Demystifying Demodulation and Enveloping

Alan Friedman, CAT IV, CRL, CMRP

Posted 10/12/2023

The following is a brief description of this short video on modulation, demodulation and enveloping in vibration. To learn more, please message me, email me at [email protected] or check out my course schedule at https://ZencoVibrations.com

You might want to have a look at my last two articles before you read this one as they cover similar topics: Liner and Non Linear Vibration and Demystifying Harmonics and Sidebands

I’d recommend pausing the video at various times to take a closer look at what is happening to make sure you understand the concepts.

The top graph in the video is a time waveform. The middle graph is an FFT or spectrum of the top graph. The video starts by showing the relationship between the two as Alan changes the frequency and amplitude of the sine wave. He then sets the frequency to 25 Hz.

Next, Alan modulates the top wave with a second sine wave. What is amplitude modulation? It is the repetitive increase and decrease in the amplitude of a signal.

Modulation in the time waveform results in sidebands in the spectrum. In the first example, 25 Hz is modulated by 4.2 Hz. In the spectrum you see the peak at 25 Hz with peaks on either side separated by 4.2 Hz (20.8 Hz and 29.2 Hz).

Next Alan adjusts the modulating frequency so you can see how the sideband spacing changes. Alan then changes the amplitude of the modulating frequency and you can see the amplitude of the sidebands in the spectrum going up and down.

The bottom plot is called a demodulated spectrum or an envelope spectrum. Demodulation, and these plots, are often used for analyzing rolling element bearings. When we demodulate the time waveform graph at the top, we get the modulating frequency. At this point in the video, the modulating frequency is at 7 Hz. The frequency being modulated is 25 Hz. What do we see in the demodulated spectrum? We see 7 Hz.

We also see harmonics of 7 Hz in the demodulated spectrum. Why? Because we are demodulating the “envelope” of the top time waveform and this envelope is not a perfect sine wave. I will post another video and article in the near future that explains this.

Next, Alan changes the shape of the modulating wave by clipping it. This leads to more harmonics in the demodulated spectrum. It also leads to more sidebands in the regular spectrum.

Alan then clips the (25 Hz) frequency being modulated. This results in harmonics of 25 Hz in the spectrum and these harmonics also have sidebands (of 7 Hz in this case). This pattern is common in AC motors, Gears and Bearings.

Demystifying Demodulation and Enveloping in Vibration – Conclusion

I often see people misunderstand and misuse demodulation plots or confuse them for regular spectra. It’s common for people to see a high 1x peak in a PeakVue or demodulated spectrum and incorrectly claim “It’s unbalance!” or they see a 2xLF peak in the demodulated spectrum and think its related to soft foot. (it is not)This is just a tiny glimpse of one of my simulators and a tiny taste of what you will learn in one of my courses.

Thinking about becoming a Certified Vibration Analyst? Read this article.

Alan Friedman

Alan, aka the Vibe Guru, has over 30 years of vibration analysis experience, He has trained 1000’s of students around the world up to Category IV. One of the things that makes Alan a great teacher is his ability to teach people where they are at. Whether you are a math challenged millwright, an engineer or a PhD, Alan will challenge you without overwhelming you. If you are interested in condition monitoring you can also check out his book: Audit It. Improve It! Getting the Most from your Vibration Monitoring Program or hire him for an on-site program audit.

Improve Basic Work Systems First

Many organizations spend too much time searching for—and starting implementation of—new reliability and maintenance concepts, and very little time on implementation and improvements of what they just started. Let me give some examples of my own observations as they relate to the statement above. Some time ago I met with a group of supervisors, planners, and craftspeople—the front line of maintenance—in a mill.

What is Vibration?

In its simplest form, vibration can be considered to be the oscillation or repetitive motion of an object around an equilibrium position. The equilibrium position is the position the object will attain when the force acting on it is zero. This type of vibration is called "whole body motion", meaning that all parts of the body are moving together in the same direction at any point in time.

Web Based Vibration Analysis Programs

A number of changes are occurring in the Condition Based Maintenance (CBM) market and in the realm of communications, Internet and database technologies that are resulting in the adoption of Internet based monitoring programs. The following paper will describe the technologies involved as well as the current market climate that is driving this change. This paper will focus on vibration analysis in particular, however, the forces involved are just as applicable to other CBM technologies and Maintenance planning packages.

Vibration & Ultrasound Technologies: A Possible Integrated Inspection Tool?

The purpose of this paper is to introduce condition monitoring and reliability engineers to the principles of using ultrasound for the assessment of machine condition. Ultrasound can be a complimentary technology to vibration, thermography and lubrication monitoring. It must be emphasized that it is rarely successful as a stand alone technology for effective machine condition assessment and subsequent required maintenance planning. This paper concentrates on the use of airborne ultrasound as a complementary technique particularly for machinery that may be inaccessible due to guards or hazardous locations.

Vibration Pen, Shock Pulse Measurement (SPM), & Vibration Analysis - What's the Difference?

There are three major types of tools for finding bearing defects using vibration techniques. Which of the tools should you chose for inspections of bearings? The best solution is often a combination of two or three of the tools, but it depends very much on how much rotating equipment you have in your plant, the criticality of that equipment, the speed of your rotating equipment, and how perceptive your organization is to training. A short description of the three techniques follows.

Vibration Analysis Reveals Poor Motor Mounting Structure

At the installation of a new 150 gal/min centrifugal pump driven by a 125 HP motor, high vibration levels were found immediately upon first start. After several days of alignment checks, pump teardowns, bearing inspections, motor uncoupled runs and vendor technical support, no solution was found. The plant maintenance manager asked the company's vibration analyst to take readings to troubleshoot the problem. After measuring extremely high vibration levels, resonance was suspected and was found at the motor support structure due to inadequate mounting design for the new installation.

The Truth Behind Misalignment Vibration Spectra of Rotating Machinery

Misalignment is probably the most common cause of machinery malfunction. Considering the importance of alignment, the vibration spectra of alignment is not well documented. Various authors have reported different spectra. The goal of this research was to determine the unique vibration signature for misalignment at varying operating and design conditions such as speed, type and level of misalignment, coupling types and machinery dynamic stiffness.

Vibration Standards and Damage Factors for Machinery

The present study is aimed at establishing the vibration standards for precision machine tools. The machine tools are first segregated and then their vibration data are analyzed for determining the normal vibration level and damage factors (DF). After refining and fixing the vibration standards obtained, they can be used to assess the machinery health.

Demystifying Demodulation and Enveloping