Frequently Asked Questions about Microphones

When using the MI-8100 Directional Microphone in combination with the SR-2200 sensor amplifier and the CF-7200 portable FFT analyzer,the following explains the unit correction value calibration required to display sound pressure level (dB) on the CF-7200 Portable FFT Analyzer.

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. When analyzing the voltage output from the SR-2200 sensor amplifier with an FFT analyzer, you can set the Y-axis of the FFT to display dB RMS values and use the unit calibration function to correctly display the sound pressure level (dB).

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

■ Unit calibration Correction Value Calculation 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

The MI-8100's sensitivity (-30.41dB re. 1 V/Pa at 1kHz) 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.

CF-7200 Portable FFT Analyzer Unit calibration Correction Values

Considering the case where 94 dB is present per 1 Pa, the following equation holds true:

If the unit calibration correction factor is K:

By setting the CF-7200's unit calibration to "166149EU/V", you can display the power spectrum in sound pressure level (dB) units.