In this measurement column, we will address frequently asked questions received by our customer support center.
We are presenting the answers.
Sound is a phenomenon in which fluctuations in pressure are transmitted as waves, and the unit of this pressure fluctuation (sound pressure) is Pa (Pascal).
A sound level meter (noise meter) is a measuring instrument that measures the loudness of sound, and its loudness is measured by the sound level.
The pressure level is displayed in [dB].
Sound level meters (noise meters) have both AC (alternating current) and DC (direct current) analog outputs.
The AC output contains a voltage signal containing the sound pressure value detected by the microphone at the moment of detection.
The voltage output to the AC output is the sound pressure value [Pa] detected by the microphone.
Because it is proportional to the sound level, when converting a voltage value to sound intensity, use the sound pressure value [dB] instead of the sound pressure level value [dB].
We need to consider this in terms of [Pa].
The voltage output to the AC output (output sensitivity) when a sound of 1 Pa is detected is:
This is determined by the level range of the sound level meter. The voltage of the voltage signal output from the AC output of the sound level meter.
When converting the value to sound pressure [Pa] and then analyzing it to determine the sound pressure level, this output sensitivity...
A value will be required.
Sound pressure level and RMS value
Let p be the effective sound pressure of a certain sound, and p be the reference sound pressure. 0 In that case, the sound pressure level L p This is given by Equation 1.
It is possible. 0 For sound in air, this is 20 μPa.
formula 1
Table 1 shows the relationship between sound pressure level and effective sound pressure. When the sound pressure level increases by 10 dB, the effective sound pressure increases.
The effective value increases by √10 = 3.16 times, and a 20 dB increase in sound pressure results in a 10-fold increase in the effective sound pressure.
The sound pressure level is generally said to be 94 dB, but it is actually 93.9794... dB.
Rounded to one decimal place, it becomes 94.0 dB.
Table 1 Relationship between sound pressure level and RMS sound pressure
| Sound pressure level [dB] | Effective sound pressure [Pa] |
| 130.0 dB | 63.2 Pa |
| 120.0 dB | 20.0 Pa |
| 114.0 dB | 10.0 Pa |
| 110.0 dB | 6.32 Pa |
| 100.0 dB | 2.00 Pa |
| 94.0 dB | 1.00 Pa |
| 90.0 dB | 0.632 Pa |
| 80 dB | 0.200 Pa |
| 70 dB | 0.0632 Pa |
| 60 dB | 0.0200 Pa |
| 50 dB | 0.00632 Pa |
| 40 dB | 0.00200 Pa |
Sound pressure level and sound time waveform
Figure 1 shows the time-domain waveforms of sounds with sound pressure levels of 110 dB (blue), 100 dB (green), 94 dB (red), and 80 dB (brown). All are sine waves with a frequency of 1 kHz.
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Figure 1: Time waveforms with different sound pressure levels
Blue: 110 dB, Orange: 100 dB, Green: 94 dB, Purple: 90 dB
The effective value of a sound with a sound pressure level of 94 dB (green) is 1 Pa, so the magnitude of the single-amplitude (0-peak) of the sine wave is √2 times that, or 1.414 Pa.
The amplitudes of sounds at sound pressure levels of 110 dB (blue), 100 dB (orange), and 90 dB (purple) are 8.944 Pa, 2.828 Pa, and 0.894 Pa, respectively. The difference in sound pressure levels between 110 dB (blue) and 100 dB (orange) is 10 dB, but the amplitude ratio of the sound pressure waveforms is 3.16 times. Note that a 10% increase in the sound pressure level does not mean a 10% increase in sound pressure.
Output voltage of the AC output of the sound level meter (normal range)
Our high-performance sound level meters LA-7500/LA-7200 offer six normal range settings: 10-80 dB, 20-90 dB, 30-100 dB, 40-110 dB, 50-120 dB, and 60-130 dB. When a sound level equal to the upper limit of the range is input to the AC output of this product, a voltage signal of 0.707 V (RMS) is output. The specifications of the sound level meter often indicate that the output voltage is RMS (effective value) as 0.707 Vrms.
