How To Choose A Sample Rate For A Required Analysis Frequency Range

The relationship between sample rate and maximum frequency that can be analysed (called bandwidth) is a factor of 0.4. Or to look at it another way the sampling rate is 2.5 times the maximum analysis frequency.

The value of 10,000 Hz is multiplied by 2.5 to allow for an anti-alias filter during the capture of the data. An anti-alias filter is set to 0.4 of the sample rate, thus the bandwidth or frequency content that can be studied is 0.4 of the sample rate.

For example, when looking to study a frequency up to 10,000 Hz what sample rate should be used?

So we multiply by 2.5…
10,000 Hz x 2.5 = 25,000 Hz

So the sample rate should be 25,000 samples per second to allow frequencies of up to 10,000 Hz to be studied.

 

Calibrating an Accelerometer with a Prosig P5000 system

Here we look at how to calibrate an accelerometer using a Prosig P5000 system.

Put the wax on the shaker top. Place the accelerometer in the axis you wish to calibrate with positive up and cable connected to P5000 with the relevant transducer class chosen.

Accelerometer calibration screen
Accelerometer calibration screen

Go to Single Channel Calibration screen.

Click on the Tone tab.

With P5000 armed turn on the shaker and monitor the sine wave on the real-time monitor and check Signal Quality as being GOOD.

With a GOOD sine wave click the Calculate button. It is recommended to click Calculate three times.

Check to see the Sensitivity change from what was originally entered when setting up the channel transducer information to the new calculated value. The calculated value should be close to the original.

DC Cal Offset is for DC level accelerometers.

Requested Excitation is for non IEPE (ICP) accelerometers such as capacitive.

After calculating the new sensitivity click on the Use button to make the change in the Transducer Sensitivity Acquisition Setup file.

NOTE: To achieve a GOOD signal reading place the shaker on a flat surface and avoid touching during operation. Make sure the surface the shaker is sitting on does not come in contact with other sources of vibrations or electrical conductance.

Reference frequency for third octave filters

A DATS user asked…

We are using the third octave band filter at very low frequencies (~1Hz)  and I noticed that the response of the filter could introduce very significant errors for short or transient signals. Looking a bit more in details at the function, the help says:

“For audio work ISO standards use a reference frequency of 1kHz not 1Hz”

Does that implies that for non-audio work, a reference frequency of 1Hz should be applied? If yes, is it possible to change this reference frequency in the dats function?

Dr Mercer replied…

Essentially there is no problem and no need to change the reference frequency provided you use Base 10 mode and not Base 2. Base 10 is the ANSI S1.11-2004 preferred scheme. Continue reading Reference frequency for third octave filters