Let us try to understand what waterfall frequency spacing is. Waterfall frequency spacing is the gap between spectral lines in an FFT plot. For example, if you had an analysis frequency of 0Hz to 100Hz and 100 spectral lines, then Frequency Spacing is 1Hz. So why is there a ‘Requested Frequency Spacing’ and an ‘Actual Frequency …

# Category Archive: signal processing

Aug 20

## What is “waterfall smearing”?

When analysing a waterfall or performing order analysis it is important to consider the frequency resolution or the frequency spacing. There is often a desire to increase the resolution to finer and finer detail. But that is a process of diminishing returns, and actually fraught with danger. And that danger is waterfall smearing.

Jul 28

## Multiplane Balancing – Zero Imbalance or Specified Imbalance

In the DATS Multiplane Balancing Software, there are two options, optimise for ‘Zero imbalance’ or optimise for ‘Specified imbalance’, but what is the objective of these two options?

May 13

## What engineering decisions are made by measuring a power spectrum?

By power spectrum we are talking about an ASD (Auto Spectral Density) or PSD (Power Spectral Density). In fact PSD is often used when really ASD would be more correct. Taking any signal and performing a frequency analysis using an ASD or a PSD will give the energy over a range of frequencies.

May 09

## What Is A Load Spectrum?

When engineers talk about the ‘Load Spectrum’ what do they mean? There is no simple answer, simple terms like load and spectrum can be used in different situations and therefore to mean different things. However the most common definition of load spectrum is as follows…

May 06

## How Do I Convert To Decibels?

Much confusion revolves around linear and non-linear numbers. The following outlines the mathematical process to convert from a number expressed in dB to a linear quantity. How do we convert to decibels and back again?

Mar 12

## What is Auto Spectral Density?

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.

Feb 05

## 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. …

Nov 11

## 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 …

Aug 29

## 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 nth order we need samples per revolution.