vibration analysis3

LESSON

2 INTRODUCTION TO MACHINERY AND NOISE

LECTURE

SUB - OBJECTIVE

At the end of the lesson the Trainee will be able to demonstrate an understanding of Introduction to Machinery and Noise.

1.0 WHAT IS VIBRATION?

Vibration is simply the motion of a machine or machine part back and forth from its position of rest.

The simplest way to show vibration is to follow the motion of a weight suspended on the end of a spring as shown in Fig. 13-2-1. This is typical of all machines since they, too, have weight and spring-like properties.

Until a force is applied to the weight to cause it to move, we have no vibration. By applying an upward force, the weight would move upward, compressing the spring. If we released the weight it would drop below its neutral position to some bottom limit of travel where the spring would stop the weight. The weight would then travel upward through the neutral position to the top limit of motion and then back again through the neutral position. This motion will continue in exactly the same manner as long as the force is reapplied. This is vibration.

WHAT CAUSES VIBRATION?

With few exceptions mechanical troubles in a machine cause vibration. To list all the possible troubles in a machine would be impossible, so we have listed only those most common problems which we know produce vibration. They are :

Unbalance of rotating parts.

Misalignment of couplings and bearings.

Ben shafts

Worn, eccentric or damaged gears.

Bad drive belts and drive chains.

Bad bearings – anti-friction type.

Torque variations.

Electromagnetic forces.

Aerodynamic forces.

Hydraulic forces.

SPECIFIC COURSE FOR ENGINEERS MECHANICAL MAINTENANCE MODULE 13VIBRATION ANALYSIS & CORRECTION LESSON 2 PAGE 1

Looseness

Rubbing

Resonance.

All of these causes can be reduced to one or a combination of five types of trouble. Either one or more parts will be unbalanced, mis-aligned, loose, eccentric or out-of tolerance dimensionally, or reacting to some external force.

Regardless of how the causes of vibration are listed, one basic thing must always be true: The cause of vibration must be a force which is changing in either its direction or its amount. It is the force which causes vibration, and the resulting characteristics will be determined by the manner in which the forces are generated. This is why each cause of vibration has its own peculiar characteristics.

Fig. 13-2-1. Vibration of a simple spring-mass system.

THE CHARACTERISTICS OF VIBRATION

A lot can be learned about a machine’s condition and mechanical problems by simply noting its vibration characteristics. What are the characteristics which identify a vibration?

Referring to the weight suspended on a spring, we can study the detailed characteristics of vibration by plotting the movement of the weight against time. This plot is shown in Fig. 13-2-2.

The motion of the weight from its neutral position to the top limit of travel back through the neutral position to the bottom limit of travel and its return to the neutral position represents one cycle of the motion. This one cycle of motion, Fig. 13-2-3, has all the characteristics needed to identify the vibration. Continued motion of the weight will simply be repeating these characteristics.

MECHANICAL MAINTENANCE MODULE 13 SPECIFIC COURSE FOR ENGINEERSLESSON 2 PAGE 2 VIBRATION ANALYSIS & CORRECTION

Fig. 13-2-2. The movement of the weight plotted against time.

VIBRATION FREQUENCY

You will note from the plot in Fig. 13-2-3 that the amount of time required to complete one cycle of the vibration is the “period” of the vibration. If a period of one second is required to complete one cycle of vibration, then during one minute the cycle will be repeated 60 times or, 60 cycles per minute. This measure of the number of cycles for a given interval of time (minute, second, hour, etc.) is the “frequency” of the vibration and is more useful than the period. For vibration work, frequency is usually expressed in terms of cycles per minute, abbreviated CPM.

Fig. 13-2-3. Characteristics of vibration.


VIBRATION DISPLACEMENT

The total distance traveled by the vibration part, from one extreme limit of travel to the other extreme limit of travel is referred to as the “peak-to-peak displacement”. Peak to peak vibration displacement is normally expressed in mils, where 1 mil equals one-thousandth of an inch (0.001 inch). In Metric units, the peak to peak vibration displacement is usually expressed in microns, where 1 micron equals one-millionth of a meter (0.000001 meter) or one-thousandth of a millimeter (0.001 millimeter).

VIBRATION VELOCITY

Since the vibrating weight, Fig. 13-2-1, is moving, it must be moving at some speed. However, the speed of the weight is constantly changing. At the top limit of the motion the speed is zero since the weight must come to a stop before it can go in the opposite direction. The speed or velocity is greatest as the weight passes through the neutral position. The velocity of the motion is definitely a characteristic of the vibration but since it is constantly changing throughout the cycle, the highest or “peak” velocity is selected for measurement. Vibration velocity is normally expressed in terms of inches per second peak. In Metric units, vibration velocity is expressed in millimeters per second peak.

VIBRATION ACCELERATION

Discussing vibration velocity, we pointed out that the velocity of the part approaches zero at the extreme limits of travel. Of course, each time the part comes to a stop at the limit of travel, it must “accelerate” to pick up speed as it travels toward the other extreme limit of travel. Vibration acceleration is another important characteristic of vibration. Technically, acceleration is the rate of change of velocity.

Referring to the motion plot, 13-2-3, the acceleration of the part is maximum (+) at the extreme limit of travel where the velocity is zero, point “A”. As the velocity of the part increases, the acceleration decreases. At point “B”, the neutral position, the velocity is maximum and the acceleration is zero. As the part passes through the neutral point, it must now “decelerate” as it approaches the other extreme limit of travel. At point “C”, acceleration is at peak (-).

