TRI-MODE LAYERED NOISE REDUCTION
The design of noise reduction system in the EVoK amplifiers is predicated on two objectives:
1) When speech is present, to reduce the background noise in frequency regions where there is only noise energy present. In the regions where speech energy is present, keep this energy intact. This function will act to enhance speech in noise.
2) In situations where no speech is present, reduce the noise to a higher degree than when speech is present, and do this in a way that does not create an annoying loudness-change effect (so-called pumping), and also does not inhibit the audibility of speech when it starts up. This function will improve comfort and decrease fatigue in the frequent noisy non-speech use situations.
PSEUDO-RANDOM NOISE LAYER
This layer of the system is designed to adjust the Noise Reduction band gains at a rate of change that is similar to the rate of change of speech syllables. This rate of change is approximately 50 msec. The Pseudo-random Noise Layer will monitor the power in a particular Noise Reduction band, to determine if that band contains fast-changing power (fast modulation rate), or only slow-changing power (slow modulation rate). If the modulation rate is fast, it is decided that the band contains important information, so the band gain is unchanged. If the modulation rate is slow, then the decision is that the Noise Reduction band does not contain
important information, and the band gain is reduced. The amount of gain reduction is proportional to the changing power measurements and the Noise Reduction parameter setting (off, low, medium, or high).
STEADY STATE NOISE LAYER
This layer is designed to help reduce noise by monitoring the longer-term changes in the listening situation. This layer identifies when the hearing aid is performing in an environment without a speech signal present. The decision is made using a long time constant, so that the layer is not triggered by every gap in speech. Instead, the layer will not be triggered until speech is absent for 10 seconds. At that time, the layer will begin to decrease the Noise Reduction band gain gradually over a period of about 10 seconds. The amount of gain reduction made by this layer depends on the Noise Reduction parameter setting. When the Steady State Noise Layer is active, it will stop reducing gain when speech is detected again. The gain will be restored gradually over 10 seconds. This provided a pleasing sound because there is not an abrupt change in gain (pumping sound).
QUICK REACTING NOISE LAYER
This is a specialty layer. In the Steady State Noise Layer, gain changes are made very slowly to avoid pumping artifacts. There is one problem with slow changing gain. If speech has been absent for a long period, the Steady State Noise Layer gain reduction will be maximum. When speech starts again, it will take 10 seconds for the gain to be restored in this layer. Therefore there is a risk that the wearer will not be able to hear the speech during this first 10 seconds. This could be an important problem to the wearer. To solve this problem, we add the Quick Reacting Noise layer. This layer also slowly reduces gain when speech is absent, but when speech starts again, it restores the gain within about 50 msec. So this layer gives some gain for this start-up period compared to having only the Steady State Noise Layer. The result is more gain during speech start-up, but also not having significant pumping sound. Two layers allow the system to have a good compromise between these two limiting conditions.
MEASURING NOISE REDUCTION ACTION
It is very difficult in a written document to demonstrate the performance of Layered Noise Reduction. Instead we have prepared a PowerPoint presentation that shows graphically the performance, and also has sound recordings to allow one to hear the difference that Noise Reduction makes.
The demonstration file can be downloaded at this link: