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Landscape and Urban Planning 125 (2014) 28–37

Contents lists available at ScienceDirect

Landscape and Urban Planning

j o ur na l ho me pag e: www.elsev ier .com/ locate / landurbplan

esearch Paper

he effects of audio–visual factors on perceptions of environmentaloise barrier performance

oo Young Hong1, Jin Yong Jeon ∗

epartment of Architectural Engineering, Hanyang University, Seoul 133-791, Republic of Korea

i g h l i g h t s

Low frequency contents of insertion loss increase noise annoyance.The influence of the esthetic values of barriers increases as noise level increases.The influence of the expected noise reduction of barriers decreases as noise level increases.Vegetation enhances the expected noise reduction and the esthetic value of noise barriers.

r t i c l e i n f o

rticle history:eceived 26 May 2013eceived in revised form 29 January 2014ccepted 2 February 2014vailable online 2 March 2014

eywords:oundscape perceptionoise barriersudio–visual interactionegetationnvironment

a b s t r a c t

As concerns on the adverse effects of noise pollution in the environment grow, the use of environmen-tal noise barriers has increased. Noise barriers are commonly considered to be public structures thataffect landscape quality. Hence, both acoustic and landscape issues need to be taken into considerationto design effective noise barriers in urban environments. This study aims to investigate the intersen-sory perceptions of noise barrier performance in terms of the spectral characteristics of noise reductioncombined with visual impressions of five different barrier types: aluminum, timber, translucent acrylic,concrete, and vegetated barriers. Illustrations of five barrier types were taken in a real urban environ-ment, and the noise reduction characteristics of these five barrier types were predicted based on theiracoustic characteristics. Noise annoyance, preconceptions regarding the noise attenuation performanceof the barrier, the esthetic preference, and the overall preference for noise barriers were assessed throughlaboratory experiments. Three different types of experiments were conducted: audio-only experiments,

visual-only experiments, and audio–visual experiments. The results of the experiments revealed thatnoise reduction, particularly low frequency components, had a dominant effect on the perception of thenoise-attenuating performance of barriers. The preconceptions of noise attenuation by barriers affectedthe overall preference for noise barriers at 55 dBA, while esthetic preferences for noise barriers weresignificant at 65 dBA. In addition, barriers covered with vegetation increased the perceived noise barrierperformance with increasing esthetic preference and preconceptions of noise reduction.

. Introduction

Noise pollution in urban spaces is a major environmental issue

hat has adverse health effects (WHO, 2000). As the demand foronstructing tranquil environments in urban settings grows, a vari-ty of noise mitigation methods, including noise barriers, earth

∗ Corresponding author at: Architectural Acoustics Lab (Room 605-1), Departmentf Architectural Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-u, Seoul 133-791, Republic of Korea. Tel.: +82 2 2220 1795; fax: +82 2 2220 4794.

E-mail addresses: st200045@hanmail.net (J.Y. Hong), jyjeon@hanyang.ac.krJ.Y. Jeon).

1 Address: Architectural Acoustics Lab (Room 601), Department of Architecturalngineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-91, Republic of Korea. Tel.: +82 2 2220 1795; fax: +82 2 2220 4794.

169-2046/$ – see front matter © 2014 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.landurbplan.2014.02.001

© 2014 Elsevier B.V. All rights reserved.

mounds, tree belts, porous road surfaces, and sonic crystals, havebeen tested to improve the sound environment. There are threemain components to consider in the controlling environmentalnoise: the noise source, the propagation path, and the receiver.For instance, the use of a porous road surface as an absorptivematerial is one of the methods used to reduce interactive noisebetween the road surface and the tire, in order to control thesource of the noise. Screening environmental noise barriers orearth berms are two of the noise mitigation methods related tothe propagation paths. Recently, sonic crystals have been inves-tigated as a method of noise mitigation (Martínez-Sala et al.,

2006; Picó, Sánchez-Morcillo, Pérez-Arjona, & Staliunas, 2012;Sánchez-Dehesa et al., 2011; Taherzadeh, Bashir, & Attenborough,2012). Sonic crystals, which are analogous to phonic crystals, con-sist of periodic arrays of scatters and are used to attenuate the

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J.Y. Hong, J.Y. Jeon / Landscape a

ransmission of sound at certain frequency bands. With respecto receivers, the spatial planning and location of noise-sensitivepaces such as offices, classrooms, and bedrooms facing away fromhe noise sources could reduce the influence of noise on receivers.

