Construction Materials For Acoustic Design
Construction Materials For Acoustic Design
Construction Materials For Acoustic Design
Unit 4
CONSTRUCTIONAL MEASURES
INTRODUCTION
If noise control has to be implemented the transmission of AIRBORNE NOISE &
STRUCTURE BORNE NOISE must be INTERCEPTED (that is) the desired acoustical
privacy must be achieved by using sound insulating walls, floors, doors / windows etc.
In the past heavy & space consuming building materials were used for the construction
of sound insulating enclosures. THE HEAVIER & THICKER THEY WERE, THE MORE
EFFICIENT THEIR SOUND INSULATION WILL BE
BUT
In contemporary buildings thick & heavy walls and floors should be avoided in order to
gain more spaces; to save construction loads, thus reducing building costs; to shorten the
construction time and to provide flexibility in design.
These requirements stimulated the demand for THIN, LIGHTWEIGHT,
PREFABRICATED, AND MOVABLE BUILDING ELEMENTS, CREATING SERIOUS
ACOUSTICAL PROBLEMS.
In the selection of a wall or floor construction, 3 factors are normally considered.
1. The existing or predictable noise level as the source side.
2. The acceptable or desirable background noise level in the receiving room.
3. The ability of the selected enclosure to reduce the exterior noise to an acceptable level.
The acceptable (or desirable) background noise level can also be expressed in terms of
NOISE CRITERION (NC) level, specified in terms of NC curves.
The basic objective is that THE TRANSMITTED PORTION OF THE EXTERIOR NOISE
BE REDUCED ON THE RECEIVING SIDE JUST BELOW BACKGROUND NOISE LEVEL,
PROVIDED THAT THE BACKGROUND NOISE IS WITHIN PERMISSIBLE RANGE.
Example: If an Ext. noise of 70dB must be reduced by using a partition below a
background noise level of 20dB (to about 15dB). The STC value of the partition should be 70 15 = 55dB.
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If the background noise level can be raised to 35dB instead of 20dB, (provided it is
still within the acceptable range) a more economical 40dB (instead of 55dB) partition will
render the TRANSMITTED NOISE INAUDIBLE.
CONTINUOUS
NOT TOO LOUD
UNOBSTRUSIVE
CARRIES NO INFORMATION such as intelligible speech or identifiable music.
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An excessive masking noise will impair audibility or intelligibility by DROWNING out faint
speech, soft music, or other very low intensity sound, one might want to hear, for example, in a
demonstration room.
WALLS/PARTITIONS:
PARTITION ISOLATION:
Optimum performance against Airborne noises can be expected from a wall only if the
following conditions are satisfied: It has a reasonable and uniformly distributed MASS over its entire area.
It is constructed both HORIZONTALLY AND VERTICALLY as an UNINTERRUPTED
COMPLETE BARRIER.
It is EFFECTIVELY SEALED around its periphery, between its elements, and around
openings made for outlets, switches, etc.
Fig. 2:- Fixtures recessed back to back in partition walls (A) should be staggered with
voids pocketed and gaps caulted and (B) to avoid direct transmission of sound through
the partition.
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It is built from structural slab to structural slab or when attached to a suspended ceiling
only adequate care have been takes for the acoustical restoration of its missing portion
above the suspended ceiling.
The sealant should be a non setting and non hardening caulking compound; Gasketing
alone is not effective as caulking.
Loose and void mortar joints should be carefully avoided in masonry wall construction
because they create noise leaks.
If no plaster is specified for masonry wall its exterior surfaces should be painted possibly with 2
layers. Gypsum boards used in dry wall construction should be carefully SEALED AND
TAPED.
Since it is necessary to double the weight of a single leaf partition to provide about 5 to
6 dB improvement in its STC rating, using a MULTIPLE PARTITION is recommended
where a high degree of sound insulation is required.
A multiple partition should include many of the useful features as recommended in
page.
Resilient layers (fiber board, glass-fiber board) semi resilient attachments or resilient
clips between individual layers and the inner framing or core of the wall are particularly
recommended. These elements are inexpensive and contribute to NR of 2 to 5 dB.
Every attempt should be made to increase the weight of a wall without rendering it too
stiff. As mentioned earlier, the stiffness of a wall tends to counteract the acoustically
beneficial effect of the mass and of the separation between individual layers of the wall.
Using sheet LEAD, a relatively heavy material, in walls / floors is recommended to
increase the weight without rendering the partition harmful stiff. (fig. below)
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WT KG AVG.
/M2
REDUCTION
DB)
490
50 dB
710
53 dB
490
50 + 53 dB
310
50 dB
170
50 dB
9 concrete block
sides)
45 dB
45 dB
36 dB
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SOUND
(T.L) (in
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The following methods can be adopted for sound insulation, for walls / partitions.
RIGID & MASSIVE HOMOGENUOUS CONSTRUCTION
PARTITIONS OF POROUS MATERIALS
HOLLOW & COMPOSITE PARTITION WALLS OR DOUBLE WALL CONSTRUCTION
RIGID & MASSIVE HOMOGENUOUS CONSTRUCTION
The sound insulation of rigid & massive homogeneous walls depends directly as
the weight of the partition / unit area.
The sound insulation thus increases with the thickness.
But the increase in insulation value of a solid wall construction is SLOW in proportion
with the increase in thickness. Hence it is not economical to increase the thickness of a
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solid wall beyond a certain limit. There is a possibility of sound getting transmitted
through cracks / holes and space left due to p---- workmanship.
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It should be noted that in cavity wall construction, air space, is better than a filling materials.
(The filler itself provides for the propagation of sound if not selected properly).
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FLOORS / CEILINGS:
While airborne noise must primarily be isolated by walls, an ACOUSTICALLY
EFFICIENT FLOOR CONSTRUCTION should provide adequate protection against
AIRBORNE & IMPACT NOISES.
A FLOOR WITH THICK CARPETING WHICH ADEQUATELY REDUCES IMPACT
NOISES IS NOT NECESSARILY SATISFACTORY AGAINST AIRBORNE NOISE.
A THICK, BARE CONCRETE SLAB WHICH GIVES SATISFACTORY INSULATION
AGAINST AIRBORNE NOISES WILL NOT PROVIDE ADEQUATE PROTECTION AGAINST
IMPACT NOISES FROM THE ROOM ABOVE.
THE SOUND INSULATION OF THE FLOORS CAN BE IMPROVED IN THE FOLLOWING
WAYS:1. A soft RESILIENT surface carpet, cork tile, rubber, or vinyl tile greatly improves
impact noise insulation of the floor (not against airborne).
2. A FLOATING FLOOR considerably improves the sound insulation against both
airborne & impact noises.
3. A solid suspended ceiling improves insulation against both airborne & structure borne
noises, depending on the weight of the ceiling & the degree of resiliency with it is
attached to the structure.
HARD, RIGID furnishes over a concrete floor WOOD, LINOLEUM, VINYL ASBESTOS
TILE, ASPHALT TILE etc. Do not improve the impact noise insulation of the floor whenever
possible, laying a CARPET is unquestionably the most sensible measure for eliminating impact
noises (footsteps).
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In all cases the floating floor assembly rests on the structural slab.
Floating wood floors should be carefully nailed to the supporting sleepers only, in order
not to short circuit the resilient blanket beneath the sleepers.
A floating concrete floor / slab should be at least 3 thick, on top of an adequately
resilient blanket. FLOATING CONCRETE SLABS ARE BETTER THAN FLOATING
WOOD FLOORS.
In order to obtain maximum efficiency it is essential not only to provide a consistent
and uninterrupted acoustical separation between floating floor and structural slab, but
also to avoid any rigid contact between floating floor and the surrounding walls.
