Table Of Contenttechnical
monograph
43
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Floyd D. Jury
Technical Consultant
Fisher Controls International, Inc.
This paper provides a basic understanding of the mecha-
nisms by which aerodynamic noise is generated in a control
valve. A fundamental approach is taken to the description of
these noise mechanisms and the terminology associated with
noise. This terminology is consistent with that used in the
IEC valve noise prediction standard.
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might state that there is noise coming from the valve,
THE CONTROL VALVE AS A NOISE SOURCE
yet there is no observer inside the valve or the flowing
fluid to hear the noise. On the other hand, it is illogical
Control valves are a major source of noise in any
to believe that there is no noise inside the valve or
industrial environment. A well designed control valve
flowing fluid, but somehow noise magically appears
can reduce its noise to an acceptable level, but in
outside the pipeline. Therefore, this manuscript will
order to ensure that an appropriate valve selection is
take a much more pragmatic approach to this issue
made for any given application, one must understand
and talk about noise inside the valve, the fluid, the
how and why control valves produce and control noise,
pipeline, etc..
and how the amount of noise produced by a control
valve can be predicted before purchasing the valve
The amount of energy contained in the pressure wave
and placing it in service.
(i.e., its sound power) also affects the ability of the ear
to register the pressure wave as sound. There are
INTRODUCTION TO NOISE
some pressure waves whose energy levels are low
enough that it does not register as sound in the human
Sound is the physiological phenomenon which occurs
ear. In other words, the pressure disturbance is below
when fluctuations in air pressure register against our
the human threshold of sound, or in common parlance,
eardrums. Our brain then interprets these fluctuations
it is not loud enough for us to hear. In other words,
as sound. Not all air pressure fluctuations, however,
how loud the noise sounds is a measure of the power
result in sound being interpreted by the brain. Only
of the sound wave.
those pressure fluctuations in the general frequency
range of 20 Hz to 20,000 Hz, for individuals with good
Although the term “loudness” is one that would feel
hearing, are detected and interpreted by the brain as
very comfortable to most of us, it is a term which will
sound.
not be used in this manuscript. “Loudness” has a
technical definition which is used in the field of archi-
Air pressure fluctuations which are outside the range
tecture, but has no significance within the scope of this
of human hearing (audible range) simply do not
discussion. We may use relative terms indicating that
register in the brain as sound. For example, normal
one noise is more (or less) loud than another, but we
changes in atmospheric pressure occur so gradually
will not attempt to quantify “loudness” as such.
that they are a much lower frequency than the audible
range and therefore cannot be heard. For this, we can
The greater the power of the pressure wave distur-
be truly thankful. Likewise, there are some sounds,
bance, the louder the sound; however, this is not a
such as a dog whistle, which are sufficiently high
linear relationship. We cannot double the power of the
enough in frequency that they are above the range of
sound wave and produce a sound which is twice as
human hearing. Most dogs, however, can hear
loud. In order to quantify how loud one sound is
frequencies well above the audible range of humans.
compared to another, we will use either the Sound
Power Level or Sound Pressure Level of the noise
Since sound is defined as air pressure fluctuations
disturbance. Both of these sound measurement
against the eardrum, the implication seems to be that
quantities follow a logarithmic relationship which we
if there is no ear, there is no sound. Every beginning
will define shortly. We measure both of these quanti-
physics student sooner or later hears the question, “If
ties in units called “decibels” or “dB.” Measuring things
a tree falls in the forest, and there is no one there to
in units of decibels (dB) is outside our range of normal
hear it; is there sound?” This type of philosophical
experience and may seem complex, or even magical,
question may be fun to debate at cocktail parties, but it
at first, but it is really quite straightforward once one
sheds little useful light. For example, imagine a control
learns the basic principles involved. Dealing with
valve in a pipeline which is passing a large amount of
decibels is so fundamental to the field of valve noise it
turbulent flow. The turbulent fluid downstream of the
is essential that we have a good grasp of this type of
valve produces vibrations in the pipe wall which in turn
measurement. If you feel uncomfortable about your
disturb the surrounding air causing air pressure
understanding of decibels as a form of noise measure-
fluctuations to eventually impinge upon the ear drum of
ment, a review of Fisher Control’s Technical Mono-
a person standing nearby the pipeline. The observer
2
graph, TM-42, “Understanding Decibels, (dB or not may be noise to some individuals, but it is sweet music
dB)” will allow you to talk about decibels like an expert. to the ears of a pilot flying a single-engine airplane
across a large body of water. Since the primary focus
The human ear does not possess the same sensitivity of this manuscript is the unwanted sound produced by
to noise at every frequency in the audible range. control valves, nearly all future references will be to
Sound at some frequencies tend to seem louder than noise.