Vibration acceleration is normally expressed in “g’s” peak, where one “g” is the acceleration produced by the force of gravity at the surface of the earth. By international agreement, the value of 980.665 cm/sec/sec = 386.087 inches/sec/sec = 32.1739 feet/sec/sec has been chosen as the standard acceleration due to gravity.

PHASE

Another important characteristic of vibration is “phase”. Phase is defined as “…..the position of vibration part at a given instant with reference to a fixed point or another vibrating part.”

In a practical sense, phase measurements offer a convenient way to compare one vibration motion with another; or, to determine how one part is vibrating relative to another part. For example, the two weights in Fig. 13-2-4 are vibrating at the same frequency and displacement; however, weight “A” is at the upper limit of travel at the same instant weight “B” is at the lower limit.


We can use phase to express this comparison. By plotting one complete cycle of motion of these two weights, starting at the same given instant, we see that the points of peak displacement are separated by 180° (one complete cycle = 360°). Therefore, we would say that these two weights are vibration 180° out of phase.

In Fig. 13-2-5, weight “x” is at the upper limit at the same instant weight “Y” is at the neutral position moving towards the lower limit. These two weights are vibration 90° out of phase. In Fig. 13-2-6, weights “C” and “D” are “in-step”. These weights are vibration in-phase.

Phase readings are normally expressed in degrees (0° to 360°) where one complete cycle of vibration equals 360°.

Fig. 13-2-4. Weights Vibration 180° out of phase.


Fig. 13-2-5. Weights vibration 90° out of phase.

SIGNIFICANCE OF VIBRATION CHARACTERISTICS

The real significance of the characteristics of vibration lies in the fact that they are used to detect and describe the unwanted motion of a machine. Each of the motion of a machine. Each of the characteristics of vibration tells us something significant about the vibration. Therefore, the characteristics might be considered to be symptoms used to diagnose inefficient operation r impending trouble in a machine.

IMPORTANCE OF VIBRATION FREQUENCY

When analyzing a machine’s vibration to pinpoint a particular problem, it is essential to know the vibration frequency. Knowing the frequency allows us to identify which part is at fault and what the problem is.

The forces which cause vibration are generated through the rotating motion of the machine’s parts. Therefore, these forces change in amount and direction as the rotating part changes its position with respect to the rest of the machine. As a result, the vibration produced will have a frequency dependent upon the rotating speed of the part which has the trouble. Thus, by knowing the frequency of the vibration, we can identify which part is at fault.

It is also important to recognize that different machinery troubles cause different frequencies of vibration. This makes it possible for us to identify the nature of the problem. For example, unbalance of a rotating part will produce a frequency of vibration equal to the rotating speed (1 X RPM) of the part. On the other hand, mechanical looseness produces a vibration at a frequency equal to twice the rotating speed (2 x RPM). Anti-friction bearings which have flaws on the raceways, balls or rolls will cause a very high frequency of vibration, usually at several times the RPM of the shaft.


Illustrated Fig. 13-2-7 is a defective antifriction bearing detected by the presence of a high frequency vibration. Its only flaw appears on the one ball. Faulty gears, also, will produce high frequency vibrations; usually at a frequency equal to the number of gear teeth times gear RPM.

Fig. 13-2-6. Weights vibrating in phase.

Fig. 13-2-7. The defective ball in this bearing caused high frequency vibration.


More detailed information on the techniques for measuring vibration frequency and identifying the various machinery problems is presented in Chapters III and IV.

IMPORTANCE OF DISPLACEMENT, VELOCITY AND ACCELERATION

The displacement, velocity and acceleration characteristics of vibration are measured to determine the amount or severity of the vibration. The displacement, velocity or acceleration of a vibration is often referre3d to as the “amplitude” of the vibration.

In terms of the operation of a machine, the vibration amplitude is the indicator used to determine how bad or good the operation of the machine may be. The greater the amplitude, the more severe the vibration.

DISPLACEMENT, VELOCITY OR ACCELERATION – WHICH SHOULD WE USE

Since the amplitude of vibration can be measured in terms of displacement, velocity or acceleration, the obvious question is; “which parameter should we use?”

Actually, the displacement, velocity and acceleration of a vibration are directly related. For example, f the peak to peak displacement and frequency of a vibration are known, the peak velocity of the vibration can be found as follows:

Where : Vpeak = Vibration velocity in inches per second peak.D = Peak to peak displacement in mils (1 mil – 0.001”)

F = Frequency in cycles per minute (CPM)

NOTE:

The find velocity in millimeters per second (metric units), D = peak to peak displacement in microns.

Further , the vibration acceleration (g’s peak) can be found as follows:

Where :

g = Acceleration due to gravity

D = Peak to peak displacement in mils (1 mil = 0.001”)


NOTE:

For finding the vibration acceleration where displacement is measured in microns the following formula is used:

The calculations referred to above for finding velocity and acceleration are not required for most vibration work; however, they are presented here simply to illustrate the important relationship between the amplitude parameters – displacement, velocity and acceleration.

