Prosig Support Blog

The place to come for support for Prosig's DATS, P8000 & PROTOR

Prosig Support Blog - The place to come for support for Prosig's DATS, P8000 & PROTOR

What Sample Rate Do I Use To Measure Whole Body Vibration?

What sample rate should I use if I am measuring whole body vibration?

Strict ISO8041 compliance imposes a minimum sample rate of 900 samples per second. This ensures the amplitude and phase characteristics of the applied weightings meet the required criteria. To take account of the anti alias hardware filter we would recommend 1200 samples/second. If non-strict compliance is used the minimum sample rate is 300 samples/second.

ISO2631-1 recommends a minimum data duration of 227 seconds. However, for greater reliability, 600 seconds is now considered essential. That is, a 600 second or longer signal will be more representative and will provide better quality results.

Relative signal levels of a sinusoid with and without background noise

In the process of looking at some order data, a question about the accuracy of the measurement of the signal level of discrete frequency signals which were close to the general noise level.  To answer this question, a small DATS worksheet was created which generated 2 signals.  The first signal was a 35 Hz sinusoid which, by itself the spectrum level was measured to be approximately -9 dB (ref 1 V) as seen in Figure 1.

Spectrum level of 35Hz sinusoid

Fig 1: Spectrum level of 35Hz sinusoid

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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.

 

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