Indoor soundscape modelling: Rethinking acoustic comfort in naturally ventilated residential buildings

The connection with the outdoor acoustic environment created by open windows has so far been one of the main impediments to the adoption of natural ventilation (NV), due to indoor noise levels easily exceeding design requirements. Starting from the apparent conflict between ventilation and acoustic comfort needs, and the potential offered by NV for low-energy cooling and ventilation, the study explores the opportunities for shaping healthy and supportive acoustic environments through sound transmitted via ventilation openings. The research question challenges the traditional approach to acoustic design, which assumes noise annoyance reduction by merely reducing decibel noise levels, drawing inspiration from the soundscape concept. Soundscape science characterises the human response to the acoustic environment in context and can help understand if and how NV may contribute to defining spaces that sound good to their occupants. The aim is to go beyond an exclusive focus on the ‘noise’ – ‘noise annoyance’ binomial, and to employ ‘wanted’ sounds as a design resource for creating acoustically pleasant environments. However, the soundscape framework, as described by ISO 12913 standard series, has been primarily developed for use in the context of urban planning. This has led to question (i) how the soundscape approach can be applied to the indoor built environment, (ii) what factors positively influence it and (iii) how it can be measured in residential buildings. A systematic literature review categorized the factors that positively influence acoustic perception in domestic environments, highlighting its strongly multi-factorial nature. Beyond noise level, a combination of acoustic and non-acoustic factors was found to affect acoustic perception, such as the urban context, house and person-related factors, socio-economic, situational, and environmental factors. The study benefited from a round of interview with experts in the field of urban soundscape, indoor soundscape, acoustic design, and public health and well-being. The collective discussion encompassed the characterization, management, and design of indoor (and indoor versus outdoor) soundscapes to identify current research gaps in the objective and subjective evaluation of the indoor acoustic environments. In response, based on a laboratory listening test, a model of perceived affective quality of indoor acoustic environments has been derived to guide the measurement and improvement of indoor residential soundscapes. During the test, 35 participants were asked to rate 20 different sound scenarios each. Scenarios were defined by combining four indoor sound sources and five urban environments, filtered through a window ajar, on 97 attribute scales. Comfort, content, and familiarity were extracted as the main perceptual dimensions explaining respectively 58%, 25% and 7% of the total variance in subjective ratings. A measurement system was proposed, based on a 2-D space defined by two orthogonal axes, comfort, and content, and two derivative axes, engagement and privacy – control, rotated 45° on the same plane. The model was tested in a large-scale online survey to assess the influences of different acoustic and non-acoustic factors on indoor soundscape dimensions, window-opening behavior, and occupant well-being. Evaluating the affective response to the indoor acoustic environment through the comfort – content model helped identifying the impacts that acoustical factors (e.g., sound typology), building (e.g., house size), urban (e.g., availability of a quiet side), situational (e.g., number of people at home), and person-related factors (e.g., noise sensitivity) determine on building occupants depending on the specific activity people are engaged with at home, reaching a more in-depth knowledge compared to appraisals based on annoyance evaluation alone. By disentangling the positive and negative contributions of sound stimuli according to people’s perception, it was possible to highlight the opportunity provided by NV to create a sense of place and enhance indoor soundscapes, providing useful masking opportunities in the presence of disturbing indoor noise sources. Results pointed to the existence of benefits from NV able to compensate for a reduced acoustic comfort in case of outdoor acoustic pollution. However, the availability of ‘positive’ urban soundscapes is essential for occupants’ well-being, and is linked primarily to access to natural sounds, but also to other commonly available urban sounds. The ‘quieter’ is therefore not always the better, but it really depends on the composition of indoor and outdoor sound types according to people’s preference and on the interaction with different domains (e.g., visual). Such evidence reinforces the role of acoustics in building and urban design, integrated with the other disciplines involved and based on multi-domain research. Overall, the doctoral study contributes to framing the ‘indoor soundscape’ concept, addressing scientific, industrial, social, and environmental implications, and suggesting future lines of research.