Frequently Asked Questions about Microphones

The following describes how to convert the output voltage (V) from the SR-2200 to sound pressure level (dB) when using the MI-8100 Directional Microphone in combination with the SR-2200 sensor amplifier.

The output of the SR-2200 sensor amplifier is a voltage value proportional to the sound pressure (Pa = Pascals). Since the sound pressure level (dB) to be converted is logarithmic, logarithmic calculations must be performed using a calculator or similar tool to convert the voltage value to sound pressure level.

Sound pressure level (dB) is an RMS value. In contrast, the time waveform output from the MI-8100 Directional Microphone via the SR-2200 sensor amplifier is an instantaneous value, not an RMS value. Therefore, the value obtained by converting the output voltage from the SR-2200 sensor amplifier to sound pressure level (dB) is only a reference value. Note that when analyzing the voltage output from the SR-2200 sensor amplifier with an FFT analyzer, the sound pressure level (dB) can be correctly displayed by using the unit calibration function by setting the Y axis of the FFT to dB RMS value display.

Because the MI-8100 is Directional Microphone, the sound pressure level measured using Directional Microphone such as a sound level meter will differ from the measured value.

■ Conversion Method

When using the MI-8100 Directional Microphone in combination with the SR-2200 sensor amplifier, the conversion from output voltage (V) to sound pressure level (dB) is performed as follows. In the following explanation, "r" in "Vr" means RMS value.

MI-8100 Directional Microphone Sensitivity

MIv (-30.41 dB re. 1 V/Pa at 1 kHz) means that when the sound pressure is 1 Pa (94 dB sound pressure level) at a frequency of 1 kHz, the output voltage is -30.41 dBVr (voltage).

SR-2200 Microphone Amplifier Output Voltage

When the gain of the SR-2200 is set to 20 dB, the output voltage from the SR-2200 is amplified by 20 dB (10 times) from the output voltage of the MI-8100. Therefore, at a sound pressure of 1 Pa (94 dB sound pressure level), the output voltage will be -30.41 + 20 dB = -10.41 dBVr.

If the SR-2200 has a gain of 20 dB, the voltage per 1 Pa is -10.41 dBVr, so converting this to voltage gives:

Therefore, the voltage output from the SR-2200 at 1 kHz and 1 Pa is 0.301 Vr.

Conversely, the conversion of the sound pressure level in dB when the output voltage of the SR-2200 is 0.0301 Vr at 1 kHz is:

Therefore, when the output voltage of the SR-2200 is 0.0301 Vr at 1 kHz, the sound pressure level is 74 dB.

The formula summarizing the above is as follows, assuming the gain setting of the SR-2200 is G (dB) and the effective output voltage is A (Vr):

（1）

■ Instantaneous value and RMS value

For a sine wave, the relationship between the amplitude V and the RMS value A (Vr) is:

（2）

If the instantaneous waveform is a sine wave, then from equation (1):

（3）

If the waveform is a sine wave, the sound pressure level in dB can be calculated by converting the amplitude V read from the waveform using equation (3). However, since the voltage read from a complex instantaneous waveform is not an RMS value, the conversion to sound pressure level in dB will be done by applying equation (3) and obtaining a "reference value calculated from equation (3)".

To accurately convert the output voltage (V) from the SR-2200 to sound pressure level (dB), it is recommended to analyze it using an FFT analyzer.