Auralization setup for auditory research
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The Simulated Open Field Environment
1. Why simulate realistic acoustic space in the laboratory?
Most psychoacoustical and audiological testing to date is being done with simple stimuli such as tones and noises, or with speech, played via headphones or a few loudspeakers in a test booth. While these approaches have led to significant insights, more research is needed to understand and alleviate the problems hearing impaired persons face in everyday listening situations, particularly when hearing aids or cochlear implants are used. A good way to study this is by testing in the situations problematic for patients, that is when multiple sounds are simultaneously present, the so-called "cocktail party effect", and when reverberation smears the sound.We have developed a system to create such difficult everyday listening situations in the laboratory, thereby allowing full control over the sound. The system makes a wide array of tests of spatial hearing and audio-visual interaction possible. Because participants sit in the free-field of an anechoic chamber, they listen unencumbered by earphones and the sound field is identical regardless of when they listen with their own ears or with hearing devices. This makes accurate comparisons between normal hearing and hearing impaired listeners possible and enables testing of hearing devices in realistic situations. Since the system simulates rooms with all their reflections, it can of course also be used to study the impact of reflections with normal hearing, for example for concert hall acoustics.
2. The setup
The first incarnation of the Simulated Open Field Environment has been created by me in the Auditory Perception Lab at UC Berkeley and a refined version was installed at the Institute of Hearing Research in Nottingham. Each consists of a number of loudspeakers arranged at ear height around the listener along the perimeter of the anechoic chamber, and the Nottingham setup also features speakers above and below the listener. Curtains can cover the loudspeakers from view and serve as projection surfaces for up to three video projectors. Three computers control the system, one handling the sound output, one creating the visual environment and one for handling input devices. Computers, amplifiers and most other equipment is mounted in racks in the control room to the anechoic chamber. A measurement microphone at the position of the listener's head and switching unit to read out the amplifier signals make calibration of the system possible. An infrared camera and an intercom allows communication with the participant.
3. Software
Experiments with the system are controlled from Matlab scripts running on the Audio-PC which makes fast prototyping of new experiments possible. Custom sound playback software allows to equalize the frequency response of each loudspeaker directly while playing the sound from Matlab's memory which makes long preprocessing of sounds unnecessary. Loudspeaker equalization filters are generated off-line by a recursive measurement procedure to achieve an impressive level accuracy of 0.3 dB between speakers and frequency response deviations of less than 1.5 dB within 200 Hz to 12 kHz.The visual environment is a stand-alone program running on a separate computer and it is completely remote controlled via network messages from the Audio-PC and the computer handling input devices. Visual objects can be placed at defined locations relative to the participant and moved in space with input devices such as a trackball. This implements the accurate and intuitive ProDePo-localization method I developed during my thesis where the listener positions a visual object to the location of the sound by turning on a trackball. The visual environment is also used to give instructions and feedback to the participant, or to implement game-like tests for children.
4. Experiments
The systems have been used for a wide array of studies, e.g. on:- Methods for studying localization ability,
- Localization ability of cochlear implant listeners,
- Localization cues used by normal hearing and cochlear implant listeners,
- The precedence effect with normal hearing listers, and those with cochlear implants or hearing aids,
- Virtual acoustics with individual and non-individual head-related transfer functions,
- The impact of reverberation on localization in rooms with cochlear implants,
- The effect of compression in hearing aids on localization and its interaction with room reflections,
- Auditory-visual interaction and the ventriloquism effect,
- Techniques for rendering panning errors between loudspeakers inaudible.