Environmental noise barriers have been widely adopted to pro-ect residents exposed to noise in urban environments. Accordingo the Korea Ministry of Environment (2011), environmental noisearriers with a total length of 1315 km were constructed at 4904

ocations by 2011. The U.S. Department of Transportation (2010)eported that forty-seven states built environmental noise barri-rs. The expenditure for constructing highway noise barriers hasteadily increased by 141 million dollars during the current period2008–2010), and the total expenditure in this period was approxi-

ately 554 million dollars. In general, the environment is perceivedy several modalities simultaneously, including, vision, sound,ouch, smell and taste (Shams, Kamitani, & Shimojo, 2002) andross-modal interactions between vision and audio were viewed ashe major modalities (Spence & Driver, 2004; Vroomen & Gelder,000). In terms of multisensory evaluation of urban environments,he acoustic comfort, visual quality, and daylight were evaluated asominant factors affecting the overall impression of urban environ-ents, whereas the effects of fragrance and odors on environmental

uality were significant (Jeon, Lee, Hong, & Cabrera, 2011). This alsoupports the idea that vision and audition were the major modal-ties to perceive urban environments. Therefore, even though therimary aim of environmental noise barriers is to mitigate noise,he design should also take visual character and visual quality intoonsideration.2 In South Korea, the Design Seoul Headquarters, ledy the mayor of Seoul, has provided design guidelines for pub-

ic structures such as benches, telephone booths, streetlights andnvironmental barriers since 2008. Regarding the design of noisearriers, the guidelines encouraged the construction of transparentarriers or vegetated barriers in urban public spaces to reduce theisual intrusion of noise barriers, which may have a negative impactn the perception of visual quality (Bailey et al., 2007; Joynt & Kang,010; Kim & Joo, 2007; Kotzen & English, 2009). In this context,nvironmental noise barrier design needs to satisfy not only noiseevel requirements but also respond appropriately to landscapeharacter and landscape quality. Thus, intersensory approaches tooise barrier design are necessary to identify effective solutions tochieve both good soundscape and landscape quality.

Research into soundscapes has emphasized the importancef contexts, including acoustic and non-acoustic factors, on theerceived sound environment (Jeon et al., 2011; Jeon, Lee, You, &ang, 2012; Maris, Stallen, Vermunt, & Steensma, 2007; Raimbault,avandier, & Bérengier, 2003; Schulte-Fortkamp, 2002), and manyesearchers have studied audio–visual interactions to evaluateoundscape quality. Carles, Barrio, and de Lucio (1999) investigatedhe interactions between various visual and sound environ-

ents and showed that congruence between sound and visualnvironments influenced subjects’ preference for the environ-ent. Similarly, Viollon, Lavandier, and Drake (2002) studied

udio–visual interactions using sound recordings and photographsn urban environments and demonstrated that an increase inrban visual scenes caused more negative soundscape perceptions.iollon (2003) also found that subjects perceived the combinationf traffic noise with vegetation as more annoying than the traffic

oise presented alone due to the incoherent combination of visualnd sound stimuli.

2 The visual character can be defined as the type of place relating to its topograph-cal, socio-cultural and natural resources, while, visual quality usually refers to theverall impression and the degree to which the visual experience satisfies the usersuch as comparing high, medium, and low.

an Planning 125 (2014) 28–37 29

Recently, Immersive Virtual Reality (IVR) technology, repro-ducing artificial environments as close as possible to realenvironments (Bailenson, Blascovich, Beall, & Loomis, 2003;Biocca & Delaney, 1995; Slater, 2009), has been employed tostudy audio–visual interaction. Ruotolo et al. (2013) assessed theaudio–visual impact of a new motorway in a rural area on cogni-tive performances and noise annoyance by simulating the pre- andpost-scenarios of the rural environment based on an IVR system.Similarly, Maffei, Iachini, et al. (2013a) explored the perception ofwind turbine sounds influenced by visual factors including the dis-tance from the wind turbines and the color of the turbines using anIVR system.

Several researchers have studied the perceptions of envi-ronmental noise barriers from the viewpoint of intersensoryinteractions. The visual impacts of noise barrier materials onperceived noise reduction have been explored in previous stud-ies. Aylor and Marks (1976) revealed that completely screeningsound sources leads to greater perceived loudness. Watts, Chinn,and Godfrey (1999) conducted laboratory experiments and cameto a similar conclusion, i.e., that a higher degree of visual screeningleads to perceptions of increased noisiness of the same noise.Maffei, Masullo, Aletta, and Di Gabriele (2013b) conducted labo-ratory experiments based on an IVR system to examine the effectsof three aspects of noise barriers on the perceived loudness andannoyance of noise: the esthetic aspect of the noise barriers, thevisibility of the noise source through the barriers, and noise lev-els at the receiver position. Similar to previous works, Maffei et al.(2013b) concluded that transparent barriers lead to less perceivedloudness and noise annoyance compared with opaque barriers, andthe effect of the visibility of the noise sources on noise percep-tion significantly increased as the noise level increased. Joynt andKang (2010) investigated how preconceptions of the noise reduc-tion behavior of noise barriers and esthetic preferences affectedthe perceived sound reduction by using five different noise barriermaterials: concrete, timber, metal, transparent acrylic, and vegeta-tion. These researchers found that the preconceptions of the noisereduction effectiveness of the various barrier materials affected theperceived sound reduction; solid and opaque materials, such asconcrete and metal, scored higher for preconceived noise reduc-tion, while a transparent barrier achieved lower rating scores interms of the prediction of noise attenuation, which affected theperceived noise attenuation. This finding implies that the barriermaterial through which traffic may be viewed, i.e. through trans-parent type barriers, was considered to be ineffective to attenuatenoise for residents even though the acoustic performance of thebarrier was similar to other barriers. Joynt and Kang (2010) alsorevealed that visual preferences for noise barriers were inverselycorrelated with the perception of noise attenuation, while statisti-cally significant correlations were not found between the predictednoise attenuation and the esthetic qualities of noise barriers. Tosummarize, the various visual factors affecting audio perceptionssuch as perceived noisiness and loudness were examined in previ-ous studies. However, the effects of the audio–visual aspects of thenoise barriers on the overall environment may be more essentialfor residents of an urban environment (Nilsson et al., 2012). Thus,the holistic assessment of the overall quality of an environment,including the audio and visual qualities, is necessary in order toevaluate the performance of the noise barrier.