Vibration amplitude readings taken for checking overall machinery condition indicate the severity of the vibration. But which is the best indicator of vibration severity: displacement, velocity or acceleration? To answer this question, consider what happens when a wire or piece of sheet metal is bent repeatedly back and forth. Eventually, this repeated bending causes the metal to fail by fatigue in the area of the bend. This is similar in many respects to the way a machine or machine compo9nent fails from the repeated cycles of flexing caused by excessive vibration. Of course, the amount of time required to fail the wire or sheet metal can be reduced by :

1. Increasing the amount of the bend (displacement). The farther the metal is bent each time, the more likely it is to fail.

2. By increasing the rate of bending (frequency). Obviously, the more times per minute the metal is flexed, the quicker it will fail.

Thus, the severity of this bending action is a function of both how far the metal is bend (displacement) and how fast the metal is bent (frequency). Vibration severity, then, appears to be a function of displacement and frequency. However, since vibration velocity is also a function of displacement and frequency it is reasonable to conclude that a measure of vibration velocity is a direct measure of vibration severity. Through experience we have found this to be basically true. Vibration velocity provides the best overall indicator of machinery condition.

Displacement and acceleration, readings are sometimes used to measure vibration severity. However, when displacement or acceleration is used, it is also necessary to know the frequency of the vibration. Charts like those shown in Fig. 13-2-8 & 13-2-9 are often used to cross-reference the displacement or acceleration with frequency to determine the level of severity. Note from Fig. 13-2-8 that a displacement of 1.0 mil occurring at a frequency of 1200 CPM (Cycles per minute) is in the “GOOD” range; however, the same displacement of 1.0 mil at a frequency of 20,000 CPM is in the “VERY ROUGH” range. Note also, that the diagonal lines dividing the zones of severity are constant velocity lines. In other words, a velocity of 0.5 inches per second peak is in the “ROUGH” range regardless of the frequency of the vibration. Referring to the chart, Fig. 13-2-9, you will note that an acceleration of 1.0 g at a frequency of 100,000 CPM is in the “GOOD” region of the chart; however, 1.0 g at a frequency of 18,000 CPM is in the “SLIGHTLY ROUGH” region.


Fig. 13-2-8. This chart can be used to cross-reference displacement with frequency to determine vibration severity.


Fig. 13-2-9. This chart can be used to cross-reference acceleration with frequency to find the zone of severity.


SEVERITY OF COMPLEX MACHINERY VIBRATION

Unfortunately, the vibration of machine is not usually simple; that is, occurring at just one frequency like the weight on the end of a spring. Most machinery vibration is complex, consisting of many frequencies. See Fig. 13-2-10. In general, the overall or total peak-to-peak displacement of the machine will be the sum of all the individual vibrations.

For example:

If the machine in Fig. 13-2-10 has 1 mil vibration occurring at 1 x RPM due to unbalance.

1 mil at 2 x RPM because of looseness.

1 mil at a high gear frequency and 1 mil at a high anti-friction bearing, frequency.

The total peak to peak displacement will probably be around 4 mils.

But, we cannot apply this reading of 4 mils to the Severity Chart, Fig. 13-2-8, because that displacement is not occurring at just one frequency. Where the vibration is complex like this, to apply displacement to the Severity Chart it would be necessary to first determine the individual displacements and their frequencies.

This is done using a vibration analyzer with a tunable filter. The analyzer’s tunable filter works in much the same way as the tuner on a radio in that is allows us to look at one frequency of vibration (radio station) while rejecting all the others.

However, even if we can measure the individual displacements and their corresponding frequencies, it should be obvious that in many cases a single filtered reading alone may not indicate the overall severity of the vibration. In reality, only an unfiltered, overall amplitude measurement will reveal the total condition of a machine. And, since velocity combines the functions of displacement and frequency, only an unfiltered velocity measurement will provide a general indication of overall vibration severity.

A SPECIAL PLACE FOR DISPLACEMENT MEASUREMENT

Although displacement readings are not widely recommended for determining overall machinery condition, there are times when amplitude readings should be taken in displacement. For example, under conditions of dynamic stress, displacement may be a better indicator of severity. Earlier, we discussed the effects of repeated bending related to the failure of a piece of wire or sheet metal, but the wire and sheet metal did property of most rigid machinery components, and that property is brittleness. Brittleness is simply the tendency of a material to break of “snap” when stressed beyond a given limit.

To illustrate the important relationship between displacement and stress, consider a very large, slow rotating machine such as a heavy mine hoist rotating at 50 RPM. Assume for a moment that this hoist is vibration 100 mils peak-to-peak displacement at a frequency of 50 CPM (1 x RPM) due to unbalance. In terms of vibration velocity, 100 mils occurring at 50 CPM is equal to a vibration velocity of only 0.26 inches per second peak. Referring to the Severity Chart.


Fig. 13-2-8, 0.26 inches per second appears in only the “SLIGHTLY ROUGH” region which may give little cause for immediate concern. However, keep in mind that the bearing of this machine is being deflected 100 mils or one-tenth of an inch. Under these conditions, failure may occur due to stress (displacement) rather than fatigue (velocity). In other words, the machine’s structure or bearing pedestal may crack or break simply because it is being bent too far rather than failing from the repeated cycles of flexing.

Because of the importance of displacement measurements at very low frequencies where stress is of major importance, it is suggested that displacement readings be taken on those machines which may be subject to low frequency vibration. A “low” frequency vibration is generally regarded as one below 600 CPM. Of course, an overall velocity measurement should also be taken to determine overall machinery condition at the higher vibration frequencies – above 600 CPM.

