ASHRAE Journal - January 2021 - 54

COLUMN ENGINEER'S NOTEBOOK

Techniques
When a room is considered too noisy or too reflective (or both), a ceiling height might be lowered as a
first response. This early design factor might replace
adding a silencer. Lowering the ceiling lowers room
reverberation time by reducing the room cubic volume,
therefore not amplifying duct noise that could produce
a sustained, elevated and louder ambient noise. When
excess reverberation is still found, treatments are carefully located to lower the room reverberation and echo
characteristics. If the ceiling grid is already absorptive,
the next choice is increasing the ceiling absorptive value
(NRC) and adding acoustic treatment with appropriate
surface areas and locations.
The first room surface to evaluate acoustically might
be the farthest surface from a talker or an AV loudspeaker. This surface often needs treatment by virtue of
its distance and travel time from a source to a listener
in a room; this distant surface can produce a late and
destructive echo due to the longer time of reflection it
fosters. Late echo reduction in a room is Job #1 for acousticians, but only after the level of quiet is selected.
The next step after determining noise criteria (NC-x)
for quiet might be determining the shape of the room.
This is more important for small rooms with a repetitive sound " flutter. " Changing the room shape can
be a small adjustment of any two parallel walls, like
a telemedicine room where talkers must be clear as
they share information or stream their conference to a
remote site.
Adjusting a simple wall dimension can be important
to tame destructive influences. A room that is 10 ft ×
10 ft × 10 ft (3 m × 3 m × 3 m) is a bad-sounding room,
as are circular rooms. One of the authors worked with a
client with a circular reception room. When he walked
up to the receptionist, he was at the center focus of
the room, and the echo impacted his sense of balance.
54

ASHRAE JOURNAL

ashrae.org

JAN UARY 2021

FIGURE 1 Effect of gaps in walls on actual sound transmission class (STC).

60
50
STC With X% Leakage Area

characteristics. These absorption factors are assembled
and calculated to arrive at the reverberation time
(RT60) for a room as a design benchmark. If the AHU
and duct supply noise are marginal to meet the desired
NC, and the room is found too reflective from hard
surfaces, the room will amplify HVAC noise, and the
NC value for quietness will increase from its scheduled loudness value. If ceilings are high, this will also
increase reflection time and noise.

40
30
20
10
0

20
0%

25

30
0.00%

35
40
45
STC With No Leaks
0.01%

50
0.10%

55

60
1%

Another example was a bank counter with a curved wall
and domed ceiling. Private discussions became focused
to a remote patron that should not hear private bank
balances from remote tellers who were amplified by the
room.
Next, flanking paths around barrier walls and over
ceilings often create privacy issues. These constructions
must maintain acoustics for private or confidentiality. If a transfer " Z " duct is used, its length should be
evaluated, and the use of an open-cell or rated lining
considered to absorb noise. Lining is almost always an
open-cell media, never closed-cell absorption. Walls
should also be checked for tight extension to the deck for
privacy needs. Supply and return duct must be designed
to minimize openings between rooms used for privacy.
Always keep Z-duct openings far away from return
grilles.
Noise leaks (Figure 1) are especially challenging with
corrugated metal decks that are not flat on the bottom.
A seal must be made, and packing flutes with absorbent
insulation is often not a final solution without adding a
solid barrier. This is because absorption, e.g., fiberglass,
does not work for high STC-rated barriers. This small
point is one of the most consistent misconceptions of
noise control. Low density, soft absorption is not a solid
barrier.
After the process above is completed, architectural
interiors and room finishes can be considered. Interior
design first determines the amount of noise abatement
needed, then calculates the required surface areas with


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ASHRAE Journal - January 2021

Table of Contents for the Digital Edition of ASHRAE Journal - January 2021

Contents
ASHRAE Journal - January 2021 - Intro
ASHRAE Journal - January 2021 - Cover1
ASHRAE Journal - January 2021 - Cover2
ASHRAE Journal - January 2021 - 1
ASHRAE Journal - January 2021 - 1a
ASHRAE Journal - January 2021 - 1b
ASHRAE Journal - January 2021 - Contents
ASHRAE Journal - January 2021 - 3
ASHRAE Journal - January 2021 - 4
ASHRAE Journal - January 2021 - 5
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ASHRAE Journal - January 2021 - Cover3
ASHRAE Journal - January 2021 - Cover4
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