No.16 AC output of Sound level meter
Sound is a phenomenon in which pressure fluctuations are transmitted as waves, and the unit of pressure fluctuations (sound pressure) is Pa (Pascal). Sound level meter is used for measuring the loudness of the sound, and its unit is sound pressure level [dB].
The analog output of the sound level meter includes an alternating current (AC) output and a direct current (DC) output. From the AC output, the instantaneous sound pressure value detected by a microphone is converted into a voltage signal and output. The value of the output voltage from the AC output is proportional to the sound pressure value [Pa] detected by a microphone. Therefore, it is necessary to use the sound pressure value [Pa] instead of the sound pressure level [dB] when converting the voltage value to the loudness of sound.
When 1 Pa of sound is detected, the output sensitivity (= how many volts are output from the AC output) is determined by the level range of the sound level meter. This output sensitivity value is required when converting the voltage value of the voltage signal output from the AC output of the sound level meter into sound pressure [Pa] and then analyzing it to obtain the sound pressure level.
Sound pressure level and root mean square value
If p represents the root mean square value of sound pressure and p0 represents the reference sound pressure, the sound pressure level Lp is obtained from Equation 1. p0 is 20 μPa for sound in the air.
Table 1 shows the relationship between the sound pressure level and the root mean square sound pressure value. When the sound pressure level increases by 10 dB, the root mean square sound pressure value increases by √10 = 3.16 times, and when the sound pressure level increases by 20 dB, the root mean square sound pressure value increases by 10 times. The sound pressure level of 1 Pa is generally said to be 94 dB, which is exactly 93.9794 dB rounded up to one decimal place.
Table1 The relationship between the sound pressure level and the root mean square sound pressure value
| Sound pressure level [dB] | RMS sound pressure value [Pa] |
|---|---|
| 130.0 dB | 63.2 |
| 120.0 dB | 20.0 |
| 114.0 dB | 10.0 |
| 110.0 dB | 6.32 |
| 100.0 dB | 2.00 |
| 94.0 dB | 1.00 |
| 90.0 dB | 0.632 |
| 80.0 dB | 0.200 |
| 70.0 dB | 0.0632 |
| 60.0 dB | 0.0200 |
| 50.0 dB | 0.00632 |
| 40.0 dB | 0.00200 |
Sound pressure level and time waveform
Figure 1 shows the time-axis waveforms with sound pressure levels of 110 dB (blue), 100 dB (orange), 94 dB (green), and 90 dB (purple). All are sine waves with frequency of 1 kHz.
Fig.1 Time waveforms with different sound pressure level
As the root mean square sound pressure value of sound pressure level 94 dB (green) is 1 Pa, the magnitude of half amplitude (0-peak) of the sine wave is 1.414 Pa, which is √2 times of that. As for the sound pressure level, 110 dB (blue), 100 dB (orange), 90 dB (purple), the half amplitude is 8.944 Pa, 2.828 Pa, and 0.894 Pa respectively. Although the difference of sound pressure level between 110 dB and 100 dB is 10 dB, the amplitude ratio of sound pressure waveform is 3.16. Please note that the sound pressure level is increased by 10%, which does not mean that the sound pressure is increased by 10%.
The output voltage of AC output (normal range)
The normal range for the LA-7500/7200 High performance Sound Level Meter can be set in 6 levels: 10 to 80, 20 to 90, 30 to 100, 40 to 110, 50 to 120 and 60 to 130 dB. If sound with the same loudness as the upper range is input, the voltage signal of 0. 707 V (root mean square value) is output from AC output. The output level shows 0.707 Vrms as the root mean square value in the specification of the sound level meter. Table 2 shows the relationship between the sound pressure level and the output voltage of the AC output when the level range of the sound level meter is 50 to 120 dB. The output voltage is not proportional to the sound pressure level. The output voltage is proportional to the Sound pressure [Pa] value. Since 0. 707 V is output when the upper range of the level range is 120 dB (20. 0 Pa), the output sensitivity of the AC output is 0. 707 ÷ 20 = 0. 03535 V/Pa. It is necessary to convert to the sound pressure value [Pa] instead of the sound pressure level [dB] before calculating.
