Acoustic Comfort Assessment Scale (ACAS-12)

1. Abstract

The Acoustic Comfort Assessment Scale (ACAS-12) is a standardized, psychometrically validated instrument developed by Karmele Herranz-Pascual and colleagues (2023) to quantify human subjective perception and pleasantness of acoustic environments within urban public spaces. Formulated in alignment with the international standard ISO 12913-1:2014 (Acoustics — Soundscape: Definition and conceptual framework), the ACAS-12 represents a significant methodological advancement from traditional decibel-centric noise assessments toward holistic, perceptual soundscape evaluations. The instrument employs a 5-point semantic differential response format consisting of twelve pairs of bipolar adjectives grounded across five foundational theoretical dimensions of environmental perception: affective quality, emotional response, dynamic composition, communicative and informational functionality, and contextual coherence.

Psychometric evaluation of the ACAS-12 demonstrates exceptional structural validity and internal consistency. Although rooted in five theoretical facets, confirmatory factor analysis (CFA) demonstrated that a robust, parsimonious single-factor model accounts for approximately 50% of the total variance across all 12 adjective pairs (explaining 44% of extracted variance via Raykov’s method and 56% via Velicer’s Minimum Average Partial test). The single-factor model exhibits excellent goodness-of-fit indices (Standardized Root Mean Square Residual [SRMR] = 0.057; Root Mean Square Error of Approximation [RMSEA] = 0.042; Incremental Fit Index [IFI] > 0.90). Internal consistency is remarkably high, supported by a Cronbach’s alpha (α) of 0.91 and a composite reliability (ρ_c) of 0.90. While a trimmed 10-item variant (ACAS-10) offers marginal statistical gains by dropping items with lower factor loadings, the full ACAS-12 is retained to preserve critical ecological nuances, particularly acoustic familiarity and continuity. Rigorously cross-validated via forward- and back-translation between Spanish and English, the ACAS-12 provides environmental psychologists, urban planners, and acousticians with an empirically sound, rapid-to-administer diagnostic tool for perceptual soundscape research, restorative design, and urban health policy.

2. Keywords

Acoustic Comfort, Soundscape Assessment, Urban Soundscapes, Environmental Psychology, Semantic Differential, ISO 12913-1, Auditory Perception, Noise Annoyance, Restorative Environments, Perceptual Acoustics, Human Factors Engineering, Psychometrics.

3. Authors

The Acoustic Comfort Assessment Scale (ACAS-12) was developed and validated by an interdisciplinary research consortium spanning environmental psychology, acoustic engineering, and clinical psychometrics:

• Karmele Herranz-Pascual — TECNALIA, Basque Research & Technology Alliance (BRTA), Derio, Spain. ORCID: 0000-0003-3930-5445. Corresponding author: [email protected].
• Ioseba Iraurgi — University of Deusto, Faculty of Health Sciences, DeustoPsych (Assessment, Clinical and Health Research Unit), Bilbao, Spain.
• Itziar Aspuru — TECNALIA, Basque Research & Technology Alliance (BRTA), Derio, Spain.
• Igone Garcia-Pérez — TECNALIA, Basque Research & Technology Alliance (BRTA), Derio, Spain.
• José Luis Eguiguren — TECNALIA, Basque Research & Technology Alliance (BRTA), Derio, Spain.
• Álvaro Santander — TECNALIA, Basque Research & Technology Alliance (BRTA), Derio, Spain.

4. Purpose

For decades, municipal acoustic governance and environmental health strategies relied almost exclusively on energetic noise indices, such as the equivalent continuous sound pressure level (L_Aeq) and the day-evening-night level (L_den). While these physical parameters are indispensable for identifying auditory risk thresholds and chronic noise exposure risks, they fail to capture the subjective human experience of acoustic environments. A quiet public square dominated by mechanical hums may generate low decibel readings yet induce profound psychoacoustic dissatisfaction; conversely, an urban plaza energized by human chatter, water fountains, and birdsong may exhibit high sound energy levels while being perceived as exceptionally pleasant, calming, and restorative. The ACAS-12 was engineered to bridge this fundamental gap, translating the theoretical paradigm shift codified by the soundscape framework into an operational, psychometrically rigorous instrument.

