Sound, as a landscape element, has not received much interest in landscape design and planning compared to vision. Listening to an environment is generally not the primary activity or interest of a person (Jennings & Cain, 2012); however information provided by the visual landscape play a great role in realizing our daily activities. The concept of soundscape has recently gained attention of planning and design disciplines where focus is generally on the visual aspect, rather than the acoustic. One of the reasons might be that most of the time designers’ and planners’ lack of scientific knowledge on acoustics. Concepts like "weighted sound levels", "absorption coefficient", and logarithmic measurements may seem unfamiliar and intimidating. Although noise mapping is quite a popular tool in environmental assessment studies, sound is rarely considered as a design element in landscape. Sound does not literally mean "noise". While some sounds can be disturbing, some sounds can give pleasure to an individual. However, sound as a sensory experience is rather different from vision. Acoustic space does not have obvious boundaries and is less precise in terms of orientation and localization (Porteous, 1996). Therefore assessment of sound as a design element is much complex than the visual dimension. Sound is an important element of a place which affects individual’s perception and understanding of an environment. People derive information from sounds, just like visual environment. Sound can act as a guide for way finding or a cautionary signal for alert. In the context of space, soundscape can be defined as the acoustic character of an environment.
Urban environments are diverse and complex acoustic environments. They include different kinds of sound resources. Therefore, outdoor acoustic environment studies are mostly concerned with urban soundscapes. Evaluation of urban soundscapes is crucial not only for noise mitigation but also to assessment of acoustic comfort which is integral to overall environmental quality.
The term of soundscape was first coined by R. Murray Schafer. In his book "The Tuning of the World" (1977), he describes the soundscape as any acoustic field of the study; it may be a musical composition or a radio program or an acoustic environment. Influenced by Gestalt figure-background relationship (see section 2.2.6), Schafer identified three elements of a soundscape; (i) keynote sounds, (ii) sound signals, and (iii) soundmarks. Keynote sounds are background sounds and can be perceived subconsciously. Schafer suggests keynote sounds might have an effect on our behavior and moods since they are permanently there, whether we hear them consciously or not. Traffic sound is often given as a keynote example for contemporary urban environments. Sound signals, are foreground sounds and are listened to consciously (e. g. sirens). Finally, soundmarks (derived from landmark) are unique to that environment or to people in the community, thus they need to be protected. These elements have established a foundation for many soundscape studies so far.
The most noticeable study on the relationship between landscape architecture and soundscape is Hedfors’s (2008) book "Site Soundscapes: landscape architecture in the light of sound". In his book, he analyzes sound in context of landscape architecture. He suggests a hypothetical model, named "the model of prominence" as a starting point for landscape architects. The model is also grounded on Gestalt figure-background relationship, like Schafer’s work. It is based on description of the sounds. In Hedford’s model figure-ground relationship is combined with two other dimensions; intensity and clarity (Figure 6). According to Hedford, a soundscape can be described as clear if prominent sounds are strongly experienced against a weak background. However, if prominent sounds are weaker than the background, then the soundscape becomes crowded. If both prominent sounds and the background are experienced equally strong, the soundscape can be described as powerful. On the contrary, if both are experienced weak, the soundscape becomes mild.
Similar to natural landscapes, natural sounds such as bird sound and water sound are highly preferred by people. This may be explained through evolutionary perspective on landscape perception as well as therapeutic effects of natural landscapes (please refer to section 2.4). In fact, relaxation is found to be an important factor for urban open soundscapes (Yang & Kang, 2005). It is known that natural sounds such as bird and water sound can help people feel relaxed (Carles et al., 1999).
Since sound perception is assumed to be a personal and therefore unique phenomenon, most researchers believe that perception of the acoustic environment is affected by personal factors such as demographics and culture. Yu & Kang (2010) found that people preferred natural sounds with increasing age and education level. Their results showed no significant correlation between preferences and occupation, and residence status. They found that gender influenced preference only for some sound types (e. g. bird sound).
Anderson et al. (1983) emphasize the importance of expectations in people’s sound evaluations (Hedfords, 2008). People might tolerate or appreciate undesirable sounds if they expect to hear them in an environment. For instance some traffic sounds were found to be appreciated in urban environments. Thus, cultural and life-style differences might play role in evaluations of environments with different sound levels.
Sound types have also been found to be related to acoustic comfort evaluations; pleasant sounds, with either high or low sound levels, are perceived to improve the acoustic comfort (Yang & Kang, 2005). The source of the sound type can also affect preference. Zhang & Kang (2007) found that while "music on the street" was rated as favorite by 46% of the participants, 15% rated for music from stores, and only 2% rated for music from cars. Perception of the soundscape is also influenced by the activity involved and hence listening situation (Jennings & Cain, 2012).
Although sound level measurements (e. g. A-weighted levels) are widely used in soundscape research, it is also indicated that perception of the acoustic environment is independent from sound levels (Jennings & Cain, 2012; Szeremeta & Zanin, 2009). Reducing sound levels do not always improve perceived quality of acoustic environment (Yang & Kang, 2005). Furthermore, elimination of negative sounds from the environment does not necessarily make the acoustic environment more positive, may even generate anxiety (Cain et al., 2011). However, Yang & Kang (2005) have found that background sound level is an important factor in evaluating soundscape in urban open public spaces; they suggest that reduced background sound level can help to create comfortable acoustic environments.
Visual perception also affects sound perception; Faburel & Gourlot (2009) found that visual images can reduce the negative effect of a sound, equivalent of up to 10dB decrease in the sound pressure level (SPL) (Solene, M., 2011). Yang and Kang (2005) also concluded that visual factors affect acoustic comfort evaluations and they suggested that interaction of visual and auditory perception work together "as an aesthetic comfort factor". Carles et al.’s (1999) study supports this idea. They presented varying combinations of visual and auditory stimuli and participants were asked to rate each image, each sound, and finally each combination. It is found that sounds in the scenes containing vegetation or abundant water were rated higher; hence they concluded that visual and acoustic information can reinforce or interfere with each other. Furthermore, people are less annoyed by the sounds when the source is not visible (Solene, M., 2011). Zhang & Kang (2007) proposed some suggestions for creation of soundscapes in urban environments. They state that if SPL is higher than 65-70 dBA, then people will feel annoyed. Figure 7 shows their design suggestions.
Fig. 7. Soundscape design strategies (Zhang & Kang, 2007).