sounds at other frequencies, even though the power
level of the sound waves are the same. For example, Sound waves cannot radiate in a vacuum. They must
sounds at the high frequency end of the audible range have some material medium to propagate the wave. In
typically do not seem as loud as sounds of the same order for sound to get from its source of origin to the
power level at middle range frequencies. Research human ear, it must eventually pass through the air as a
has shown that the human ear is most sensitive to pressure wave which registers against the human
sounds at a frequency of 1000 Hz. eardrum. Before it can get to the ear, however, it may
have to pass through one or more additional material
In order to have an equitable way to compare the effect media. Any reduction in the eventual sound pressure
of two sounds on the human ear, scientists have fluctuations at the ear which is caused by the noise
developed a weighting scale (called “A-weighting”) passage through the different media, or the boundaries
which adjusts the dB sound level at other frequencies between these media is called “transmission loss.”
to a level which would be the perceived equivalent to a
sound at 1000 Hz. To determine the amount of If we think of the valve as a point source of noise
adjustment, the power level of a sound at 1000 Hz is sending out spherical sound waves in all directions, we
adjusted until it sounds equally as loud as the test can recognize that only the sound radiated in a direc-
noise. The test noise is then assigned the value of the tion which can arrive at the observers ears is effective
sound power level of the 1000 Hz sound, regardless of at producing perceived noise. Noise generated in the
the actual power level of the test noise. These units of fluid at the valve must then be transmitted through the
adjusted power level are then listed as dB(A) or dBA pipe due to vibration induced in the pipe wall. Due to
and are typically used when dealing with government reflections of the sound waves from the inner surface
regulations which limit the amount of sound to which of the pipe, only the portion of the energy in the sound
human beings may be exposed. In other words, two wave which produces a radial component against the
sounds which appear to be equally loud would have pipe wall will be effectively transmitted through the
the same dB(A) number regardless of their frequency pipe. The rest will be called “transmission loss.”
or actual power level. Finally, as the pipe wall vibration induces pressure
fluctuations in the air surrounding the pipe, these
Naturally, we would expect that some of the things we sound waves must radiate outward to the observer.
have discussed so far, such as the A-weighting, The type of waves generated would be classified
frequency sensitivity of the human ear, threshold of primarily as a “line” source which would radiate sound
hearing, and the audible range of sound, etc. will tend waves radially outward from the surface of the pipe.
to be unique to a specific human being at a specific Additional transmission losses occur through the air as
point in time. Since it is not possible to deal with noise these waves expand and spread the noise energy over
prediction on this individualistic basis, scientists have an ever-increasing surface area of the sound wave.
developed a statistical representation of an average, Thus, additional transmission losses will occur which
27 year old, American male. So when anyone refers to are dependent upon the observer’s distance from the
how noise affects the human ear, they are referencing pipe.
this statistically derived model.
NOISE LEVEL
Noise is simply unwanted sound. As you might
suspect, there are many psychological, cultural, and When someone asks us how loud a given noise is, the
contextual factors which determine whether a particu- first response should be, “As compared to what?”
lar series of air pressure waves are called sound or Typically, when we measure anything, we usually
noise. Even the same sound experienced by the same measure it with respect to some reference level. For
individual may undergo different classification depend- example, when we talk about a given pressure in a
ing upon the context in which the sound is heard. For vessel, we are actually talking about the pressure level
example, the sound emanating from an aircraft engine above or below atmospheric pressure (i.e., the refer-
3
ence level). Likewise, with noise or sound we also as the name of the parameter being calculated. This is
have a reference level. In this manuscript, we will use similar to saying that m2 = (length)x(width) as the area
the letter (L) to represent noise “level.” (A) of a rectangle. We need to recognize that this
incorrect practice exists, but we should refrain from
The level of the noise is, of course, a function of the perpetuating it.
power generated by the noise producing disturbance.
