A polytrihedral dome for acoustic retroreflection, and its application to creative-arts practice-led research

Abstract

This paper examines acoustic retroreflection from a polytrihedral dome that was used in creative-arts practice-led research as a unique, high voice support singing practice space. The polytrihedral dome is investigated with laboratory measurements, field recordings of singing practice, and by finite-difference time-domain (FDTD) simulation of sound reflections within the polytrihedral dome and other kinds of domes for comparison. Measurements and FDTD simulations show that the polytrihedral dome is retroreflective, especially in the 2 kHz octave band and higher, and provides high levels of voice support. Oral-binaural measurements of voice support exhibit a 4 dB/octave spectral slope from the 500 Hz to the 4 kHz octave bands. The power spectrum of singing is measured and used to weight voice support, yielding measured mean values of + 3.5 dB (maximum + 6 dB). This value is substantial partly because the on-axis singing energy is greatest in the 2 kHz octave band where retroreflection is particularly evident. The polytrihedral dome was installed at a remote acoustically pristine location for sustained use in singing practice. In-situ field measurements of singing practice show reduced voice levels (room effect) from the dome’s voice support, contrasted with increased voice levels (Lombard effect) at a second practice setting, a waterfall. Average voice output between the two environments as measured by a contact microphone differ by 9 dB while the level at the ears remains approximately the same. Using these approaches to investigate reflections in the polytrihedral dome, this paper documents retroreflective acoustic design that, on a larger scale, might ameliorate sound focussing problems in domed room designs.