Table 2 shows the relationship between the measured sound pressure level and the AC output voltage when the sound level meter's level range is 50-120 dB. The output voltage is not proportional to the sound pressure level. The output voltage is proportional to the sound pressure [Pa] value. At the upper limit of the level range, 120 dB (20.0 Pa), 0.707 V is output, so the AC output sensitivity is 0.707 ÷ 20 = 0.03535 V/Pa.
Please note that the calculation is not simply 0.707 ÷ 120 = ... when the upper limit of the level range is 120 dB and the value is 0.707 V. You need to convert the sound pressure level value [dB] to sound pressure value [Pa] before calculating.
Table 2 Relationship between sound pressure level and RMS sound pressure (50-120 dB range)
| Sound pressure level [dB] | Effective sound pressure [Pa] | RMS 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 shows the output sensitivity values of the AC output when the sound level meter is set to a different level range.
Table 3 Relationship between Level Range and Output Sensitivity
| Level range | Effective sound pressure [Pa] at the upper limit of the range | AC output sensitivity [V/Pa] |
| 60-130 dB | 63.2 Pa | 0.01118 V/ Pa |
| 50-120 dB | 20.0 Pa | 0.03535 V/ Pa |
| 40-110 dB | 6.32 Pa | 0.1118 V/ Pa |
| 30-100 dB | 2.00 Pa | 0.3535 V/ Pa |
| 20-90 dB | 0.632 Pa | 1.118 V/ Pa |
| 10-80 dB | 0.200 Pa | 3.535 V/ Pa |
When performing analysis by inputting the AC output of a sound level meter into an analysis device, the analysis device should be calibrated using an acoustic calibrator or the sound level meter's reference signal. The calibration value (V/EU) of the analysis device after calibration should closely match the values in Table 3. If the output sensitivity obtained from the sound level meter's level range and the calibration value of the analysis device differ significantly, it is due to an incorrect connection method or calibration procedure, or a malfunction of the device, and the cause needs to be investigated.
Output voltage (wide range) of the AC output of the sound level meter
Our high-performance sound level meters, the LA-7500/LA-7200, have a wide range of 20-130 dB. When a sound level equal to the upper limit of the level range is input to the AC output of these products, a voltage signal of 2.238 V (RMS) is output. The specifications of the sound level meter indicate that the output voltage is RMS (effective value) as 2.238 Vrms.
Table 4 shows the relationship between the sound pressure level of the sound measured in the 20-130 dB range and the output voltage of the AC output. The output voltage [V] is not proportional to the sound pressure level value [dB]. The output voltage is proportional to the sound pressure value [Pa]. At the upper limit of the level range, 130 dB (63.2 Pa), 2.238 V is output, so the output sensitivity of the AC output is 0.03535 V/Pa.
Table 4 Relationship between sound pressure level and RMS sound pressure (wide range)
| Sound pressure level [dB] | Effective sound pressure [Pa] | Output voltage RMS value [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 | 0.000200 Pa | 0.00707 mV |
summary
In this article, we discussed the relationship between sound pressure level [dB] and sound pressure [Pa], as well as their relationship to the voltage value output to the AC output.
The voltage output of a sound level meter (noise meter) is proportional to the sound pressure [Pa] detected by the microphone. Therefore, sound intensity should be considered in terms of sound pressure [Pa] rather than sound pressure level [dB]. Furthermore, the relationship between sound pressure and output voltage (output sensitivity) depends on the level range of the noise meter.
Regarding the AC output voltage of the sound level meter, we have explained the relationship between the level range and output voltage of the high-performance sound level meters LA-7500/LA-7200. While the settable level ranges differ for our other sound level meters, the underlying principle is the same.
(Excerpt from the email newsletter issued on December 19, 2017)
