ASHRAE Journal - January 2021 - 52

COLUMN ENGINEER'S NOTEBOOK

TABLE 2 Acoustic design begins with the source, path and receiver.

TABLE 1 Sound pressure levels for various sound sources.
SOURCE OF SOUND

SOUND
PRESSURE, dBA

Handgun (Arm's Length), Pain Threshold

130

Very Loud Nightclub, Two-Stroke Chainsaw

110

Typical Rock Concert

100

High School or Band Class (High School Band Class 4 Hour
Exposure Limit for OSHA Hearing Protection)

Very Loud Restaurant, Alarm Clock

Loud TV in Hotel Room w/ Complaints, Car in Next Lane at 50 mph

Conversational Speech at 6 ft

Business Call Center, Open Office

Executive Office, Home, Birds Chirping

Good Conference or Sleeping Room (Whisper Near Ear)

Quiet Rural Area, Mosquito

Good Recording Studio; Slow, Shallow Breathing

Threshold of Hearing (Young Person)

Anechoic Chamber

-10

IMEG site tests/calibrated readings.

compromise this very quickly. The clear market trend
is to improve rooms and limit arbitrary design compromise in acoustic design. But many projects still live
with a degree of acoustic compromise with permanent
embedded shortcomings. A more circumspect approach
that is not defined by a product alone requires a balanced viewpoint with acoustic metrics that are applied
well.
Room acoustics are inextricably tied to designed AV
systems. AV systems need good acoustics because AV
technology has limitations built in, including bandwidth
and compression artifacts that regularly degrade speech
intelligibility if the room is not providing a good acoustic
environment. A room with a good acoustic signature
includes low background noise and barrier control even
before absorption is added. Fuzzy wall panels alone are
no longer " the acoustics " for a space.
Architects are now challenged more often to predict
room acoustics before a building is built. Most people
reading this are engineers, and we (the authors) aspire
to move the reader to the level where you can look at
architectural plans and say, " This will not be a good
acoustic environment for your client. " Engineers
can be thought leaders and should assist their architects in as many areas as possible, especially where
52

ASHRAE JOURNAL

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JAN UARY 2021

1.
2.
3.
4.

First Determine How Loud it Needs to Be for a Receptor?
Quiet The Source, First!
Find Path Leaks
Break Vibration Paths
* Use springs or pads, placed first at the AHU.
* Float and/or decouple the floor.
* Use air voids, gaps. Separated studs.
* Use decouplers. Beware of recoupling with only resilient channel.

acoustics and noise control are a key metric for speech
intelligibility.
The first task in room acoustics design is to assess programming and select the acoustic metrics appropriate
for the needs of listeners connected to it. For example,
noise control for an on-air broadcast should address
exterior building barriers more stringently than a
simple office or retail building might. The International
WELL Building Institute's WELL guidelines ask for an
STC barrier for building shells so urban noise is abated.
Yet the abatement chosen does not depend on the building skin or the guideline, but on how loud the noise
source is first. For example, if you are near an airport,
train or other very loud source, the envelope is much
more important.

Acoustics 101
Defining a room's " source-path-receiver " criteria is
Acoustics 101. One should apply noise control at the
source first, then along the path and for the need of the
receivers. But there is a sequence. It always begins with
how loud the source is. Then how quiet do you need it to
be at the receivers? Only after these questions are evaluated are the wall barrier assemblies and silencers in the
path considered. This process of design is counter to the
usual noise abatement request for " What product do I
use? " or " Give me a wall barrier rating. " Product selection alone does not assess source-path-receiver design
methods. Again, the first question instead is simply how
loud or quiet must it be. The best design results are usually from source-path-receiver design methods and are
often the lower-budget solution with more accurate and
refined results.
Only after the noise source is identified and receptors
are characterized is the path solution dealt with. One
must find the weakest barrier or the " leaks " in the path
first. These often include a too-low sound transmission class (STC) barrier value for window glazing, or

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ASHRAE Journal - January 2021

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