The primary purpose of the ACAS-12 is to quantify human subjective perception and acoustic comfort in urban open public places, including plazas, pedestrian corridors, urban parks, pocket gardens, and civic promenades. Rather than treating sound solely as an unwanted pollutant (“noise”) that must be eliminated or mitigated, the ACAS-12 conceptualizes the acoustic environment as an environmental resource that can support cognitive restoration, physiological relaxation, social cohesion, and place attachment. By quantifying human perceptual judgments along standardized semantic polarities, the instrument enables researchers and spatial designers to determine not only whether an urban setting is perceived as comfortable, but also the specific qualitative drivers that foster or compromise that comfort.

In applied and research contexts, the ACAS-12 fulfills several complementary objectives:

• Urban Planning and Soundscape Design: It provides municipal planners, landscape architects, and urban designers with actionable diagnostic data to conduct pre-occupancy evaluations, guide urban renewal projects, and execute post-occupancy assessments of civic interventions (e.g., pedestrianization, green infrastructure installation, or traffic calming).
• Environmental Psychology and Public Health: It serves as an empirical instrument to evaluate how acoustic comfort interacts with psychological well-being, mental fatigue reduction, stress alleviation, and restorative environment mechanisms as described by Attention Restoration Theory.
• Standardized Cross-Cultural Benchmarking: Developed with rigorous linguistic equivalence between its Spanish origin and English adaptation, the ACAS-12 enables comparative international soundscape benchmarking under the overarching auspices of the ISO 12913 series.
• Policy Formulation and Quality-of-Life Metrics: The tool allows local governments to establish subjective environmental comfort thresholds alongside statutory decibel maps, supporting multi-criteria urban liveability monitoring.

5. Psychological Construct

The overarching construct captured by the ACAS-12 is Acoustic Comfort within urban environments. Within modern environmental psychology, acoustic comfort is defined not as the mere absence of intrusive noise, but as a holistic psychological state of satisfaction, positive hedonic appraisal, cognitive compatibility, and emotional well-being elicited by an acoustic environment in relation to a person’s expectations, activities, and contextual milieu. Grounded in the extensive psychometric literature on soundscape perception, the ACAS-12 was conceived through an iterative developmental trajectory that consolidated five theoretical dimensions into a singular, highly cohesive general factor.

The twelve bipolar adjective pairs of the ACAS-12 originate from these five theoretical dimensions:

1. Hedonic Tone and Emotional Well-Being

This core affective dimension evaluates the fundamental emotional valence experienced by an individual within the sound environment. It reflects the primary psychological appraisals of valence and arousal described in Russell’s circumplex model of affect:

• Unpleasant vs. Pleasant (Item 1): Represents the broad hedonic appraisal of the auditory environment, serving as the quintessential affective anchor for perceived acoustic comfort.
• Stressful vs. Relaxing (Item 2): Captures the autonomic and emotional restorative capacity of the soundscape, distinguishing between sound environments that provoke sympathetic nervous system arousal versus those that foster parasympathetic recovery and emotional restoration.

2. Soundscape Dynamics and Temporal Fluctuations

This dimension addresses the temporal structure, acoustic intensity, and perceptual steadiness of the auditory scene. Temporal characteristics significantly influence psychological stress, cognitive distraction, and auditory fatigue:

• Noisy vs. Peaceful, Calm (Item 3): Evaluates subjective loudness integrated with tranquility, indexing whether auditory events overwhelm sensory processing or facilitate psychological equilibrium.
• Intermittent vs. Uninterrupted (Item 10): Measures the perceived continuity of the sonic background versus sudden, irregular auditory interruptions, which are known to trigger sensory orienting reflexes and cognitive disruption.

