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D. PROVISIONS FOR TAB IN SYSTEM DESIGN
balance the hydronic portion of the system. Additional
information on testing, adjusting and balancing can
be found in other SMACNA and NEBB manuals listed
on the publications page of this manual.
If a SMACNA Contractor wants to become more proficient and more involved in the testing, adjusting and
balancing of environmental or HVAC systems, it is
recommended that consideration be given to becom-
ing a Certified TAB Contractor of the National Environmental Balancing Bureau (NEBB). NEBB has a
comprehensive home study course designed to educate qualified personnel, particularly those in management positions, to direct and be responsible for
the TAB operations of the firm. Information about the
study course and NEBB membership can be obtained from the SMACNA or NEBB National Offices
or local chapters.
Duct systems, unless adequately designed, will act
as large "speaker tubes" and will transmit noise
throughout the building. The direction of the airflow
has little to do with the transmission of the noise.
When confined in a duct, sound transmits just as
effectively upstream in a return air duct as it does
downstream in a supply air duct.
Adequate noise control in a duct system is not difficult
to achieve during the design of the system, providing
the basic noise control principles are understood.
This chapter provides the principles, terms and design data required by the designer of a duct system.
Additional information relating to noise control and
acoustical principles can be found in books and technical papers listed under "References" at the front of
the manual. The National Environmental Balancing
Bureau (NEBB) also has two publications and a home
study course on Sound and Vibration.
It is recommended that those not familiar with terms
used in duct noise control design study Section B"Definitions" before proceeding further. It is suggested that those with past experience in this type of
work also read Section B to become acquainted with
the new terms.
Mechanical equipment noise is one of the major
sources of unwanted noise in a building. The primary
considerations given to the selection and use of mechanical equipment in buildings have generally been
only those directly related to the intended use of the
equipment. However, with the trend towards light
weight building structures and variable-volume air
distribution systems, the noise generated by mechanical equipment and its effects on the over all acoustical environment in a building must also be considered. Thus, the selection of mechanical equipment
and the design of equipment spaces should not only
be undertaken with an emphasis on the intended
uses of the equipment, but also with a desire to provide acceptable noise and vibration levels in the occupied spaces of the building in which the equipment
Over the past 15 years ASHRAE Technical Committee TC 2.6, Sound and Vibration Control, has spon-
sored research that has greatly expanded the available technical data associated with HVAC acoustics.
These data, all of which have been included in this
chapter on noise control, have greatly expanded the
ability of designers to make more accurate calculations related to the acoustical characteristics of HVAC
Absorption Coefficient: For a surface, the ratio of
the sound energy absorbed by a surface of a medium
(or material) exposed to a sound field (or to sound
radiation) divided by the sound energy incident on the
surface. The stated values of this are to hold for an
infinite area of the surface. The conditions under
which measurements of absorption coefficients are
made must be stated explicitly. The absorption coefficient is a function of both angle of incidence and
frequency. Tables of absorption coefficients usually
list the absorption coefficients at various frequencies,
the values being those obtained by averaging over all
angles of incidence.
Aerodynamic Noise: also called generated noise,
self-generated noise; is noise of aerodynamic origin
in a moving fluid arising from flow instabilities. In duct
systems, aerodynamic noise is caused by airflow
through elbows, dampers, branch wyes, pressure reduction devices, silencers and other duct components.
Airborne Noise: Noise which reaches the observer
by transmission through air.
Attenuation: The transmission loss or reduction in
magnitude of a signal between two points in a transmission system.
Background Noise: Sound other than the signal
wanted. In room acoustics, it is the irreducible noise
level measured in the absence of any building occupants when all of the known sound sources have
been turned off.
Breakout Noise: The transmission or radiation of
noise through some part of the duct system to an
occupied space in the building.
Decibel (dB): The unit "bel" is used in telecommunication engineering as a dimensionless unit for the
logarithmic ratio of two power quantities. The decibel
is one-tenth of a bel. Therefore:
Flanking (Sound) Transmission: The transmission
of sound between two rooms by any indirect path of
Forward Flow: Forward flow occurs when air flows
and noise propagates in the same direction, as in an
air conditioning supply system or in a fan discharge.
