Data Sheet

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12 EasyVR 3 (Plus) User Manual (1.0.17)
Microphone
The microphone provided with the EasyVR 3 module is an omnidirectional electret condenser microphone
(Horn EM9745P-382):
 Sensitivity -38dB (0dB=1V/Pa @1KHz)
 Load Impedance 2.2K
 Operating Voltage 3V
 Almost flat frequency response in the range 100Hz – 20kHz
The microphone circuit is optimized for use at ARMS_LENGTH (default, about 60cm) or FAR_MIC distance
settings.
If you use a microphone with different specifications the recognition accuracy may be adversely affected.
Differences in rated load impedance and sensitivity can be compensated to a certain extent by changing
the microphone gain. This can be done in several ways:
 Replacing the internal gain resistor R4 (1.2kΩ)
 Adding an external resistor Rx going in parallel with R4 (it can only reduce gain, useful for
HEADSET distance settings)
 Removing the internal resistor R4 and using only the external resistor Rx
MIC
VM
RET
3V
AGND
R4
1.2kΩ
Module
Internals
External
Microphone
+
-
Rx
Optional
Microphone circuit
Modifying gain resistance
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:
1. 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);
2. 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;
3. 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
3
.
3
Converting μBars to Pascal: microphone manufacturers specify the sensitivity referencing to μBars or
Pascal. If the microphone sensitivity is referenced to μBars, simply add 20 dB to the rating. For example, -
58 dB/μBars + 20dB = -38 dBV/Pa.