A SPECIAL PLACE FOR ACCELRATION MEASUREMENT

Vibration acceleration measurements…………………………………….satisfactory forces being applied to the machine, and relatively large forces can occur at high frequencies even though the displacement and velocity of the vibration may be small. This is clearly indicated by noting that acceleration is a function of the displacement and frequency squared.

For example, consider a machine with a measured vibration of 1.0 mil peak-to-peak displacement at a frequency of 6000 CPM. This corresponds to a velocity reading of 0.3 inches per second peak which may be considered “SLIGHTLY ROUGH” for general machinery. Fig. 13-2-8. This also corresponds to a vibration acceleration of 0.5 g. Next, consider a vibration 0.00001 mil peak-to-peak displacement occurring at a frequency of 600,000 CPM. Although this vibration also corresponds to a velocity reading of 0.3 inches per second peak, it also represents a vibration acceleration of 50 g’s which, according to the chart in Fig. 13-2-9, is “VERY ROUGH”. For the vibration occurring at a frequency of 6000 CPM, failure would most likely result from fatigue (velocity); however, at the higher frequency of 600,000 CPM, failure would most likely result from the excess forces (acceleration) being applied. Excessive force may result in breakdown of the lubrication and ultimate surface failure of bearings.

Generally, vibration acceleration measurements are recommended for vibration frequencies above 60,000 CPM, although experience has shown that velocity measurements can also be used.

IMPORTANCE OF PHASE

Phase, defined earlier, provides a convenient way to compare one motion with another. Comparing the relative motion of two or more parts of a machine or diagnosing specific machinery defects. For example, if analysis reveals that the vibration of a machine is “out-of-phase” with base or foundation vibration, we may want to look for loose mounting bolts, improper grouting or other signs of looseness between the machine and its base.

Phase measurements are also important for balancing. When the machinery problem is unbalance, being able to measure phase allows us to balance the part quickly and easily without trial and error techniques. Usually, parts can be balanced in-place eliminating the need for costly disassembly.


HOW MUCH VIBRATION IS TOO MUCH

Since vibration amplitude (displacement, velocity or acceleration) is a measure of the severity of the trouble in a machine, your next question may be; “how much vibration is too much?” To answer this question, it is important to keep in mind that our objective should be to use vibration checks to detect trouble in its early stages for scheduled correction. The goal is not to find out how much vibration a machine will stand before failure, but to get a fair warning of impending trouble so it can be eliminated before failure.

Absolute vibration tolerances or limits for any given machine are not possible. That is, it is impossible to select a vibration limit which, if exceeded, will result in immediate machinery failure. The development of mechanical failure is just far too complex for such limits to exist. However, it would be impossible to effectively utilize vibration as an indicator of machinery condition unless some questionnaires are available, and the years of experience of those familiar with machinery and machinery vibration has provided some realistic guidelines.

Table 13-2-1. Vibration Velocity (Inches/Second Peak)


Fig. 13-2-10. Most machinery vibration is complex, consisting of many frequencies.

You will recall that vibration velocity provides a direct measure of machinery condition for the intermediate vibration frequencies 600 to 60,000 CPM. The velocity values in Table 13-2-1 are offered as a guide for overall (unfiltered) velocity readings. When vibration amplitude is measured in displacement or acceleration, the charts in Figs. 13-2-8 & 13-2-9 apply to machinery such as motors, fans blowers, pumps and general rotating machinery where vibration does not directly influence the quality of a finished product. Amplitude readings should be those taken on the bearings or structure of the machine.

Of course, the vibration tolerances suggested in these references will not be applicable to all machines. For example, so machines such as hammer mills or rock and coal crushers will inherently have high levels of vibration. Therefore, the values selected using these guides should be used only so long as experience, maintenance records and history proves them to be valid.

For machines such as grinders and other precision machine tools where vibration can affect the quality of a finished product, the “Guide To Vibration Tolerance For Machine tools,”, Fig. 13-2-11, may be used. Applying vibration tolerances to machine tools is rather easy because they can be based on the machine’s ability to produce a certain size or finish tolerance. The values shown in Fig. 13-2-11 are the result of years of experience with vibration analysis of machine tools, and represent the vibration levels for which satisfactory parts have been produced. Of course, these values may vary depending on specific size and finish tolerances required. A comparison of the normal pattern of vibration on the machine and the quality of finish and size control required will reveal what level of vibration is acceptable.


The first time the quality of finish or size control deteriorates, an unacceptable vibration level would be indicated. The initial values selected from Fig. 13-2-11, can then be modified to the new, more realistic ones.

Making the decision to correct a condition of vibration is often a very difficult one indeed, especially when it involves downtime of critical machinery. Therefore, when establishing acceptable levels of machinery vibration, experience and factors such as safety, labor costs, downtime costs and the importance of a machine’s operation to your company’s profits must be considered.

Fig. 13-2-11. Tentative guide to vibration tolerances for machine tools.

WHAT IS NOISE?

Psychologically, noise can be defined as simply “unwanted sound”. This includes anything from the drip of a faucet in the middle of the night to the mighty roar of a rocket as it leaves the launch pad. It can even include music if it disturbs the listener. To be considered “noise” all that is required is that it be sound that is undesirable to the listener.

Technically, sound (or noise) is a pressure oscillation in the air which radiates away from the source. To visualize the generation of sound is slow motion, consider that happens to the air surrounding our familiar vibration weight, Fig. 13-2-12. As the weight moves downward, the air molecules ahead of the weight are being pushed together or compressed. The air molecules adjacent to the weight push against air molecules and so on in succession.


