Starkey Research & Clinical Blog

Considerations for Directional Microphone Use in the Classroom

Directional Benefit in Simulated Classroom Environments

Ricketts, T., Galster, J. and Tharpe, A.M. (2007)

This editorial discusses the clinical implications of an independent research study. The original work was not associated with Starkey Laboratories and does not reflect the opinions of the authors.

Classroom acoustic environments vary widely and are affected by a number of factors including reverberation and noise from within the classroom and adjacent areas. Signal-to-noise ratio (SNR) is known to affect speech perception for children with normal hearing and those with hearing loss (Crandell, 1993; Finitzo-Hieber & Tillman, 1978). Because listeners with hearing loss typically require more favorable SNRs to achieve the same performance as normal hearing listeners, hearing-impaired students are particularly challenged by high levels of classroom noise.

FM systems are often recommended as a method for improving SNR in the classroom. However, they may not effectively convey voices other than the teacher’s, so children may be less able to hear comments or questions from other students.  The additional bulk of ear level FM systems may prompt reluctance to wear the FM system, as the student may perceive this as calling attention to their hearing loss.  Because of these and other potential limitations of FM systems, the use of hearing aids with directional microphones is an opportunity to improve SNR for hearing-impaired children.

The benefits of directional microphones for speech perception in the presence of background noise are well known for adults (Bentler, 2005; Ricketts & Dittberner, 2002; Ricketts, Henry & Gnewikow, 2003). Research has shown that children also benefit from directionality in laboratory conditions (Gravel, Fausel, Liskow & Chobot, 1999; Hawkins, 1984; Kuk, et al., 1999), but more information is needed on the effect of directional microphone use in classroom environments.  The study summarized in this post evaluated directional microphone use in simulated classroom situations and the subjective reaction to omnidirectional and directional modes by children and parents.

The authors recruited twenty-six hearing-impaired subjects ranging in age from 10-17 years participated in the experiment.  All but two had prior experience with hearing aids.  Subjects were fitted bilaterally with behind-the-ear hearing instruments that were programmed with omnidirectional and directional modes.  Digital noise reduction and feedback suppression features were disabled and all participants were fitted with unvented, vinyl, full-shell earmolds.

This study consisted of three individual experiments. The first investigated directional versus omnidirectional performance in noise in five simulated classroom scenarios:

1) Teacher Front – speech stimuli presented in front of the listener.

2) Teacher Back – speech presented behind the listener.

3) Desk Work – speech presented in front of the listener, the listener’s head oriented down toward desk

4) Discussion – three speech sources at 0 and 50-degree azimuth (left and right), simulating a round table discussion

5) Bench Seating – speech presented at 90-degree azimuth (left and right)

Speech recognition performance was evaluated in each of these scenarios using a modified version of the Hearing in Noise Test for Children (HINT-C, Nilsson, Soli & Sullivan, 1994). Speech stimuli were initially presented at 65dB SPL for the five test conditions. Noise was presented from four loudspeakers positioned 2 meters from each corner of the room. For conditions 1-3, the noise level was 55dB. For conditions 4 and 5, noise levels were fixed at 65dB.

A second experiment examined the performance of omnidirectional versus directional modes in the presence of multiple talkers. Monosyllabic words from the NU-6 lists (Tillman & Carhart, 1966) were randomly presented at 63dB SPL from speakers positioned 1.5 meters, surrounding the listener at three angles: 0 degrees (in front of listener), 135 degrees (back right) and 225 degrees (back left). Noise was presented at 57dB SPL, which again yielded an SNR of 6dB.

Not surprisingly, the results of the first experiment showed that directional performance was significantly better than omnidirectional performance for Teacher Front, Desk Work and Discussion conditions, but was significantly worse for the Teacher Back condition.  There was no significant difference between omnidirectional and directional modes for the Bench Seating condition.  In the Bench Seating condition, however, subjects were not specifically instructed to look at the speaker. If some subjects did look at the speaker and others did not, individual differences between omnidirectional and directional modes may have been obscured on average.  Improved performance was generally noted as the distance between speaker and listener decreased. This is consistent with previous studies with adult listeners, which showed increased directional benefit with decreasing distance (Ricketts & Hornsby, 2003, 2007).

The second experiment yielded no significant difference in performance between omnidirectional and directional modes when speech was in front of the listener. When speech was presented behind the listener, omnidirectional mode was significantly better than directional in both the back-right and back-left conditions.  The authors surmised that the directional benefit may have been reduced because subjects were told that all of the talkers were important and because 2/3 of the talkers were behind them, they may have been more focused on speech coming from the back.

The current study offers insight into the potential benefit of directional microphones for classroom environments. An FM system remains the primary recommendation for improving signal-to-noise ratio of a teacher’s voice, but overhearing other students and multiple talkers can be compromised by FM technology.  Additionally, because of social, cosmetic or financial concerns FM use may not be feasible for many students. Therefore, directional hearing instruments will likely continue to be widely recommended for hearing-impaired schoolchildren. This study reported a directional benefit ranging from 2.2 to 3.3 dB, which is consistent with studies of adult listeners (Ricketts, 2001).  Therefore, directional microphone use in classrooms may indeed be beneficial, as long as the teacher or speaker of interest is in front of the listener. However, for round table or small group arrangements, directionality could be detrimental, especially when talkers are behind the listener.  The authors point out that many school scenarios involve multiple talkers or speech from the sides and back, so directional microphone benefit may be limited overall.

The results of these experiments underscore the importance of counseling for school-age hearing aid users, as well as their parents and teachers. It is common practice to recommend preferential seating close to the teacher in the front of the classroom. Improved performance with decreases in distance from the speech source, in this and other studies, shows that this recommendation is particularly important for hearing aid users, whether or not they are in a directional mode. Furthermore, hearing-impaired students should be instructed to face the teacher so they can benefit from directional processing as well as visual cues. This should also be discussed in detail with teachers so that efforts can be made to arrange classroom seating accordingly.

An incidental finding of the first experiment showed that performance for the Desk Work condition was better than the Teacher Front condition, even though the distance between speaker and listener was comparable.  In the Desk Work condition, subjects were instructed to work on an assignment on the desk as they listened. Therefore, the listener’s head position was pointed slightly downward, which may have resulted in more optimal, horizontal positioning of the microphone ports, increasing directional effect. This finding demonstrates the importance of selecting the proper tubing or wire length, to position the hearing aid near the top of the pinna and align the microphone ports along the intended plane.

Overall, directional processing improved performance for speech sources in front of the listener and reduced performance for speech sources behind the listener. The instruments in this study were full-time omnidirectional or directional instruments, so it is unknown how automatic, adaptive directional instruments would perform under similar conditions. Because of the prevalence of automatic directionality in current hearing instruments, this is a question with important implications for school-age hearing aid users.  Perhaps automatic directionality could provide better overall access to speech in many classroom environments, but controlled study is needed before specific recommendations can be made.


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