We call this the “acoustic power” or “sound power” and COMPARING NOISE SOURCES
we designate it as (W ). The standard noise reference
a
level is the lowest sound power which can just barely We don’t always want to compare a particular noise
be detected by the average person with average source with the reference threshold of hearing. Some-
hearing. This statistically derived threshold of hearing times we may want to compare one noise source (W )
1
is called the “reference sound power” and is desig- to a second noise source (W ). We can use our noise
2
nated as (W ). Both W and W are typically measured level definition here as well; i.e., the following equation
o o a
in units of Watts of power. The statistically derived can tell us how the noise power level of source (W )
2
value for the reference sound power is W = 10-12 compares to the noise power level of source (W ).
o 1
Watts.
∆L = 10 Log (W /W ) (Units are “dB”) (4)
w 10 2 1
As we increase the acoustic power we increase how
loud the noise sounds to us, but not in a linear fashion. If the answer is positive, it means that W is so many
2
For example, if we were to double the sound power, dB louder than W , and if the answer is negative, it
1
we would not get a noise that sounds twice as loud. means that W is not as loud as W .
2 1
Due to this nonlinear nature, as well as due to the wide
range of sounds to which the human ear can respond, There is another comparison which is often of special
it is convenient to use logarithms when dealing with interest to us. Let’s look at what would happen if we
noise measurements. Likewise, it is typical to quantify were to actually double the power produced by the
how loud a noise is compared to a reference level noise source; i.e.,
which is chosen as the threshold of hearing. Thus, the
Sound Power Level is defined as: ∆L = 10 Log (W /W ) = 10 Log (2W /W )
w 10 2 1 10 1 1
= 10 Log (2) = 10(0.301) = +3.01 ≅ +3 dB (5)
10
L = Log (W /W ) (Units are “Bels”) (1)
w 10 a o
The usual custom, as inferred above, is to round the
The units of measurement for this noise level are 3.01 value to an even 3. In almost all noise control
“Bels;” named, of course, after Alexander Graham problems it makes little sense to deal with small
Bell, the grandfather of acoustic science. As it turns fractions of decibels. The precision of 0.1 dB is rarely
out, however, the units of Bels are normally too large required and noise levels are nearly always best stated
for practical usage. Consequently, acoustic research- only to the nearest decibel.
ers quickly began using a smaller unit of noise mea-
surement called a “decibel (dB)” which equals one- The above result means that if we started out with a
tenth of a Bel. We can easily convert from Bels to dB noise level of 80 dB and we doubled the sound power,
as illustrated in the example below. Imagine a noise the noise level would increase only to 83 dB. By the
level such that same token, if we were to cut the sound power in half,
the noise level would decrease only to 77 dB.
L = 8 Bels = (8 Bels)(10 dB/Bel) = 80 dB (2)
w
COMBINING NOISE SOURCES
It is easy to conclude from this example, that if we
wished to rewrite the equation (1) in terms of dB In a typical industrial environment, a control valve is
(decibel) units, it would become seldom the only noise source present in a particular
location; i.e., there are usually motors, compressors,
L = 10 Log (W /W ) (Units are “dB”) (3) turbines, other valves, other machines, etc. which are
w 10 a o
contributing to the general noise level. Although
You will often see this equation incorrectly written as control valves can generate a significant amount of
dB = 10 Log (W /W ) which can be somewhat confus- noise, they often are not the major noise source in a
10 a o
ing since it is actually using the units of measurement given area. The noise level in a particular area is the
4
result of combining the noise generated by each Table 1 below provides a practical aid in helping to
source in the vicinity. When doing this, however, it is combine two different noise sources or to aid in
important to recognize that dB’s don’t add; the determining how much improvement would be gained
powers add. by quieting one of several different noise sources.
Imagine that you are standing halfway between two Table 1: Combining Two Point Noise Sources
identical valves installed in two different pipelines.