In this manner, the “zone of compression” radiates away from the source in much the same way that motion is transmitted when we line up a row of dominos and then strike the first one. See 13-2-12. Each succeeding domino (or air molecule) transmits the motion down the line. Although the individual air molecules move only very slightly, this motion can be transmitted over great distances.

At the same time the weight, Fig. 13-2-11, moves downward compressing the air ahead of it, avoid or partial vacuum is being created behind the weight. This region of partial vacuum is referred to as the zone of “rarefaction”. This region of rarefaction radiates away from the source by successively drawing in air molecules from the adjacent area in attempt to equalize the void created.

Thus, the simple downward motion of the weight created a zone of compression and a zone of rarefaction radiating away from the source. As the weight moves upward, a zone of compression is created above the weight while a zone of rarefaction is created below the weight. Continued vibration of the weight will produce corresponding zones of compression and rarefaction as illustrated in Fig. 13-2-13.

Fig. 13-2-12. Downward motion of the weight creates a zone of compression ahead of the weight.


Fig. 13-2-13. A row of dominos illustrates how sound is transmitted. Each falling domino (oscillating air molecule transmits the motion down the line.)

WHAT CAUSES NOISE?

Noise or sound waves can be generated in three ways. The most common of these is the vibration of solid structures such as machines, or wall panels which alternately compress and rarefy the air in contact with these structures. Fig. 13-2-14. Therefore, the same causes of machinery vibration (listed in this Chapter under “What causes Vibration?) are also source of noise.

Noise can also be generated by the movement of air over solid structures. A typical example is shown in Fig. 13-2-15. Even though the structure itself does not vibrates, the moving air can still create loud sounds. A well known example of this noise generator is the pipe organ. The flow of air over fan blades or through ventilation grills are other examples.

The third mechanism of noise generation comes from the turbulent moving of fast moving air with relatively slow moving air in which no solid structures are involved. An example of this which is commonly heard is the noise from a jet engine, Fig. …. This type of noise only becomes important when the air (or other gas) is moving at very high velocity.

All three of these sound generating mechanisms are commonly found in many types of industrial machinery; and by recognizing them, it is often possible to reduce them to acceptable levels.

THE CHARACTERISTICS OF NOISE

Like vibration noise has a number of characteristics which are needed to define or describe it. By examining the pressure oscillations created by our vibration weight in Fig. 13-2-13, we can better understand some of the characteristics of noise.

PROPGATION VELOCITY

It was mentioned earlier that the zones of compression and rarefaction radiate outward or away from the noise source. The speed or velocity that sound waves radiate is called the propagation velocity © or simply the speed of sound. The speed of sound in air at standard temperature and pressure is a constant of approximately 1130 feet per second (346 meters per second).


Do not confuse the velocity of sound with the velocity of vibration. Vibration velocity is a measure of the amplitude of vibration, whereas the velocity of sound is a constant which is essentially independent of sound amplitude.

FREQUENCY

Another characteristic of sound is frequency (f). It can be loosely defined as the number of sound waves or regions of compression which move past a fixed point during a specified period of time such as a minute or second. For example, if 100 regions of compressed air molecules move past a fixed point (such as a microphone) in one minute, we will be observing a sound with a frequency of 100 cycles per minute.

Sound frequency measurements taken for the purpose of relating sound to human hearing are generally expressed in cycles per second (CPS). However, within the last several years, to achieve standardization on an international basis, it has been agree to use the term “Hertz” (abbreviated Hz) in honor of the German physicist, rather than cycles per second.

Sound frequencies measured for relating to machinery problems are normally expressed in cycles per minute (CPM) for direct correlation with rotating speeds and multiples of rotating speeds which are expressed in RPM. Sound frequencies which are expressed in cycles per minute can be easily converted to Hz (cycles per second) by simply dividing CPM by 60 (Hz = CPM/60).

With regard to human hearing, sound frequencies are generally broken down into three categories:

1. Infrasonic – sounds which are at frequencies below the range of human hearing (i.e., less than 15 Hz)

2. Audio-Sonic – sounds which are at frequencies within the range of human hearing (i.e. 15 Hz to 20,000 Hz), and

3. Ultrasonic – sounds which are at frequencies above the range of human hearing (i.e., greater than 20,000 Hz).

With industrial noise we are almost always concerned only with audio-sonic noise because this is the noise that has the greatest effect on personnel.

To get a better feel for the various noise frequencies, refer to the piano keyboard, Fig. 13-2-……Note that the lowest frequency note on the piano is at approximately 27 Hz and the highest frequency note 4186 Hz.

WAVELENGTH

Referring again to fig. 13-2-14, you will note that the zones of compression have a definite spacing. This distance between regions of compression is called wavelength. The Greek symbol Lambda () is generally used to represent wavelength.

There is a unique relationship which exists between the frequency (f), propagation velocity © and wavelength () characteristics of sound.


This relationship can be expressed by the simple equation:

This expression says that sound frequency is inversely proportional to wavelength. Or, very high frequencies sounds have short wave length whereas low frequency sounds have relatively long wavelengths.

Fig. 13-2-14. Continued oscillation of the weight generates corresponding zones of compression and rarefaction radiating away from the source.

Fig. 13-2-15. The vibration of solid structures is the most common source of sound.


Fig. 13-2-16. Sound may be generated by the movement of air over a solid structure.

