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.
When working in the synchronous/angle domain, how many samples per revolution are required to study a particular order.
To study the nth order we need samples per revolution. Continue reading
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
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
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.
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
A simple question should have a simple answer and that answer is “No”.
As usual, however, life is never that simple. Continue reading
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
ODS = Motion of structure vibrating naturally (for example bridge vibration) or when excited by an unknown force (for example generator vibration). Prosig’s Structural Animation software performs ODS analysis.
OMA = Modal Analysis
Prosig’s Modal Analysis software performs OMA. Continue reading
Transfer Path Analysis (TPA) provides the route of the noise or vibration through the structure, not just which sources cause the response, but the route taken. This is useful for developing and improving components like engine anti-vibration mountings or suspension anti-vibration mountings. Continue reading