3. Cognitive Clarity and Information Processing

Human auditory cognition actively parses acoustic scenes into meaningful auditory streams. This dimension reflects the cognitive ease with which individuals navigate their environmental soundscape:

• Confusing, Chaotic vs. Clear, Accurate (Item 4): Gauges auditory scene organization and cognitive legibility, assessing whether overlapping sound events produce cognitive sensory overload or allow distinct, organized auditory parsing.
• No Informative vs. Informative (Item 9): Measures the semiotic utility of the sonic environment, determining whether sounds convey meaningful environmental, orientation, or situational cues to the listener.

4. Social Interaction and Communicative Functionality

Acoustic spaces serve as functional platforms for human interaction, community life, and interpersonal communication:

• Hinders Conversation vs. Facilitates Conversation (Item 8): Assesses functional speech intelligibility and communicative comfort, measuring whether the acoustic milieu supports effortless interpersonal discourse or forces increased vocal effort and cognitive strain.
• Boring vs. Fun (Item 5): Gauges emotional engagement and experiential vibrancy, differentiating sterile or depressogenic environments from stimulating civic spaces.
• Monotonous vs. Lively (Item 7): Measures dynamic auditory variability and vitality, reflecting the soundscape’s capacity to sustain engagement without descending into cacophony.

5. Contextual Congruence, Familiarity, and Naturalness

Perception of environmental sounds is mediated by cognitive expectations, ecological validity, and place identity:

• Artificial vs. Natural (Item 6): Reflects the fundamental dichotomy between anthropogenic/technological mechanical sounds (e.g., motor traffic, HVAC systems) and biophonic/geophonic natural sounds (e.g., wind, foliage, water, birdsong), which are strongly linked to evolutionary restorative preferences.
• Unknown vs. Familiar (Item 11): Captures cognitive schemata and perceptual predictability; familiar acoustic signatures provide a sense of psychological safety and ontological security.
• Inappropriate for the Surroundings vs. Appropriate for the Surroundings (Item 12): Evaluates ecological and contextual congruence, indexing whether the sounds align seamlessly with the visual, cultural, and functional typology of the public space.

6. Theoretical Framework

The ACAS-12 is anchored at the intersection of three major theoretical traditions: modern soundscape theory as codified by ISO 12913-1, environmental psychological theories of attention restoration and stress recovery, and the psycholinguistic measurement paradigm of the semantic differential technique pioneered by Charles E. Osgood.

1. The ISO 12913 Soundscape Conceptual Framework

Historically initiated by R. Murray Schafer (1977) and Barry Truax (1984) through the World Soundscape Project, soundscape studies shifted acoustic research from treating sound exclusively as an unwanted physical emission toward treating it as a perceived perceptual construct. In 2014, the International Organization for Standardization formally defined a soundscape as the “acoustic environment as perceived or experienced and/or understood by a person or people, in context” (ISO 12913-1:2014). This framework articulates that auditory perception is not a direct linear readout of acoustic energy; rather, sound sources pass through the physical acoustic environment, undergo sensory audition, and are subsequently filtered through cognitive, affective, memory, and sociocultural matrices. The ACAS-12 operates directly at this perceptual-cognitive interface, translating the conceptual requirements of ISO 12913-1 into a standardized psychometric measurement scale.

2. Restorative Environmental Theories

The development of the ACAS-12 is deeply grounded in environmental restorative theories, predominantly Attention Restoration Theory (ART) formulated by Rachel and Stephen Kaplan, and Stress Recovery Theory (SRT) developed by Roger Ulrich. ART posits that directed cognitive attention is a finite, exhaustible resource that becomes fatigued in demanding urban settings characterized by high cognitive load, sensory chaos, and unpredictable stimuli. Exposure to environments with high “soft fascination” (such as natural sounds of water, rustling leaves, and bird calls) permits directed attention mechanisms to rest and recover. SRT complements this by demonstrating that natural, non-threatening sensory inputs trigger rapid neuroendocrine and parasympathetic recovery from physiological stress. The ACAS-12 items (specifically contrasting stressful vs. relaxing, artificial vs. natural, and chaotic vs. clear) operationalize these exact restorative mechanisms, providing empirical markers of restorative auditory potential in civic architecture.

