MODULE

123
MANAGING NOISE
IN DUCTWORK
PROUDLY SPONSORED BY
SYSTEMS
In many air conditioning systems, ductwork is used to reticulate
the conditioned air to and from the spaces to be served.

The noise level requirements of the areas to be served can affect

Skills summary the layout of the ductwork, the size of the ducts and the extent
of any noise treatment required. The capital cost of the ductwork,
the running costs and the cost of providing space for the ductwork
■ What? can all be affected as a consequence.
Noise in ductwork systems.
Because of the computational effort involved, the noise level
■ Who? requirements are often given scant attention.
Graduate mechanical engineers,
HVAC&R technicians, maintenance This Skills Workshop looks at how noise is generated in ductwork
personnel, and practitioners involved systems and transmitted to the air conditioned or adjacent spaces.
in the design and/or day-to-day
It also looks at strategies you can use to reduce ductwork noise.
operations of HVAC&R systems.

Noise in Generation of Noise in ductwork systems is generated by:

noise in ducts • The fan
ductwork systems Noise in ductwork systems is often a very • Air terminals
Noise from the equipment, ducts, or air outlets important aspect of ductwork design. • VAV boxes
is an important part of the comfort equation. Poorly executed designs can cause noise • Dampers and other fittings
The placement of equipment must consider the generation by the ductwork system, which
impact of equipment noise during operation and • Ducts.
may lead to complaints.
may involve consideration of noise and vibration.
The noise passes down the ductwork system and
Adding bends and straight duct and reducing
enters the room, where the acoustical properties of
turbulence can reduce noise
the room determine the final noise level.
As air speed increases in duct systems, so does
the noise level. Ducts are sized to maintain the Noise and air velocity
maximum velocity of air without adding noise. There is a direct relationship between duct
air velocity and the noise generated by the
airflow. All other things being equal, increasing
air velocity causes more turbulence and more
noise. Table 1 provides some guidelines on
recommended maximum duct velocity for specific
noise outcomes, but designers should always
select and check appropriate duct sizes to suit the
specific project’s noise criteria. The table is only
applicable to systems greater than 1,000L/s and
provides absolute maximum duct velocities, not
Adding bends and straight duct to reduce noise recommended design velocities.

Table 1 – Maximum duct velocities and noise effects

Main supply
Required Riser velocity Branch duct Run-out Return duct
duct velocity
NR level (m/s) velocity (m/s) velocity (m/s) velocity (m/s)
(m/s)
50 10.0 10.0 8.0 6.0 8.0
45 10.0 9.0 7.0 6.0 7.0
40 10.0 8.0 7.0 5.5 7.0
35 10.0 7.5 6.5 5.0 6.5
30 9.0 6.5 5.5 4.0 5.5

July–August 2019 | www.airah.org.au/nation | HVAC&R Nation | 15

HVAC&R Skills Workshop

Noise from outlets Medium-high frequency noise • Attenuation effect of fittings

The type of air outlet chosen and its placement Medium-frequency and high-frequency noise
• Changing the layout to introduce more bends
in the space will have an impact on noise levels. tends to be easier to attenuate than low-frequency
or longer duct runs
noise.
Air that leaves an air outlet at a higher velocity
than the outlet is intended to handle will create an Lined or double‑walled ducts can provide good • Fan selection to change fan noise
undesirable whistling or hissing noise. Improperly attenuation. • Inserting silencers
placed or selected air outlets can also create drafts
in the occupied zone, which is a comfort issue. Lengthen duct runs if necessary (on the non-index • Changing the acoustical properties of the room.
run) to provide additional attenuation.
When selecting an air outlet, consult the To avoid noise issues:
Silencers can be effective at attenuating specific
manufacturer’s performance data, which will list a
targeted sound frequencies. • Start with quiet equipment
comparative noise criteria (NC) rating
Silencers • Locate air-handling equipment in less sensitive
Noise frequencies areas
Duct-mounted silencers can be very effective at
Low-frequency noise
attenuating airborne noise in ducts. • Allow for proper fan outlet conditions
Low-frequency noise can break out of or break into
a duct and be transferred with the airstream. Silencers are generally designed and tested to • Address “system effect”
provide a specific “Insertion loss”.
For rectangular ducts: • Use radiused elbows where possible
Pressure drops need to be considered, particularly
• Use rectangular ducts where breakout noise is on the index run. • Larger ductwork reduces velocity and reduces
beneficial generated noise
Reactive silencers have low pressure drops.
• Do not use where break-in noise is a concern. • Avoid abrupt changes in layout
Dissipative silencers have no fill, they use baffles
Round ducts: and chambers to attenuate noise. • Place dampers away from outlets
• Do not allow as much breakout • Provide flexible connections to equipment.
• Do not allow as much break-in.
Controlling noise
Noise is controlled in a variety of ways. There are various calculations involved in
Acoustic liners: determining noise levels from a ductwork system.
• Internally lining ductwork with acoustic
• Thicker liner typically attenuates lower absorbing insulation (bare ducts provide These calculations are complex and invariably
frequencies. limited attenuation) require the use of a computer program.

