Abstract
Objectives:Although directional microphones on a hearing aid provide a signal-to-noise ratio benefit in a noisy background, the amount of benefit is dependent on how close the signal of interest is to the front of the user. It is assumed that when the signal of interest is off-axis, users can reorient themselves to the signal to make use of the directional microphones to improve signal-to-noise ratio. The present study tested this assumption by measuring the head-orienting behavior of bilaterally fit hearing-impaired individuals with their microphones set to omnidirectional and directional modes. The authors hypothesized that listeners using directional microphones would have greater difficulty in rapidly and accurately orienting to off-axis signals than they would when using omnidirectional microphones.Design:The authors instructed hearing-impaired individuals to turn and face a female talker in simultaneous surrounding male-talker babble. Participants pressed a button when they felt they were accurately oriented in the direction of the female talker. Participants completed three blocks of trials with their hearing aids in omnidirectional mode and three blocks in directional mode, with mode order randomized. Using a Vicon motion tracking system, the authors measured head position and computed fixation error, fixation latency, trajectory complexity, and proportion of misorientations.Results:Results showed that for larger off-axis target angles, listeners using directional microphones took longer to reach their targets than they did when using omnidirectional microphones, although they were just as accurate. They also used more complex movements and frequently made initial turns in the wrong direction. For smaller off-axis target angles, this pattern was reversed, and listeners using directional microphones oriented more quickly and smoothly to the targets than when using omnidirectional microphones.Conclusions:The authors argue that an increase in movement complexity indicates a switch from a simple orienting movement to a search behavior. For the most off-axis target angles, listeners using directional microphones appear to not know which direction to turn, so they pick a direction at random and simply rotate their heads until the signal becomes more audible. The changes in fixation latency and head orientation trajectories suggest that the decrease in off-axis audibility is a primary concern in the use of directional microphones, and listeners could experience a loss of initial target speech while turning toward a new signal of interest. If hearing-aid users are to receive maximum directional benefit in noisy environments, both adaptive directionality in hearing aids and clinical advice on using directional microphones should take head movement and orientation behavior into account.
Highlights
Modern hearing aids often incorporate directional microphones that can provide a substantial signal-to-noise ratio (SNR) improvement—and typically a corresponding speech-intelligibility benefit—over omnidirectional microphones (Bentler 2005)
The Causes of Increased Movement Complexity We suggest the observed effects of directional microphones on latency and complexity are due to two primary factors
We have previously demonstrated that the SNR for spatially separated targets and noises at one ear is dependent on head angle (Brimijoin et al 2012)
Summary
Modern hearing aids often incorporate directional microphones that can provide a substantial signal-to-noise ratio (SNR) improvement—and typically a corresponding speech-intelligibility benefit—over omnidirectional microphones (Bentler 2005). For signals and noises in a spatially complex environment, the potential benefit of a directional hearing aid is critically dependent on the listener’s head already being pointed in the correct direction. On-axis listening with directional microphones (i.e., with the head pointed at the signal source) has been shown to increase intelligibility relative to omnidirectional microphone listening (for a review, see Ricketts & Dittberner 2002). This difference is termed the directional benefit. When the angle between the listener and signal of interest is larger than about 60°, depending on the directional pattern, intelligibility drops substantially below omnidirectional performance Off-axis orientation between 90 and 135° leads to dramatic directional deficits in word recognition scores in quiet as well as decreases in puretone detectability (Kuk et al 2005)
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