# Exploring sound installation reactivity: Acoustic niches in public space

Valérian Fraisse, Nadine Schütz, Coralie Vincent, Marcelo M. Wanderley, Catherine Guastavino, Nicolas Misdariis

► To cite this version:

Valérian Fraisse, Nadine Schütz, Coralie Vincent, Marcelo M. Wanderley, Catherine Guastavino, et al.. Exploring sound installation reactivity: Acoustic niches in public space. International Symposium on Music and Room Acoustics (ISMRA 2025), Acoustical Society of America, May 2025, La Nouvelle Orléans, LA, United States. pp.022002, 10.1121/2.0002189. hal-05504062

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Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
**International Symposium on**
**Music and Room Acoustics**

24-28 May 2025
Loyola University
New Orleans, Louisiana

**Architectural Acoustics:**
**Interdisciplinary**

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# Exploring sound installation reactivity: Acoustic niches in public space

**Valérian Fraisse**

McGill University Schulich School of Music; STMS Ircam-CNRS-SU; Centre for Interdisciplinary Research in Music Research and Technology, Montreal, QC, H3A 1E3, CANADA; valerian.fraisse@mail.mcgill.ca

**Nadine Schütz**

STMS Ircam-CNRS-SU; Institute of Landscape and Urban Studies, ETH Zurich Paris, Île-de-France, FRANCE; nadine.schuetz@ircam.fr

**Coralie Vincent**

STMS Ircam-CNRS-SU, Paris, Île-de-France, FRANCE; coralie.vincent@ircam.fr

**Marcelo Mortensen Wanderley**

McGill University Schulich School of Music; Centre for Interdisciplinary Research in Music Media and Technology Montreal, Montreal, Quebec, CANADA; marcelo.wanderley@mcgill.ca.

**Catherine Guastavino**

School of Information Studies McGill University; Centre for Interdisciplinary Research in Music Media and Technology, Montreal, Quebec, CANADA; catherine.guastavino@mcgill.ca

**Nicolas Misdariis**

STMS Ircam-CNRS-SU, Paris, Île-de-France, FRANCE; nicolas.misdariis@ircam.fr

While the soundscape approach gains increased research attention, it remains underexplored in urban design and sound art. Sound installations in public spaces exist within dynamic acoustic environments, where the interplay between pre-existing sounds and introduced artistic elements influences the auditory experience in complex ways. Yet, there is a lack of tools and methods to evaluate their perceptual impact and inform artistic practice. The present work is part of a research-creation collaboration on Niches Acoustiques, a permanent sound installation designed by sound artist Nadine Schütz for a public square in Paris. The installation explores the acoustic niche hypothesis by adapting its sonic content in reaction to fluctuations in the acoustic environment. We report on a laboratory study focusing on the installation's reactivity, following up an initial study that linked composition strategies to soundscape perception. Specifically, we present the implementation of variations in the installation's responsiveness – such as amplifying, attenuating, or transforming its content – in reaction to the ambient conditions. These variations serve as a framework for evaluating the impact of reactivity across different listening scenarios.
# 1. INTRODUCTION

The study of the human auditory experience of urban environments is gaining increased awareness through the soundscape approach¹,². However, little attention has been given to how sound installations affect public space soundscapes³. Exploring this relationship could be a valuable step towards addressing the longstanding neglect of sound in urban design⁴,⁵. Specifically, a deeper understanding of how sound installation art shapes urban soundscapes would help understand its potential role in urban soundscape design⁶. This also requires providing artists with the means to incorporate soundscape research methods into their creative practice.

Sound installations in public spaces interact with ever-changing urban sound environments characterized by vast temporal, spatial, and spectral dynamics⁷. Thus, it is worth investigating how introduced artistic elements coexist with the pre-existing sound environment, both acoustically and semantically. To investigate the matter, it may be interesting to consider the concept of *acoustic niches*. Drawn from the fields of bioacoustics and acoustic ecology, this concept explains how animal species coexist within mature ecosystems by differentiating their vocalizations in both spectral and temporal dimensions⁸. In urban soundscape research, the concept of *niche* more generally translates into “an occurrence of a sound emission at the moment that is the most favorable and that offers a particularly well-adapted place for its expression”⁹.