Fig. 13-2-17. The turbulent mixing of fast moving air with slow moving air is another source of sound.

Fig. 13-2-18. The lowest frequency note on the piano is approximately 27 Hz: the highest frequency note is 4186 Hz.


IMPORTANCE OF NOISE FREQUENCY AND WAVELENGTH

Knowing the frequency of noise (and indirectly the wavelength) is important for a number of reasons. First, noise frequency is the key to identifying a noise source, just like vibration frequency helps us identify the vibration problem in a machine. The most common source of noise is the vibration of solid structures such as machines or wall panels. The rate or frequency at which these structures vibrate determines the frequency of the noise which is radiated. Thus, a 100 Hz (6000 CPM) sound wave.

Secondly, the disturbing frequencies of noise being generated also determine, in many cases, which method or methods must be used to control the noise. For example, although noise may be generated by machinery vibration, it may be that the level of vibration is relatively low, indicating that no significant mechanical problems exist. In such a case, the noise being generated may be classified as inherent to machinery operation. If this is the case, other control measures may be required to reduce the noise level. These other measures may include noise barriers or acoustic enclosures which restrict or actually absorb the sound. The types and configuration of materials used will be determined to some extent by the noise frequencies (wavelengths) to be controlled.

Finally, wavelength is an important factor to consider when positioning the microphone for measurement. This consideration is discussed in more detail in this Chapter under Sound Fields, and again in Chapter III under Positioning the Microphone.

NOISE AMPLITUDE

Sound was defined earlier as a pressure oscillation in the air. Actually, we live in a sea of air where the pressure all around us due to the atmosphere is approximately 14.7 pounds per square inch (1030 grams per square centimeter). A measure of noise amplitude, then, is a measure of how far the air pressure (compression0 and then sinks below atmospheric pressure (rarefaction). The maximum amount by which the pressure differs from atmospheric pressure is called the pressure amplitude of sound.

Sound pressure amplitudes are usually expressed in “microbars”, where one microbar equals one millionth of a bar; and one bar represents one atmosphere. In some instances sound pressure is expressed in dynes per square centimeter, where one dyne per square centimeter equals one microbar.

Compared to atmospheric pressure, the sound pressure amplitudes which we hear are rather small. For example, the faintest sound we can hear is approximately 0.0002 microbars (0.002 dynes per square centimeter) or, two ten-thousandths of a millionth of one atmosphere.

This sound pressure is referred to as the acute threshold of hearing. On the other hand, the loudest sounds that the human ear can tolerate are approximately 2000 microbars. This is called the threshold of pain.


THE DECIBEL (dB)

As you can see, the ear responds to a rather tremendous dynamic range of sound pressure amplitudes – from 0.0002 to 2000 microbars or a ratio of 10 million to one. To measure sound amplitude using this extremely long linear range would be very inconvenient. Therefore, to simplify our measurements, a logarithmic decibel scale is used instead.

By international agreement, sound pressure level (SPL) in decibels is defined as follows:

Where the standard reference pressure is the acute threshold of hearing of 0.0002 microbars (or 0.00032 dynes/cm²); and the measured sound pressure is the sound amplitude of interest. With this equation it is possible to calculate SPL if the measured sound pressure in microbars or dynes/cm² is known, or we can calculate the sound pressure in microbars or dynes/cm² if the SPL is known. However, such exercises are not normally necessary since noise measurement and engineering evaluations are almost always done in terms of decibels.

From the equation above, the acute threshold of hearing becomes 0 dB, and the threshold of pain 140 dB. Without a doubt, a scale from 0 to 140 is much more convenient to understand and work with than a scale from 0.0002 to 2000. In addition, the logarithmic dB scale better represents the response of the human ear to changes in sound amplitude. For example, it is unlikely that we would be able to detect a change in sound amplitude from, say, 1000 microbars to 1010 microbars. However, under controlled conditions we probably could detect a 1 dB change. The ear generally considers a 3 dB increase as a “noticeable” difference in loudness; 6 dB a significant increase; 10 dB is twice as loud; and 20 dB is very much louder.

Decibels are as easy to measure and use as mils, inches, microns or millimeters once you get a feel for them. To assist you in becoming familiar with the decibel (dB) unit of sound amplitude the charts in Fig. 13-2-19 and 13-2-20 provide examples of noise levels typical of common-place noise sources. Fig. 13-2-19 shows representative decibel levels encountered by people during the course of a day from common noise sources other than industrial. Fig. 13-2-20 provides examples of noise levels from industrial sources.

Of course, the values shown in Figs. 13-2-19 & 13-2-20 are only presented as typical noise levels. Each of these sources can vary over a wide range depending upon a number of factors. For instance, a large diesel truck may have a typical level of 90 dB when measured at a distance of 20 feet when traveling 50 miles per hour on a flat, straight road. If the truck is going up hill, however, the additional power for the engine may cause the noise to increase to 95 dB. If, in addition, the engine muffler is defective, the noise may increase to 100 dB.

Assuming that the truck noise is made in an open area with no nearby buildings or large structures to reflect the sound, and the position of the microphone is 40 feet, rather than 20 feet from the truck, the measured level will be about 6 dB lower. That is, it will be 84 dB instead of 90 dB.