Noise reduction using the noise barriers was taken into accountas one of the important factors to assess the perceived noise reduc-tion. Joynt and Kang (2010) evaluated five different noise levelsfrom 71.6 to 91.6 dBA in increasing steps of 5 dBA to examine

the influence of the higher or lower noise level on the percep-tions of noise barrier effectiveness according to barrier materials.Maffei et al. (2013b) conducted a laboratory experiment usingthree different noise levels in steps of 6 dB according to Maekawa’s

30 J.Y. Hong, J.Y. Jeon / Landscape and Urban Planning 125 (2014) 28–37

Table 1Acoustic characteristics of different types of noise barriers at a 1/1 octave band: AC (absorption coefficient), TL (transmission loss, dB) and IL (insertion loss, dB).

Material Frequency [Hz]

63 125 250 500 1k 2k 4k

Timber AC 0.3 0.4 0.6 0.9 0.8 0.6 0.6TL 7.2 9.0 13.0 33.1 36.0 41.0 41.0IL 6.5 8.2 13.2 18.1 20.9 22.7 23.3

Aluminum AC 0.4 0.5 0.7 0.7 0.6 0.6 0.6TL 4.5 10.1 27.8 32.7 34.7 34.9 34.9IL 3.8 9.5 17.5 20.6 21.3 20.9 21.0

Translucent acrylic AC 0.2 0.3 0.9 0.9 0.8 0.6 0.4TL 8.3 10.7 14.1 39.9 44.8 46.1 45.5IL 8.1 10.3 13.8 18.8 21.1 21.5 22.1

Concrete AC 0.1 0.1 0.5 0.8 0.6 0.6 0.6TL 19.5 31.5 38.2 58.0 50.0 51.2 52.3IL 15.4 16.6 18.1 19.8 20.5 20.8 21.2

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of the photomontages during the experiments. The width of theroad was approximately 30 m, and the average vehicle speed onthis road was 60 km/h. The original sound pressure level for the

Vegetation AC 0.3 0.4

TL 8.1 10.8

IL 8.5 10.9

urve (Maekawa, 1968). However, the ranges of the spectralharacteristics of noise reduction for the barriers were not takennto account in the studies. Although Watts et al. (1999) testedhe spectral differences of noises according to the types of screensncluding conifers, metals and willows, they only compared therends of the average noisiness rating as a function of the A-eighted sound pressure level (LAeq) and the loudness level for the

alidation of the appropriate indicator of the noise exposure. There-ore, objective noise reduction as well as spectral characteristics inerms of noise barrier types should be considered when planningnvironmental noise barriers.

In this study, we examined the intersensory perceptions of noisearrier performance with respect to the objective acoustic prop-rties of the barrier and the subjective impressions of the noisearrier by receptors. In terms of the acoustic characteristics of thearriers, audio stimuli were manipulated based on the measuredbsorption coefficients and the transmission losses of the noise bar-ier materials so that the spectra of the acoustic stimuli varied withoise barrier materials and their noise annoyance was evaluated. Inerms of the visual aspects, the esthetic preferences and preconcep-ions of the noise reduction capabilities of the various noise barriersere assessed. Finally, the dominant factors affecting the overall

nvironmental qualities were explored based on the audio–visualharacteristics of five different noise barrier materials in laboratoryxperiments at different noise levels.

. Methodology

.1. Stimuli

.1.1. Acoustic stimuliFive noise barrier materials, namely timber, aluminum,

ranslucent acrylic, concrete, and vegetation, were chosen as rep-esentative of materials widely used in urban spaces (Joynt & Kang,010; Kotzen & English, 2009). The timber barrier was made ofimber batons with transparent acrylic, and the aluminum barrieronsists of partially absorptive material with a perforated alu-inum skin.Preliminary field measurements of the acoustic environment

ere conducted to design experiments. Ten environmental noisearriers were installed in urban areas mainly consisting of apart-

ents and schools facing onto roads with traffic. Sound pressure

evels behind and in front of the barriers were measured 2 m awayrom the barriers for 3 min, and the LAeq, 3 min behind and in frontf the barriers ranged from 54.6 to 63.7 dBA and 65.1 to 80.7 dBA,

0.5 0.6 0.6 0.6 0.814.5 40.3 51.3 55.5 56.514.4 19.5 21.1 21.3 21.9

respectively. The average height of the ten barriers was 4.8 m,and the standard deviation was 1.1 m. The in situ measurementprocedures of an insertion loss of a noise barrier are specified inISO-10847. However, the measurement of insertion losses in urbanenvironments is limited in comparing environmental barrier per-formances because the barrier height, width, types of traffic, andtraffic volumes are difficult to control. Hence, in this study, theacoustic performance of the barriers with respect to sound absorp-tion and transmission loss were evaluated in the laboratory. Theabsorption coefficients and transmission losses of the various bar-rier materials, listed in Table 1, were measured in a reverberationchamber based on ISO 354 and ISO 140-3, respectively.