This concept is central to the design of Nadine Schütz’s permanent sound installation *Niches Acoustiques*. The installation, which will be deployed in the forecourt of the new Judicial Court of Paris, France, seeks to operationalize the *acoustic niche* concept by reacting to its surrounding acoustic environment. We call the process for an installation to respond to external conditions *reactivity*—sometimes referred to as *adaptivity*—, to distinguish it from *interactivity*, where there is a reciprocal action between an installation and its user(s)¹⁰. The integration of reactivity into the sound installation stems from the artistic intention to compose with and within the existing environment, taking into account not only spatial features but also the acoustic and semantic characteristics of all surrounding sonic phenomena. This is also motivated by the artist’s exploration of the perceived ‘naturalness’ of sonic landscapes, particularly in relation to their temporal variability, which is considered at two scales. First, through scenario-based compositions that respond to distinct ambient conditions observed on site—linked to different times of day, days of the week, and corresponding usage patterns of the space. At this level, the sound content is curated and composed by the artist with regard to both structural and semantic parameters, allowing the added soundscape to remain contextually responsive. Second, through the real-time acoustic reactivity of these compositions to the temporal and the spectral dynamics of the pre-existing sonic environment at a given instant. This dual mechanism of reactivity enables the added sounds to emerge in a way that integrates harmoniously with the existing environment, avoiding perceptual masking or overload and fostering a balanced sonic interaction.

The present research is part of a research-creation collaboration to inform the design of the *Niches Acoustiques* installation and evaluate its impact on the soundscape on-site. This collaboration unfolds over multiple stages, which include field recordings, laboratory experiments, and *in situ* soundscape evaluations¹¹. We previously conducted recordings in the public space¹² and developed a sound art simulation tool to assess installations in laboratory settings¹³. The simulation tool was then used in a first laboratory experiment to compare the soundscape impacts of elementary composition sketches of *Niches Acoustiques* within a steady background sound environment³. This previous laboratory experiment helped better understand potential soundscape impacts of public space sound installations based on different composition strategies (e.g. abstract sounds led to stronger alterations of soundscape familiarity and variety). However, it had limited practical implications for the design of the sound installation: the simulation of the pre-existing environment was limited to one soundscape representative on an average activity level on-site, and the compositions tested where—non-reactive—elementary sketches to be used as building blocks of a more complex final composition.
Based on the findings of the previous experiment and stemming from the artistic rationale of *Niches Acoustiques*, we propose a follow-up study to understand the impact of more developed compositions sketches on a wider range of soundscape variations. In addition, we report on strategies for implementing the acoustic reactivity of the composition to the pre-existing sound environment. We report on the study's methodological development with a particular emphasis on two elements:

1. The creation of laboratory scenarios that are representative of the public space dynamics over time in terms of sound environment and activities.

2. The implementation of laboratory conditions simulating the installation's acoustic reactivity.

We first introduce the research project in Section 2, including the methods for soundscape simulation and evaluation. In Section 3, we focus on the elaboration of scenarios that will serve as baseline soundscapes in the experimental conditions and represent different time periods and/or space uses in *situ*. In Section 4, we report on the laboratory implementation of the installation's acoustic reactivity. Specifically, we report on three approaches for implementing spectrotemporal reactivity: 1) *data-driven tracked* approach whereby reactivity automatically follows the spectrotemporal trends of the pre-existing sound environment for all compositions and scenarios; 2) *data-driven inverse* approach which operationalizes the concept of *acoustic niche*, whereby compositions occupy spectral or temporal niches of the pre-existing sound environment; 3) *artistic* approach whereby the artist fine-tunes the reactivity for each composition and scenario.

Finally, Section 5 discusses the refinement of the experimental conditions. It includes the rationale and methods for selecting reactivity parameters, a summary of two pilot studies, and mastering of the conditions. Results of the study will be presented in a forthcoming publication.