3. Osgood’s Semantic Differential Paradigm

Methodologically, the ACAS-12 is built upon the semantic differential technique developed by Osgood, Suci, and Tannenbaum (1957). Osgood demonstrated that affective meaning across cultures and sensory modalities consistently clusters along three fundamental dimensions: Evaluation (good/bad, pleasant/unpleasant), Potency (strong/weak, intense/mild), and Activity (active/passive, fast/slow). Subsequent psychoacoustic researchers (such as Östen Axelsson, Catherine Guastavino, and Jian Kang) adapted semantic differentials to soundscape research, showing that human perceptual appraisals of environmental sound are dominated by two primary orthogonal axes: Pleasantness (Hedonic Evaluation) and Eventfulness (Activity/Dynamics). The ACAS-12 refines this lineage by presenting twelve carefully chosen, mutually reinforcing bipolar pairs on a 5-point graded continuum, enabling respondents to articulate nuanced perceptual judgments without imposing complex technical jargon.

7. Validity

The validity of the ACAS-12 was established through comprehensive empirical investigations conducted across heterogeneous urban public spaces in Spain, encompassing open urban squares, pedestrian streets, recreational waterfronts, and civic green areas. The validation process adhered to rigorous psychometric protocols governing construct, structural, content, and cross-linguistic validity.

Construct and Structural Validity

Construct validity was established through confirmatory factor analysis (CFA) applied to cross-sectional field datasets collected in situ. The empirical data corroborated the scale’s structural validity, establishing that the 12 semantic differential pairs converge reliably to measure the target construct of perceived acoustic comfort. In structural equation modeling, the hypothesized unifactorial structure yielded fit indices that satisfied or surpassed standard psychometric cutoffs:

• Absolute and Incremental Fit: The Incremental Fit Index (IFI) and the absolute fit index both exceeded the conventional 0.90 threshold, indicating robust model specification.
• Residual-Based Fit Indices: The Standardized Root Mean Square Residual (SRMR) was 0.057, well below the stringent 0.08 cutoff for acceptable model fit. The Root Mean Square Error of Approximation (RMSEA) was 0.042 (with a 90% confidence interval extending well below 0.05), indicating a close approximate fit of the single-factor model to empirical data.

Item Contribution and Structural Refinement

During the CFA evaluation, researchers analyzed individual item factor loadings. While the majority of items exhibited strong factor loadings converging on the global acoustic comfort construct, two specific pairs—Item 10 (intermittent vs. uninterrupted) and Item 11 (unknown vs. familiar)—demonstrated comparatively lower relative factor contributions. To evaluate the impact of these items, a trimmed 10-item model (ACAS-10) was tested using Satorra-Bentler robust estimation:

• χ²_SB = 49.83, df = 35, p = 0.049
• χ²_SB / df = 1.42 (substantially below the recommended threshold of 2.0 to 3.0)
• Normed Fit Index (NFI) = 0.99; Non-Normed Fit Index (NNFI) = 0.99
• Comparative Fit Index (CFI) = 0.99; Incremental Fit Index (IFI) = 0.99
• SRMR = 0.034; RMSEA = 0.026 (90% CI: 0.000 to 0.050)

Despite the statistical superiority of the 10-item model, Herranz-Pascual et al. (2023) recommended retaining the full 12-item instrument (ACAS-12). The rationale rests upon ecological and theoretical validity: items such as “unknown vs. familiar” and “intermittent vs. uninterrupted” provide indispensable qualitative insight into the cognitive mechanisms of environmental familiarity, place identity, and temporal stability, which are prominently highlighted in environmental psychology and soundscape literature.

Cross-Linguistic and Translation Validity

The scale was originally conceived and formulated in Spanish. To ensure international applicability, an English version was developed using a rigorous back-translation protocol. A bilingual multidisciplinary panel conducted forward translation, blinded back-translation by independent native English translators, and consensual reconciliation. Semantic, idiomatic, experiential, and conceptual equivalence were verified across all bipolar adjective descriptors.

