HVAC noise
& vibration
control
Noise and vibration control
of heating, ventilation and
air conditioning (HVAC)
systems is vital for providing
comfortable and safe
environments for occupants.
Excessive noise levels lead to
poor communication, fatigue
and stress. High noise levels
can even cause hearing loss,
especially when exposed
for extended periods of
time. Excessive HVAC noise
and vibration is usually
due to poor design, poor
installation and sometimes
poor maintenance.
Proper system design and
equipment selection avoids
the need for costly and
disruptive remedial works.
Good design, coupled
with carefully supervised
installation and regular
maintenance, including
testing, is the most effective
way to ensure that a
building is free from noise
and vibration problems.
Basic noise
criteria
The most commonly used
descriptors of HVAC noise
are as follows:
NOISE CRITERION (NC):
The NC curves were
established in the United
States for rating indoor
noise, and mainly from air
conditioning equipment.
They consist of a set of
criteria curves extending
from 63 to 8,000 Hz which
use a tangency rating
procedure. The criteria
curves define the limits of
octave band spectra that
46
Saying that HVAC noise and vibration could be attributed to
issues like poor design, wrong selection of equipment, incorrect
installation methods and lack of maintenance, Edward Adamczyk
demonstrates that proper care and attention during all stages of a
project can help mitigate the problems.
must not be exceeded to
meet occupant acceptance
in spaces. The NC rating
can be obtained by plotting
the octave band levels for a
given noise spectrum against
the NC curves. The noise
spectrum is specified as
having an NC rating, which
is the same as the lowest NC
curve, which is not exceeded
by the spectrum.
NOISE RATING (NR): The
NR curves are a set of
internationally agreed octave
band Sound Pressure Level
(SPL) curves, based on the
concept of equal loudness,
as referenced in ISO/R 1996
(Acoustics – Assessment
of noise with respect to
community response). They
represent the human’s
perception of noise, where
less annoyance is caused by
low frequency noise than by
higher frequency noise at the
same sound pressure level.
Hence, higher levels may be
tolerated for low frequency
noise. The NR curves are
slightly less sensitive at
low frequencies and more
sensitive at high frequencies
than the corresponding NC
curves.
dBA (A-Weighted Noise
Level: The sound pressure
level determined when using the
frequency-weighting network
A) The A-weighting network
modifies the electrical
response of a sound level
Climate Control Middle East September 2013
Noise
limits
The table below presents
typical requirements from
different international design
standards and guidelines
documents which advise on the
maximum permissible noise
levels in different room types:
Room Type
dBA
Hospitality
 Guestrooms and suite
 Meeting rooms and
banquet spaces
 Corridors and lobbies
 Service and support areas
35
35
45
45
Health Care
 Patient rooms
 Wards
 Operating theatres
 Corridors and lobbies
30
35
35
40
Residential
 Living spaces
 Resting spaces
 Bathrooms, kitchens,
utility rooms
40
35
45
Commercial
 Cellular offices
 Meeting rooms and
executive offices
 Open plan
 Corridors and lobbies
35
30
40
45
Religious
 Main praying / assembly
halls
35
Education
 Classrooms
 Large lecture rooms with
sound reinforcement
 Large lecture rooms
without sound
reinforcement
 Laboratories
30
30
25
35
meter so that the sensitivity
of the meter varies with
frequency in approximately
the same way that the
sensitivity of the human
hearing system varies with
frequency. As the human ear
has a non-linear frequency
response, it is less sensitive
at low and high frequencies
and most sensitive in
the mid-frequency range
between 1 kHz and 4 kHz.
The A-weighting is applied
to measured or calculated
SPLs so that these levels
correspond more closely to
the response of the human
ear. A-weighted SPLs are
often denoted dB(A) or dBA
or dB LA. It has been found
that, in the majority of cases,
the difference between the
dBA level and NC/NR curves
is approximately 5 to 6 units,
i.e. NC/NR 25 ≈ 30/31 dBA.
