Effects of reverberation and amplification on sound localisation

Communication often takes place in reverberant spaces making it harder for listeners to understand speech. In such difficult environments, listeners would benefit from being able to locate the sound source. In noisy or reverberant environments hearing-aid wearers often complain that their aids do not sufficiently help to understand speech or to localise a sound source. Simple amplification does not fully resolve the problem and sometimes makes it worse. Recent improvements in hearing aids, such as compression and filtering, can significantly alter the Interaural Time Difference (ITD) and the Inter-aural Level Difference (ILD) cues. Digital signal processing also tends to restrict the availability of fine structure cues, thereby forcing the listener to rely on envelope and level cues. The effect of digital signal processing on localisation, as felt by hearing aid wearers in different listening environments, is not well investigated. In this thesis, we aimed to investigate the effect of reverberation on localisation performance of normal hearing and hearing impaired listeners, and to determine the effects that hearing aids have on localisation cues. Three sets of experiments were conducted: in the first set (n=22 normal hearing listeners) results showed that the participants’ sound localisation ability in simulated reverberant environments is not significantly different from performance in a real reverberation chamber. In the second set of four experiments (n=16 normal hearing listeners), sound localisation ability was tested by introducing simulated reverberation and varying signal onset/offset times of different stimuli – i.e. speech, high-pass speech, low-pass speech, pink noise, 4 kHz pure tone, and 500 Hz pure tone. In the third set of experiments (n=28 bilateral Siemens Prisma 2 Pro hearing aid users) we investigated aided and unaided localisation ability of hearing impaired listeners in anechoic and simulated reverberant environments. Participants were seated in the middle of 21 loudspeakers that were arranged in a frontal horizontal arc (180°) in an anechoic chamber. Simulated reverberation was presented from four corner-speakers. We also performed physical measurements of ITDs and ILDs using a KEMAR simulator. Normal hearing listeners were not significantly affected in their ability to localise speech and pink noise stimuli in reverberation, however reverberation did have a significant effect on localising a 500 Hz pure tone. Hearing impaired listeners performed consistently worse in all simulated reverberant conditions. However, performance for speech stimuli was only significantly worse in the aided conditions. Unaided hearing impaired listeners showed decreased performance in simulated reverberation, specifically, when sounds came from lateral directions. Moreover, low-pass pink noise was most affected by simulated reverberation both in aided and unaided conditions, indicating that reverberation mainly affects ITD cues. Hearing impaired listeners performed significantly worse in all conditions when using their hearing aids. Physical measurements and psychoacoustic experiments consistently indicated that amplification mainly affected the ILD cues. We concluded that reverberation destroys the fine structure ITD cues in sound signals to some extent, thereby reducing localisation performance of hearing impaired listeners for low frequency stimuli. Furthermore we found that hearing aid compression affects ILD cues, which impairs the ability of hearing impaired listener to localise a sound source. Aided sound localisation could be improved for bilateral hearing aid users, if the aids would synchronize compression between both sides