IN THIS ISSUE - Academy of Dispensing Audiologists

DIAGNOSIS & TREATMENT
More than ten years have passed since hearing aids
with directionality experienced a revival in the industry.
Today, most hearing aids dispensed have directionality and enhancements to this
basic feature have made the only proven way to help with hearing in noise when
using hearing aids even more effective for listeners. This article will focus on
some new developments that have solved some problems that remained with this
effective feature.
Although directionality in hearing aids is a proven feature improving the signalto-noise
ratio for patients in noisy environments (Killion et al., 1998; Valente et
al., 1995; and Walden et al., 2000), many recent improvements have made this feature
even more useful to hearing aid users. Directionality has been available in
hearing aids since the early 1970s, but the late 1990s marked a resurgence of this
feature in hearing aids. Directionality of the 1990s was improved over the 1970s
with increased directivity indexes (better performance) and the ability to switch
the hearing instrument from omni directional to directional settings either manually
or automatically. Recent developments in directional-microphone hearing
aids that have tried to further improve user benefit by solving some lingering problems
associated with the feature. This article will detail these developments including
the emergence of wireless technology which provides the opportunity for even
greater help when listening in noise.
Split-Band Directionality
problem solved: the sound quality in the directional setting is
either noisy or tinny.
The benefit of using directionality in noisy environments has been demonstrated
in both the real-world and laboratory, listening in the directional setting has often
come at the cost of sound quality. Inherent in the design of directional hearing
aids is a low-frequency roll-off which occurs because low-frequency sounds have
similar phase relationships between the front and rear microphones. To accommodate
for the decrease in audibility caused by this roll-off in the directional setting, a
boost in low-frequency amplification is usually applied (this is called equalization).
Equalization causes the internal noise floor of the hearing aid to increase and ultimately
can detract from the benefit of the directional setting (Ricketts and Henry,
2002). The result of not compensating for this low frequency roll-off though has the
undesired effect of making the hearing aid sound quality tinny in the directional
setting. Thus, traditional designs have the trade off of either being too noisy or too
tinny rather than sounding natural.
A solution for this problem is to process the sound in the hearing aid the same
way it is processed by a normal-hearing listener. This processing, called split-band
directionality, approximates the unaided ear’s natural directional characteristics.
Figure 1 illustrates how the open ear and split band directionality are similar. The
KEMAR response for four frequencies is shown on the left in the polar plot. For the
two lower frequencies the response is essentially omnidirectional while the higher
frequencies are directional to the front. The panel on the right presents the same
AUDIOLOGY PRACTICES n VOL. 3, NO.2 15