Communication acoustics in classroom environments : on the use of assistive listening devices

Sammanfattning: Assistive listening devices (ALDs) are used in classrooms to assist communication for students with hearing loss. An ALD, a system of external microphones, transmits sound directly to the students' hearing aids. The signal is coupled to the hearing aid using a radio frequency (FM) or an induction loop (IL) system. Using a switch on the hearing aid, the students can listen to the signal from the hearing aid microphone or the ALD signal received by a FM-receiver or a telecoil. An interest in the impact of ALD solutions on student communication and interaction prompted the work reported in this thesis. The thesis evaluates how the quality of classroom ALDs can be optimized in terms of the concept communication acoustics. Aspects of room acoustics, sound quality, and binaural hearing were explored.The methodical approach was based on self-assessment using questionnaires, interviews and listening tests. The empirical data in Paper I, II, and III consisted of responses from 25 students (10-20 years old) who were attending classes for the hard-of-hearing. In Paper I, the hearing aid microphone (M) and telecoil (T) mode were assessed using a questionnaire. When the hearing aid was in T mode, audibility increased: speech intelligibility was improved and less listening effort was required. Better awareness was achieved using M mode. The students could better hear sounds in the environment around them and participate in conversations - classified as non-teaching - when the ALD was not used. An important feature of sound quality was the distinction of sounds, which is the ability to recognize additional characteristics of a speech sample, e.g., the ability to identify students by voice and judge the mood of students from their voice. Hearing aids also offer a combined mode where the signals from the internal microphone and the telecoil/FM are mixed. In Paper II, different hearing aid mode combinations were assessed using a combined approach where the different combinations were self-rated in a questionnaire and compared in a listening test. The result supports the finding that a combination of M and T mode is a feasible compromise between audibility and awareness. The students were active in their use of different hearing aid modes and aware of the advantages and disadvantages of the alternatives. The hearing strategies varied in different classroom settings and for different degrees of hearing loss, findings that emphasize the importance of individual adjustments. In Paper III, binaural aspects of hearing and ALDs were assessed in a listening test. A binaural model was compared to an omni-directional microphone. No advantage in speech intelligibility and listening effort was found using a binaural ALD. ALD design and characteristics can be evaluated using room acoustic modelling and auralization in different room acoustic conditions. In Paper IV and V, auralization and binaural reproduction techniques used in Paper II and III were investigated. Aspects of binaural and spatial hearing were assessed in normal-hearing subjects. Auralization is a reliable method to render a binaural listening experience in a classroom environment: the performance was equal to that of using artificial head recordings. The method used for binaural reproduction - a two-loudspeaker cross-talk cancellation system - introduces distortion in reproduced interaural differences. The binaural advantages in speech intelligibility were reduced when compared to headphone reproduction. The interaural differences were sufficiently reproduced in the frequency region of ALDs (300-4k Hz); the use of cross-talk cancellation for hearing aid and ALD evaluation is to be further studied.High sound quality matches students' expectations and demands. To take an active part in the communication in the classroom, students expect to hear sounds in the classroom that they perceive as adequate. However, the students with hearing loss required speech signals with significantly reduced noise and competing speech levels. Today, students have to make a compromise between audibility and awareness. Any alternative, however, could make communication in the classroom difficult. Different classroom settings and sound environments as well as individual factors of preference and degree of hearing loss affect their decision.