Frequency: The number of vibrations or waves or
The referenced power for sound power level is 10-12 cycles of any periodic phenomenon per second. In
noise control of duct systems, our interest lies in the
audible frequency range of 20 to 20,000 cycles per
In noise control work, the decibel notation is used to
second. The United States has adopted the internaindicate the magnitude of sound pressure and sound
tional designation of "hertz" (Hz) for cycles per secpower.
Combining Decibels: In sound survey work, it is
Frequency Spectrum: A representation of a comfrequently necessary to combine sound pressure
plex noise which has been resolved into frequency
level readings. An example would be to evaluate the
The most commonly used components
effect of adding a noise source in a room where the
bands and 1/3 octave bands.
noise level is already considered borderline. Since the
Level: The logarithm of the ratio, expressed in decdecibel scale is logarithmic, decibel values cannot be
ibels, of two quantities proportional to power or enadded directly. The correct procedure is to convert
ergy. The quantity which is the denominator of the
the decibels to intensity ratios, add the intensity raratio is the standard reference quantity.
tios, and reconvert this sum into decibels.
Directivity Factor: The ratio of the sound pressure
squared at some fixed distance and direction divided
by the mean-squared sound pressure at the same
distance averaged over all directions from the source.
Dynamic Insertion Loss: The dynamic insertion
loss of a silencer, duct lining, or other attenuating
device is the performance measured in accordance
with ASTM E 477 when handling the rated airflow. It
is the reduction in sound pressure level, expressed
in decibels, due solely to the placement of the sound
attenuating device in the duct system.
Mass Law: The law relating to the transmission loss
of sound barriers which says that in part of the frequency range, the magnitude of the loss is controlled
entirely by the mass per unit area of the barrier. The
law also says that the transmission loss increases 6
decibels for each doubling of the frequency or for
each doubling of the barrier mass per unit area.
Noise Criterion (NC) Curves: Established 1/1 octave band noise spectra for rating the amount of noise
of an occupied space with a single number.
Noise: Sound which is unpleasant or unwanted by
End Reflection: When a duct system opens abruptly
into a large room, some of the acoustic energy at the
exit of the duct is reflected upstream with the result
that the amount of the acoustic energy radiated into
the room is reduced. This decrease in radiated energy increases as the frequency decreases.
1/1 Octave Band: A range of frequencies where the
highest frequency of the band is double the lowest
frequency in the band. The band is specified by the
center frequency. The preferred octave bands are
designated by the following center frequencies: 31.5,
63, 125, 250, 500, 1000,2000, 4000, 8000, 16,000.
First Acoustically Critical Room: Most duct systems service a number of rooms. The room that has
the shortest duct run from the fan is usually exposed
to more fan noise than rooms further away from the
fan. If this "first" room has the same noise criterion
(NC) or a lower NC value than rooms further away
from the fan, it may be assumed that, if the acoustical
attenuation of the duct system from the fan to this
"first" room satisfies the requirements for this "first"
room, it also satisfies the acoustical requirements for
rooms further away from the fan.
Preferred Noise Criterion (PNC) Curves: The
PNC curves are a proposed modification of the older
NC curves. These PNC curves have values that are
about 1 dB lower than the NC curves in the four
octave bands at 125, 250, 500, and 1000 Hz for the
same curve rating numbers. In the 63 Hz band, the
permissible levels are 4 or 5 dB lower; in the highest
three bands, they are 4 or 5 dB lower.
Reverse Flow: Occurs when noise propagates and
air flows in opposing direction, as in a typical returnair system.
Room Absorption: The product of average absorption coefficients inside a room and the total surface
area. This is usually expressed in sabins.
Room Criterion (RC) Curves: The RC curves are
similar to NC or PNC curves. However, they have a
slightly different shape to approximate a well balanced, bland-sounding spectrum whenever the
space requirements dictate that a certain amount of
background noise be maintained for masking or other
Room Effect: The difference between the sound
power level discharged by a duct (through a diffuser
or other termination device) and the sound pressure
level heard by an occupant of a room is called the
Room Effect. The Magnitude of the Room Effect depends upon the amount of the sound absorption in
the room (sabins), the distance between the termination duct and the nearest observer and the directivity factor of the source.
Sabin: The unit of acoustic absorption. One sabin is
one square foot of perfect sound absorbing material.
Sone: One sone is defined as the loudness of a 1000
Hz tone having a sound pressure level of 40 dB. Two
sones is twice as loud as the 40 dB reference sound
of one sone, etc.
Sound Power Level (Lw): the fundamental characteristic of an acoustic source (fan, etc.) is its ability
to radiate power. Sound power level cannot be measured directly; it must be calculated from sound pressure level measurements. The sound power level of
a source (Lw) is the ratio, expressed in decibels, of
its sound power divided by the reference sound power
which is 10-12 watts.
A considerable amount of confusion exists in the relative use of sound power level and sound pressure
level. An analogy may be made in that the measurement of sound pressure level is comparable to the
measurement of temperature in a room; whereas, the
sound power level is comparable to the cooling capacity of the equipment conditioning the room. The
resulting temperature is a function of the cooling capacity of the equipment and the heat gains and losses
of the room. In exactly the same way, the resulting
sound pressure level would be a function of the sound
power output of the equipment together with the
sound reflective and sound absorptive properties of
Given the total sound power output of a sound source
and knowing the acoustical properties and dimensions of a room, it is possible to calculate the resulting sound pressure levels.
Sound Pressure: Sound pressure is an alternating
pressure superimposed on the barometric pressure
by sound. It can be measured or expressed in several
ways such as maximum sound pressure or instantaneous sound pressure. Unless such a qualifying
word is used, it is the effective of root-mean-square
pressure which is meant.
Sound Pressure Level (Lp): A measure of the air
pressure change caused by a sound wave expressed
on a decibel scale reference to a reference sound
pressure of 2 x 10-5 Pa or 0.0002 microbar.
Sound Transmission Class: Sound transmission
class is the preferred single figure rating designed to
give a preliminary estimate of the sound insulating
properties of a barrier.
Structure-Borne Noise: A condition when the
sound waves are being carried by a portion of the
building structure. Sound waves in this state are inaudible to the human ear since they cannot carry
energy to it. Airborne sound can be created from the
radiation of the structure-borne sound into the air.
1. Sound Levels
The most common parameter which is used to give
an indication of "loudness" is the sound pressure
level, Lp. Sound pressure level, Lp (dB), is defined as:
where Prms is the root-mean-square value (rms) of
acoustic pressure (Pa). Pref is the reference sound
pressure and has a value of 2 x 105 Pa or 0.0002
microbar. This amplitude was selected because it is
the amplitude of the sound pressure that roughly corresponds to the threshold of hearing at a frequency
of 1000 Hz.
Intensity level, L, (dB), is defined as:
where I is acoustic intensity (watts/m2).Iref is the reference intensity and has a value of 10 12 watts/m2.
Sound power level, Lw (dB), is defined as:
where W is sound power (watts). Wref is the reference
sound power and has a value of 10- 12 watts.
Noise reduction, NR (db), with respect to HVAC systems, is:
where when adding sound pressure levels:
and when adding sound power levels:
where Lp(1) is the sound pressure level (dB) of the
sound entering a duct element and Lp(2) is the sound
pressure level (dB) of the sound coming out of the
Insertion loss, IL (dB), is:
Figure 11-1 is a nomogram that can be used to add
two sound pressure levels or two sound power levels.
When examining the sound propagation in a HVAC
system, it is necessary to subtract noise reduction,
insertion loss, or transmission loss values from given
sound power levels at different points in the system.
When this is done,
where Lp(w/o)is the sound pressure level (dB) at a
point without a specific duct element inserted and
Lp(w) is the sound pressure level (dB) at the same
point with the duct element inserted.
Transmission loss, TL (dB), is:
where Win is the sound power (watts) of sound entering a duct element and Wout, is the sound power
(watts) of the sound exiting the duct element.
Often it is necessary to add the sound pressure levels
at a point in a room from several sound sources or
to add the sound power levels at a specific point in a
duct system associated with different duct elements.
When adding sound power or sound pressure levels,
the total level, LT (dB), is:
where Lw1 is the sound power level before a duct
element and Lw2 is the sound power level after the
The following sound power levels exist at a point in a
duct system and the IL values are associated with a
duct element that exists at the point.
Determine the sound power levels that exist after the
Number to be Added to
Higher Sound Pressure or Sound Power
Sound Pressure or Sound Power Levels, dB
Figure 11-1 NOMOGRAM FOR ADDING TWO SOUND PRESSURE OR SOUND POWER LEVELS
Table 14-35 in Chapter 14 shows the recommended
NC levels for several activity areas. The lower NC
levels in the table should be used in buildings where
high quality acoustical environments are desired.
The upper levels should be used only for situations
where economics or other conditions make use of the
lower values impractical. Table 14-36 shows telephone
use and listening conditions as a function of NC levels.
The following 1/1 octave band sound pressure levels
were measured in a laboratory work area. What is
the NC rating of the noise in the work area?
The following sound pressure levels are given at a
specified point within a room.
Determine the total sound pressure level.
2. Noise Criterion Curves
Noise criterion curves are shown in Figure 11-2.
These curves apply to steady noise and specify the
maximum noise levels permitted in each 1/1 octave
band for a specified NC curve. For example, if the
noise requirements for an activity area call for a NC
20 rating, the sound pressure levels in all eight 1/1
octave frequency bands must be less than or equal
to the corresponding values for the NC 20 curve.
Conversely the NC rating of a given noise equals the
highest penetration of any of the 1/1 octave band
sound pressure levels into the curves. If the farthest
penetration falls between two curves, the NC rating
is the interpolated value between the two curves.
Figure 11-3 shows a plot of the above data relative to
the NC curves. Since the 1/1 octave band sound
pressure level in the 500 Hz 1/1 octave band penetrates to the NC 55 curve, the NC rating of the work
area is NC 55.
3. Room Criterion Curves
HVAC noise is often the primary type of background
noise that exists in many indoor areas. Experience
has indicated that when HVAC background noise is
present the use of NC levels has often resulted in a
poor correlation between the calculated NC levels
and an individual's subjective response to the corresponding background noise. As a means of overcoming this, the room criterion (RC) curves shown in
Figure 11-4 can be used. There are four factors that
should be considered when assessing HVAC system
background noise: (1) level, (2) spectrum shape or
balance, (3) tonal content, and (4) temporal fluctuations.
There are two parts to determining the RC noise rating associated with HVAC background noise. The
first is the calculation of a number which corresponds
to the speech communication or masking properties
of the noise. The second is designating the quality or
character of the background noise. The procedure for
determining the RC rating is:
1. Calculate the arithmetic average of the 1/1 octave band sound pressure levels in the 500 Hz,
1,000 Hz and 2,000 Hz 1/1 octave frequency
1/1 Octave Band Center Frequency -
Figure 11-2 NOISE CRITERION CURVES
bands. Round off to the nearest integer. This is
the RC level associated with the background
2. Draw a line which has a -5 dB/octave slope
which passes through the calculated RC level
at 1,000 Hz. For example, if the RC level is RC
32, the line will pass through a value of 32 dB
at the 1,000 Hz 1/1 octave band. This value may
not be equal to the value of the 1/1 octave band
sound pressure level of the background noise
in the 1,000 Hz 1/1 octave band.
3. Determine the subjective quality or character of
the background noise.
