This post covers how to upsample and downsample data and the possible pitfalls of this process. Before we cover the technical details let us first explain what we mean by upsample and downsample and why we may need to use it.…

## RMS From Time History And FFT Spectrum

The RMS of a time history is a measure of the signal’s overall energy and is often used when extracting features from a signal for prognosis and trending of vibration data. For a time history which has been digitised at…

## A Different Way Of Averaging – Time Synchronous Averaging

Assuming one wants a frequency spectrum from an acquired time measurement, it is generally accepted that averaging of a signal in the time domain is not very useful due to the randomness of the start of the time block used…

## What Is A Hammer Test Or Hammer Impact Test?

This is often also known as Modal Testing. It is a method of testing that allows us to calculate the natural frequencies (modes), modal masses, modal damping ratios and mode shapes of a test structure. This is commonly done using either…

## What is Resonance? (Part 2)

This article is a follow on from What Is Resonance? (Part 1) and answers some of the issues not covered in that post. How do you find the resonant frequency in the real world? What do you do when there…

## Wide Band Integrators – What Are They?

For some time now it has been conventional ‘wisdom’ that using time based digital integration may cause amplitude errors in the result and that these get worse as the frequency increases. As a result of this, integration using Omega arithmetic has been prevalent by using Fourier Transforms of the signal. This, of course, remains a valid approach and is particularly useful if the data is already in the frequency domain, which was its prime purpose.

## Where Does The Noise Come From?

[latexpage] When one thinks of noise in a signal it is generally associated with having been added in some way to the amplitude of a signal. This is not always the case. The two sinewaves shown in Figure 1 appear…

## Understanding Filter Characteristics

Recently when discussing with an engineering student the characteristics of filters, it became clear that some confusion exists around this subject area. This note attempts to explain the differences between types of filter and the effects of the parameters of those…

## A Simple Frequency Response Function

The following article will attempt to explain the basic theory of the frequency response function. This basic theory will then be used to calculate the frequency response function between two points on a structure using an accelerometer to measure the…