FOCUS MICROPHONE SYSTEM
Automatic Directionality Algorithm Description
The EVoK amplifier has the feature of automatically switching between omni microphone input and directional microphone input. This feature is designed to smoothly switch to the directional mode when there are strong sound inputs from the side and rear. When the sound source is primarily from the front, the amplifier switches back to the omni mode. This feature allows, for example, hearing aids to benefit from directional microphone performance in noisy situations while providing omni input during quiet listening environments. We refer to this feature as auto-direct.
The auto-direct feature works with hearing aids built with two omni microphones that have ports located 5 mm to 14 mm apart. The ports need to be aligned so that the line connecting the rear microphone to the front microphone is approximately horizontal and points to the front. This is the same requirement of a fixed directional microphone system that is constructed with two omni microphones.
The automatic feature works by estimating the conditions where directionality would provide benefit.
Figure 1 shows the decision process. The algorithm forms two main calculations that are the start of this process. The first calculation is the power from the directional microphone (referred to as “FRONT” in the flow chart). This is formed by delaying the rear microphone appropriately and subtracting it from the front microphone The second calculation is the power in the “OMNI” microphone. For this calculation, the output of the front microphone is first filtered to match the low frequency roll-off inherent in the directional microphone. The power of this filtered output is calculated.
The basic concept is to move to the directional mode under conditions where directionality gives a significant reduction in power. This is determined by comparing the two power levels described above. When the “FRONT” power is much less than the “OMNI” power the directional mode is chosen. When the powers are nearly equal then the omni mode is used. The threshold of the power comparison (referred to as “X” in the flow chart) depends somewhat on the absolute power level. The details of the threshold are given in the chart.
After the decision of which mode to use has been made, there is still one last step. The decision is smoothed so that there is gradual transition between modes. The time constant of the transition is several seconds. The final result is that when there is a strong noise in the rear and sides of the listener the amplifier switches to the directional mode and the interfering noise is attenuated. Note also that when the switch occurs to the directional mode there is a change in the frequency response in addition to the omni/directional change. In the directional mode there some low frequency attenuation. This can
be helpful in highlighting the high frequencies of speech during challenging listening situations.
Examples of Operation of Automatic Directionality
Some examples of the operation of the auto-direct may help to clarify the operation. Consider the case where the only sound source is near-by and directly in front of the hearing aid user. In this case the “FRONT” power is nearly equal to the “OMNI” power. The result is a switch to the omni mode. This is advantageous since it reduces the internal microphone noise. Next, consider when a strong noise source that suddenly appears to the rear of the aid user. Then the “FRONT” power is much less than the “OMNI” power. The algorithm switches to directional mode for this case. The switch occurs over several seconds. The user will hear the noise source gradually become attenuated. This scenario can be simulated in most normal rooms with loud speakers two to three feet from the user. It is a way to test and demonstrate the auto-direct feature.