The subjective rating of background noise associated
with the RC level can be classified as follows:
1. Neutral: Noise that is classified as neutral has
no particular identity with frequency, It is usually
1/1 Octave Band Center Frequency - Hz
11-3 NC LEVEL FOR EXAMPLE 11-3
bland and unobtrusive. Background noise
which is neutral usually has a 1/1 octave band
spectrum shape similar to the RC curves in
Figure 11-4. If the 1/1 octave band data do not
exceed the RC curve by 5 dB the background
noise is neutral and a "(N)" can be placed after
the RC level.
2. Rumble: Noise that has a rumble has an excess of low-frequency sound energy. If any of
the 1/1 octave band sound pressure levels below the 500 Hz 1/1 octave band are more than
5 dB above the RC curve associated with the
background noise in the room, the noise will be
judged to have a "rumbly" quality or character.
If the background noise has a rumbly quality,
place a "(R)" after the RC level.
3. Hiss: Noise that has an excess of high-frequency sound energy will have a "hissy" quality.
If any of the 1/1 octave band sound pressure
levels above the 500 Hz 1/1 octave band are
more than 3 dB above the RC curve, the noise
will be judged to have a hissy quality. If the
background noise has a hiss quality, place an
"(H)" after the RC level.
4. Tonal: Noise that has a tonal character usually
contains a humming, buzzing, whining, or whistling sound. When a background sound has a
tonal quality, it will generally have one 1/1 octave band in which the sound pressure level is
noticeably higher than the other 1/1 octave
bands. If the background noise has a tonal character, place a "(T)" after the RC level.
Background noise which has a 1/1 octave band spectrum that falls within the limiting boundaries identified
with rumble and hiss and which has no tonal components is classified as neutral.
It is desirable to have background noise that has a 1/
1 octave band spectrum that has a neutral character
or quality. If the noise spectrum is such that it has a
rumble, hiss or tonal character, it will generally be
judged to be objectionable. If the background noise
has a neutral quality, the NC levels specified in Tables
14-35 and 14-36 can be used to indicate the desired
RC levels in different indoor activity areas.
The 1/1 octave band sound pressure levels of background noise in an office area are given below:
Determine the RC level and the corresponding character of the noise.
The RC level is determined by obtaining the arithmetic average of the 1/1 octave band sound pressure
Octave Band Center Frequency - Hz
Figure 11-4 ROOM CRITERION CURVES
levels in the 500 Hz, 1,000 Hz, and 2,000 Hz 1/1
octave bands, or
Thus, the RC level is RC 33. The 1/1 octave band
sound pressure levels for the background noise are
plotted in Figure 11-5. The RC 33 curve (level in 1,000
Hz 1/1 octave band is 33 dB) is shown in the figure.
A dashed line 5 dB above the RC 33 curve for frequencies below 500 Hz and a dashed line 3 dB above
the RC 33 curve for frequencies above 500 Hz are
also shown in the figure. An examination of the figure
indicates that at frequencies below the 250 Hz 1/1
octave band, the 1/1 octave band sound pressure
levels of the background noise are 5 dB or more
above the RC 33 curve. Thus, the background noise
has a rumble character. The 1/1 octave band sound
pressure levels above 500 Hz are equal to or below
the RC 33 curve, so there is no problem at these
frequencies. The RC rating of the background noise
is RC 33(R).
GENERAL INFORMATION ON
DESIGN OF HVAC
Several general factors should be considered when
selecting fans and other related equipment and when
designing air distribution systems to minimize the
noise transmitted from different components of the
system to the occupied spaces which it serves. They
1. Air distribution systems should be designed to
minimize flow resistance and turbulence. High
flow resistance increases the required fan
pressure, which results in higher noise being
generated by the fan. Turbulence increases the
flow noise generated by duct fittings and dampers in the air distribution system.
2. A fan should be selected to operate as near
as possible to its rated peak efficiency when
handling the required quantity of air and static
pressure. Also, a fan should be selected which
generates the lowest possible noise but still
meets the required design conditions for
which it is selected. Oversized or undersized
fans which do not operate at or near rated
peak efficiencies result in substantially higher
3. Duct connections at both the fan inlet and outlet should be designed for uniform and straight
air flow. Failure to do this can result in severe
turbulence at the fan inlet and outlet and in
flow separation at the fan blades. Both of these
can significantly increase the noise generated
by the fan.