## 2. OVERVIEW OF THE PROJECT

Researchers and sound artist designed the follow-up study together and all aspects of the methods are the result of joint decisions. The experiment uses a soundscape simulation tool previously developed and validated through listening tests³,¹³ and implemented in Cycling '74 Max¹⁴. Max is a visual programming language designed for music and multimedia applications, well-suited for developing listening experiments due to its capacity to develop customized graphical user interfaces. Overall, the experiment aims to evaluate soundscapes in the presence of different compositions and compare these evaluations to baseline soundscapes without added sounds. The research design is illustrated in Figure 1 and involves four steps as follows:

1. **Reproduction of the pre-existing sound environment.** To reflect variations of the pre-existing soundscape, different baseline soundscapes were edited from Higher-Order Ambisonics (HOA) recordings in the form of loops associated with specific narratives. These loops represent a range of scenarios (corresponding to different times of day, days of the week, and corresponding usage patterns of the space), and are described in Section 3. Similarly to the previous laboratory experiment³, sound environment reproduction is calibrated and then padded with a -4 dB factor to better match the *in situ* loudness¹⁵.

2. **Reactivity implementation** on both the temporal and spectral features of the scenario loops, based on laboratory measurements reflecting listening conditions. It is described in detail in Section 4.

3. **Added sounds**—monophonic composition extracts of the future sound installation—are auralized after reactivity implementation, using IRCAM's EVERTims framework¹⁶. The auralization involves a combination of image source beamforming with the aid of a 3D model of the space and reverberation using a Feedback Delay Network¹⁷. Before auralization, compositions are filtered temporally and spectrally based on measurements of the scenario loops to mimic and assess the acoustic reactivity of the future sound installation (see Section 4).

4. Representation of the resulting soundscape over a loudspeaker array for **soundscape evaluation** by participants familiar to the public space. The experiment is conducted at IRCAM's studio 4, an acoustically treated listening room, over a hemispherical dome of 24 Amadeus PMX 4 speakers. Methods for soundscape evaluation are similar to those used in a previous study¹⁸ and are only briefly synthesized here for conciseness. They consist in momentary evaluation of the soundscapes using
<figure><img src="image_3.png" /><figcaption>**Figure 1** – Flowchart of the research design. The sound environment (left), is simulated from HOA recordings. Added sounds (right) are based on monophonic compositions of the sound installation, first fed into an acoustic reactivity module based on laboratory measurements (center), and then auralized with a 3D modelling of the space. Both the sound environment and the added sounds are presented in the listening room using HOA streams for soundscape evaluation (bottom).</figcaption></figure>

semantic scales derived from previous research³,¹⁸. These evaluations are followed by semi-structured interviews¹⁸.

## 3. SCENARIOS

The first methodological development was to create different scenarios from recordings of the pre-existing sound environment *in situ* to reflect different variations over time (of the day, of the week) and corresponding patterns of space use. While previous research evaluates sound installation or other soundscape intervention on a single baseline “neutral” or “average” soundscape, the concept of acoustic niches calls for considering compositions as reacting temporally and spectrally to the pre-existing soundscape, therefore requiring different baseline soundscapes to fully explore this reactivity. We thus first identified different scenarios reflecting different uses of the space and corresponding sound events.

To do so, we first conducted spatial recordings *in situ* at different time periods (Section 3.A), and then created the different Scenarios, made of a narrative and a Baseline loop (Sections 3.B and 3.C). Both the
<figure><img src="image_4.png" /><figcaption>Figure 2 – Map of the forecourt of the Judicial court with positions of the future installation's speakers and recording positions for both measurement campaign (Campaign 1: Spring 2021; Campaign 2: Spring 2022).</figcaption></figure>

scenarios' loops and narratives were created through an incremental process, using joint listening sessions with the first and second author.

## A. FIELD RECORDINGS

The recordings used for creating the scenarios were collected across two measurement campaigns in spring 2021 and spring 2022. We conducted punctual HOA recordings using an mh Acoustics' Eigenmike 32 together with a B&K 2250 sound level meter for calibration, both at sitting ear height (1.3 m). For both campaigns, the protocols described below were conducted across several measurement sessions spanning 2 to 4 hours and covering different activity levels, including weekday morning, weekday afternoon, weekday evening, weekend morning, and weekend evening (only during the first campaign).

The first campaign was conducted for a previous laboratory experiment³,¹²,¹³. During this campaign, we performed 5-minutes recordings at 18 measurement points (see map in Figure 2), across five sessions¹². Recordings from this campaign were used for two scenarios involving specific events that only happened then: construction work across the street, and a large gathering. For the other scenarios, we conducted a second campaign in spring 2022 involving 10 minutes recordings—to facilitate the creation of 3-minute scenario loops—and this time only at 8 measurement points (see map in Figure 2). Notably, the 18 measurement points used in the previous campaign were filtered out based on their distance from each other and from the installation's speakers, but also on their relevance in relation to the different uses of the forecourt reported in the previous study³. For this second campaign, recordings were conducted across 4 measurement sessions.

## B. PROCESS FOR CREATING THE SCENARIOS

We first identified relevant activities done in the forecourt based on those mentioned in semi-structured interviews conducted at the end of the previous experiment³ with users of the space (residents and local workers). During the interviews, participants mentioned using the court mostly as a transit space for people working in the area (e.g. lawyers and magistrates at the Judicial Court) or living in the neighborhood, many by commuting through the local metro station (see Figure 3). Occasionally, participants reported taking a break, eating/drinking, waiting there, and a few others reported idiosyncratic uses (e.g. playing
with children, making professional calls, people watching). We also identified activities based on potential uses (i.e. activities that are uncommon but could be fostered in the future such as leisure), and uses that we observed in situ (e.g. guards securing the space, people waiting for a trial).

In parallel, we identified periods representative of the forecourt's soundscape variations that could be associated with these activities. These corresponded to the measurement sessions described above but with a higher granularity (e.g. weekday evening – afterwork). Some periods were also related to time-specific events (e.g. protests in relation to a hearing, presence of sounds from nearby construction work). Listening areas were identified based on the list of activities but also in relation to the speakers' position to explore different listening situations. We relied on areas rather than fixed positions because it was more relevant in terms of space use, adjacent measurement points being *de facto* not differentiable. For instance, a recording between 8:30 and 9:30 am close to the metro entrance could be associated with a weekend morning – rush hour period. Recordings were then pre-screened via a first series of listening sessions leading to the selection of a few potential candidates for each identified period.

Finally, we refined the identified periods (initially 10), selected relevant excerpts and edited the final scenario loops through another series of joint listening sessions. Specifically, we identified relevant periods according to their general tone and dominant sound sources, and edited the selected moments along the following procedure:

1. Removing parasite sounds (e.g. clicks/coughs/drones).

2. Shortening the 10 minutes recordings to fit within a loop (3 to 4 minutes) by selecting the parts that correspond to the target ambience.

3. Removing sonic elements that could reinforce the identification of repetitions between the conditions of a given scenario (e.g. intelligible speech).

4. Selecting and looping final excerpts: cutting a piece at one end and adding it to the other end using crossfading between them to create a Baseline loop of 3 minute 10 seconds. This length, corresponding to the conditions' duration, has been determined based on previous research³,¹⁸ to be an optimal duration for proper narrative evolution of the scenarios and the associated compositions while being representative of the public space's soundscape¹⁹.

<figure><img src="image_5.png" /><figcaption>**Figure 3 – Map of the forecourt showing the future installation's speakers and scenarios' area and orientation.**</figcaption></figure>
## C. SCENARIOS NARRATIVES

To contextualize listening for each scenario, we wrote a short narrative describing an activity and time period that is relevant for nearby residents and local workers. Baseline excerpts were edited in the form of a loop, to retain the possibility of choosing a fixed or random onset time for presentation in the experiment.

Scenarios are listed in Table 1 with their associated narrative and a brief description of sound sources. Listening area and orientation for each scenario are shown in Figure 3. The first three scenario narratives are representative of typical space uses, namely commuting to—**Rush hour**—and from—**Afterwork**—work on a weekday, or taking a lunchbreak—**Lunchtime**. The activity suggested in the fourth scenario—**Quiet time**—was less common but provides an interesting variation for the composition (a calmer, more meditative moment). The fifth and sixth scenarios—**Large gathering** and **Construction**—were associated to site-specific events.

