Abstract
Retroreflection is rarely used as a surface treatment in architectural acoustics but is found incidentally with building surfaces that have many simultaneously visible concave right-angle trihedral corners. Such surfaces concentrate reflected sound onto the sound source, mostly at high frequencies. This study investigated the potential for some Indian stepwells (stepped ponds, known as a kund or baori/baoli in Hindi) to provide exceptionally acoustically retroreflective semi-enclosed environments because of the unusually large number of corners formed by the steps. Two cases—Panna Meena ka Kund and Lahan Vav—were investigated using finite-difference time-domain (FDTD) acoustic simulation. The results are consistent with retroreflection, showing reflected energy concentrating on the source position mostly in the high-frequency bands (4 kHz and 2 kHz octave bands). However, the larger stepped pond has substantially less retroreflection, even though it has many more corners, because of the greater diffraction loss over the longer distances. Retroreflection is still evident (but reduced) with non-right-angle trihedral corners (80°–100°). The overall results are sufficiently strong to indicate that acoustic retroreflection should be audible to an attuned visitor in benign environmental conditions, at least at moderately sized stepped ponds that are in good geometric condition.
Highlights
In architecture, there are some notable acoustic phenomena that can provide fascinating auditory experiences for a listening visitor
The overall results are sufficiently strong to indicate that acoustic retroreflection should be audible to an attuned visitor in benign environmental conditions, at least at moderately sized stepped ponds that are in good geometric condition
Stepwells are designed for water access, not for acoustics; they are certainly not designed for optimum acoustic retroreflection
Summary
There are some notable acoustic phenomena that can provide fascinating auditory experiences for a listening visitor. Steps can be a source of acoustic intrigue, as exemplified by the chirp-like diffraction effects at the Mayan Chichén Itzá pyramid [1–3], or the efficient sound transmission over arrays of stepped seats in the Epidaurus theatre [4,5] These and other acoustic phenomena can take the listener’s spatio-temporal experience beyond the limits of vision, opening up another mode of experiencing space and time as an expression of the architectural form. The present paper investigated another step-related acoustic phenomenon in distinctive and sometimes monumental and ancient architecture: acoustic retroreflection in some Indian stepwells This phenomenon arises in cases where there are many simultaneously visible concave trihedral right-angle corners and has been previously studied in building façades [6–8]. FR/2, the diffraction loss slope is 6 dB/octave, and Rindel [14] suggests fR/2 as a practical frequency above which diffraction loss at normal incidence is of minor importance
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