90 Chapter 4
use window units that get shut off when quiet is needed!
If this solution is not acceptable, and central distributed
systems must be used, the designer must understand that
success will require significant expense and engi-
neering. The design of HVAC systems is best left to
professional mechanical engineers. No better prepara-
tion for this responsibility can be obtained than from
carefully studying the American Society of Heating,
Refrigeration, and Air-Conditioning Engineers
(ASHRAE) publications.16,17,18
It is important to understand that HVAC systems
found in most residences or even in light commercial or
office spaces are totally inadequate for use in noise crit-
ical spaces. Unlike residential systems that often use
high efficiency systems that deliver low volumes of
cold air at high velocities, low noise systems require
high volume, low velocity delivery. Many commercial
systems utilize supply ducts and the return relies on
leakage under doors or common ceiling plenums. In
order to achieve low noise, both the supply and return
must be individually ducted to each room.
4.4.1 Location of HVAC Equipment
From the standpoint of sound room noise, the best loca-
tion for the HVAC equipment is in the next county.
Short of this, a spot should be selected that isolates the
inevitable vibration of such equipment from the
sound-sensitive area. A good situation is to have the
equipment mounted on a concrete pad completely
isolated from the structure. In this way, the noise
problem is reduced to handling the noise coming
through the ducts, a much simpler task than fighting
structure-borne vibration.
4.4.2 Identification of HVAC Noise Producers
The various types and paths of HVAC noise producers
are identified in Fig. 4-43. This figure provides an inter-
esting study in flanking paths. It is important to
remember that there will be relatively little noise reduc-
tion unless all of the paths are controlled. A represents
the sound room. B represents the room containing the
HVAC system. Looking at the noise sources as
numbered, 1 and 2 represent the noise produced by the
diffusors themselves. The noise is produced by the air
turbulence that is created as the air moves through the
diffusor. Many diffusors have a noise rating at a given
air flow, and the only element of control in this case is
selecting the design with the best rating. Don’t forget
that this applies to the return grille as well as the supply
diffusor. Arrows 3 and 4 represent essentially fan noise,
which travels to the room via both supply and return
ducts and is quite capable of traveling upstream or
downstream. The delivery of fan noise over these two
paths can be reduced by silencers and/or duct linings.
Sizing the ductwork properly is also a means of
combating fan noise since sound power output of a fan
is fixed largely by air volume and pressure. Arrow 5
represents a good example of a flanking path that is
often missed. Depending on how the ceiling in both of
the rooms is constructed, the sound from the HVAC
unit can travel up through the ceiling in the HVAC
room and comes down into room A. Of course the way
to control path 5 is to make sure that the ceilings in both
rooms are well built, massive enough to control low
frequency vibrations, and of course, airtight. Arrow 6
represents that path where the sound can travel through
gaps or holes inadvertently left in the partition. This has
already been discussed in Section 4.3.11 and in Fig.
4-17. Number 7 represents the sound that can travel
straight through a poorly built wall. Numbers 8, 9, and
10 represents the paths that the structure-borne vibra-
tions can take through the structure. We will deal with
isolation issues in the next section. Finally, 11 and 12
represent what is called break-in noise. This is what
happens when sound enters or breaks into a duct and
travels down it, radiating into the room.4.4.3 Vibration IsolationThe general rule is first to do all that can reasonably be
done at the source of vibration. The simple act of
mounting an HVAC equipment unit on four vibration
mounts may help reduce transmitted vibration, may be
of no effect at all, or may actually amplify the vibra-
tions, depending on the suitability of the mounts for the
job. Of course, if it is successful it would drastically
reduce or eliminate paths 8, 9, and 10 in Fig. 4-43. The
isolation efficiency is purely a function of the relation-
ship between the frequency of the disturbing source fd to
isolator natural frequency fn, as shown by Fig. 4-44. If
fd=fn, a resonance condition exists, and maximum
vibration is transmitted. Isolation begins to occur when
fd /fn is equal to or greater than 2. Once in this isolation
range, each time fd /fn is doubled, the vibration transmis-
sion decreases 4–6 dB. It is beyond the scope of this
treatment to go further than to identify the heart of both
the problem and the solution, leaving the rest to experts
in the field.