be calculated twice, using signal recorded with-, Nb , and without thebarrier, N. In order to compare the loudness of two signals one has to examine the ratio between two
numbers, N and N b. If the ratio N/N b equals to e.g. 2, it means that the signal recorded without thebarrier is twice as loud as the signal recorded with the barrier and so on. This loudness ratio is used in
the present study as a measure of the insertion loss of the acoustic barrier,(3)‫ܮܫ‬ே ൌ  ܰΤܰ௕ Ǥ

3. METHOD
A psychoacoustic experiment in which the listeners assessed the loudness of noise signals without
and with the barrier, for different initial sound level values, i.e. without barrier, was carried out. The
aim of the experiment was to verify the hypothesis that the effectiveness of the acoustic barrier,expressed by the volume ratio of signals (3), is better correlated with the subjective assessment oflisteners than effectiveness expressed by differences in sound levels, as in eq. (1). That’s why bothmeasures of effectiveness were compared with the subjective ratings.
3.1 Signals and measurement set up
Noise signals with- and without the acoustic barrier were recorded in both laboratory (fullycontrolled environment) as well as in real life conditions. Hereafter an anechoic chamber recordings(Institute of Acoustics, AMU) are used. The acoustic barrier and dummy heads used in this study arepresented at Fig.1.

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Figure 1 – The model of acoustic barrier and dummy head used for signal recording
Stationary noise signal was generated by the Brüel & Kjar type 4224 sound source. It was withe
noise weighted by traffic noise spectrum, according to ISO 1793-1, which was controlled with areference microphone. Signals were recorded at two points, for seven input sound levels (Table 1).
Duration of sound sample used in psychoacoustic experiment was set to 3 seconds.
Table 1 – The sound level values L pA [dB] without and in the presence of a barrier recorded in two, M1
and M2 , observation points
Position
M1
Position
M2

Barrier

Lmax-18

Lmax-15

Lmax-12

Lmax-9Lmax-6

Lmax-3

Lmax

without

56.5

59.4

62.3

65.3

68.4

71.2

74.0

with

34.3

37.0

39.9

42.8

45.7

48.5

51.4

Barrier

Lmax-18

Lmax-15

Lmax-12

Lmax-9Lmax-6

Lmax-3

Lmax

without

57.3

60.2

63.2

66.1

69.1

72.1

75.1

with

38.0

40.9

43.8

46.6

49.6

52.4

55.3

The model of acoustic barrier was built of chipboard (size: 2,7 x 5,0 x 0,02 m). Sound source waslocated at H s=1.8m above the “floor” level and in the distance Ds=1.3m from the barrier. Signals wererecorded at two distances of the dummy head and sound level meter microphones: M 1=2.6m (firstobservation point) and M 2=4.9m (second observation point). Two distances were selected to take into
account differences in sound spectrum which comes from diffraction on small barrier edges. The
dummy head and microphones were located at H M = 1.8 m above the floor level. The signals wererecorded using Neumann KU100 dummy head.
Based on these recording both measures of the insertion loss, ILA and ILN , were calculated.
3.2 Procedure Psychoacoustic experiment
The psychoacoustic experiment was fully controlled by computer software (Matlab’s environment).
Signals were played back by the sound card RME DIGI 96 PRO and trough an equalizer PEQ IV.1 tothe headphones Sennheiser HD 600. The headphone presentation was diotic with the subject sitting in7714

a sound-proof booth. Listeners were asked to judge the loudness of 14 signals, each sample of 3s
duration and calibrated to obtain sound pressure level as of Table 1. During 15-minute session eachlistener was exposed to 140 random order signals (14 signals repeated 10 times). Each session wasrepeated three times. The sequence of the stimuli was randomized. All in all, each signal was repeated
30 times. The loudness estimation of the signals was conducted with the use of the Absolute Method ofEstimation (AME).
The following instruction was given to the subject: Estimate the loudness of each signal by
assigning a positive number which in your opinion represents the loudness of the sound . Negative
numbers or zero should not be used.
Ten listeners with normal hearing participated in the experiment. No subject had to be excluded dueto hearing loss.

4. RESULTS
4.1 Calculated values of the insertion loss
The calculations were performed for 7 different input values of sound pressure levels. The resultsof these calculations are presented in Fig. 2 (for the insertion loss of the acoustic barrier expressed as
the level difference) and in Fig. 3 (for the insertion loss of the acoustic barrier expressed as a loudnessratio).

Figure 2 – Insertion loss of the acoustic barrier expressed as the difference in sound levels, ILA [dB], fordifferent input signal levels, at two locations, M1 and M2

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Figure 3 – Insertion loss of the acoustic barrier expressed as a loudness ratio, ILN, for scenarios as in Fig. 2
As expected the insertion loss, calculated as the difference of the sound levels with - and withoutbarrier, IL A, does not depend on the level of the presented signal (Fig. 2), with the differences in therange of 0,5 dB. The results are different when the insertion loss of the barrier is defined as a loudness
ratio, ILN . In this case, IL N depends on the sound level of the signal. The value of this measuredecreases when the levels of presented signal increases (Fig. 3).
4.2 Results of the psychoacoustic experiment
The geometric means of the perceived loudness were calculated for each individual subject (30repetitions for each signal). The term “perceive loudness” means the estimate of loudness performedby the subject. The term “loudness” itself means in this study calculated value of the loudness
according to ISO532B standard. As an example of individual data, the results obtained for two subjects
are presented in Figs. 4-5. The upper curve in each diagram refers to the perceived loudness of thesignal recorded without the barrier, while the lower curve represents the perceived loudness of thesignal recorded with barrier.