The resilient blanket beneath the floating slab must be adequately protected by a water
proof membrane (roofing felt or polyethylene) against moisture originating from the
pouring of the floating slab.
Protection of the water proof membrane (by plywood) may also be necessary against
such potential damages as rupture during reinforcement of concrete pouring.
All pipes, duets, conduits, etc; which penetrate through floating floors should be isolated
all around to avoid short circuiting between the structural floor & floating floor.
When floating floors are used to protect certain areas against impact noises from above,
it is important to make sure that no impact noise enters the protected area through any
other path.
SUSPENDED CEILINGS
Suspended ceilings attached to the structural floor contribute substantially to the sound
insulation of a floor against both airborne and impact noises. In order to increase their
effectiveness the following recommendations should be observed;
1. The ceiling membrane should weigh not less than 5/6 s/sft (25kg/m 2). If an absorbent
blanket (mineral or glass wool) is used in the air space above the ceiling, the weight of
the ceiling membrane can be somewhat reduced.
2. The ceiling membrane should not be too rigid.
3. Direct paths of noise transmission through the ceiling should be avoided by the use of
solid, airtight membrane.
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4. Gaps between ceiling and surrounding structure should be sealed, thus avoiding noise
penetration through direct air paths.
5. The air space between ceiling membrane and structural floor should be increased to a
reasonable maximum & an isolation blanket placed in the air space.
6. The number of points of suspension from the structural floor above should be reduced
to a minimum. Resilient hangers are preferable to rigid ones.
When a suspended ceiling is used to improve the airborne or impact sound insulation of a
floor, the application of a light weight and sound absorptive but sound transparent acoustical
ceiling is useless and should be avoided.
The ceiling membrane should consist of a minimum solid cement plaster layer with
hermetically sealed joints all around. If the reduction of undesirable noises within a sound
insulated room is required, the installation of sound absorptive treatment along the underside
of the solid ceiling membrane should be considered.
Various types of hangers are used in resiliently suspended ceilings. A semi resilient
separation between structural floor and suspended ceiling achieved by the use of resilient
caps does not contribute nearly as effectively to the sound insulation of floor ceiling
assemblies as fully resilient hangers do.
Walls built only of up to suspended ceiling height offer a serious reduction in TL. It is
essential that the space between suspended ceiling and structural slab above the line of the
partition wall be adequately sealed.
To avoid noise transmission above partition built up to suspended ceiling height.
a) Walls should be constructed up to the structural slab
b) Or an adequate sound barrier be installed in the ceiling space
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Lightweight, Hollow care wooden doors are dimensionally unstable and can warp, destroying
the seal along the perimeter of the door.
Sealing components that can be used:
Rubber
Foam rubber
Foamed plastic strips
Gaskets
Adjustable or self aligning stops
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The sealing components should be installed so that they are slightly compressed
between door and stop, when the door is in a closed position.
A most effective alternative is to install drop-bar draft excludes called AUTOMATIC
THRESHOLD CLOSERS.
If doors must have an unusually high degree of sound insulation they are built in such a way
that
1) There is a separation between the two faces of the door, carried through
uninterruptedly from edge to edge in both directions.
2) In addition damping treatments are inserted between individual layers of the doors.
The use of automatic door closer is recommended whenever, feasible and applicable in
order to avoid the annoying sound of doors slamming. Similarly door chimes are preferable to
door knockers.
Doors always reduce the overall TL of walls in which they are installed. In order to keep this
reduction to a minimum, it is recommended that THE DIFFERENCE BETWEEN THE TL OF
THE WALL AND THAT OF THE DOOR SHOULD NOT EXCEED 5 TO 10DB.
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While planning, the positions of the doors should be staggered. So that noise from one
room will not penetrate directly across another. Staggering increase the distance thereby
reducing the amount of sound that reaches the door.