<table><caption>Table 1 - Scenarios: narratives and general description. Narratives are translated from French by the first author.</caption><thead><tr><td>Name</td><td>Narrative</td><td>Description</td></tr></thead><tbody><tr><td>Rush hour</td><td>It's a weekday morning, around rush hour. You're on your way to work.</td><td>Dominant traffic; few other sound sources; a few isolated human voices; quite loud</td></tr><tr><td>Lunchtime</td><td>It's lunchtime on a weekday. You sit on a chair next to the trees. You eat your lunch.</td><td>A few passers-by and birds; people eating; background traffic; moderately loud</td></tr><tr><td>Afterwork</td><td>It's early evening on a weekday. You wait for a moment near the metro entrance, chatting with your colleagues or watching passers-by before heading home.</td><td>Dominant human presence (footsteps, suitcases, voices); average traffic noise; moderately loud; eventful</td></tr><tr><td>Quiet time</td><td>It's late morning on a weekend. You take a stroll and stop on the forecourt for a break.</td><td>Low traffic noise; few passers-by; some leisure activities (jogging, scooter); uneventful</td></tr><tr><td>Large gathering special scenario</td><td>It's a weekday afternoon. You're on your way to the forecourt when you see a gathering.</td><td>Average traffic noise masked by dominant human voices</td></tr><tr><td>Construction special scenario</td><td>It's a weekday morning. You stop near the forecourt, heading towards the center of Paris. There is construction work across the street.</td><td>Dominant mechanical sources (traffic, circular saw, hammer); very loud and eventful</td></tr></tbody></table>

## 4. ACOUSTIC REACTIVITY

The second methodological development was to operationalize the concept of *acoustic niche* through spectrotemporal acoustic reactivity. For each scenario, we filtered the corresponding composition in the laboratory to mimic the acoustic reactivity of the future sound installation on-site.

### A. APPROACHES TO REACTIVITY

While the initial compositional focus was on salient sound events, findings from a previous experiment³ revealed the importance of a continuous sonic background to support perceptual coherence and to address the contrasting layers of the existing acoustic context—for example, the constant but low-level background hum of the Parisian ring road versus the more proximal and dynamically variable engine noise from Avenue de la Porte de Clichy (see Figure 3). In response, a dual-layer structure was developed for the added sounds: a stable, cohesive *middleground* layer—composed from recorded natural sound textures such as rustling foliage—acts as a perceptual ‘glue’, while a *foreground* layer features emergent, salient sound events, derived from recordings in nearby public spaces and reinterpreted in abstract and tonal forms³. This layering strategy, in combination with the concept of *acoustic niches*, provides a theoretical and compositional framework for implementing differentiated reactivity across layers—in contrast to more generic, data-
driven models that apply homogeneous reactivity across all layers and scenarios. By designing distinctive reactivity patterns for each layer to occupy a distinct temporal and spectral niche, the artistic approach strives for nuanced responsiveness and perceptual clarity, enabling a context-sensitive auditory integration with the existing urban soundscape.

To explore the potential benefits of such a tailored approach for reactivity to the composition layers and context, we identified three strategies for implementing spectrotemporal reactivity: *data-driven tracked reactivity*; *data-driven inverse reactivity*; and *artistic reactivity*. *Data-driven reactivity* refers to an implementation that is undifferentiated between scenarios and the different composition layers, using *tracked*—reactivity follows spectrotemporal trends—versus *inverse*—compositions occupy *acoustic niches*—filters. Parameters are chosen by the artist but are applied to the composition overall and remain unchanged throughout the scenarios. In contrast, *artistic reactivity* refers to an implementation tailored to the compositions and scenarios using different kinds of *tracked* and *inverse* filters. In this approach, the artist selects different parameters for each listening scenario and applies them differently to the *foreground/middleground* layers of the compositions. Here the reactivity is curated to the nuances of the scenarios and the compositions, considering not only spectrotemporal parameters but also semantic features.

To implement the three strategies, we developed a workflow for the artist to easily fine-tune reactivity parameters. The general process for implementing acoustic reactivity along spectral and temporal approaches is illustrated in Figure 1. Starting from the scenario loops, it involves three steps, which are formalized in Sections 4.B and 4.C:

1. Measurements of the scenario loops representative of listening conditions. Scenarios are recorded in the calibrated listening room at the sweet spot and ear level (1.3 m) using a B&K 2250 sound level meter.

2. Retrieval of indicators (specifically, temporal and spectral acoustic energy) from the scenario loops measurements using Python packages SciPy²⁰ and NumPy²¹.

