Cochlear SPrint Sound Processor Informações Técnicas descarregar pdf (Página 36)

SECTION ONE | CLINICAL GUIDANCE

1.11 MICROPHONE SENSITIVITY AND VOLUME SETTING

1.11.1 Microphone Sensitivity

The setting of the microphone sensitivity control determines the minimum

input signal level required for stimulation. At higher sensitivity settings, less

acoustic energy, i.e. a lower SPL, is required to cause stimulation. At lower

sensitivity settings, greater acoustic energy, that is a higher SPL, is required to

cause stimulation (see Figure 9).

When the sensitivity is set optimally, the AGC circuit uses the most recent peak

value in the signal to determine the instantaneous gain. With the onset of a high

level signal, the AGC rapidly decreases the gain so that the peak value of the

signal is set to a ﬁxed value. The AGC slowly increases the gain after the high-

level peak has passed. The sensitivity control range on the Freedom and CP810

sound processors is equivalent to that on the SPrint sound processor, i.e. ranging

from 0-20. The default setting is 12. With the default settings (T - SPL = 25,

C - SPL = 65), 25 dB SPL sound input stimulates at T-level and sounds at or above

65 dB SPL stimulate at C-level. Recipients should be counselled to use the default

sensitivity setting in a quiet environment. If the recipient consistently sets the

microphone sensitivity above or below the optimal setting, it means that overall

C-levels in the MAP are not set appropriately and general speech understanding

will be compromized. If the sensitivity is set too low, the gain applied to the

incoming signal is reduced. In this case, conversational speech levels cause

stimulation below C-level and softer sounds will not be heard. If the sensitivity

is set too high the gain applied to low-level inputs (such as background noise) is

increased. The AGC operates on higher-level input signals such as speech and the

result is a decrease in the perceived signal-to-noise ratio (SNR). This decreased

SNR makes it difﬁcult for the listener to accurately discriminate and recognize

Figure 9: Acoustic

energy required to

cause stimulation

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