Consider further that you measure a noise level of 60 dB Difference dB Difference Between
dB from each valve, when it is operating by itself. Now, Between Two Sources Total Noise and Louder Source
you want to know what the noise level would be if you
operated both valves simultaneously. Since the 0 3.01
powers add (not the dB’s), we know that the total 1 2.54
resultant power would be twice as much as before. 2 2.12
From the example above, we can see that the new 3 1.76
noise level would only be 63 dB, not 120 dB. 4 1.46
5 1.20
What happens when we combine noise sources of two 6 0.97
different levels? Consider the example of the two 7 0.79
valves above, but this time while one valve produces 8 0.64
60 dB of noise by itself, the other produces 100 dB by 9 0.52
itself. It turns out that 100 dB is so much louder than 10 0.42
60 dB that the combination is essentially still 100 dB. 11 0.33
We know that the result is really larger than 100 dB, 12 0.27
but the difference between the resultant overall noise 13 0.22
level and the noise generated by the 100 dB valve by 14 0.17
itself is negligible. This may seem strange at first, but 15 0.14
the mystery should disappear when we consider the 16 0.11
following. 17 0.09
18 0.07
Recalling that powers add rather than dB’s, we can 19 0.06
solve equation (3) above in reverse to discover that 20 0.05
100 dB represents a power of 0.01 Watt (Don’t forget
to take into account the reference power). On the You can use Table 1 to combine any number of noise
other hand, 60 dB represents a power of only 0.000001 sources. You can do this by combining any two
Watt. When we combine these two noise sources to sources, then combining the result with another
get a total of 0.010001 Watt we can see that there is source, etc. As an example of how this can be done,
indeed negligible difference between 0.010001 and consider the following example:
0.01. These two examples allow us to develop two
useful rules of thumb: EXAMPLE 1: Use table 1 to combine four point noise
sources which generate noise levels of 102 dB, 96 dB,
RULE NO. 1: When a secondary noise source is 108 dB, and 102 dB respectively. Determine your
combined with a louder noise source, the overall answer to the nearest dB.
resulting noise level cannot exceed 3 dB greater than
the loudest source by itself. This maximum would ANSWER: First combine 102 dB and 102 dB to get
occur only when the secondary noise source became 105 dB. Next, combine this 105 dB with the 108 dB.
equally as loud as the loudest primary source. The difference is 3 dB. From Table 1 then, 1.76 dB
must be added to 108 dB to get 109.76 dB. The
RULE NO. 2: When trying to reduce the overall noise, difference between 109.76 dB and the remaining 96
determine which noise sources are the loud, dominant dB source is 13.76 dB. We could interpolate between
ones and correct them. Negligible improvement will be 0.17 dB and 0.22 dB in the table; however, to the
attained by eliminating a minor noise source. Even nearest dB the combined noise level is 110 dB.
eliminating one of two equal, dominant sources will
only result in a 3 dB improvement.
5
SOUND POWER VERSUS SOUND PRESSURE Equation (7) reminds us that when we double the
sound pressure, we actually quadruple the sound
Although the acoustic power (W ) generated by a noise power. Now, when we double the sound pressure in
a
disturbance is related to how loud the noise sounds, it equation (8) we must quadruple the sound power in
is not always easy to directly measure the sound equation (3) in order to be consistent. Thus, in both
power. On the other hand, it is relatively easy to cases, we will get a 6 dB increase in noise level. This
measure the noise induced air pressure disturbances leads us to another rule:
(p ) that exist in the vicinity of our ears. From a
d
pragmatic point of view, we are often more interested RULE NO. 3: The noise level of any given noise
in noise at the point where the noise pressure fluctua- disturbance which exists in the environment will result
tions impact our ears than we are at the actual source in exactly the same number of dB’s, regardless of
of the noise. All of this leads us to the conclusion that whether we use the sound power or sound pressure to
it would be useful to understand how sound power and determine it, as long as we remember and account for
sound pressure are related to each other and to the the relationship described by equation (7).
noise level.