More mass in the duct walls is better at attenuating • Increasing duct size will reduce air Further information can be found in the AIRAH
lower frequencies. velocity and noise DA03 Ductwork for Air Conditioning manual.

Table 2 – Comparative NC values

Communication environment Typical occupancy
Extremely quiet environment;
Broadcasting studios, concert halls,
<NC25 suppressed speech is quite audible;
music rooms.
suitable for acute pickup of all sounds.
Very quiet office;
Residences, theatres, libraries,
NC 30 suitable for large conferences;
executive offices, director’s rooms.
telephone use satisfactory.
Quiet office;
satisfactory for conference at a 5m table; Private offices, schools, hotel guestrooms,
NC 35
normal voice 3 to 10m; courtrooms, churches,hospital rooms.
telephone use satisfactory.
Satisfactory for conference at a 2 to 3m table;
NC 40 General office, labs, dining rooms.
telephone use satisfactory.
Satisfactory for conference at a 11/2 to 2m table;
Retail stores, cafeterias, lobby areas,
normal voice 1 to 2m,
NC 45 large drafting and engineering offices,
raised voice 2 to 4m;
reception areas.
telephone use occasionally difficult.
Unsatisfactory for conferences of more than 2 or 3 persons;
normal voice 300mm to 600mm, Computer rooms, stenographic pools,
>NC 50
raised voice 1 to 2m; print machine rooms, process areas.
telephone use slightly difficult.

16 | HVAC&R Nation | www.airah.org.au/nation | July–August 2019

 HVAC&R Skills Workshop

Designing ductwork to Branch takeoffs

minimise noise regeneration Radius or bevel branch takeoffs are preferable to Y or straight takeoffs.
See figure 3.
Maintain the air velocity in the duct as low as practicable. The design of fittings
can have a significant impact on system resistance and fan operating energy
requirement. Providing low pressure drop and laminar flows across ducts and NOISIEST BETTER QUIETEST
fittings will minimise the potential for regenerated noise.

Elbows ½D

Use long radius elbows with full radius turning vanes D

for maximum noise reductions.

Figure 3 – Guidelines for minimising regenerated noise in takeoffs.

Tees
Keep airflows as laminar as possible, use turning vanes at all diverging T
connections.

NOISIEST BETTER QUIETEST

Turning vanes

Figure 1
No vanes Two square elbows Dual radius
to direct back-to-back split
Long radius elbows without vanes and square elbows the airflow
with short vanes are satisfactory for most applications.
Square elbows without vanes should be avoided. See figure 2. Figure 4 – Guidelines for minimising regenerated noise in duct tees.

NOISIEST BETTER QUIETEST

Offsets
Avoid using Z-offsets without turning vanes.
D Gradual offsets (15° maximum) are preferable to Z-offsets with turning vanes.
See figure 5.
>
½
D

Square Long Radius* Square with Long Long Radius NOISIEST BETTER QUIETEST
Trailing Edge with Vanes
Vanes**
15° Max.

15° Max.

Square with
Short Vanes*
7.5° Max.
* Airflow velocity and proximity of upstream and downstream
fittings and fans determine which type is preferable.
** Trailing edge length should be at least 3 times the vane spacing.

Figure 2 – Guidelines for minimising regenerated noise in elbows. 15° Max.

Figure 5 – Guidelines for minimising regenerated

noise in transitions and offsets.

This month’s Skills Workshop has been adapted from AIRAH’s Professional Certificate
in HVAC&R Fundamentals, an online training course designed to fast-track
HVAC&R technical knowledge and skills. Go to www.airah.org.au/PCHVAC_R

PROUDLY SPONSORED BY Next month: Controls fundamentals

July–August 2019 | www.airah.org.au/nation | HVAC&R Nation | 17