CENTRA Active: A new receiver-in-canal solution designed to ...

V
Research results
VI
Fitting
guidelines
6
Open loop gain (OLG) measurement
To understand the OLG technique, refer back to
Figure 2. This illustration shows that the sound
travels through the hearing aid to the receiver
and after it is delivered in the ear canal, a portion
of the sound leaks back to the microphone
and is picked up again. It is possible, therefore
to think of sound as a “closed loop” (from microphone
to microphone) for all types of hearing
instruments. When the OLG testing is con-
ducted, however, the hearing aid signal processing
path is split between microphone and
receiver. That is, the microphone signal is not
forwarded to the receiver. Hence, the loop
is opened, which is why it’s referred to as an
“open loop” measure.
Now imagine what happens in the “closed loop”
if there is no attenuation of the sound that
leaves the receiver. As soon as any amount of
gain is applied, the sound will be amplified by
this amount in each run through the closed
loop. Because of the speed of sound, this continuous
looping effect quickly drives the gain
to the maximum output of the hearing instrument.
If there is a dominant frequency, there
will be a whistle, or what has come to be
known as feedback.
Given this knowledge of the physics of sound,
it becomes clear that the amount of attenuation
in the feedback path directly indicates how
much gain can be applied without feedback.
Hence, if we simply measure the attenuation
between the ear canal SPL and the microphone,
we know the maximum stable gain; this in fact
is the OLG measurement. For example, if a tone
was generated with a known output of 80 dB
SPL, and the microphone picks up this tone via
the feedback path and measures its level to be
10 dB SPL, then, the maximum stable gain
would be 70 dB at this frequency. It’s important
to understand that this value is unrelated to the
maximum gain provided by the hearing aid
used to conduct the measurement. It simply indicates
the maximum stable gain that potentially
could be delivered, given the ear acoustics,
earmold plumbing and the microphone
location of that particular fitting.
These recordings were then played back-to-back to
the participants using a paired-comparison paradigm.
By using recordings, the participants were blinded to
the products themselves, and factors such as size of
the hearing instrument, comfort of the fitting, etc.,
were not confounding variables. In each round, all
products were compared to each other, and in each
comparison, the participant was instructed to pick
a “winner” (the hearing instrument that they would
prefer to use). In the case that two instruments
sounded the same, the subject was to indicate that
as well. The complete round-robin procedure was
repeated two times, resulting in a total of 18 comparisons
for each hearing instrument.
Fig. 7 Preference in direct comparison
Wins / %
50
40
30
20
10
0
Figure 7: Percent of wins in paired comparison test for three
different listening conditions. Data represents pairings where
differences were noted.
The results of this research are illustrated in Figure 7.
Shown are the percent of wins for each product for
each listening condition. As all devices were premium
products, in several of the comparisons, no difference
could be detected. However, observe that
when differences were noted, CENTRA Active consistently
had the most wins for all three listening conditions.
These findings reveal that even when compared
to the highest level of competition, the signal
processing and acoustics of CENTRA Active have a
distinct advantage.
Fitting guidelines
In general, fitting decisions and procedures concerning
CENTRA Active are similar to other members of
the CENTRA family. There are some specific issues to
consider for CENTRA Active, however, that will be
discussed in this section.
Fitting range
CENTRA
Active
HI #2
Speech in noise
Speech in quiet
Music
HI #3
HI #4
As shown in Figure 8, CENTRA Active is appropriate
for a wide range of hearing losses. In observing the
fitting range, it’s important to consider that the upper
limit of the fitting range for Siemens hearing