The insertion loss of each noise barrier type at 1/1 octaveband was then predicted using the Environment Noise Predictionand Design Program (ENPro) acoustic simulation software basedon the measured transmission loss and absorption coefficient ofeach barrier type, as listed in Table 1, and the barrier’s geometry.ENPro calculated the sound attenuation during outdoor propaga-tion considering direct diffracted and reflected paths according toISO 9613. As shown in Fig. 1, the height of a barrier was fixed at5 m, taking into account the averaged barrier height in the fieldmeasurements, and the receiver was positioned 2 m behind thebarrier. Road traffic noise from a 10-lane dual carriageway in Seoulcharacterized by high traffic flow was recorded using a binauralmicrophone (Type 4101, B&K) and a digital audio recorder (Fos-tex, FR-2). The head orientation was coherent with the orientation

Fig. 1. Scheme of the simulated conditions of the laboratory experiments.

J.Y. Hong, J.Y. Jeon / Landscape and Urban Planning 125 (2014) 28–37 31

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ig. 2. Photomontages for the different types of noise barriers: (a) timber, (b) aluminover, (g) concrete with dense vegetation cover, (h) translucent acrylic with sparse

-min recording was 75.8 dBA. For the laboratory experiments, a s audio sample of road traffic noise, with continuous vehicle flownd a small variation in noise levels, was excerpted from the 3-minecording.

.1.2. Visual stimuliVarious landscape simulation methods have been used in previ-

us studies to assess the visual impact of noise barriers. Watts et al.1999) used photographs taken at listening positions with varyingevels of vegetation cover. Joynt and Kang (2010) presented subjects

ith video recordings of selected locations for contextual real-sm. Kim and Joo (2007) evaluated subjects’ visual preferences forifferent noise barriers using a photomontage method. Similarly,affei, Masullo, et al. (2013b) created virtual noise barrier images

sing graphical 3D-modeling programs. The use of photographsnd video recordings is difficult to control for the multiplicity ofnteractive factors such as the background content, angles of view,

ifferent weather conditions, and the sky ratio, which may affecthe subjective responses to the barrier. Jeon, Kim, Cabrera, andassett (2008) investigated the effect of audio–visual interactionn seat preferences in an opera house. They used visual images of

) concrete, (d) translucent acrylic, (e) vegetation, (f) concrete with sparse vegetationation cover, and (i) translucent acrylic with dense vegetation cover.

the stage view from different seats, where the angle of view wasdifferent for each seat. It was found that the visual preferences forcertain stage views affected the perception of audio–visual pre-ferences. This indicates that the angle of view, distance and scaleinfluences the perception of the visual images. It was also revealedthat sky ratio affects the perception of openness in urban spaces(Jeon, Hong, & Lee, 2013; Hong & Jeon, 2013). In particular, main-taining the same view and camera angle is required to comparethe landscape qualities of the images (Tveit, 2009; Ode, Tveit, &Fry, 2010). Therefore, in this study, the photomontage method wasused to create visual images of the noise barriers.

Photographs of barriers in urban spaces were taken using a dig-ital camera (Canon EOS 300D) with the same angle of view to allowfor the superimposition of barrier images on the background con-tent (Fukahori & Kubota, 2003; Todorova, Asakawa, & Aikoh, 2004).Photomontages of the five types of barriers were then created basedon these photographs using Adobe® Photoshop® CS4 software; the

photomontages are shown in Fig. 2. The viewpoint angle was fixedto prevent the viewing angle from influencing the results.

To investigate the effect of the visual aspects of vegetation onthe perception of noise barrier performance, different densities

32 J.Y. Hong, J.Y. Jeon / Landscape and Urb

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Fig. 3. Spectral characteristics of acoustic stimuli at 55 dBA.

f ivy, which is one of the plants most widely used as a climbingall cover on urban noise barriers, were superimposed on the

ranslucent and concrete barrier images, which were judged to behe best and worst in terms of esthetic preference (Kim & Joo, 2007;

affei et al., 2013b), respectively, as shown in Fig. 2(f–i). The rea-on that the translucent and concrete barriers were selected was toxamine the effect of two levels of vegetation (dense and sparse) onhe perception of barrier performance at two significantly differentevels in terms of visual preference. All of the barrier images com-letely screened the sound sources (vehicular traffic) to avoid theffect of visibility of the sound source (traffic) on noise perceptionAylor & Marks, 1976; Watts et al., 1999). In total, nine visual noisearrier photomontages were created for use in the experiments.