CAUTION: Although equations (8) and (3) are closely
It is a fundamental fact of nature that “power” is always related to each other as we have indicated above, we
proportional to the square of the “potential.” In electri- should be careful about concluding that these two
cal systems, for example, we all know from our high equations are always equal to each other because as
school physics that electrical power is proportional to we shall shortly see, they are not.
the square of the electrical potential (E). Likewise, the
acoustic power is proportional to the square of the The relationship between sound power and sound
sound pressure. We can express this as an equation pressure is much like the relationship between mass
where C is a constant of proportionality involving the flow and pressure in a pipeline. As we proceed down
area (A),1 the density (ρ), and the speed of sound (c). a long pipeline, the mass flow is the same at every
point even though the pressures will be different due to
ρ friction losses, changes in pipe diameter, changes in
W = (A/ c) (p )2 (6)
a d temperature, etc. Likewise, sound power will be the
W = C (p )2 (7)
a 1 d same as it “flows” outward from the source, but the
sound pressure will vary at different distances from the
If we combine this fact with another fundamental fact
source. To illustrate, let’s take the simplest case of a
about logarithms [Log Xa = aLog X], we can write two
point source of noise.
different “level” equations which are simply two differ-
ent ways of looking at what is essentially the same
If a sudden noise disturbance, such as an explosion,
phenomenon.
occurs at a point it will generate a spherically shaped
sound pressure wave which emanates outward from
L = 10 Log (W /W ) (3)
pwr 10 a o the source in all directions. Even though the total
power of the wave is the same at every point away
L = 20 Log (p /p ) = 10Log (p /p )2 (8)
pres 10 d o 10 a o from the source, this total power is being spread
uniformly over an increasing surface area of the
NOTE: In both equations, the units are “dB”.
spherical wave. Thus, the Watts/m2 will decrease
dramatically at any point on the spherical surface as
The statistically derived value for the reference sound
the surface area increases when we move further and
pressure is p = 2x10-5 Pa.
o further from the source.
NOTE: L is often written as L and is called “Sound
pres p Since the sound pressure at any point is proportional to
Pressure Level.”
the Watts/m2 at that point, rather than the total power, it
follows that the sound pressure, which is what we
Equation (8) tells us that if we double the sound
typically measure, will be different depending upon how
pressure we will get a 6 dB increase in noise level;
far away we are from the noise source. This situation
whereas equation (3) tells us that if we double the
is analogous to the phenomenon we witnessed as
sound power we will get only a 3 dB increase in noise
children when we tossed a rock into a quiet pond. As
level. At first, this sounds inconsistent to some indi-
the rock entered the water it would produce a distur-
viduals until they realize that we are not talking about
the same thing in these two cases.
6
bance which generated a circular wave which traveled RULE NO. 4: If the log equation contains a power
outward from the source. As we watched, we would term ratio such as (W /W ), the expression is most
1 2
see the amplitude of these waves decrease as they certainly sound power level. If the log equation con-
moved further and further from the source. Even tains a pressure ratio and an area term; e.g., (p 2A /
1 1
though the total power in each wave was staying the p 2A ), the expression is also sound power level,
2 2
same, it was getting dispersed over the rapidly increas- because as equation (6) reminds us, the square of the
ing circumference of the wave circle. Eventually this pressure acting over a specific area is actually a
total wave power would get so dispersed that there measure of power. Only when the log equation
would be no detectable amplitude of the wave left. contains a pressure ratio, but is stripped of the area
relationship can we consider the equation as sound
The dispersal of power and corresponding decrease in pressure level. The following examples should help to
sound pressure as we move away from the source is illustrate this rule:
the reason why it is important to have a standard
reference point for measuring and comparing the L = 10Log (W /W ) Sound power level
10 1 2
sound pressure levels of noise sources. If we don’t L = 10Log (p 2/p 2) Sound pressure level
10 1 2
measure them at the same distance from the source L = 10Log (p 2A /p 2A ) Sound power level
10 1 1 2 2
each time, there would be no basis for comparison. L = 10Log (p /p )2 Sound pressure level
10 1 2
The standard distance for measuring is typically one L = 20Log (p /p ) Sound pressure level
10 1 2
meter from the source. L = 10Log [(p /p )2(A /A )] Sound power level
10 1 2 1 2
We can see now that equation (3) may tell us that the NOISE FREQUENCY AND WAVELENGTH
noise power level at the source is perhaps 100 dB,
while at the same time we may measure a sound Sound, from the point of view of control valve noise,
pressure level at the one meter reference point of only consists of pressure disturbances produced and
85 dB. Of course, the sound pressure level would be propagated by waves in solid or fluid media. For
even less as we moved further from the source, even practical purposes, the most important type of wave is
though the TOTAL sound power level would still be 100 the simple harmonic or sinusoidal wave shown in
dB. Figure 1. This is a snap-shot picture of one cycle of a
simple harmonic sound wave.
