An Auditory Illusion of Proximity of the Source Induced by Sonic Crystals

RESEARCH ARTICLE An Auditory Illusion of Proximity of the Source Induced by Sonic Crystals Ignacio Spiousas, Pablo E. Etchemendy, Ramiro O. Vergara, Esteban R. Calcagno, Manuel C. Eguia* Laboratorio de Acústica y Percepción Sonora, Universidad Nacional de Quilmes, Bernal, Buenos Aires, Argentina * Abstract OPEN ACCESS Citation: Spiousas I, Etchemendy PE, Vergara RO, Calcagno ER, Eguia MC (2015) An Auditory Illusion of Proximity of the Source Induced by Sonic Crystals. PLoS ONE 10(7): e0133271. doi:10.1371/journal. pone.0133271 Editor: Christian Friedrich Altmann, Kyoto University, JAPAN Received: February 5, 2015 Accepted: June 25, 2015 In this work we report an illusion of proximity of a sound source created by a sonic crystal placed between the source and a listener. This effect seems, at first, paradoxical to naïve listeners since the sonic crystal is an obstacle formed by almost densely packed cylindrical scatterers. Even when the singular acoustical properties of these periodic composite materials have been studied extensively (including band gaps, deaf bands, negative refraction, and birrefringence), the possible perceptual effects remain unexplored. The illusion reported here is studied through acoustical measurements and a psychophysical experiment. The results of the acoustical measurements showed that, for a certain frequency range and region in space where the focusing phenomenon takes place, the sonic crystal induces substantial increases in binaural intensity, direct-to-reverberant energy ratio and interaural cross-correlation values, all cues involved in the auditory perception of distance. Consistently, the results of the psychophysical experiment revealed that the presence of the sonic crystal between the sound source and the listener produces a significant reduction of the perceived relative distance to the sound source. Published: July 29, 2015 Copyright: © 2015 Spiousas et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: This work was supported by Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) (http://www.conicet.gov.ar/), PIP 114200801-00016 (MCE, ROV). This work was also supported by Universidad Nacional de Quilmes (UNQ) (http://www.unq.edu.ar), PUNQ 976/11 (MCE, ROV, IS, PEE). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Introduction The task of reconstructing the auditory space from the pressure waves arriving at the eardrums is one of the most challenging operations that our brains can perform. We have learned to localize and segregate sound sources integrating several temporal and spectral cues, and comparing them between our ears [1]. Also, we are able to detect and localize sources even in the presence of reverberation (precedence effect [2]), and competing sounds (cocktail party effect [3]). However, under certain circumstances, our brains can be tricked, and the localization and spatial features of the sources can be manipulated without changing the actual sources of sound. An echo from a distant wall, a reflection from a curved surface or the formation of a “creeping wave” along a wall (as happens in the whispering gallery effect [4, 5]), dramatically alters our spatial perception of the sound sources. In this last case, the speech from a distant talker might be heard as being emitted from a nearby location or even from inside our heads. PLOS ONE | DOI:10.1371/journal.pone.0133271 July 29, 2015 1 / 23 Auditory Illusion Induced by Sonic Crystals Competing Interests: The authors have declared that no competing interests exist. Auditory illusions are scarcely investigated but, as many other illusions, they can provide insights into how the brain organizes perceptual information. Moreover, the vast majority of the illusions studied are related to temporal [6] and spectral attributes of sound, and only very few to spatial effects. Among the spatial effects studied are the precedence effect [2], the saltation effect [7] and the “in-head” localization illusion [8] (that produces an effect similar to that observed in the whispering galleries). On the other hand, spatial auditory illusions have been present since ancient times in building acoustics through the (sometimes intentional) utilization of curved walls and resonances in closed spaces. Examples of this are some greek amphiteaters [9] and early christian churches [10], and ancient galleries and courts [11–13]. However, these provide static and highly idiosyncratic examples of spatial auditory illusions. Therefore, one can ask if these illusions can be studied in a more systematic way. Here, we report and study an auditory spatial illusion that takes place in a real acoustic space, but that can also be controlled in a laboratory environment: the changes in the apparent distance to a sound source that is located behind a sonic crystal. Sonic crystals are essentially periodic structures that can block, shape and manipulate the propagation of sound [14]. Preliminary observations in our laboratory revealed that sonic crystals can also modify the apparent localization of acoustics sources, and subjective reports pointed to the existence of an “in-head” effect similar to those produced by curved walls. Sonic Crystals The first evidence of the unusual acoustical properties of a sonic crystal came from the measurements performed on a minimalist sculpture, designed by Eusebio Sempere and exhibited at the Juan March Foundation in Madrid [14]. This seminal work showed that the repetition of rigid cylinder rods arranged in a lattice, inhibited the sound transmission for certain frequency ranges (band-gaps), in an analogous way to the already known effect of photonic crystals on light [15]. In its simplest realization, a sonic crystal (SC) consists in an bi-dimensional array of rigid cylinders in air. As an example, the SC slab employed in this work is displayed in Fig 1a. The particularity of this kind of composite material that make it interesting for studying perceptual effects is that it displays a large variation of its acoustical properties only by changing its geometrical configuration [16]. Sonic crystals have been extensively studied due to their singular transmission and reflection properties. These properties extend from acoustic band gaps [17] to negative refraction [18], negative bi-refraction [19] sound focusing [20] and imaging [21]. When a sound wave impinges a sonic crystal slab the resulting behavior dramatically changes depending on its frequency: it can be blocked (in the band-gap frequency range), refracted, diffracted, splitted or focused. The phenomenon of focusing b (...truncated)