You can calculate the overall microphone gain resistance using the formula below:
Rs is the optimal microphone gain resistance
I is the impedance rating of the microphone
G is the desired overall system gain, defined as follows:
- If the module is configured for headset microphone distance (typically a few centimeters from the user’s mouth), then the overall system gain should be -49 dB (0dB=1v/Pa@1KHz);
- If the module is configured for arms_length microphone distance (typically 60-90 cm from the user’s mouth – this is the default setting of EasyVR), then the overall system gain should be -44 dB;
- If the module is configured for far_mic microphone distance (up to about 3 meters from the user’s mouth), then the overall system gain should be -43 dB.
S is the sensitivity rating of the microphone you want to use, and it is specified in –dB in the microphone’s specification.
- The optimal gain resistance for the bundled microphone at arms_length distance is:
Use the closest standard 5% resistor to Rs. In this example, it would be 1.1 kΩ. The EasyVR uses a 1.2 kΩ resistor to allow use of “FAR” settings without replacing the internal resistor.
Sometimes you might also need to compensate some gain loss for a voltage lower than the microphone ratings (using a larger resistor value sets a higher input gain).
- The gain resistance for the bundled microphone at HEADSET distance would be:
In this case you may just add an external 1.2 kΩ resistor to get a gain resistance of 600 Ω (close enough).
Please note that improper acoustic positioning of the microphone will reduce recognition accuracy. Many mechanical arrangements are possible for the microphone element, and some will work better than others. When mounting the microphone in the final device, keep in mind the following guidelines:
1. Flush Mounting – The microphone element should be positioned as close to the mounting surface as possible and should be fully seated in the plastic housing. There must be no airspace between the microphone element and the housing. Having such airspace can lead to acoustic resonance, which can reduce recognition accuracy.
2. No Obstructions, Large Hole – The area in front of the microphone element must be kept clear of obstructions to avoid interference with recognition. The diameter of the hole in the housing in front of the microphone should be at least 5 mm. Any necessary plastic surface in front of the microphone should be as thin as possible, being no more than 0.7 mm, if possible.
3. Insulation – The microphone should be acoustically isolated from the housing if possible. This can be accomplished by surrounding the microphone element with a spongy material such as rubber or foam. The provided microphone has this kind of insulating foam. The purpose is to prevent auditory noises produced by handling or jarring the device from being “picked up” by the microphone. Such extraneous noises can reduce recognition accuracy.
4. Distance – If the microphone is moved from 15 cm to 30 cm from the speaker’s mouth, the signal power decreases by a factor of four. The difference between a loud and a soft voice can also be more than a factor of four. Although the internal preamplifier of the EasyVR compensates for a wide dynamic range of input signal strength, if its range is exceeded, the user application can provide feedback to the speaker about the voice volume (see appendix Error codes).