In valve noise studies, we are often talking about
looking at changes or reductions in noise levels. It is
reassuring to note that a 10 dB reduction in noise is
always a 10 dB reduction whether we are talking about
sound power level or sound pressure level. For
instance, in the hypothetical example above, a 10 dB
reduction in noise would reduce the sound power level
at the source from 100 to 90 dB, while the sound
pressure level measurement at the standard reference
point would be reduced from 85 dB to 75 dB.
Figure 1: Simple harmonic sound wave
One final point should be made about the differences
between sound power level and sound pressure level.
For illustration purposes, we can think of this as a
Some people get very confused about this issue and
single-frequency sound wave traveling through the air
tend to make their distinctions between the two cases
in the x-direction. The ordinate measures the ampli-
based on things such as the coefficient in front of the
tude of the air pressure disturbance above and below
Log term (i.e., 10 or 20), or they make the distinction
the ambient atmospheric pressure. It is this varying
based on whether the equation contains a power term
pressure against our ear drums that registers as
(W) or a pressure term (p). Neither of these distinc-
sound.
tions by themselves can lead one to the correct
conclusion. Equation no. (8) makes it clear that the
Figure 1 only shows one complete cycle of the sound
coefficient method is unreliable. The following rule of
wave. The physical distance spanned by this one
thumb can assist you in making a definitive distinction
complete cycle is called the wavelength (λ). Actually,
between sound power level and sound pressure level:
the distance between any two corresponding points on
7
the wave is the wavelength. Typically, the wavelength
is measured in meters (m). Noise Frequency Spectrum
8
The wave propagation corresponds to the motion of r, satt 6
the whole sinusoidal figure to the right along the x-axis ew
wo 4
with a velocity (c) which is the speed of sound in the air PoMicr 2
(meters/second). 0
0 100 200 300 400 500 600 700 800 900 1000
The number of complete wavelengths (cycles) which Frequency, Hz
pass any given point on the x-axis per second is the
frequency (f) of the wave in cycles/sec (Hertz). Figure 2: Noise Frequency Spectrum
By concentrating on the dimensions of these funda- For the type of noise generated in a control valve, the
mental factors, we can logically combine them in such noise power spectrum tends to be more of a “hay
a way as to derive the fundamental relationship stack” shaped curve, such as shown in Figure 3, rather
between wavelength (λ) and frequency (f). In other than the type of irregularly shaped curve shown in
words, Figure 2.
(cycles/sec)(meters/cycle) = (meters/sec)
f
p
(f)(λ) = (c) which is often written as
f = c/λ (9)
Noise
Power
Thus, we arrive at the logical conclusion that the Level
shorter the wavelength of the sound, the higher the
frequency. As we shall see later, this is a fact which is
of great importance in noise abatement using drilled-
hole valve trim.
NOISE FREQUENCY SPECTRUM
Frequency (Hz)
Industrial noise, including control valve noise, is rarely
Figure 3: “Hay Stack” Noise Power Spectrum
a pure tone such as that shown in Figure 1. The noise
is usually made up of a whole band of frequencies,
Thus, as the noise being generated in the valve
with varying sound power at each frequency. In order
changes, the power spectrum curve rarely changes its
to represent this type of noise, a noise frequency
basic shape, but it may move up or down as the peak
spectrum is often used. Webster’s dictionary defines a
noise power level changes, and it may move right or
spectrum as any collection of radiant energy arranged
left as the frequency (f ) changes at which the peak
in order of their wavelengths. In noise studies, it is p
power occurs.
