The Auto Spectral Density or Auto RMS spectrum analyses uses Fourier Transforms to process optionally overlapped sections of the input data. The result of each Fourier analysed section is called a periodogram. We then process all the resulting periodograms to produce a spectral result. Continue reading

# Category Archives: signal processing

# 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.

# How Many Samples Per Revolution Are Required To Study A Particular Order

When working in the synchronous/angle domain, how many samples per revolution are required to study a particular order.

To study the n^{th} order we need samples per revolution. Continue reading

# How do I find the natural frequency of an inlet manifold?

There are a number of ways to find a resonant frequency of a part like an automotive inlet manifold. Here are three different types of popular test technique. But which one should you use and why? Continue reading

# How do I perform vibration analysis on a cylinder head and inlet manifold?

We were asked the following question…

I want to perform some cylinder head and inlet manifold vibration analysis, what should I do?

First we need to consider sensor selection Continue reading

# 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

# There are tachometer signals available on the vehicle CAN-bus. Can I use these signals instead of connecting my own tachometer signal?

A simple question should have a simple answer and that answer is “No”.

As usual, however, life is never that simple. Continue reading

# What is Source Contribution Analysis (or SCA)?

How do you measure the causes of a noise or vibration with respect to several sources? Which source is causing what part of the response?

For example, how does the noise inside the cabin of a vehicle relate to the engine noise or wheel hub noise and vibration? Continue reading