3. Automatic transfer of acoustic indicators into Reaper²² sessions using the Python package reathon²³. Reaper is a digital audio production software suitable for multitrack edition and diffusion. For each scenario, indicators are converted into automation envelopes (temporal reactivity) and 12-band equalizers (spectral reactivity). In the Reaper sessions, parameters can be adjusted both for temporal and spectral reactivities. These Reaper sessions were used by the artist to fine-tune reactivity parameters during the next stage (see Section 5).

## B. TEMPORAL REACTIVITY

Temporal reactivity is implemented by applying automation envelopes to the added sounds in Reaper via Python. First, moving averages in the form of decibel relative to full scale Root Mean Square (RMS<sub>dBFS</sub>) values of the signal s (corresponding to the scenario loops measurements) are computed as described in Eqs. 1 and 2, with a specified window size $\delta_t$ in seconds, $f_s$ being the signal's sampling rate. In situ, this would roughly correspond to a normalized equivalent level $L_{Zeq,\delta_t}$ of the pre-existing sound environment.

$$ RMS_{\delta_t}[n] = \sqrt{\frac{1}{\delta_t \times f_s} \sum_{n=i-\delta_t \times f_s}^{i-1} s^2[n]} \quad (1) $$

$$ RMS_{dBFS,\delta_t}[n] = 20 \times \log_{10} \left( \frac{RMS_{\delta_t}[n]}{\max(RMS_{\delta_t})} \right) \quad (2) $$

For the *tracked* temporal reactivity, these RMS values are multiplied by an intensity coefficient $n_t$, as shown in Eq. 3. For the *inverse* temporal reactivity, they are inverted and multiplied by $n_t$ as shown in Eq. 4. Values can also be time-shifted with a time factor $\Delta_t$.

$$ temp_{tracked,n_t,\delta_t,\Delta_t}[n] = n_t \times RMS_{dBFS,\delta_t}[n - \Delta_t \times f_s] \quad (3) $$
$$ \text{temp}_{\text{inverse},n_t,\delta_t,\Delta_t}[n] = -n_t \times (\text{RMS}_{\text{dBFS},\delta_t}[n - \Delta_t \times f_s] - (\min(\text{RMS}_{\text{dBFS},\delta_t})) \quad (4) $$

Then, these temp values are sent into a Reaper project via reathon in the form of automation envelope points with a refresh rate of $\delta_t$ and a time shift of $\Delta_t$. To smoothen the curve, the points are interpolated in Reaper with a built-in piecewise cubic curve: for each segment $[\text{temp}[n-1], \text{temp}[n]$, the smoothed values are computed as described in Eq. 5, where $t_k = \frac{k}{\delta_t \times f_s}$ the normalized position within the segment, and $f(t_k) = 3t_k^2 - 2t_k^3$ the cubic easing function. The resulting envelope automation takes the form described in Eq. 5, either for the tracked or inverse temporal filters. Examples of automation envelopes are shown in Figure 4.

$$ \text{env}_n[k] = f(t_k) \cdot (\text{temp}[n] - \text{temp}[n-1]) + \text{temp}[n-1], \quad k \in [1, \delta_t \times f_s] \quad (5) $$

<figure><img src="image_6.png" /><figcaption>Figure 4 – Rush hour scenario: Example of automation envelopes applied to the added sounds before auralization for temporal reactivity on a 60 second section. Top: tracked reactivity; Bottom: inverse reactivity. The grey curve represents the temporal envelope of the pre-existing sound environment.</figcaption></figure>

## C. SPECTRAL REACTIVITY

To implement spectral reactivity, we applied equalizer filters to the added sounds, which were also implemented in Reaper via Python. We implemented static filters, as we assumed through previous measurements¹² that the average spectrum of the pre-existing sound environment would only slightly fluctuate over short time periods. The spectral filters were a modified version of Reaper’s built-in 12-band parametric equalizer, a JSFX plug-in²⁴ which uses biquad filters based on R. Bristow-Johnson’s EQ cookbook²⁵. Specifically, we switched the filter’s frequency bands’ center frequencies so that they can correspond to our measurements. Filters are implemented with a Q factor of 1.

