In this measurement column, we will address frequently asked questions received by our customer support center.
We are presenting the answers.
The current situation is
The sound pressure data (in Pa units) with A-weighting applied is recorded as a WAV file.
I want to know the average sound pressure level for a specific section of that data.
Oscope Basic is already installed.
This is what it will be.
Oscope Basic does not include features for 1/N octave analysis or fundamental frequency analysis.
With these two analysis functions, you can display an average graph and easily obtain the average value.
Masu.
What should I do if I only have the Oscope Basic functionality installed?
It's okay. With a little effort, you can calculate the average sound pressure level.
- Method for calculating from the standard deviation (or RMS value) using statistical processing functions.
- Method for obtaining the result from the RMS value calculation in signal processing.
I will introduce two methods.
1. How to use statistical processing functions
First, specify the range for which you want to calculate the average.
Open the custom statistics processing menu.
Statistical processing will be performed.
The calculated value will be displayed. We will use the standard deviation from this value.
Standard deviation: The square root of the mean of the squared differences (between individual data points and the mean) within an interval.
RMS value: Standard deviation: The square root of the mean of the squared values of the individual data points within an interval.
Reason for not using RMS values: Sound pressure level values cut off the DC component (average value).
If the average value of the data is off from 0 (there is a DC offset), subtract the average value.
We'll use the standard deviation method, which involves calculating the sum of squares.
The standard deviation is 0.0205791
The sound pressure level is 20 * (0.0205791 / 0.00002)
= 60.248 dB
0.00002 (20 μPa) is the 0 dB reference value when calculating sound pressure levels.
2. Method using the result of RMS calculation
In signal processing, the effective value calculation requires the instantaneous sound pressure level.
However, the average value within the interval cannot be obtained.
The instantaneous sound pressure level values are saved in text format, and the average value is calculated using Excel.
Opens the RMS calculation for signal processing.
Select "All sections" for the route.
You specify the interval when you export it to a text file later.
The frequency weighting correction used this time is FLAT. The sound pressure waveform already has an A-weighting applied.
Masu.
The time waveform of the RMS value was obtained.
Save in text format.
From exporting the data file,
Select the CSV file.
Select the file and run it.
The file to be exported is not the original waveform, but a data file containing the RMS values.
A CSV file will be created.
Open it in Excel.
Converts dB values to squared values.
The value is squared when multiplied by 10 (dB / 10).
The average of the squared values for each data point is converted back to dB values.
10 * LOG(mean of squared values)
= 60.254 dB
The average value was obtained using two methods.
If the original sound pressure waveform is only available as a FLAT characteristic waveform and you want the average value in A-weighted mode, you can use RMS calculation.
Then, select A-weighting for frequency weighting and run the process.
Since A-weighting cannot be applied to the sound pressure waveform, it cannot be calculated from the standard deviation in statistical calculations.
I have introduced two methods above.
(Excerpt from the email newsletter issued on September 25, 2020)