8. Reliability

The reliability of the ACAS-12 has been extensively examined through classical test theory metrics and modern structural equation modeling indices of internal consistency.

Internal Consistency

The ACAS-12 exhibits excellent internal consistency across diverse urban field conditions. The scale achieved an overall Cronbach’s alpha (α) of 0.91, indicating a high degree of inter-item correlation and minimal measurement error. To complement Cronbach’s alpha—which assumes tau-equivalence and can underestimate or overestimate reliability in psychological scales—the authors calculated Raykov’s composite reliability (ρ_c), which yielded an equally robust coefficient of 0.90. These values comfortably surpass the standard threshold of 0.70 for research instruments and the stringent 0.85 threshold recommended for diagnostic and evaluative decision-making tools.

Item Homogeneity and Variance Distribution

Item-total correlation analyses confirmed that all twelve adjective pairs contribute coherently to the latent construct of perceived acoustic comfort. No individual item deletion resulted in an increase in Cronbach’s alpha above 0.91, indicating that every item contributes meaningfully to the shared variance without introducing extraneous psychometric noise. The scale avoids severe ceiling or floor effects in normative public space evaluations, offering balanced discriminative power across quiet restorative zones and high-energy civic centers.

9. Factor Analysis

The structural configuration of the ACAS-12 was scrutinized using both exploratory factor extraction heuristics and rigorous confirmatory factor analysis (CFA).

Factor Dimensionality

Although the ACAS-12 was intentionally constructed using items sampled across five distinct conceptual domains (hedonic tone, dynamic temporal features, cognitive clarity, social interactivity, and context congruence), dimensionality analyses established that a single-factor model best accounts for the shared variance among the 12 items. This demonstrates that in urban open spaces, citizens’ qualitative appraisals of diverse acoustic attributes coalesce into a unified global appraisal of acoustic comfort.

Variance Explained

The proportion of variance accounted for by this unifactorial structure was evaluated using multiple methodological algorithms:

• Velicer’s Minimum Average Partial (MAP) Test: The MAP test, an empirical criterion for determining the number of retaining factors, confirmed a one-factor solution explaining 56% of the total variance.
• Raykov’s Variance Extraction: When computing extracted variance through structural equation modeling formulations, the latent factor accounted for 44% of the extracted variance.
• Overall Model Variance: Across estimation procedures, the general acoustic comfort factor consistently accounts for approximately 50% of the empirical variance across all twelve adjective pairs.

Summary of Confirmatory Factor Models

The table below provides a psychometric comparison between the comprehensive 12-item ACAS model and the refined 10-item model evaluated by Herranz-Pascual et al. (2023):

Fit Index	Reference Threshold	ACAS-12 Model	ACAS-10 Model
χ2SB / df	≤ 2.0 – 3.0	Acceptable	1.42 (49.83 / 35, p = 0.049)
Comparative Fit Index (CFI)	≥ 0.95	> 0.90	0.99
Incremental Fit Index (IFI)	≥ 0.90	> 0.90	0.99
Normed Fit Index (NFI)	≥ 0.95	> 0.90	0.99
Non-Normed Fit Index (NNFI)	≥ 0.95	> 0.90	0.99
SRMR	≤ 0.080 (excellent ≤ 0.050)	0.057	0.034
RMSEA (90% CI)	≤ 0.060 (excellent ≤ 0.050)	0.042	0.026 (0.000 – 0.050)

While the ACAS-10 demonstrated minor statistical fit enhancements, the researchers explicitly retained the ACAS-12 for standard deployment. The inclusion of Items 10 and 11 provides crucial environmental diagnostic information that informs urban architectural remediation, compensating for the minor difference in numerical fit.

