1. Introduction

Ecological validity was introduced in the 1980s to evaluate the outcomes of a laboratory experiment focused on visual perception [1]. Ecological validity describes the degree to which results obtained in a controlled laboratory experiment are related to those obtained in the real world [2]. The discussion of the ecological approach regarding its internal validity and experimental control began in the 1980s with cognitive and behavioral psychology research [1,3], and these two factors are still significant factors in the design and undertaking of an ecological approach study. Under laboratory conditions, researchers should give participants corresponding environmental cues and instructions to enable the reactivation of the cognitive processes of participants that were determined in actual situations [4]. For high ecological validity, the findings in the laboratory can be generalized into real-life settings [2]. As a simulated technology, Immersive Virtual Reality (IVR) places the user inside an experience, which allows the impact on participants of a new environment with complex social interactions and contexts to be assessed [5,6,7]. In 2001, Bishop et al. [8] reported their non-IVR assessments of path choices on a country walk, and they agreed that faster computers and better display systems make the virtual environment experience more credible. Thus, low ecological validity resulting from non-sufficient immersiveness could be a limiting factor for the generalizability of data collected from laboratory experiments. The need for more research that explores applications of perceptual simulations in general and related questions of validity and reliability has been stressed ever since the emergence of environmental simulation as a research paradigm.

Ecological validity has been conceptualized into two approaches: verisimilitude and veridicality. Verisimilitude refers to the extent of similarity of a virtual experience to relevant environmental behaviors [9]; it reflects the similarity of the task demands between the test in the laboratory and the real world [10]. This approach attempts to create new evaluation assessments with ecological goals [11]. Veridicality refers to the degree of accuracy in predicting some environmental behaviors [12,13]; the establishment of veridicality is required to assess the results from the laboratory test and the measures in the real-world. There are some limitations for both approaches. One limitation of the veridicality approach is that, for those conditions which are not likely to be reproduced in the real world or that have a high cost, the outcomes from real-world measures cannot correlate with experiment results. When using the verisimilitude approach alone, no empirical data are needed to claim that the evaluation is similar to real life settings [11].

Virtual reality has revealed a functional rapprochement that fuses the boundary between the laboratory and real life [5]. Through multisensory stimuli with experimental control, participants tend to respond realistically to virtual situations as if they were in a real environment [14,15,16,17]. The responses to a virtual environment are generated when place illusion (PI) and plausibility illusion (Psi) occur at the same time [14,15,16]. The ecological approach studies based on virtual reality provide controlled dynamic presentations of background narratives to enhance the affective experience and social interactions [3,18]. From a methodological viewpoint, environmental conditions and test results can be ecologically validated through virtual reality technologies according to a subjective evaluation framework. Numerous researchers have examined ecological validity in different topics and fields with the comparison of a virtual environment and real life [19,20,21,22,23].

Spatial audio is a technique of creating sound in a 3D space; then, a listener can hear the sound from any direction in a sphere [24]. Because of this feature, it is often combined with virtual reality to render auditory stimuli. For the auralization of spatial audio in case of binaural reproduction, head-tracking is required for the reproduction of a dynamic sound field based on the real-time position of the head within Euclidean space. Binaural recordings only reproduce the sound field of both ears at the time of recording, which shows the incompatibility between binaural recordings and head-tracking during auralization. Ambisonics is a sound reproduction technique used for recording and playing-back spatial audio, and it is based on the spherical harmonic decomposition of the sound field [25]. Ambisonics enables a listener to experience a spatially-accurate perception of the sound field [26], and this reproduction technique was originally introduced by Gerzon [27]. In the case of first-order Ambisonics (FOA)—currently the most widespread Ambisonics recording technique—the signals are recorded as four audio channels, most usually in the so-called A-format. The audio files needed to reproduce such recordings are known as B-format audio files, which are converted from the A-format. The B-format can be decoded into any speaker array matching the needs of dynamic auralization under Immersive Virtual Reality (IVR), including higher-order Ambisonics (HOA). HOA has a higher spatial resolution based on higher-order spherical harmonics [25]. Head-related transfer function (HRTF) is considered as a frequency response describing the sound pressure transformation from a free field point source to the eardrum [28]. When this filtering is not applied with the listener’s own HRTF (acknowledging individual head size, auricle size and shape, etc.) [29,30,31], front–back and elevation confusions in localization typically occur [32].

For many urban sound assessment studies, in situ surveys have been widely applied as a conventional method to evaluate certain sound environments [5,33,34]. In soundscape or noise assessment studies, researchers expect the presentation of controlled experimental conditions to participants; e.g., recorded audio and reconstructed visual stimuli in a listening room. Therefore, researchers introduced laboratory tests to validate their research questions with human participation. All simulations under laboratory conditions attempt to represent some aspects of the environment as accurately as possible to assess human responses. In urban noise prediction and soundscape assessment research, an audio–visual system is a conventional and valid approach to render essential information or cues during human participation. The audio–visual interaction influences the perception of the soundscape and global environment, as shown in previous studies [35,36,37,38,39,40,41]. For interior spaces with VR techniques, several studies have assessed the evaluation of indoor noise protection with head-mounted displays (HMDs) [42], the main uses of auralization [43], the influence of visual distance [44], the use of water features [45] and the spatial representations of visually impaired participants [46,47]. The urban sound environment in this review refers primarily to sound sources originating outdoors or in urban public spaces, and it reflects, to some extent, the mobility of people and the multifunctionality of urban spaces.

The evaluated multisensory method shows enormous significance in helping participants to perceive environments holistically. The reproduction system of listening tests needed to be adapted to the purpose of the study to allow the subjects to treat the test samples as potentially familiar experiences through cognitive processes elaborated in actual situations. With the aid of immersive virtual reality, the installation of laboratory conditions was performed with the aim of reproducing urban sound environments and presenting a multisensory experience to participants. A subjective test of immersive virtual reality reproduction in urban sound environment assessments would show high veridicality if it correlated well with measures of perceptual responses in the real world.

The concept of ecological validity has been extended from psychological experiments to the domain of complex sound environment perception. It is not only related to the evaluation methods during laboratory tests, but also closely associated with the developing IVR technologies. Attempting to establish a standardized soundscape evaluation protocol with high veridicality under an immersive virtual environment has a broader impact on the practice of soundscape planning and design. The research on soundscape standardization has discussed the definitions, variety of contexts, evaluation methods and reporting requirements [48,49].

The International Organization for Standardization Technical Standard (ISO TS) 12913-2 [34] introduced two common recording techniques in soundscape research: binaural and Ambisonics. The standard states that if some environmental factors are not present or differ during playback, the outcomes could possibly result in different impressions to those received in the original context. In terms of the statement of ISO TS 12913-2 [34], the validity of these auralization techniques combined with other environmental factors still presents some uncertainty. The ISO TS 12913-3 [50] stated that the key factors to consider when conducting ecologically valid laboratory studies are the effect of memory, the duration of exposure to each of the stimuli and the auditory immersiveness. As a multisensory tool, IVR could deliver more environmental stimuli than conventional 2D rendering methods. A comparison of the ecological validity using IVR for urban sound environments with different reproduction techniques and research topics was made.

2. Discussion

This entry is adapted from the peer-reviewed paper 10.3390/acoustics3010003