.2. Experimental design

Two main factors were taken into account to design the labora-ory experiments: the first factor was the effects of the frequencyharacteristics of the noises, which were determined by thensertion losses of different barrier types, on noise annoyance. Theecond factor was the influence of the visual aspects of the noisearrier materials, including the esthetic preferences and the pre-icted noise attenuation, on the perception of the overall quality ofhe environment. To examine the audio–visual interactions, labora-ory experiments were undertaken consisting of three sessions: (1)

visual-only session, (2) an audio-only session, and (3) a combinedudio–visual session in which images were provided along withhe sound stimuli during the experiment. Considering the ranges ofound pressure levels behind the barriers from the preliminary fieldeasurements, road traffic noises behind the barriers were kept at a

onstant SPL of 55 or 65 dBA in the laboratory experiments, but onlyhe spectral characteristics were varied in terms of the insertionoss of each barrier at each octave band. Fig. 3 illustrates the spec-ral characteristics of acoustic stimuli in the second experiment, inhich the A-weighted equivalent sound level, LAeq, behind the bar-

iers was fixed at 55 dBA. The audio sample of the road traffic noiseas manipulated to reflect each barrier’s insertion loss by using

he octave-band graphic equalizer filter function within Adobe®

udition® software. The sound pressure levels for the acoustictimuli presented to the subjects were verified through an acousti-al analysis of the recordings of presenting sounds. The recordings

ere conducted using a dummy head (Brüel&Kjær 4100) and aeadphone (Sennheiser HD 600). LCeq − LAeq, and Zwicker’s loud-ess of acoustical stimuli are listed in Table 2. The loudness of thecoustic stimuli was calculated using Pulse Software, ver. 12.6

an Planning 125 (2014) 28–37

(Brüel & Kjær) in the same manner as a previous study (Jeon et al.,2012).

2.3. Procedure

In total, 20 university students participated in the exper-iments. Previous studies that dealt with the perception ofaudio–visual stimuli have used laboratory experiments witha similar number of participants (Hong & Jeon, 2013; Jeon,Lee, You, & Kang, 2010; Jeon et al., 2013; Joynt & Kang,2010). In addition, the participants were asked to take partin the experiments twice to enhance the reliability of thedata. The participants all had normal hearing and consisted of 13males and 7 females with an age distribution of 19–29 (mean: 24.3,standard deviation: 2.4) years.

A paired comparison method was adopted to evaluate theannoyance caused by noise, esthetic preferences, preconceptionsof barrier attenuation, and overall preferences concerning the envi-ronment. In the audio-only session, paired combinations of thefive stimuli resulted in 10 pairs without reversal. The durationof each stimulus was 4 s, with 1-s silent intervals between thestimuli. After the presentation of each pair, subjects were askedto respond to the following question: “Which stimulus would bemore annoying if you were exposed to it in an urban space?” In thevisual-only session, 36 pairs consisting of noise barrier images werepresented to the subjects. In this session, where visual stimuli werepresented without sound, the visual preferences and preconcep-tions regarding the noise attenuation capability of the barriers wereassessed by asking the subjects to respond to the following ques-tions: “Which noise barrier would be more aesthetically preferable toyou in an urban area?” and “Which noise barrier would be more effec-tive at attenuating noise?” In the audio–visual session, nine visualimages combined with attenuated road traffic noises from the pre-vious tests were used to construct 36 pairs of audio–visual stimuli.Subjects were asked to select the preferred environment for eachpair with regard to both visual and acoustic qualities with the fol-lowing question: “Which stimulus would be more preferable if youwere exposed to it in an urban space?” The subjects were allowedto listen to the pairs of stimuli as many times as they wanted toanswer the questions.

During the experiments, acoustic stimuli were presentedthrough open-type headphones (Sennheiser HD 600), and visualstimuli were projected onto a white screen (1.9 m wide and 1.4 mhigh) using a beam projector (Sony VPL-CX6). The experimentswere conducted in a testing booth (4 m × 3 m) in which the back-ground noise level was less than 20 dBA.

3. Results

3.1. Audio-only condition

Scale values were calculated based on Thurstone’s law of com-parative judgment Case V (1927). The results of the scale valuesof annoyance from the audio-only tests are shown in Fig. 4. It wasfound that the acoustic stimuli of timber and aluminum barrierswere evaluated as relatively more annoying than those from theother materials, while the concrete barrier had the lowest scalevalues for annoyance. A two-way ANOVA test was carried out toexamine the effects of barrier types and noise levels, and the resultsare listed in Table 3. A significant effect of the barrier material wasfound at the 0.01 level, whereas the effect of the noise level wasnot significant. A post hoc Tukey’s test indicated that the acoustic

stimulus of the aluminum barrier was the most annoying of all thebarriers (p < 0.01), whereas that of the concrete barrier was the leastannoying. The interaction between the noise barrier type and noiselevel was also significant (p < 0.05). Simple main effects (SME) were

J.Y. Hong, J.Y. Jeon / Landscape and Urban Planning 125 (2014) 28–37 33

Table 2Acoustic characteristics of acoustic stimuli: (a) 55 dBA and (b) 65 dBA.

Timber Aluminum Translucent acrylic Concrete Vegetation

LCeq − LAeq (dB) (a) 10.7 12.1 9.2 5.5 9.2(b) 10.7 12.1 9.2 5.5 9.2

Loudness (sone) (a) 11.2 11.2 11.1 10.5 11.1(b) 21.9 22.0 21.7 20.4 21.7

Table 3Summary of a two-way ANOVA for the scale values of annoyance in the audio-only condition with factors of barrier types and SPLs, and the results of a simple main-effectanalysis of data obtained from Experiment II.