HVAC
system noise
assessment
Noise assessments for
HVAC systems involve
the examination of the
source (equipment) and
its transmission paths to
a receiver point (noisesensitive space). There
are multiple paths, which
include the supply, return,
and noise breakout from
the equipment casing. It is
paramount that all noise
(and vibration) paths are
reviewed and assessed.
Since noise takes the path
of least resistance, even a
small acoustic weakness in
a system will significantly
affect the noise level
generated in a space. Any
short-circuiting of vibration
will similarly result in
structure-borne transmission,
which can subsequently be
re-radiated as airborne noise
in a space.
The following is a list of
common noise and vibration
issues:
 Wrong selection of
equipment
 Insufficient space for
implementing noise
control, such as duct
attenuators
 Incorrect locations for
attenuators and duct
fittings, which increase
airflow generated noise
 High velocities in ducts,
which increase airflow
generated noise and cause
duct vibration
 Misalignment of ducts and
flexible connections, which
result in increased air
turbulence
 Poor balancing of systems
with the overuse of
dampers
 Excessive vibration caused
by inadequate vibration
isolation of equipment and
adjoining ductwork and
pipework.
 Poor sealing of service
penetrations and open
return air routes which
cause crosstalk and reduce
speech privacy between
adjacent spaces
 Lack of room acoustic
treatments, which
increase reverberation and
accentuate the room noise
levels
 Flanking sound
transmission around
structures through indirect
paths
The above issues can be
resolved by considering the
following solutions:
 Select low-noise equipment
 Include sound attenuators,
lined ducts/plenums and
duct lagging
 Design duct systems with
smooth transition elements
and correct air flows to
minimise turbulence
 Provide vibration isolation,
as prescribed by the
equipment manufacturers
and as per guidelines given
by ASHRAE and CIBSE
 Clean areas after
installation,
ensuring
that
there
are no debris
or temporary
protectors still
connected to
the equipment
and/or
isolators
 Select grilles,
registers and
diffusers with
low pressure drops and
low airflow velocities
 Control crosstalk using
lined ducts, return air
boots or attenuators
 Control flanking sound
transmission at all
junctions/interfaces.
Sustainable
design
There are several sustainable
design credit systems used
around the world, such
as BREEAM (Building
Research Establishment
Environmental Assessment
Method) used in the UK,
CASBEE (Comprehensive
Assessment System for
Building Environmental
Efficiency) used in Japan,
Green Star used in Australia,
HKBEAM (Hong Kong
Building Environmental
Assessment Method), LEED
(Leadership in Energy and
Environment Design) used in
the US and internationally.
In the UAE, the Green
Building Regulations are
used in Dubai, and Estidama
is used in Abu Dhabi, while
Qatar has the GSAS (Global
Sustainability Assessment
System).
The above assessment
methods recognise the need
for good control of acoustics,
noise and vibration, which
will result in additional
credit ratings for buildings.
There are many acoustics
products available on the
THErE ArE MAny ACoUSTICS
prodUCTS AVAIlAblE on THE
MArKET THAT HAVE bEEn dESIGnEd
To MEET SUSTAInAblE bUIldInG
rEqUIrEMEnTS, UTIlISInG
rECyClEd MATErIAlS or wITH low
lEVElS oF VolATIlE CHEMICAlS
market that have been
designed to meet sustainable
building requirements,
utilising recycled materials
or with low levels of
volatile chemicals. Acoustic
consultants can use their
knowledge of locally
available materials and
products to advise on such
matters.
construction administration,
site supervision and testing
services. The overall result
is a project that meets the
client’s requirements, for
which the contractors have
fulfilled their obligations in
achieving the design intent.
Conclusion
With proper care and
attention during all stages
of a project, acoustics,
noise and vibration control
can be achieved to avoid
the problems highlighted
in this article. An acoustic
consultant appointed at
an early stage can set
appropriate standards for the
project, ensure compliance
with local codes and/
or international practices,
prepare specifications,
review tenders and provide
The writer is the Head of
Acoustics and Principal
Consultant at PMK
International. He can be
contacted at edward.a@
pmkllc.com
More on acoustics in the next issue.
September 2013
www.climatecontrolme.com
47