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

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IV<br />

Acoustic<br />

advantages<br />

4<br />

Ga<strong>in</strong> / dB<br />

Fig. 3 REOG of open and closed dome<br />

20<br />

15<br />

10<br />

5<br />

0<br />

-5<br />

-10<br />

-15<br />

REUG<br />

Dome closed<br />

Dome open<br />

-20<br />

100 500 1000 4000 10000<br />

Figure 3: Illustration of the real ear occluded ga<strong>in</strong> (REOG) for<br />

both the open and closed domes of <strong>CENTRA</strong> <strong>Active</strong>. The real<br />

ear unaided ga<strong>in</strong> (REUG) is shown for comparison.<br />

Figure 3 shows the real ear unaided ga<strong>in</strong> (REUG) for<br />

the open ear <strong>canal</strong>. Observe that there is 13-17 dB<br />

peak <strong>in</strong> the 2000 <strong>to</strong> 4000 Hz range. In a closed hear<strong>in</strong>g<br />

aid fitt<strong>in</strong>g this open ear advantage is elim<strong>in</strong>ated,<br />

as the <strong>canal</strong> is closed and resonant properties are<br />

altered. However, observe <strong>in</strong> the dark blue curve of<br />

Figure 3, when the <strong>receiver</strong> fitted <strong>in</strong> the open dome<br />

is placed <strong>in</strong> the <strong>canal</strong>, all the open ear effects rema<strong>in</strong>.<br />

The curve for the closed dome shows that there is<br />

nearly no difference <strong>to</strong> the REUG below 1 kHz, but<br />

significant attenuation at higher frequencies. The<br />

follow<strong>in</strong>g sections exam<strong>in</strong>e the consequences of<br />

these acoustic properties on occlusion and feedback<br />

stability.<br />

Occlusion reduction<br />

Frequency / Hz<br />

In addition <strong>to</strong> improved high frequency amplification,<br />

another desired benefit of <strong>CENTRA</strong> <strong>Active</strong> is the<br />

reduction of the occlusion effect. The occlusion effect,<br />

often described as a “hollowness” of one’s own<br />

voice, is known <strong>to</strong> be bothersome, and <strong>in</strong> some <strong>in</strong>stances,<br />

prevents <strong>in</strong>dividuals from us<strong>in</strong>g amplification.<br />

Physically, the occlusion effect relates <strong>to</strong> an <strong>in</strong><br />

<strong>in</strong>crease of sound pressure level at low frequencies<br />

<strong>in</strong>side the ear <strong>canal</strong>. While the curves shown <strong>in</strong> Figure<br />

3 suggest that the ear <strong>canal</strong> is not significantly<br />

occluded below 1 kHz, this is not a direct measure of<br />

the “occlusion effect,” as REOG is measured with an<br />

external signal.<br />

Fig. 4 Occlusion with open and<br />

closed dome<br />

Occlusion / dB<br />

12<br />

10<br />

8<br />

6<br />

4<br />

2<br />

0<br />

-2<br />

250 500<br />

Figure 4: Occlusion measurements for <strong>CENTRA</strong> <strong>Active</strong> fitted<br />

with both the open and closed dome. Results from MacKenzie<br />

2006 for occluded, 2 mm and OC tips shown for comparison.<br />

To confirm that <strong>CENTRA</strong> <strong>Active</strong>’s ear pieces do not<br />

cause an occlusion effect, test<strong>in</strong>g was conducted<br />

with ten <strong>in</strong>dividuals (five males and five females).<br />

Each subject was fitted with the most appropriate<br />

dome size and wire length. The amount of occlusion<br />

was measured by subtract<strong>in</strong>g the level <strong>in</strong> the<br />

open ear <strong>canal</strong> from the level <strong>in</strong> the test condition<br />

(open dome, closed dome). Subjects vocalized an<br />

“ee” dur<strong>in</strong>g the measurements <strong>to</strong> generate an “<strong>in</strong>ternal”<br />

test signal. The results of this test<strong>in</strong>g, us<strong>in</strong>g<br />

the open and closed dome are shown <strong>in</strong> Figure 4.<br />

For comparison, these mean f<strong>in</strong>d<strong>in</strong>gs are displayed<br />

with the mean data from MacKenzie and colleagues<br />

(MacKenzie et al. 2004), which illustrate the expected<br />

amount of occlusion for an occluded and open<br />

fitt<strong>in</strong>g, and a vented fitt<strong>in</strong>g (2-mm vent). No significant<br />

differences between female and male subjects<br />

were observed. Note that with <strong>CENTRA</strong> <strong>Active</strong>, negligible<br />

occlusion is present. Even with the tighter fitt<strong>in</strong>g<br />

closed dome, average occlusion is no more than<br />

2-3 dB – a value that would not be bothersome nor<br />

even noticeable for most patients.<br />

Smooth frequency response<br />

Frequency / Hz<br />

Occluded<br />

2 mm vent<br />

Open <strong>canal</strong><br />

Dome open<br />

Dome closed<br />

1000<br />

There is a direct relationship between sound quality<br />

and the smoothness of the frequency response. The<br />

<strong>receiver</strong> and coupl<strong>in</strong>g system of <strong>CENTRA</strong> <strong>Active</strong> were<br />

optimized <strong>to</strong> provide a smooth response, with extended<br />

amplification <strong>in</strong> the higher frequencies.<br />

Figure 5 illustrates measures taken <strong>in</strong> the ear of the<br />

KEMAR for both the open and closed domes. Notice<br />

that the frequency response of <strong>CENTRA</strong> <strong>Active</strong> is<br />

smooth throughout the frequency range, with high<br />

frequency amplification extend<strong>in</strong>g <strong>to</strong> 8000 Hz.

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