When a particularly high degree of acoustical privacy is required for a room, the need for a
highly effective and expensive sound insulating door may be avoided by using 2 moderately
sound insulating doors or a SOUND LOCK. All walls, floors, ceilings within a sound lock should
be rendered sound absorptive, in order to achieve better results.
MOVABLE PARTITIONS:
Depending on the basic functions the movable partitions should meet the following
requirements.
They should combine visual separation with acoustical privacy to the required degree.
They should be capable of dividing an area into 2 or more spaces, easily and quickly,
without the need for additional manpower.
They should require a minimum of maintenance and servicing.
Their shape should conform to any unavoidable irregularities in the floor without loss of
acoustical privacy.
The elements should not warp / twist.
The bottom seal should not warp the finished (carpeted floor).
The surface treatment should provide for variety in the interior.
WINDOWS:
Like doors, windows constitute weak components in exterior walls and enclosures because
Their surface weight is much less than that of the exterior wall.
Their connection with the wall, unless adequately sealed, constitutes a direct path for
the penetration of exterior noise.
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window, particularly at low frequencies, the TL increases with increased distance between
panes.
If a reasonable distance cannot be secured between the panes, it is advisable to
increase the thickness that is the weight of the panes.
The diagram below illustrates the range of sound insulation of single & double glazed
windows.
Single glazing
3mm,6mm,13mm.
Double glazing
Range of sound insulation of single and double glazed windows. Value on the left represent
windows that can be opened, values on the right represents fixed window.
In an air-conditioned buildings, the TL of fixed windows with thick double panes well spaced
and structurally isolated from each other may approximate that of the surrounding wall.
CONTROL WINDOW BETWEEN A RADIO OR RECORDING STUDIO AND A SOUND CONTROL ROOM.
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Adding sound absorbing treatment around the perimeter between the panes, mounting panes
in an elastic material and eliminating parallelism between panes will result in a reasonable
increase in the TL of a window.
Each glass should be of a different thickness or weight per unit area. These methods of
increasing the sound insulation quality of windows are used in the installation of control &
observational windows, in radio, TV, and recording studios.
INSULATION OF SANITARY FITTINGS:
For improved insulation in the case of Sanitary Fittings the following guidelines can be
followed:
In planning a toilet care should be taken so that the W/C is not placed against the
bedroom wall or a living room wall. In case they have to be against such walls, the
walls must be acoustically treated.
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The W/C pan & cistern should be insulated. The pan should be made to rest upon a thin
pad of felt, linoleum, cark, rubber or any other suitable materials.
Cisterns should not be fixed directly to walls, but should be fixed upon insulators fixed to
the brackets.
All sanitary pipes should be properly wrapped, where they pass through a wall or floor
and held in insulated clips.
NOISE FROM PLUMBING SYSTEMS:
Plumbing noises are seldom excessively loud; never the less they can be annoying and
embarrassing. The following are the main sources of plumbing noises:
The best method is to suppress the noise at source (ex) by installing supply & drain pipes as
far as possible from quiet areas of the building or by installing quietly operating fixtures.
The next step is to prevent noise from penetrating from the pipe into the structure
Transmission noise along the pipe can be reduced considerably by inserting a flexible pipe
between the source and the metal pipe.
If noise is transmitted through a pipe, the amount of noise that is radiated by the pipe itself
is negligible; most of the disturbing sound is radiated by the building structure (partitions, slabs,
ceilings, etc) to which the pipe is attached. To eliminate this noise radiation, pipes should be
attached to massive structural elements (masonry walls) or resiliently mounted (ie) adequately
insulated from their supports by being wrapped in
o
o
o
o
FELT
RUBBER
NEOPRENE
NATURAL WOOL OR SOME OTHER SUITABLE MATERIAL.
If such materials cant be incorporated, the noise conducting pipes should be screened
from the affected rooms by building them into suitable ducts or shafts. It should also be made
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sure that these ducts in fun do not create problems by transmitting airborne noise from on part
of the building to another.
In high rise buildings the use of pressure regulators or reducers should be considered.