10. Instrument / Measurement Tool

• Test Type: Original psychometric rating scale; environmental assessment questionnaire.
• Measurement Paradigm: 5-point Semantic Differential format.
• Item Count: 12 bipolar adjective pairs (with a validated 10-item research variant).
• Target Construct: Perceived Acoustic Comfort and Soundscape Pleasantness.
• Target Population: General human public space users (adults and adolescents capable of evaluating environmental stimuli).
• Context of Administration: In situ field soundwalks, momentary public space surveys, laboratory psychoacoustic listening tests, and virtual reality urban simulations.
• Administration Time: Approximately 2 to 4 minutes (including the mandatory 1-minute initial listening period).
• Available Languages: Spanish (native) and English (fully back-translated and validated).
• Scoring Format: Each adjective pair is rated along a continuum from 1 to 5:
  • Score 1: The negative adjective on the left describes the acoustic environment very well.
  • Score 2: The negative adjective on the left describes the acoustic environment well, but not as strongly as 1.
  • Score 3: Neutral / Uncertain / Neither adjective clearly predominates.
  • Score 4: The positive adjective on the right describes the acoustic environment well, but not as strongly as 5.
  • Score 5: The positive adjective on the right describes the acoustic environment very well.
• Scoring and Interpretation Procedures:
  • Overall Scale Score: Calculated by summing all 12 item responses (range: 12 to 60) or computing the mean item score across the 12 items (range: 1.0 to 5.0).
  • Directionality: Higher numerical scores unequivocally denote higher acoustic comfort, greater environmental pleasantness, enhanced tranquility, and positive soundscape appraisal.
  • Cutoff Guidelines: Mean scores > 3.5 indicate positive acoustic comfort; mean scores between 2.5 and 3.5 indicate neutral or mixed acoustic environments; mean scores < 2.5 indicate poor acoustic comfort characterized by annoying, chaotic, or stressful soundscapes.
  • Individual Item Profiling: In addition to the global comfort score, individual item profiles (e.g., radar charts of the 12 semantic axes) provide diagnostic insights into specific design deficits, such as speech interference (Item 8) or excessive artificiality (Item 6).

11. Permissions & Fee and Test Year

• Publication Year: 2023.
• Developers: Karmele Herranz-Pascual, Ioseba Iraurgi, Itziar Aspuru, Igone Garcia-Pérez, José Luis Eguiguren, and Álvaro Santander.
• Copyright & Licensing: Published under the Creative Commons Attribution License (CC-BY 4.0).
• Usage Permissions: Free open access. The scale may be reproduced, distributed, adapted, and used without commercial royalty fees for academic, research, teaching, and urban planning purposes, provided the original authors and primary publication are fully credited.
• Commercial Use: Permitted under CC-BY 4.0 guidelines provided proper attribution is maintained.

12. References

• Axelsson, Ö., Nilsson, M. E., & Berglund, B. (2010). A principal components model of soundscape perception. The Journal of the Acoustical Society of America, 128(5), 2836–2846. https://doi.org/10.1121/1.3493436
• Herranz-Pascual, K., Iraurgi, I., Aspuru, I., Garcia-Pérez, I., Eguiguren, J. L., & Santander, Á. (2023). Development of the Acoustic Comfort Assessment Scale (ACAS-12): Psychometric properties, validity evidence and back-translation between Spanish and English. PLoS ONE, 18(2), Article e0281534. https://doi.org/10.1371/journal.pone.0281534
• International Organization for Standardization. (2014). Acoustics — Soundscape — Part 1: Definition and conceptual framework (ISO Standard No. 12913-1:2014). https://www.iso.org/standard/52161.html
• Kaplan, R., & Kaplan, S. (1989). The experience of nature: A psychological perspective. Cambridge University Press.
• Osgood, C. E., Suci, G. J., & Tannenbaum, P. H. (1957). The measurement of meaning. University of Illinois Press.
• Schafer, R. M. (1977). The tuning of the world: Toward a theory of soundscape design. Alfred A. Knopf.
• Truax, B. (1984). Acoustic communication. Ablex Publishing Corporation.
• Ulrich, R. S., Simons, R. F., Losito, B. D., Fiorito, E., Miles, M. A., & Zelson, M. (1991). Stress recovery during exposure to natural and urban environments. Journal of Environmental Psychology, 11(3), 201–230. https://doi.org/10.1016/S0272-4944(05)80184-7

13. Items of the Scale