This measurement column addresses frequently asked questions received by our customer support center and provides answers to those questions.
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 displayed as sound pressure level [dB].
Sound level meters (noise meters) have both AC (alternating current) and DC (direct current) analog outputs. The DC output is a voltage signal associated with the instantaneous sound pressure level value [dB] measured by the sound level meter.
Regarding instantaneous sound pressure level
The instantaneous sound pressure level Lp displayed on the sound level meter is calculated by applying frequency weighting (A/C/Z) and time weighting (FAST/SLOW) to the waveform of instantaneous pressure fluctuations (instantaneous sound pressure) to obtain the effective value of sound pressure p, and then calculating the value from the effective value of sound pressure using Equation 1. Here, p0 is the reference sound pressure, which is 20 μPa for sound in air.
Formula 1
Some sound level meters have a DC output function to allow the instantaneous sound pressure level measured by the sound level meter to be input into an external device. This function outputs the measured instantaneous sound pressure level value as a DC voltage signal.
Sound level meters have the ability to calculate instantaneous sound pressure levels, as well as average values (time-averaged sound level, equivalent sound level), maximum values, and minimum values within a given measurement period. However, these calculated values cannot be output to a DC output.
Output voltage of the DC output of the sound level meter
The main specifications for the DC output of our high-performance sound level meters LA-7500 / LA-7200 are as follows:
Output level: 2,500 V ± 20 mV (full scale, 1 MΩ load)
Scale factor: 0.250 V ± 20 mV (per 10 dB)
Update interval 1ms
The normal range of this sound level meter has six settings: 10-80 dB, 20-90 dB, 30-100 dB, 40-110 dB, 50-120 dB, and 60-130 dB. The wide range has one setting: 20-130 dB. In any range, the relationship between the instantaneous sound pressure level and the DC output voltage is determined by the upper limit of the level range (full scale).
When a sound at the same volume as the upper limit of the level range (full scale) is input, the DC output will be 2.5 V. Additionally, the output voltage changes by 0.25 V for every 10 dB change in sound pressure level.
Table 1 shows the relationship between the instantaneous sound pressure level and the DC output voltage when the sound level meter's level range is 50-120 dB. The AC output voltage, which we introduced last time, is also shown in the same table.
Table 1 Relationship between instantaneous sound pressure level and DC/AC output voltage (50-120 dB range)
| Instantaneous sound pressure level [dB] | DC output voltage [V] | RMS value [V] of AC output voltage |
| 130.0 dB | 2.75 V | 2.238 V |
| 120.0 dB | 2.50 V | 0.707 V |
| 114.0 dB | 2.35 V | 0.3535 V |
| 110.0 dB | 2.25 V | 0.2238 V |
| 100.0 dB | 2.00 V | 0.0707 V |
| 90.0 dB | 1.75 V | 0.02238 V |
| 80.0 dB | 1.50 V | 7.07 mV |
| 70.0 dB | 1.25 V | 2.238 mV |
| 60.0 dB | 1.00 V | 0.707 mV |
| 50.0 dB | 0.75 V | 0.2238 mV |
This sound level meter calculates sound pressure level values up to 10 dB above the upper limit of its level range, so the maximum output voltage of the DC output is 2.75 V.
Furthermore, the voltage change per 10 dB is 0.25 V, and the voltage change per 1 dB is 0.0025 V. The output voltage does not become 0 V when a sound corresponding to the lower limit of the level range (50 dB) is input.
The DC output voltage of this sound level meter has an accuracy of ±20 mV. 20 mV is equivalent to 0.8 dB. Please note that if you input the DC output voltage signal into an external device and convert it to a sound pressure level using that device, an error of ±0.8 dB or less will be present.
Formula for converting DC output voltage to instantaneous sound pressure level.
If we use the full-scale output voltage and scale factor specifications, the conversion formula becomes complex. Therefore, Table 2 shows a modified formula for converting the DC output voltage to instantaneous sound pressure level, making it easier to calculate.
Even if the level ranges are different, the conversion formula is determined by the upper limit of the level range. The normal range of 60-130 dB and the wide range of 20-130 dB are different ranges, but in both cases, the conversion formula with an upper limit of 130 dB can be used.
Table 2. Upper limit of the level range and the conversion formula from output voltage to instantaneous sound pressure level.
| Upper limit of the level range | Formula for converting to instantaneous sound pressure level |
| 130 dB | Sound pressure level [dB] = Voltage value [V] × 40 + 30 |
| 120 dB | Sound pressure level [dB] = Voltage value [V] × 40 + 20 |
| 110 dB | Sound pressure level [dB] = Voltage value [V] × 40 + 10 |
| 100 dB | Sound pressure level [dB] = Voltage value [V] × 40 + 0 |
| 90 dB | Sound pressure level [dB] = Voltage value [V] × 40 - 10 |
| 80 dB | Sound pressure level [dB] = Voltage value [V] × 40 - 10 |
| 70 dB | Sound pressure level [dB] = Voltage value [V] × 40 - 10 |
Regarding switching analog output terminals
Our high-performance sound level meters, the LA-7500 and LA-7200, have two analog outputs. The first analog output is for AC output only. The second analog output (AC/DC OUT terminal) can output one of the following signals: through output, DC output, or AC-Z output. When using the DC output signal, please switch the analog output setting on the sound level meter.
Some of our other sound level meters also require switching the output settings to use a DC output signal. Please refer to the Analog Output section in the instruction manual.
summary
This time, we introduced the relationship between instantaneous sound pressure level [dB] and the voltage value output to the DC output.
Regarding the DC 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.
The DC output provides the instantaneous sound pressure measured by the sound level meter as a voltage signal, which can be used to record and determine the instantaneous sound pressure level by inputting it into an external device. Using the DC output may require changing the analog output settings on the sound level meter itself. Also, changing the level range of the sound level meter will change the conversion formula, so it is necessary to use it with a fixed level range.
(Excerpt from the email newsletter issued on February 21, 2018)