more conventional to deal with the frequency rather
than the wavelength. Thus, the noise frequency
Strouhal, an early experimenter in turbulent flow,
spectrum is a band of frequencies displayed in order of
discovered that in general, the tone frequency of noise
their frequencies. The audio spectrum, as was stated
due to turbulent flow was proportional to flow velocity
earlier, is typically defined as the band of frequencies
and inversely proportional to some characteristic
from 20 Hz to 20,000 Hz. These frequencies are
dimension. To turn this proportionality into an equa-
normally plotted along the horizontal axis of a chart,
tion, he invented what is known as the Strouhal
while the corresponding sound power for each fre-
number (S ). Thus, the frequency equation becomes
quency is plotted along the vertical axis as shown in T
S U
Figure 2. f = T vc (10)
D
j
Since our noise studies typically deal with valve flow,
the velocity we need becomes the velocity at the vena
8
contracta (U ), and the characteristic dimension means that in this frequency range there is negligible
vc
associated with that velocity becomes the diameter of difference between the actual measured sound
the jet at the vena contracta (D). pressure level and the A-weighted noise level.
j
When experimenters plotted noise power level versus Since aerodynamic noise in a valve is generated
frequency as shown in Figure 3, it was discovered that primarily in this same frequency range of 600 Hz to
the peak frequency of the “hay stack” noise spectrum 10,000 Hz, the A-weighting factor is essentially unity.
typically occurred around a Strouhal number of two Thus, whether we predict the noise level in dB or in
tenths (i.e., S = 0.2). dB(A), we will arrive at approximately the same
T
number when we are dealing with aerodynamic noise.
A-WEIGHTING OF NOISE LEVEL (dB(A))
On the other hand, hydrodynamic noise in a valve can
In literature concerning noise, we often encounter the have appreciable energy at frequencies below 600 Hz.
terms dBA or dB(A). These terms both refer to what is Because the human ear is much less sensitive to noise
known as A-weighted noise, but dB(A) is the preferred at these lower frequencies than it is at 1000 Hz, the A-
designation. Weighting a noise measurement consists Weighting factor is going to be considerably less than
of modifying a measured sound pressure level by unity. Therefore, when dealing with hydrodynamic
some weighted factor which is a function of frequency. noise, it is extremely important to take into account the
The A-weighting factors are intended to adjust actual, A-weighting factor.
measured sound pressure levels at each frequency
band to the sensitivity of the human ear. GOVERNMENT STANDARDS FOR
NOISE EXPOSURE
The numerical value of the A-weighting factor at any
frequency is determined by how loud a noise sounds Society values people and their health and welfare. In
compared to how loud a 1000 Hz tone appears to be. our democracy, it is considered to be the proper role of
In other words, at 1000 Hz, the A-weighting factor is government to ensure each citizen’s health and welfare
unity (1.0). The sound pressure level of a 1000 Hz through various regulations and programs. When it
tone is adjusted until it appears to be equally as loud as became apparent to various governments in the 1960’s
the unknown noise source. If the sound pressure level that industrial noise pollution was becoming a serious
of the 1000 Hz tone measures 105 dB when that match problem, federal and state governments began devel-
occurs, we say the unknown source “sounds like” 105 oping legislation to establish standards and penalties
dB, regardless of what its sound pressure level would which are intended to protect people from noise
measure. For example, if we listen to a sound at 50 pollution.
Hz, which to us appears to be just as loud as the 105
dB was at 1000 Hz, we say the 50 Hz tone sounds like
105 dB regardless of the measured sound pressure
ALLOWABLE SOUND
level. In this case we would say that the 50 Hz A-
PRESSURE LEVEL (dBA)
weighted noise level is 105 dB(A).
120
If two or more sounds at different frequencies sound
110
equally loud, they are the same dB(A), regardless of
100
what their individual sound pressure levels may be. 90
A
Government standards are written in terms of dB(A) B 80
d 70
since they are really more concerned about how loud
60
the noise actually sounds, rather than with what the
50
sound pressure level actually is. 40
0 1 2 3 4 5 6 7 8
A-weighting of noise level is a much more significant
Hours
factor for hydrodynamic noise (noisy liquid flow) than
for aerodynamic noise (noisy gas flow). The reason for
this has to do with the sensitivity of the human ear.