To generate the spectral filters, we retrieved A-weighted third-octave bands equivalent levels ranging from 63 Hz to 16 kHz for each scenario and computed the corresponding octave band equivalent levels $L_{Aeq,band}$ every two third-octave bands from 80 Hz to 12.5 kHz. These values are then normalized by subtracting them to the logarithmic average of all the considered third-octave bands $L_{Aeq,mean}$ (i.e., all octave bands from 63 Hz to 16 kHz), and divided by a spectral intensity factor $n_f$. These values are directly used as filter gains for the equalizer along their corresponding center frequency $f_c$ for the tracked spectral filters.
filter (Eq. 6), while their negatives are used for the inverse spectral filter (Eq. 7). Examples of filter gains are shown in Figure 5.

$$ filter_{tracked,n_f}[f_c] = \frac{L_{Aeq,mean} - L_{Aeq,band}}{n_f} \qquad (6) $$

$$ filter_{inverse,n_f}[f_c] = -\frac{L_{Aeq,mean} - L_{Aeq,band}}{n_f} \qquad (7) $$

<figure><img src="image_7.png" /><figcaption>Figure 5 – Afterwork scenario: Example of filter gains computed from sound level measurements. Each filter gain is computed from the three octave bands surrounding its center frequency.</figcaption></figure>

# 5. SELECTING EXPERIMENTAL CONDITIONS

This section reports on the process of refining experimental conditions (in preparation for the follow-up study) along the three proposed strategies for implementing reactivity: *data-driven tracked reactivity*; *data-driven inverse reactivity*; and *artistic reactivity*.

Because of the wide range of variables under study, we needed to narrow down the conditions in terms of scenarios; presence of the added sounds; presence of the reactivity; reactivity parameters (i.e. filter intensities and time shifts: $\Delta_t$, $\delta_t$, and $n_t$ for the temporal reactivity and $n_f$ for the spectral reactivity). We also report on the process for normalizing and mastering the final laboratory conditions.

## A. ADJUSTING REACTIVITY PARAMETERS

Reactivity parameters ($n_t, n_f, \Delta_t$) corresponding to the three strategies—*data-driven tracked reactivity*; *data-driven inverse reactivity*; and *artistic reactivity*—were all selected by the artist through dedicated listening sessions. Prior to these sessions, we chose a windows size $\delta_t$ of two seconds to implement the temporal reactivity to allow progressive but not instantaneous evolutions of the composition’s envelope. Then, Reaper sessions including the six scenario loops and different intensities of the temporal and spectral filters ($n_t \in [1,2,3]$; $n_f \in [2,3,4,6]$) were provided to the artist to set the appropriate clusters of parameters for each strategy/condition. Based on the listening sessions, the artist set higher filter intensities for the *data-driven tracked reactivity* ($n_t = 3$; $n_f = 3$) than for the *data-driven inverse reactivity* ($n_t = 2$; $n_f = 4$). In contrast, *artistic reactivity* applies *inverse temporal filters* and *tracked spectral filters* for the *foreground layers* of the compositions, as well as *tracked temporal filters* and *inverse spectral filters* for the *middleground layers* of the compositions, with different intensities based on the scenario.
In Parallel, a dedicated Max¹⁴ patch allowed the artist to compare different time shifting settings ($Δ_t \in [2s, 4s, 6s]$) once intensities of the filters have been retained. Notably, it was chosen for the *data-driven reactivity* conditions to have a more immediate reactivity ($Δ_t = 2s$) while a longer time shift has been retained for the *artistic reactivity* ($Δ_t = 4s$), following an intention from the artist to make reactivity less seamless in this second case.

## B. PILOT STUDIES

To refine the experimental setup, we conducted two pilot studies using different subsets of conditions. The dependent variables and the structure of the listening tests—applied in both the pilot studies and the main experiment—are derived from, and very similar to, a previous laboratory study¹⁸. Detailed descriptions will be provided in a forthcoming publication on the main study.

For the *data-driven* reactivities, we initially wanted to assess spectral and temporal reactivity separately, resulting in 7 conditions per scenario (*2 data-driven tracked reactivity*; *2 data-driven inverse reactivity*; *artistic reactivity*, no reactivity, no added sound). However, the first pilot study (with 6 participants) using a single listening scenario—**Lunchtime**—revealed minimal differences across conditions. We thus reduced the number of conditions by combining spectral and temporal approaches for the *data-driven* reactivities (*tracked and inverse*), leading to a total of 5 conditions per scenario.