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Relative loudness [log AME]

3.0
2.5
2.0

y = 0.08x + 1.81
R2 = 0.98

1.9

1.9

2.1

2.1

2.2

2.3
1.7

2.4

1.8

1.5
1.5
1.0

y = 0.26x + 0.15
R2 = 0.92

1.3
1.1
0.8
without barrier

0.5

0.1with barrier

0.0
Lmax-18

Lmax-15

Lmax-12

Lmax-9Lmax-6

Lmax-3

Lmax

Relative sound level [dB]

Figure 4 – Perceived loudness scales expressed as a log of the AME judgments of the signals recorded
without (the upper curve) and with the barrier (the lower curve) for subject AD

Relative loudness [log AME]

2.0

1.5

1.0

0.5

1.8

y = 0.11x + 1.11
R2 = 0.99

1.6

1.9

1.7

1.4
1.3

1.3

1.3
0.9

y = 0.26x + 0.15
R2 = 0.92
0.6

1.0

1.1

0.8

without barrier

0.3with barrier0.0
Lmax-18

Lmax-15

Lmax-12

Lmax-9Lmax-6

Lmax-3

Lmax

Relative sound level [dB]

Figure 5 – Perceived loudness scales expressed as a log of the AME judgments of the signals recorded
without (the upper curve) and with the barrier (the lower curve) for subject AC
Application of the Absolute Magnitude Estimation (AME) method assumed that subjects could use
a different numerical scales in their loudness judgments. It allows to present the results as ageometrical mean for all subjects. On the other hand, a statistically significant difference betweensignals are possible to determine applying ANOVA method. This method however, requires
normalization of each subjects data. In this study the estimates were normalized to the maximum value,different for each subject. As a result it was shown that there are statistically significant differences
between stimuli [F(6,54)=14.4, p=0].

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In Fig. 6 insertion loss of acoustic barrier expressed as a perceived loudness ratio calculated onrelative loudness estimates is presented for 7 stimuli and all subjects participated in the experiment.
Assuming as a starting point the insertion loss of the acoustic barrier for a signal of maximum level(Lmax), i.e. taking ILN (Lmax) = 0%, it is possible to calculate the relative ILN expressed in % ofILN (Lmax). This is presented at Fig. 7.

7.0

Percived ILN

6.0
5.0
4.0
3.0

ADRSAGRPJTJSKP
ACMMKD

2.0
1.0
Lmax

Lmax-3

Lmax-6

Lmax-9Lmax-12 Lmax-15 Lmax-18

Relative sound level [dB]

Figure 6 – Subjective insertion loss of acoustic barrier for ten subjects, expressed as a perceived loudnessratio for seven signal levels
250

ADRS

Relative percived IL N [%]200AGRP
JT

150

JSKP
AC

100

MMKD

500

Lmax-3

Lmax-6

Lmax-9Lmax-12

Lmax-15

Relative sound level [dB]

Lmax-18

Figure 7 – Relative insertion loss of the acoustic barrier for ten subjects, expressed in % in relation to theinsertion loss of Lmax signal

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5. FINAL CONCLUSIONS
Proposed measure of the insertion loss of acoustic barrier, expressed as the signal loudness ratio,
has a following properties:
 the insertion loss, ILN depends on the absolute sound level of the signal. This conclusion
comes from the results of loudness calculation (Fig. 3) and listeners’ subjective assessment(Fig. 7)
 insertion loss expressed as both loudness ratio as well as listeners’ subjective assessment
decreases as the sound pressure level increases,
 one can notice a substantial subjective rise in the insertion loss for lower sound pressurelevels.
The psychoacoustic experiment based on a stationary noise signal confirms the findings of theprevious numerical calculations [5]. According to this measure insertion loss of the barrier, IL N , forlight vehicle passby (low sound pressure level) and heavy vehicle passby (high sound pressure level),
will be different. It is not observed for the existing measure of insertion loss, ILA. This conclusion haspractical potential in noise barrier design.
In the next step the above results should be confirmed based on real life recordings, eg. vehicles
moving at different speeds, what is the subject of current research.

ACKNOWLEDGEMENTS
This work would not have been possible without the support of the Institute of Acoustic AdamMickiewicz University and my attendance on this conference would not have been possible without
the financial support of AECOM company.

REFERENCES
1. E. Zwicker, H. Fastl, Psychoacoustic, Springer – Verlag, Berlin 1990.
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Applied Acoustics, 1992;58:351 – 377.
3. G. R. Watts, L. Chinn, N. Godfrey. The effects of vegetation on the perception of traffic noise. AppliedAcoustics, 1999;586:39 – 56.
4. R. Makarewicz, Sound in Environment, OWN, Poznan 1996 (in Polish).
5. M. Nowak. Insertion loss of acoustics barriers with random edge profile. XXX Zimowa Szkoła
Zwalczania Zagrożeń Wibroakustycznych, Gliwice-Ustroń, 2001; 198 – 206 (in Polish).
6. G. R. Watts, S. N. Chadler – Wilde, P. A. Morgan. The combined effects of porous asphalt surfacing andbarriers on traffic noise. Applied Acoustics,1999;58:351 – 377.
7. HEAD acoustics software, ArtemiS, v. 4.00.300.
8. ISO 532: 1975 Method of Calculating loudness level, Geneva, Switzerland.

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