If the complete elimination of plumbing noise is necessary, noise producing fixtures should be
installed on FLOATING FLOORS OR RESILIENT UNDERLAYS.
NOISE FROM VENTILATING & AIR CONDITIONING SYSTEMS:
In rooms where listening to speech or music is important, the noise level created by an air
handling unit should be about 5 to 15 db below the desired level of background noise in order
to avoid interference of ventilation noise.
In certain rooms (like offices, hospital rooms, restaurants etc) the aim in the control of
mechanical noise is not to eliminate the noise totally but to create a balanced acoustical
environment. The noise should be reduced only to the degree necessary to allow the
anticipated activity in the room to take place comfortably. To reduce mechanical noise below
this level is wasteful. Also excessive noise reduction will remove the masking noise that is
necessary to keep the external noise at bay.
The noise control of Air handling units should start, therefore, with a CRITICAL
DETERMINATION OF CRITERIA FOR THE DESIRED BACKGROUND NOISE LEVELS IN
ALL THE ROOMS, DEPENDING ON THE PARTICULAR ACTIVITIES IN EACH.
SOURCES OF MECHANICAL NOISE & VIBRATION:
Noises encountered in mechanical systems can be grouped as follows:1) Mechanical equipment noise caused by individual ventilating and air conditioning units
and by the fans, motors, compressors, pumps, etc; of large centralized systems.
2) Self noise caused by high velocity air flow, pulses created by the fan blades, airflow
around sharp bends or turbulences created by grilles, diffusers, dampers, and pressure
regulators.
3) CROSS TALK from are space to another, for example speech sound that enters supply
or return air grille in are room travels through the duct and emerges in a nearby room
through another grille, entirely irrespective of the direction of airflow.
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4) Noise transmitted from exterior sources via an exposed part of the duct into the building.
NOISE REDUCING COMPONENTS:
In the control of ventilating and air-conditioning noise attention must be given to the 3 principal
noise-producing parts of the systems
1) The central mechanical equipments.
2) The supply and return ducts.
3) The supply and return grilles.
The installation of quietly operating equipment, far removed from noise-sensitive rooms or
floors is an important point to reminder in striving to reduce mechanical noises. Additional
methods are as follows:
1)
2)
3)
4)
5)
6)
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METHODS OF REDUCING NOISE BY USING SOUND ABSORPTIVE LINING IN THE DUCTWORK OF AIR
HANDLING SYSTEMS.
THERMAL INSULATION installed along the OUTSIDE SURFACE of a duet wall will
contribute, to a certain degree, to the TL of the duct wall.
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Sound absorbing materials such as glass-fiber or mineral fiber boards installed along the
inside of rectangular or tubular duets will increase the attenuation of noise along the duct.
Sound absorbing materials used for duct lining should
The incorporation of a PLENUM (an expanded section of the duct) into the duct system with a
sound absorptive interior surface contributes to the reduction of noise within the ductwork.
Plenums chambers are used when a large number of smaller ducts are fed by are main supply
fan.
Ducts with small cross sections are more effective noise attenuators than those with larger
cross sections.
Since the noise level close to the duct exit is greater than at a distance from the grille, the duct
opening in auditorium has to be as far as possible from the listeners.
ATTENUATION OF MECHANICAL NOISE:
In order to achieve the required attenuation of ventilating and air conditioning noise below the
acceptable background noise level in a room, the following steps should be considered.
1) The noise level produced by individual units of the equipment at the source must be
determined.
2) The attenuation of this noise level due to ducts, walls, floors, ceilings, distance between
supply grille and occupant etc., must be calculated next; that is the noise level at the
occupants position has to be checked.
3) For each room to be served by the air-handling system, the NC level must be
established below which the noise level will eventually have to be reduced.
4) If the mechanical noises have not been reduced to the required extent, that is, below
NC levels, further attenuation will have to be achieved, by additional measures like.