Factor Sum of squares Degrees of freedom Mean square F p-Value

(a) Barrier type 36.84 4 9.21 29.90 0.0055 dBA 21.19 4 5.30 17.10 0.0065 dBA 19.79 4 4.95 15.97 0.00

(b) SPLs 0.05 1 0.01 0.02 0.90Timber 2.23 1 1.23 3.97 0.25Aluminum 7.26 1 0.01 0.03 4.30Translucent acrylic 17.28 1 0.40 1.29 1.30Concrete 0.76 1 0.01 0.03 4.30Vegetation 4.91 1 2.50 8.06 0.05

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a higher esthetic preference for concrete barriers with dense vege-

(c) Interaction 4.13 4

Residual error 58.52 190

Total 99.50 199

alculated to investigate the significance of these interactions, andhe results are summarized in Table 3. In terms of the SME of barrierypes, significant differences between acoustic stimuli were foundt noise levels of both 55 and 65 dBA.

The relationship between acoustic indicators and scale valuesor noise annoyance were investigated with a multiple regressionnalysis. Loudness and LCeq − LAeq, representing sound intensity sen-ation (Zwicker & Fastl, 2006) and the low frequency content ofcoustic stimuli, respectively, were selected as acoustic indicatorso explain the subjective noise annoyance responses. LAeq was notonsidered because A-weighted sound pressure levels were fixedn the experiments. The results of the multiple regression analy-is are listed in Table 4. It was found that the standardized partialegression coefficients of LCeq − LAeq were statistically significant atoth 55 and 65 dBA (p < 0.01), while the effects of loudness wereot significant.

.2. Visual-only condition

In the visual-only session, scale values of esthetic preferencesnd preconceptions of noise attenuation for nine barrier imagesere calculated as shown in Fig. 5. The black and white bars indicate

ig. 4. Scale values of annoyance according to acoustic stimuli in an audio-onlyession. Error bars indicate the standard deviations.

1.03 3.35 0.010.31

the scale values of esthetic values and preconceptions of the noisereduction ability of the various barrier types, respectively. In thissection, the effects of esthetic preferences and preconceptions ofthe effectiveness of noise barriers were investigated.

3.2.1. Esthetic preferences regarding noise barriersMean differences among esthetic values for barrier images were

significant in a one-way ANOVA [F(8, 179) = 21.16, p < 0.01]. Tukey’spost hoc tests were carried out to examine the significance of dif-ferences in visual preferences according to barrier type. The resultsshow that esthetic preference values for aluminum and concretebarriers were significantly lower than those for the other barriertypes, whereas significant mean differences were not found amongtimber, translucent acrylic, and vegetated barriers. Regarding thevisual effects of vegetation, vegetation-covered translucent barri-ers did not increase visual preference significantly, but subjects had

tation cover than concrete barriers with no climbing ivy (p < 0.05).

Fig. 5. Scale values of the esthetic preferences and preconceptions of noise atten-uation according to visual stimuli: timber (Ti), aluminum (Al), translucent acrylic(Tr), concrete (Co), and vegetation (Ve). L and H in parentheses denote low and highdensities of vegetation cover, respectively. Error bars indicate standard deviations.

34 J.Y. Hong, J.Y. Jeon / Landscape and Urban Planning 125 (2014) 28–37

Table 4Regression coefficients for acoustic indicators from a multiple regression analysis of noise annoyance: (a) 55 dBA and (b) 65 dBA.

(standardized) t Correlations R2

Zero Partial Part

(a) Loudness −0.26 −1.35 0.61 −0.10 −0.08 0.43LCeq − LAeq 0.93** 4.37 0.62 0.31 0.25

(b) Loudness −0.55 −2.03 0.52 −0.51 −0.125 0.33LCeq − LAeq 1.10** 4.04 0.56 0.29 0.25

** p < 0.01.

Table 5Summary of two-way ANOVA results for scale values of annoyance in the audio–visual condition with the factors of barrier type and SPL.

Factor Sum of squares Degrees of freedom Mean square F p-Value

Barrier type 80.71 8 10.09 46.29 0.00SPL 0.00 1 0.00 0.00 1.00

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Residual error 74.53 342

Total 156.49 359

.2.2. Preconceptions of noise barrier performanceAs illustrated in Fig. 5, preconceptions of the noise abatement

bilities of the barriers were different from the visual preferencesf noise barrier types. The mean differences in the preconceptionsf the performance of the various barrier types were statisticallyignificant [F(8, 179) = 8.53, p < 0.01]. Subjects predicted thatoncrete and vegetated barriers would perform better than thether barrier types. Similar to the case in a previous study (Joynt

Kang, 2010), the translucent barrier was judged to be the leastffective in reducing noise.

The effect of differences in vegetation cover on the preconcep-ions of noise barrier efficacy was also examined by a series of meanomparison tests. An analysis of the test results revealed that denseegetation cover on translucent and concrete barrier fac ades signif-cantly increased the preconception of the noise attenuation abilityf these noise barriers at the p < 0.01 level. Covering noise barri-rs with vegetation can therefore significantly enhance subjects’erception of noise barrier performance.

