The recording equipment consists of an audio playback system and a separate recording system. The playback system consists of a Zoom H4n digital audio recording device, a JBL car audio power amplifier and a custom-built dodecahedron loudspeaker mounted on a PA speaker stand. A 12-Volt motorcycle battery is used as the power supply for the power amplifier. The recording system consists of a Zoom H6 six-channel audio recorder and four Neumann KM 183 omnidirectional condenser microphones. The microphones are mounted on a custom-built microphone stand to form a tetrahedron microphone array. The dodecahedron speaker has an approximately omnidirectional radiation pattern both horizontally and vertically. This enables to capture the echoes from all surrounding reflective surfaces with a single impulse response recording.
The study sites are situated in the wilderness without nearby roads or access to mains electricity. Moreover, the weather conditions are often challenging with temperatures even below -10° C. Therefore, special consideration has been paid to the amount and weight of the equipment as well as the setup time.
The main excitation signal is a logarithmic sine wave sweep played through the dodecahedron loudspeaker. Additional excitation signals consist of short fixed frequency signals, noise bursts as well as pre-recorded human voice. The recorded sine wave sweep is post-processed with a deconvolution algorithm, which produces a four-channel impulse response of the acoustic space.
Angle-of-arrival estimation is performed with a custom-written computer program. There, the deconvolved impulse response is processed further using inter-channel cross correlation to find the arrival time of the discrete echoes for each of the four audio channels. The arrival angle of the echoes is then calculated with trigonometric functions using the arrival time differences between the audio signals of the individual microphones in the tetrahedron microphone array.