In this measurement column, we will address frequently asked questions received by our customer support center.
We are presenting the answers.
This time, we will introduce how to use the calculation function.
- I want to know the sound pressure level of the device alone, after removing the sound pressure level of the measurement environment (background noise).
To reduce background noise levels, subtract the power spectrum. - Although the vibration is being excited by a vibration exciter, the frequency response function of the vibration exciter is not flat, so the measurement target
The true frequency characteristics cannot be observed.
To remove the characteristics of the vibration exciter, you would divide the frequency response function.
(This can also be handled with the equalization function.)
1. Removing background noise involves subtracting sound pressure levels.
If LT is the sound pressure level including background noise when the machine is operating, and LB is the background noise level, then the machine
Sound pressure level of noise generated by the machine: L S teeth,
𝐿s=10⋅ log (10𝐿T/10-10𝐿T/10)
You can calculate it like this.
10 LT/10 and 10 LB/10 are the squared values of the effective sound pressure, respectively.
Please refer to the background noise compensation section under "12-3 dB difference (power difference)" on the following page.
Ono Ono Sokki- Technical Report: What is a Sound Level Meter? - 12. Calculation of Decibels (dB)
The power spectrum is the square of the RMS value, so simply subtracting the power spectrum is not enough.
It can be processed using calculations.
Graph 1 shows the power spectrum of the saved background noise level, and Graphs 2 and 3 show the current state
Display the current power spectrum and register the calculation formula in Graph 3 as PWR1-DISP1.
Now, Graph 3 is a graph of sound pressure levels with background noise removed.
Arithmetic formula:
The shape of the power spectrum around 100 Hz to 300 Hz is clear, and the overall value is also low noise.
The resulting value has had the sound component removed.
2. Calculation of the frequency response function
The characteristics of the vibration exciter can be removed by dividing the frequency response function.
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Measured frequency characteristics
The characteristics of the vibration exciter and the characteristics of the device overlap.
The measurement results will reflect the characteristics of G and H.
If the characteristics (G) of the vibration exciter alone are measured in advance, then (the characteristics of the vibration exciter on the exciter table) can be measured.
The measurement result is G・H, so
H = Measurement result / G
Then, the frequency response function of the device alone can be determined.
Graph 1 displays the frequency response function data of the vibration exciter that was saved.
Graphs 2 and 3 show the current frequency response function.
Graph 3 registers the calculation formula FRF1,2 / DISP1.
The corrected frequency response function is shown in Graph 3.
The same process can be easily performed using the equalize function.
Open the equalization function in the analysis settings menu.
Display the frequency response function you want to remove and activate it.
Click the EXEC button for FRF data registration.
Now, we have registered the frequency response function that we want to remove.
Activate the frequency response function you want to equalize.
Turn on equalization. (The button will turn blue.)
The graph is processed and the data is displayed.
The advantage of equalization processing is that, once registered, the calculation can be performed without displaying the frequency response function.
That's it. Please use arithmetic and basic arithmetic operations appropriately.
(Excerpt from the email newsletter issued on July 15, 2020)