This reduction is caused by the fact that sound waves tend to spread out uniformly in all directions (unless a large surface such as a high wall causes the sound to be reflected) and their amplitude decreases as they get further from the source. Under these conditions, the sound pressure level will decrease 6 dB each time the distance from the source is doubled. Thus, the truck noise measured as 90 dB at 20 feet will have the following approximate levels at other distances:

40 feet 84 dB80 feet 78 dB160 feet 72 dB320 feet 66 dB

If the sound measurement is made very close to the source (i.e., at distances closer than the width or height of the source, and particularly when closer than 3 feet) large variations in the measured sound pressure level are likely to be noted with small changes in microphone position. In such cases, the 6 dB reduction will not apply. This is caused by interfering sound waves coming from different parts of the machine. This characteristic of sound is discussed in greater detail in the following paragraphs on SOUND FIELDS.

Fig. 13-2-19. Common non-industrial noise levels.


Fig. 13-2-20. Typical noise levels found in industry.

SOUND FIELDS

Before taking noise measurements, it is important to realize that there are three types of sound “fields”. These fields have a major effect on the sound pressure levels measured; therefore, understanding what these fields are will greatly simplify data measurement and interpretation.

A simple sound source such as a vibrating sphere which expands and contracts uniformly sends out sound waves which radiate in all directions in much the same way water waves radiate when a stone is thrown into a quiet pool of water. See Fig. 13-2-21.

When the source of sound becomes more complex (for example: two vibrating spheres), the sound waves themselves become more complex. As shown in fig. 13-2-22, they begin to form interference patterns; particularly at points close to the source. At these interference points the measured sound pressure levels can be much greater, or much less, than at points away from them.


This region around the sources where the interference patterns are most pronounced is called NEAR FIELD and can extend out to a distance of 4 times the largest dimension of the vibrating source. However, in many cases these effects become insignificant at distances considerably closer to the source.

Another characteristic of a NEAR FIELD is the movement of part of the sound energy laterally along the vibrating surface as well as radially away from it. This is illustrated in Fig. 13-2-23. The part of the energy which moves laterally does not radiate away, but merely sloshes back and forth along the surface. This effect may extend outward from the source a distance of about one acoustic wavelength ().

Beyond the near field is a second region called the FAR FIELD. In this region the sound waves are reasonably free of interference and reactive sound energy. Thus, the measured sound pressure levels will not vary considerably with small changes in microphone position but will decline steadily as the microphone is moved further away from the source.

If the sound waves encounter a hard surface which causes them to reflect, a third field called a REVERBERANT FIELD is created. This reverberant field is most pronounced in the region close to the reflecting surface, and generates interference patterns similar to the near field. See Fig. 13-2-24.

It is desirable (when possible) to take noise measurements in the FAR FIELD. In the far field, the measurements will be more consistent and positioning the microphone will be less critical. It is important that microphone position not be critical when repetitive measurements are to be made as part of a machinery noise survey program.

The dimensions of the regions in which these three different fields exist are, of course, dependent on the size and configuration of the sound source, the location of reflecting surfaces, and the frequencies of noise which the source generates. As a rule of thumb, the higher the frequency of noise, the smaller the near field region will be. In addition, the closer the reflecting surface is to the sound source, and the larger its size, the greater is the reverberant field.

Because of all these variables involved, it is not possible to give specific dimensions which can be measured off to determine the location of each field. It is possible, however to obtain a general feel for these fields experimentally. These experimental techniques are discussed in Chapter III of this text under Positioning The Microphone.


Fig. 13-2-21. A stone thrown into a quiet pond illustrates the propagation of sound.

Fig. 13-2-22. A NEAR FIELD sound region may be complex patterns resulting with interference from multiple sound sources.


Fig. 13-2-23. In the NEAR FIELD, some of the sound energy may move laterally back and forth along the vibration surface.

Fig. 13-2-24. Three types of acoustic fields.


DIRECTIONAL NOISE SOURCES

There is one additional characteristic of noise fields worth mentioning because knowledge of it can aid in noise measurement and data interpretation. This is sound field directionality. What this means is that not all noise sources radiate the same level of noise in all directions. If they radiate more noise in one direction than another, they are said to be directional noise sources. As a matter of fact, most machines are directional noise sources. Fig. 13-2-25 illustrates this concept. From a measurement standpoint, this indicates that more than one measurement is needed in order to obtain an idea of the sound being radiated by the machine. Suggested noise measurement procedures are covered in Chapter III.

Fig. 13-2-25. A directional noise source, such as an automobile, radiates more noise in one direction than another.

WEIGHTING NETWORKS

One other important concept which deserves comment, is the human response to noise. Many times noise measurements are taken to determine the effect on human hearing. However, it is important to recognize that the normal human ear does not hear all sounds equally well. The human perception of sound is dependent on the amplitude and frequency of the noise.

In general we hear sound best at approximately 4,000 Hz, but rather poorly at very low and very high frequencies. This is illustrated 13-2-26 which displays the “Equal Loudness Contours”. These contours simply illustrate the way the human ear responds to various amplitudes and frequencies of sound. For example, a sound at a frequency of 40 Hz would have to be 60 dB to be equally loud as a 20 dB sound at 1,000 Hz. Note that as noise amplitude increases to higher levels, the ear beings to hear all frequencies of sound about equally well.


Since sound is often measured in conjunction with a problem related to human hearing, it has been found desirable in many cases to use instrumentation which measures sound in a manner similar to the human ear. However, as shown in the Equal Loudness Contours, the human ear does not hear equally well at all frequencies and amplitudes.

