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 magnitude 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. The DC output outputs the voltage signal associated with the instantaneous sound pressure level value [dB] measured by the sound level meter.

 

Instantaneous sound pressure level

The instantaneous sound pressure level Lp is obtained by applying frequency weighting (A / C / Z) and time weighting (FAST / SLOW) to the waveform of the instantaneous pressure fluctuation (instantaneous sound pressure) to obtain the root mean square value of sound pressure p, and calculated by Equation 1 with p . p0 is the reference sound pressure, which is 20 μPa for sound in the air.

 

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Some sound level meters have a function called DC output in order to transfer the instantaneous sound pressure level measured by the sound level meter to an external device. This function outputs the measured instantaneous sound pressure level as DC voltage signals.

The Sound level meter also has functions to calculate the average value (time average sound level, equivalent noise level), maximum value, minimum value, etc. However, they cannot be output as DC output.



DC output

The DC output of our sound level meter LA-7500/7200 are as follow:

Output level : 2.5 V ±20 mV (when range full-scale input, 1MΩ loaded)
Scale factor : 0.25 V±10 mV/10 dB
Interval        : 1 ms

The normal range can be set in 6 stages; 10-80 dB, 20-90 dB, 30-100 dB, 40-110 dB, 50-120 dB and 60-130 dB. The wide range is 20-130 dB only. In either range, the upper range of the level range (full scale) determines the relationship between the instantaneous sound pressure level and the DC output voltage.

When the magnitude of sound is same as the upper range of the level range (full scale), 2.5 V is output as the DC output. In addition, the output voltage changes by 0.25 V for every 10 dB in sound pressure level.

Table 1 shows the relationship between the instantaneous sound pressure level and the output voltage of the DC output when the level range is 50-120 dB. The output voltage of the AC output is also shown in the same table.



Table1 The relationship between the instantaneous sound pressure level and the AC/DC output voltage (50-120 dB range)

Instantaneous sound pressure level [dB] DC output voltage[V] Root mean square value of AC output voltage [V]
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 the sound pressure level up to 10 dB greater than the upper range of the level range, thus the maximum output voltage of the DC output is 2.75 V.

The voltage change per 10 dB is 0.25 V, and the voltage change per dB is 0.0025 V. For sounds equivalent to the lower range of the level range (50 dB), the output voltage does not become 0 V.

The accuracy of the output voltage of the DC output of this sound level meter is within ± 20 mV. 20 mV corresponds to 0.8 dB. Please note that when the voltage signal output by DC is input to an external device and converted to the sound pressure level, there is an error of ± 0.8 dB or less.



Equation to convert output voltage of DC output to instantaneous sound pressure level

Since the conversion equation becomes complicated if the output voltage at full scale and the specification value of the scale factor are used as they are, the equation that makes it easy to convert the output voltage of the DC output to the instantaneous sound pressure level is shown in Table 2.
Even if the level range varies, the conversion equation is calculated based on the upper range of level range. The normal range 60-130 dB and the wide range 20-130 dB have different ranges, but the upper range of each level range is 130 dB, and its conversion equation is used.



Table 2 Upper range of level range and conversion equation from output voltage to instantaneous sound pressure level

Upper range Conversion equation
130 dB Sound pressure level[dB] = Voltage[V]×40 + 30
120 dB Sound pressure level[dB] = Voltage[V]×40 + 20
110 dB Sound pressure level[dB] = Voltage[V]×40 + 10
100 dB Sound pressure level[dB] = Voltage[V]×40 + 0
90 dB Sound pressure level[dB] = Voltage[V]×40 - 10
80 dB Sound pressure level[dB] = Voltage[V]×40 - 10
70 dB Sound pressure level[dB] = Voltage[V]×40 - 10


Selection of Analog output terminal

The LA-7500/7200 High performance Sound Level Meter has two analog outputs. One is for only AC output from AC OUT, another is for one of AC-Z/DC/Through from AC/DC OUT terminal. When using the DC output signals, switch the analog output setting of the Sound level meter. Some of our other Sound level meter also require switching of the output setting to use DC output signals. Check the analog output section in the instruction manual.



Summary

I explained the relationship between the instantaneous sound pressure level [dB] and the output voltage of DC output. As for the output voltage of DC output, explained the relationship between the level range of LA-7500/7200 Sound level meter and output voltage. Although the level range of our Sound level meter is set respectively, the conversion concept is the same. With DC output, the measured instantaneous sound pressure is output as voltage signal, which can be taken into an external device and used to record and judge the instantaneous sound pressure level. When using DC output, need to change the analog output setting on the main body of Sound level meter. Also, need to fix the level range when changing the level range of Sound level meter as the conversion equation is changed.

 

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