ASHRAE Journal - December 2020 - 55

COLUMN ENGINEER'S NOTEBOOK

FIGURE 4 STC 50 values are all similar, but widely vary in performance.

Steel Stud, CMU and Glass Walls
80

Impact Insulation Class (ICC)

Transmission Loss (dB)

60
50
40
30
20
STC 50 Contours
CMU: STC 50
Steel Stud: 1+1 Gyp - NRC Avg. of 40-STC 49
Operable: Glass - STC 51

10

31.5
40
50
63
80
100
125
160
200
250
315
400
500
630
800
100
0
125
0
160
0
200
0
250
0
315
0
400
0
500
0
630
0

0
Frequency (Hz)

FIGURE 5 Steel stud wall showing effects of insulation and added layers of gypsum.

STC - Steel Stud Walls
80
70
60
Transmission Loss (dB)

A fourth important metric is impact insulation class (IIC) (Figure 7). The current architectural style of hard exposed floors affects
IIC because high heel shoes on hard floors
or dropping weights from a training center
are the most common issues. It cannot easily be fixed during a later design phase and
especially not if construction has begun. Early
detection is the cure for IIC, and if not considered early it will increase acoustic risk, especially in a multifamily ownership condition.
Reducing IIC may only add a few millimeters
to an inch of thickness to abate and improve
structural vibration to an occupied space
below. The IIC value is notable on stairs tied
to a structure where impact on stair treads is
not easily decoupled from an adjacent tenant
wall.
A common misconception is that a concrete
floor is perceived to be solid and massive with
high STC values. But concrete has a very low,
ineffective IIC value. For example, a high
heeled shoe will drive a sleeper below crazy!
This is because solid concrete is a very effective medium for sound travel. People often
construe concrete as a good IIC for impact
when they are thinking of STC barrier instead.
Improperly selected IIC design will prevent
sleep in a hotel room and can result in an
angry guest the next morning who refuses
to pay the bill. Sleepers can trace a walker
on their ceiling as they lay awake at night.
Carpet improves IIC, but design styles are
trending toward hard exposed floors. As
marketing departments invariably try to
publish the highest IIC value they can, they
often show high performance assemblies,
implying high IIC values that may not be
used in the project. Hence, the risk and
confusion of this metric is high, and it represents the type of risk that a lack of knowledge
of acoustics can create.

70

50
40
30
20
10
0
31.5
40
50
63
80
100
125
160
200
250
315
400
500
630
800
100
0
125
0
160
0
200
0
250
0
315
0
400
0
500
0
630
0

of a wall with non-contiguous framing separated by an air gap. Thicker walls with added
mass are considered improvements.

Frequency (Hz)
STC 50 Contour
Steel Stud: 2+2 Gyp - Avg. of 2 - STC 56
Steel Stud: 1+1 Gyp - Avg. of 40 - STC 49
Steel Stud: 1+1 Gyp - No Insulation - STC 38

D ECEM BER 2020

ashrae.org

ASHRAE JOURNAL

55


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ASHRAE Journal - December 2020

Table of Contents for the Digital Edition of ASHRAE Journal - December 2020

Contents
ASHRAE Journal - December 2020 - Intro
ASHRAE Journal - December 2020 - CT1
ASHRAE Journal - December 2020 - CT2
ASHRAE Journal - December 2020 - Cover1
ASHRAE Journal - December 2020 - Cover2
ASHRAE Journal - December 2020 - 1
ASHRAE Journal - December 2020 - Contents
ASHRAE Journal - December 2020 - 3
ASHRAE Journal - December 2020 - 4
ASHRAE Journal - December 2020 - 5
ASHRAE Journal - December 2020 - 6
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ASHRAE Journal - December 2020 - 8
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ASHRAE Journal - December 2020 - 12
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