A second pilot study (with 3 participants) using 5 scenarios—all but **Construction**, which was deemed too disruptive—, revealed the need to further reduce the number of scenarios as the experiment was too long, leading to auditory fatigue resulting in more negative evaluations towards the end. Based on these findings, we opted for 5 conditions and 3 scenarios (**Rush hour**, **Afterwork**, and **Quiet time**) only for a total of 15 trials.

## C. NORMALIZATION AND MASTERING

As explained in Section 2, the scenario loops were presented with a gain of -4dB relative to the levels measured on-site. Otherwise, the normalization and mastering of the compositions aim to control for loudness across different composition of a given scenario while reflecting the mastering process envisaged by the artist once the installation will be deployed on-site. For each scenario, we applied the following procedure:

1. Pre-normalization of the reference (non-reactive) compositions using Reaper. Specifically, we normalized the middleground and foreground layers of each composition separately based on LUFS values (-30 dB LUFS). This was set as a point of departure to make the next steps easier for the artist.

2. Mastering by the artist through listening sessions of the auralized reference compositions, to mimic the protocol carried out during sound installation's deployment. As for normalization, *middleground* and *foreground* layers are mastered separately.

3. Normalization of the conditions involving reactivity with their corresponding reference condition previously mastered, using Reaper. This time, normalization was done on the overall compositions (*middleground* and *foreground* layers combined). We chose to normalize these conditions based on LUFS values to reflect the fact that reactivity should be automated in the future installation.

## 6. CONCLUSION

The present methodology is part of a greater research-creation collaboration around the design and evaluation of *Niches Acoustiques*¹¹. In the previous study, we found relationships between elementary composition strategies (e.g. whether sounds could be clearly identifiable) and their impact on soundscape (e.g. more abstract sounds had a stronger impact on soundscape familiarity), in a steady sound environment³. Building from these results, we seek to evaluate more complex compositions in various scenarios representative of the studied space. Through what could be thought of as an urban interpretation of the *acoustic niche* concept, we also implement reactivity of the compositions to the pre-existing sound environment using 3 different approaches, namely *data-driven inverse reactivity*, *data-driven tracked reactivity*, and *artistic reactivity*,
A plurality of factors pertaining to different fields (acoustics, soundscape studies, sound installation art) were considered in this methodological development. This diversity of factors led us to consider correlates of parameters for developing the experimental conditions, resulting from constant back-and-forth and joint decisions between the research team and the sound artist. For example, each composition was designed for a scenario reflecting specific time periods and activities on-site and is not relevant for others. Therefore, instead of aiming for a factorial design, we opted for a subset of conditions that was relevant not only on theoretical grounds but also regarding artistic and practical concerns.

Overall, the contributions from this paper are chiefly methodological, as we present a method for evaluating the soundscape impact of acoustically reactive sound installations. These methods are intended to be (partially) transferable to other laboratory studies in the broader field of soundscape research. For instance, the development of scenarios that are representative of variations of the space (both sonically and semantically) could be transferred to other soundscape evaluations, regardless of the presence of a specific intervention. Indeed, while most laboratory soundscape evaluations focus on comparing soundscapes across sites, the proposed methodology could be used to compare the soundscapes of a given space across times and patterns of use.

Similarly, our conceptual and technical considerations on the acoustic reactivity of the installation could help inform the design of other (electroacoustic) soundscape interventions. This process also has practical implications to inform the design of *Niches Acoustiques*: the artist investigated different compositions and tested reactivity implementation in relation to a range of scenarios.

The follow-up study will be reported in a forthcoming publication. The results will inform the composition of the permanent installation *Niches Acoustiques*. Specifically, the findings will explore the impact of different compositions curated for different scenarios, considering variations of the sound environment on-site. Furthermore, we will characterize the effect of different reactivity approaches on soundscape evaluations.

## ACKNOWLEDGEMENTS

The authors are thankful to the participants in the pilot studies for giving their time and sharing their experience.

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