.3. Audio–visual condition

Fig. 6 shows the scale values of the preference for the overalludio–visual environment for the different noise barrier types.he results of a two-way ANOVA of scale values for overall

ig. 6. Scale values of the overall preference for the environment according to com-ined audio–visual stimuli: timber (Ti), aluminum (Al), translucent acrylic (Tr),oncrete (Co), and vegetation (Ve). L and H in parentheses denote the low andigh densities of vegetation cover, respectively. Error bars indicate the standardeviations.

0.16 0.72 0.670.22

preferences are listed in Table 5. The main effect of the barriertype was significant (p < 0.01), whereas that of the noise level wasnot determined. No interaction between the barrier type and thenoise level was found. Subjects rated the vegetated barrier as themost preferable in the audio–visual condition when the LAeq of thestimuli was constant at 55 or 65 dBA, followed by concrete, wood,translucent acrylic, and aluminum barriers. The results of a posthoc Tukey’s test indicated that the scale values of vegetation andconcrete were significantly higher than those of the other barriermaterials. Aluminum and translucent barriers had low scale valuesfor overall preference, but there was no significant mean differ-ence between these two barrier types. Vegetation cover improvedthe overall preference for concrete barriers (p < 0.01), and densevegetation cover also significantly improved the overall preferencefor translucent barriers (p < 0.05).

4. Discussion

4.1. Effects of the spectral characteristics of noise barriers onannoyance

It was found that subjects based their annoyance with roadtraffic noise on the spectral characteristics of the noise. In partic-ular, the road traffic noise associated with aluminum and timberbarriers, which had dominant low frequencies below 250 Hz, wasassociated with higher annoyance scale values, whereas the roadtraffic noise associated with a concrete barrier, which had thelargest noise reduction at low frequencies, had the lowest annoy-ance scale value.

The regression analysis listed in Table 4 supports that the rela-tive level of acoustic stimuli at low frequencies played an importantrole in the assessment of noise annoyance when sound pressurelevels were constant behind the barriers. This closely relates to theprevious study (Nilsson, Andéhn, & Lesna, 2008) in which it wasfound that road traffic noise behind barriers was perceived to bemore annoying than road traffic noise with no barrier due to theincrease in the relative level of low frequency sound behind thebarriers. With regard to the SME for noise levels, 55 and 65 dBA,a statistically significant difference was found only for the acous-tic cue of the vegetated barrier. This indicates that variations innoise levels may not affect the scale value of annoyance for acousticstimuli.

Even though the effect of spectral characteristics onnoise annoyance was statistically significant, the frequencycharacteristics of acoustic stimuli do not provide clear evidencewith which to judge noise annoyance because the regression model

J.Y. Hong, J.Y. Jeon / Landscape and Urban Planning 125 (2014) 28–37 35

Table 6Regression coefficients for factors from a multiple regression analysis of overall preference: (a) 55 dBA and (b) 65 dBA.

(standardized) t Correlations R2

Zero Partial Part

(a) Noise annoyance −0.29** −5.14 −0.43 −0.36 −0.28 0.49Esthetic preference 0.23** 4.14 0.12 0.30 0.22Preconceptions 0.56** 9.84 0.61 0.60 0.53

(b) Noise annoyance 0.34** 5.10 0.19 0.53 0.34 0.71Esthetic preference 0.70** 10.45 0.72 0.79 0.69Preconceptions −0.26** −3.75 −0.43 −0.41 −0.25

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rom the audio-only experiments at 55 and 65 dBA explained 43%nd 33% of the variability in noise annoyance, respectively, asisted in Table 4. Therefore, barrier designers should considerhe amount of the insertion losses of noise barriers before takinghe low frequency noise reduction capability of the barrier intoccount.

.2. Effects of subjective factors of the noise barriers on theerception of overall quality

A multiple regression analysis was conducted to examine theontributions of perceptual factors, including esthetic preferences,reconceptions of noise attenuation capability, and noise annoy-nce, to the overall preferences at different noise levels. Theegression coefficients for the independent variables are listedn Table 6. In the experiment at 55 dBA, the regression modelxplained 49% of the variability in the overall preference. The pre-onception of the noise-reducing ability of the barrier was theost dominant factor affecting the overall environmental prefer-

nce; the standardized regression coefficients for preconception,oise annoyance, and visual preference were 0.56, −0.29, and 0.23,espectively. However, in the experiment performed at 65 dBA,he contribution of the esthetic preference for the barrier to theverall impression was the greatest among perceptual factors.he coefficient of the determination of the regression model was.71. These results imply that different factors dominate at differ-nt noise levels, suggesting that respondents derived their overallmpression based on preconceptions of the barrier noise attenu-tion performance at 55 dBA of road traffic noise because a roadraffic noise level of 55 dBA is relatively acceptable. In contrast,hen the road traffic noise level was increased to 65 dBA, the sub-

ects assessed barrier performance grounded on visual preferencesor the types of barrier materials. This indicates that the precon-eptions of the noise attenuation performance of barriers playedess of a role when the road traffic noise level was higher than5 dBA.

