Frequently Asked Questions about Microphones

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

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

The sound pressure level (dB) is an effective value. In contrast, the time waveform output from the MI-8100 Directional Microphone through the SR-2200 sensor amplifier is an instantaneous value and not an effective value. Therefore, the value obtained by converting the output voltage from the SR-2200 sensor amplifier to sound pressure level (dB) is merely a reference value. Additionally, when analyzing the voltage output from the SR-2200 sensor amplifier with an FFT analyzer, setting the Y-axis of the FFT to display dB effective values allows for correct display of sound pressure level (dB) through the unit calibration function.

The MI-8100 is a Directional Microphone, so there is a difference in the sound pressure level values compared to those measured using a sound level meter (omnidirectional) or other omnidirectional microphones.

Concept of conversion

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 considered as follows. In the explanation below, the 'r' in 'Vr' indicates the root mean square value.

MI-8100 Directional Microphone Sensitivity

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

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

When the gain of the SR-2200 is 20 dB, the voltage per 1 Pa is -10.41 dBVr, so converting this to voltage;

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

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

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

The organized formula above states that if the gain setting of the SR-2200 is G (dB) and the effective output voltage is A (Vr);

（1）

■ Instantaneous value and effective value

In the case of a sine wave, the relationship between the peak amplitude V and the effective value A (Vr) is as follows:

（2）

If the instantaneous waveform is a sine wave, then from Equation (1);

（3）

When the waveform is a sine wave, the peak amplitude V read from the waveform can be used to calculate the sound pressure level in dB using equation (3). However, since the voltage read from a complex instantaneous waveform is not the effective value, the conversion to sound pressure level in dB will be based on the "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 use an FFT analyzer for analysis.