The response of the human ear to noise is fairly flat in Figure 4: Federal Noise Regulation
the frequency range of 600 Hz to 10,000 Hz. This
9
Figure 4 shows the allowable sound pressure level by implosion of vapor bubbles formed in the cavitation
which the USA government will allow an individual to process. Cavitation occurs in valves controlling liquids
be exposed to for any given duration. The allowable when the service conditions are such that at some
noise level can be louder if the individual is only going point within the valve the pressure drops below the
to be exposed briefly than if he or she must work a full vapor pressure causing bubbles to form, while the
8-hour shift in the noisy environment. Notice also that static pressure downstream of the valve is greater than
the standard uses A-weighted sound levels (dB(A)) the vapor pressure causing the vapor bubbles to
since the actual sound pressure level is not as impor- implode and release energy.
tant as how loud it actually sounds to the human ear.
As the fluid velocity increases due to the restriction
There are three elements of noise standards which are formed by the valve trim parts, vapor bubbles are
essential considerations when dealing with industrial formed in the region of minimum static pressure
noise problems. These are people, noise level, and (highest velocity) and are subsequently collapsed or
exposure time. If there are no people involved, there is imploded as they pass downstream into the pressure
no problem as far as the government standards are recovery region. Noise is produced by the energy
concerned. So, one way to address the issue is to dissipation of the imploding bubbles. Noise produced
fence off and isolate noisy equipment from people. In by cavitation in a valve has a broad frequency range,
airports, for example, the majority of people are however, it can have appreciable energy at frequencies
isolated from the jet noise either by fences, noise below 600 Hz. Cavitation noise is often described as a
barriers, or by the insulated walls of the terminal rattling sound similar to that which would be anticipated
building. Employees who can’t be completely isolated if gravel were in the fluid stream.
wear ear protectors to insulate their ears from damag-
ing noise. Since the amount of hearing damage done Cavitation may produce severe damage to the solid
by noise is a function of how long an individual is boundary surfaces that confine the cavitating fluid.
exposed to the noise, various governments have Generally speaking, noise produced by cavitation is of
developed regulations which relate noise level to secondary concern. In addition, test results and field
exposure time. experience indicate that noise levels from non-cavitat-
ing liquid applications are quite low and generally
THE CONTROL VALVE AS A NOISE GENERATOR would not be considered a noise problem.
Noise is the result of energy dissipation in the control AERODYNAMIC VALVE NOISE
valve. The major sources of control valve noise are
mechanical vibration of components, hydrodynamic The major source of aerodynamic valve noise (i.e.,
noise, and aerodynamic noise. noise resulting from gas flow) is a by-product of a
turbulent gas stream. A control valve controls gas flow
MECHANICAL NOISE by converting potential (pressure) energy into turbu-
lence. Most of the energy is converted into heat;
Vibration of valve components is a result of random however, a small portion of this energy is converted
pressure fluctuations within the valve body and/or fluid into sound. It is possible to determine an acoustical
impingement upon the movable or flexible parts. efficiency factor (η) which indicates how much of the
Noise that is a by-product of vibration of valve compo- initial energy in the flowing medium is converted into
nents is usually of a secondary concern and may even sound. This acoustical efficiency factor varies as the
be beneficial since it warns that conditions exist which valve service conditions change, and this changes the
could produce valve failure. Mechanical vibration has flow patterns through the valve. The IEC valve noise
for the most part been eliminated by improved valve prediction standard defines five different flow regimes
design and is generally considered a structural prob- which affect the acoustical efficiency factor. Since the
lem rather than a noise problem. Accordingly, me- conditions which exist in each of these flow regimes
chanical noise is not addressed by the IEC noise result in slightly different noise generation mecha-
standard. nisms, it is important that we understand the flow
regimes that have been defined by the standard.
HYDRODYNAMIC VALVE NOISE
The major source of hydrodynamic noise (i.e., noise
resulting from liquid flow) is cavitation which is caused
10