Joynt and Kang (2010) found that the mean differences in ratingcores in terms of perceived noise levels for the different noise bar-iers were statistically significant when 71.6 and 76.6 dBA of roadraffic noise were presented; the subjects consistently judged thathe concrete and metal barriers reduced noise to a greater extenthan other barriers at 71.6 and 76.6 dBA. This result is somewhat inontrast to the results from this study, even though Joynt and Kang2010) did not specifically analyze the contributions of the pre-onception and esthetic preferences to the perceived loudness ofoise at difference noise levels. It can be explained that the presenttudy concentrated on the evaluation of the overall environmentoncerning both sound and visual perceptions, whilst Joynt and

ang (2010) focused on the assessment of the perceived noisettenuation for the perception of noise barriers. This suggestshat noise barrier design strategies for selecting barrier mate-ials might be different whether the design target of a barrier

is to reduce the perceived noisiness or to improve the per-ception of the overall environment. In addition, environmentalnoise barrier designers should carefully consider barrier mate-rials when attempting to enhance the holistic quality of theenvironment by taking into account the perceptual influence ofthe esthetics and preconceptions of different noise levels behindbarriers.

4.3. Effects of vegetation on the visual aspects of barriers

In the present study, the vegetated barrier obtained high scoresin terms of esthetic preference and the preconception of noiseattenuation ability. However, the subjects’ preconceptions of thenoise barrier performance of a vegetated barrier in the previousstudy (Joynt & Kang, 2010) was relatively lower than that in thepresent study, mostly likely due to the use of a different type ofvegetation; Joynt and Kang (2010) used deciduous vegetation with-out many leaves, while the authors presented subjects with imagesof barriers covered with ivy with a high density of leaves. Thisimplies that the density of leaves and the types of vegetation couldbe important factors with which to evaluate the visual aspects ofvegetation.

It was also shown that the use of barriers covered with ivyor vegetation could enhance the perception of the environmentalnoise barriers. In particular, the visual preference for the con-crete barrier was significantly increased when the barrier wascovered with climbing ivy. This indicates that the effect of a veg-etated fac ade to improve the visual preference of a barrier mightbe significant for a barrier with poor visual quality. It was foundthat the use of vegetation on the translucent barrier could alsoenhance its esthetic preference and preconception of noise atten-uation ability from the experiments. However, transparent andtranslucent barriers are usually designed to transmit light andbe less obstructive so that adding vegetation on the surfaces oftransparent or translucent barriers might not be realistic becausecovering vegetation on the surface could negate these characteris-tics.

The conclusion that the use of barriers with climbing wall cover,or vegetation barriers could enhance perception of barrier per-formance is somewhat in contrast to a previous study (Viollon,2003), which showed that vegetation combined with traffic noisewas more annoying to subjects due to the lack of congruencebetween the visual and audio stimuli. In the present study, subjectsdid not appear to consider a vegetated barrier or a vegetation-covered concrete wall to be vegetation, such as trees and shrubbery,but as materials that could be used as noise barriers. Kotzenand English (2009) stressed the importance of planning to designenvironmental noise barriers to improve landscape quality. They

also emphasized that planting strategies should be planned bytaking the concept of the overall noise barrier and landscapedesigns into account. This indicates that solid and opaquevegetation-covered barriers could be useful as noise barrier

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aterials and could enhance the perceptions of noise barrier per-ormance.

. Conclusions

In this study, the acoustical characteristics of noise barriersnd their visual perceptions were investigated through laboratoryxperiments. Regarding the objective acoustic performance of thearriers, it was determined that the perceived noise annoyance was

nfluenced by the spectral variance of noise transmitted througharriers. In particular, the low frequency content of noise signif-

cantly affected the perception of noise annoyance for constantoise levels. The contribution of perceptual factors including noisennoyance, esthetic values and the predicted noise attenuation ofarrier types on the assessment of overall environmental qualityas examined at different noise levels. It was shown that the pre-

onception of noise barrier performance according to the barrieraterial significantly affected the environmental quality at 55 dBA

f road traffic noise, whereas the contribution of the esthetic val-es of barrier materials to the overall impression of environmentas greater than other factors. In addition, it was revealed that

egetation cover had a significant effect on subjects’ perceptionsf noise barriers. In particular, concrete barriers with climbing ivyere perceived to be more attractive and enhanced the predictedoise attenuation performance as well as the overall environmen-al quality. These findings suggest that using vegetation to cover

barrier fac ade could be a practical means of enhancing not onlyhe perceived overall environmental quality but also the perceivedoise barrier performance. This study could therefore provideseful approaches to understanding both acoustic and visual per-eptions of barrier materials to find optimal solutions and strategiesor environmental noise barrier design.

cknowledgements

This research was supported by the International Research &evelopment Program of the National Research Foundation oforea (NRF) funded by the Ministry of Education, Science andechnology (MEST) of Korea (2011-0001776). This funding madeossible our participation as a non-EU member in the collaborativeroject HOSANNA funded by the European Community’s Seventhramework Programme (FP7/2007-2013) under grant agreemento. 234306.

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