Sound measuring instruments approximate the response of the human ear by means of electronic weighting networks which serve to shape the instrument’s response in a manner similar to that of the human ear. The weighting networks most commonly incorporated in noise measuring instruments are the “A” , “B” and “C” weighting networks.

The characteristic response of these three weighting networks can be noted from Fig. 13-2-27. The “A” weighting best approximates the human ear at low noise levels (that is, below 55 dB); the “B” weighting at medium noise levels (55 to 85 dB) and the “C” weighting above 85 dB.

Generally, noise level readings taken to establish compliance with noise control legislation are taken using the “A” weighting network regardless of the sound level. This has the advantage of simplifying measurements, and it has also been found that hearing damage correlates very well with “A” weighting measurements.

The “C” weighting is used where there is interest in relatively low frequency sound and for noise analysis where a relatively flat response is desirable. The “B” weighting is not used to any great extent except in specialized studies related to hearing.

Fig. 13-2-26. Equivalent sound level contours illustrate the interrelationship of sound level (amplitude) and frequency to the response of the human listener.


Fig. 13-2-27. “A” “B” and “C” weighting networks are used in sound measurement instruments which measure sound in a manner similar to the

human ear.

HOW MUCH NOISE IS TOO MUCH?

Today there are many federal state, provincial and local regulations in force to protect workers from hearing loss resulting from prolonged exposure to excessive levels of noise. And, until recently, industry’s primary concern with noise has been hearing conservation. However, there are other reasons to be concerned with machinery noise. First, it is well recognized that certain work areas such as offices, conference rooms or libraries require low levels of noise – well below the noise levels considered dangerous.

Secondly, noise has become recognized as another parameter of machinery operation which can be measured to detect impending mechanical troubles in machinery.

NOISE LEVELS ESTABLISHED BY LEGISLATION

Where persons are exposed to certain high noise levels for extended periods of time, it has been found that permanent hearing loss can occur. Within the past several years sufficient knowledge has been accumulated to define acceptable limits on noise levels, frequencies, and duration of exposure with the intent of protecting a majority of persons from permanent hearing loss.


A typical set of limits is shown in 13-2-28. According to this chart, an employee is permitted to work in a noise environment of 90 dBA up to 8 hours per day. When the noise exceeds this level, the duration of exposure is reduced, until at 115 dBA, only 15 minutes exposure is permitted. Where an employee works in different noise level areas at different times during a day, the allowable duration must be calculated as shown.

Fig. 13-2-28. Typical noise exposure limits.


ACCEPTABLE NOISE LEVELS FOR DIFFERENT WORK AREAS

Numerous experimental studies have shown that workers efficiency can be affected to varying degrees by noise. Tasks requiring a great deal of mental concentration and exactness are affected by noise to a significantly greater degree than others. For instance, even relatively moderate noises tend to dilate the pupils of the eyes, which results in a change in the eye’s focus. Thus, a watchmaker, and others who do close work, will experience difficulty in working where periodic loud noise is present.

In many cases the ability to verbally communicate is the governing criterion for acceptable noise levels. Here again, there is some variability; levels required for a conference room will differ from those required in a machine shop, or in the engine room of ship.

Where high noise levels occur in areas where work is less exacting and more routine, acceptable levels are governed by noise which can cause personnel fatigue and inefficiency. Fig. 13-2-29 indicates some general guidelines on levels of acceptability for noise in different areas and conditions.

Fig. 13-2-29. General guidelines for acceptable noise levels.


NOISE LEVELS THAT SIGNAL MACHINERY TROUBLE

To date, there are few references available that provide information on inherent or “normal” noise levels for specific types of machines such as compressors, fans, blowers, diesel engines, etc. Although there are many references available on typical noise levels for various types of machines, there are so many variables involved that make the establishment of specific noise levels. A machine located in a small steel building may have a much higher noise level than it would if located in a large open area. Similarly, a machine locate din close proximity to other large machines may have a higher level of noise. The reason, of course, is the fact that steel walls, large nearby machines and other reflective surfaces tend to create a reverberant field which has an additive effect on the noise level.

In addition, the machinery foundation or supporting structure may influence the measured noise levels. Machines transmitting their vibration into surrounding structures create higher noise levels because of the additional noise radiated by the vibration of walls, floor, ceiling windows, etc. An identical machine mounted on resilient isolators such as coil springs, rubber, cork, etc., will most likely show a lower level of noise due to the reduction of structure borne vibration.

A change in operating condition will often produce a corresponding change in machinery noise. For instance, a fan operating at 900 RPM may have a noise level completely different when operating at 1200 RPM. In addition, machinery noise levels often change with a change in load.

The significant effects of reverberation, structure borne vibration load and speed on a machine’s measured noise level make the establishment of standard noise levels for specific types of machinery practically impossible. For this reason, noise level measurements taken to detect impending mechanical problems must be based on detecting an appreciable increase in the normal noise level established for each individual machine. With this in mind, it is important to point out that an appreciable increase in noise may be an increase of only 2,3,4 or more dB. Noise levels in dB do not add or accumulate the way vibration amplitudes do. For example, an increase of only 6 dB is actually a doubling of the sound pressure. Therefore, when taking your periodic noise level measurements for preventive maintenance, keep in mind that an increase in the measured noise level from, say 85 dB to 89 dB may be a signal of developing mechanical trouble.


vibration analysis3

Documents