Table 2 The relationship between the sound pressure level and the root mean square sound pressure (50 to 120 dB)
| Sound pressure level [dB] | Root mean square value of sound pressure [Pa] | Root mean square value of output voltage [V] |
|---|---|---|
| 130.0 dB | 63.2 Pa | 2.238 V |
| 120.0 dB | 20.0 Pa | 0.707 V |
| 114.0 dB | 10.0 Pa | 0.3535 V |
| 110.0 dB | 6.32 Pa | 0.2238 V |
| 100.0 dB | 2.00 Pa | 0.0707 V |
| 90.0 dB | 0.632 Pa | 0.02238 V |
| 80.0 dB | 0.200 Pa | 7.07 mV |
| 70.0 dB | 0.0632 Pa | 2.238 mV |
| 60.0 dB | 0.0200 Pa | 0.707 mV |
| 50.0 dB | 0.00632 Pa | 0.2238 mV |
Table 3 The relationship between the level range and output sensitivity
| Level range | Upper range of root mean square value of sound pressure [Pa] | Output sensitivity of AC output [V/Pa] |
|---|---|---|
| 60 to 130 dB | 63.2 Pa | 0.01118 V/ Pa |
| 50 to 120 dB | 20.0 Pa | 0.03535 V/ Pa |
| 40 to 114 dB | 6.32 Pa | 0.1118 V/ Pa |
| 30 to 100 dB | 2.00 Pa | 0.3535 V/ Pa |
| 20 to 90 dB | 0.632 Pa | 1.118 V/ Pa |
| 10 to 80 dB | 0.200 Pa | 3.535 V/ Pa |
When inputting the AC output of the sound level meter to the analyzer, the reference signal of sound calibrator or sound level meter is used for calibration of the analyzer. The calibration value (V / EU) of the analyzer after calibration should be approximately the same as the value in Table 3. If the output sensitivity obtained from the level range of the sound level meter and the calibration value of the analyzer are significantly different, it is necessary to confirm if the connection method or calibration procedure is incorrect or the system is out of order.
The output voltage of AC output (wide range)
The wide range for the LA-7500/7200 High performance Sound Level Meter is 20 to 130 dB. If sound with the same loudness as the upper range is input, the voltage signal of 2.238 V (root mean square value) is output from AC output. The output level shows 2.238 Vrms as the root mean square value in the specification of the sound level meter.
Table 4 shows the relationship between the sound pressure level and the output voltage of the AC output when the level range of the sound level meter is 20 to 130 dB. The output voltage is not proportional to the sound pressure level. The output voltage is proportional to the sound pressure [Pa] value. Since 2.238 V is output when the upper range of the level range is 130 dB (63.2 Pa), the output sensitivity of the AC output is 0. 03535 V/Pa.
For the above purpose, the 3-ch sensor amplifier PS-1300 is often used. Therefore, I have introduced the difference between the AC output and DC output of the sensor amplifier in this column.
Table 4 The relationship between the sound pressure level and the root mean square sound pressure (wide range)
| Sound pressure level [dB] | Root mean square value [Pa] | Root mean square of output voltage [V] |
|---|---|---|
| 130.0 dB | 63.2 Pa | 2.238 V |
| 120.0 dB | 20.0 Pa | 0.707 V |
| 114.0 dB | 10.0 Pa | 0.3535 V |
| 110.0 dB | 6.32 Pa | 0.2238 V |
| 100.0 dB | 2.00 Pa | 0.0707 V |
| 90.0 dB | 0.632 Pa | 0.02238 V |
| 80.0 dB | 0.200 Pa | 7.07 mV |
| 70.0 dB | 0.0632 Pa | 2.238 mV |
| 60.0 dB | 0.0200 Pa | 0.707 mV |
| 50.0 dB | 0.00632 Pa | 0.2238 mV |
| 40.0 dB | 0.00200 Pa | 0.0707 mV |
| 30.0 dB | 0.000632 Pa | 0.0223 mV |
| 20.0 dB | 0.000200Pa | 0.00707 mV |
Summary
I explained the relationship between sound pressure level [dB] and sound pressure [Pa] and the relationship with output voltage value in this column.
Since the value of the voltage output to the AC output of the sound level meter is proportional to the sound pressure [Pa] of the sound detected by the microphone, the loudness of the sound is captured by the sound pressure value [Pa] instead of the sound pressure level value [dB]. The relationship between the sound pressure value and the output voltage (output sensitivity) depends on the level range of the sound level meter.
Regarding the output voltage of the AC output of the sound level meter, I explained the relationship between the level range and the output voltage of the high performance Sound Level Meter LA-7500/7200. Although the level range of other sound level meter is different, the concept is same as what I explained.
(Y·K)