4. Care should be exercised when selecting duct
silencers to attenuate supply or return air
noise. Duct silencers can significantly increase
the required fan static pressure. When a rectangular duct silencer is used, it may be necessary to line the duct for a distance of at least
ten feet beyond the silencer with a minimum
one inch thick fiberglass duct lining to reduce
high frequency regenerated noise associated
1/1 Octave Band Center Frequency - Hz
Figure 11-5 RC LEVEL FOR EXAMPLE 11-4
with the silencer. For some applications,
acoustically lined sound plenums may be used
in the place of duct silencers.
5. Fan-powered mixing boxes associated with
variable-volume air distribution systems
should not be placed over or near noise-sensitive areas.
6. Air flowing by or through elbows or duct branch
take-offs generate turbulence. To minimize the
flow noise associated with this turbulence,
whenever possible, elbows and duct branch
take-offs should be located at least four to five
duct diameters from each other. For high velocity systems, it may be necessary to increase this distance to up to ten duct diameters in critical noise areas.
7. Near critical noise areas, it may be desirable
to expand the duct cross-section area to keep
the air flow velocity in the duct as low as possible. This will reduce potential flow noise associated with turbulence in these areas.
8. Turning vanes should be used in large 90 degree rectangular elbows. This provides a
smoother transition in which the air can
change flow direction, thus reducing turbulence.
9. Grilles, diffusers and registers should be
placed as far as possible from elbows and
10. Dampers in grilles, diffusers and registers
should not be used for balancing.
Table 14-37 lists several common sound sources associated with mechanical equipment noise. Anticipated sound transmission paths and recommended
noise reduction methods are also listed in the table.
Airborne and/or structure-borne noise can follow any
or all of the transmission paths associated with a
specified sound source.
With respect to the quality of sound associated with
HVAC system noise in an occupied space, fan noise
generally contributes to the sound levels in the 63 Hz
through 250 Hz 1/1 octave frequency bands. This is
shown in Figure 11-6 as curve A. Diffuser noise usually contributes to the overall HVAC noise in the 250
Hz through 8,000 Hz 1/1 octave frequency bands.
This is shown as curve B in Figure 11-6. The overall
sound pressure levels associated with both the fan
and diffuser noise is shown as curve D. The RC level
of the overall noise is RC 36. The RC 36 curve is
superimposed over curve D. As can be seen by comparing the RC curve with curve D, the classification
of the overall noise is neutral. Curve D represents
what would be considered acceptable and desirable
1/1 octave band sound pressure levels in many occupied spaces.
In order to effectively deal with each of the different
sound sources and related sound paths associated
with a HVAC system, the following design procedures
1. Determine the design goal for HVAC system
noise for each critical area according to its use
and construction. Use Table 14-35 to specify
the desirable NC or RC levels.
2. Relative to equipment that radiates sound directly into a room, select equipment that will
be quiet enough to meet the desired design
3. If central or roof-mounted mechanical equipment is used, complete an initial design and
layout of the HVAC system, using acoustical
treatment where it appears appropriate.
4. Starting at the fan, appropriately add the
sound attenuations and sound power levels
associated with the central fan(s), fan-powered mixing units (if used), and duct elements
between the central fan(s) and the room of
interest to determine the corresponding sound
pressure levels in the room. Be sure to investigate the supply and return air paths. Investigate possible duct sound breakout when
central fans are adjacent to the room of interest or roof-mounted fans are above the room
5. If the mechanical equipment room is adjacent
to the room of interest, determine the sound
pressure levels in the room associated with
sound transmitted through the mechanical
equipment room wall.
6. Add the sound pressure levels in the room of
interest that are associated with all of the
sound paths between the mechanical equipment room or roof-mounted unit and the room
7. Determine the corresponding NC or RC level
associated with the calculated total sound
pressure levels in the room of interest.
8. If the NC or RC level exceeds the design goal,