MACHINERY NOISE:
The following are notorious sources of machinery noises:
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Diesel generators
Pumps
Compressors
Cooling Towers
Motors
Pneumatic Device
Electric circuits equipment
Elevator hoist equipment
Swimming pool equipments.
The required degree of noise control in rooms close to the machinery will depend on
In order to provide adequate noise reduction between the mechanical room & adjoining
occupancies the following PATHS have to be checked:
Airborne Paths between the noise equipment room and the adjoining occupancies.
Structure Borne Paths between vibrating equipment and adjoining areas.
Duct Borne Paths for transmission of fan noise and airflow noise into adjacent serviced
by the equipment.
To secure the required ATTENUATION of machinery noise somewhat below NC levels, in the
rooms close to an equipment room, the following MEASURES should be considered:
VIBRATION:
The increased use of THIN & LIGHTWEIGHT building constructions, the demand for HIGH
PRESSURE & HIGH SPEED AIR CONDITIONING SYSTEMS, and THE ACCOMMODATION
OF LARGE, CENTRALIZED MECHANICAL EQUIPMENT ROOMS have all increased the
occurrence of VIBRATION in modern buildings.
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VIBRATION:- The movement of a structure or a other solid body caused by same alternating
FORCE, may be transmitted readily to distant parts of the structures and RE-RADIATED from
large surfaces (walls, ceilings, floors) as irritating noise. It may even be transmitted to other
buildings.
Vibration may have the following EFFECTS:
The source of vibration usually has a predominant frequency at which it vibrates, called THE
DISTURBING FREQUENCY. The resilient mount with the weight of the equipment on it will
have its own RESONANT FREQUENCY OR NATURAL FREQUENCY OF OSCILLATION, at
which it will oscillate if given a deflect ran and then allowed to move on its own.
The more deflection in the system, the lower its natural frequency. The degree of vibration
isolation provided by the resilient mount depends on the ratio of the driving frequency and the
natural frequency. The natural frequency of the resilient mount must be at least twice as low as
the driving frequency if any vibration isolation is to be obtained. No vibration isolation will be
achieved if the natural frequency of the resilient mount is higher than the disturbing frequency.
If the natural frequency & the disturbing frequency are equal, or nearly equal, the resilient
mount will make the situation works that is more vibration will be transmitted than if no resilient
mount were used at all.
The amount of deflection of the resilient mount resulting from the dead weight of the supported
load is called STATIC DEFLECTION or STATIC DISPLACEMENT. Extra care should be taken
in the selection of a resilient mount if the frequency of vibration is low.
The transmission of vibration from one structure to another can be avoided by interposing a
resilient element called a VIBRATION ISOLATOR between the 2 structures.
The VIBRATION ISOLATOR can be any one of the following:
Resilient floor mount (steel spring, rubber, cork, felt, neoprene, easterner, fiber
glass).
Resilient ceiling hanger.
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DUCT LAGGING:
Duct lagging may include enclosing the duct in gypsum board or insulation wrapped in
sheet lead.
Duct lagging using a gypsum board enclosure (left) or lead-wrapped around insulation (right).
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ISOLATION HANGERS:
MUFFLERS:
Sound attenuators also called mufflers, silencers or sound traps are prefabricated devices
designed to provide much greater sound attenuation over a wider frequency range than would
be provided by equal length of lined duct.
They provide greater attenuation because the distance between absorptive layers is much
smaller than in a lined duct.
Mufflers are normally available in both rectangular and circular cross sectional shapes with
absorptive baffles or splitters.
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CROSS TALK:
Crosstalk is unwanted sound which is transmitted from one to another through short,
common air ducts.
To reduce crosstalk, main duct runs should be located above corridors, with individual
branches extending to each space.
Return air transfer ducts to plenum spaces above ceilings should have duct liner
installed, and there should be an elbow in the duct .
The sound transmission loss through common ducts should exceed by more than 5 dB ,
the transmission loss of the
common wall between rooms.
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