ASHRAE Journal - January 2021 - 55

COLUMN ENGINEER'S NOTEBOOK

appropriate locations. Finishes may include hard panels
with perforation, or a hard diffusion treatment located
over talkers for a conference room ceiling. Note, if the
acoustician ﬁrst starts his acoustic design with only interior treatments, a scope gap may already exist.
It is important for hard reﬂecting surfaces that support speech, instead of absorbent materials to be
located above and behind most talkers. To reduce
the reverberation time, presentation rooms might
incorporate absorbent wall panels placed along one
rear wall and then extend only through one corner to
the next wall. This treatment is likely in addition to
the selection of a higher absorption value (NRC) for
acoustical ceiling tiles. Absorptive wall panels can be
used to truncate a destructive reﬂection path between
walls-and despite having an absorbent ceiling still in
the room.
When AV technology is added, loudspeaker integration to a room is important and assessed for location,
aim and seat coverage. For AV, we turn down the speaker
loudness when possible, then increase loudspeaker density and coverage. This aids speech clarity. Any large or
powerful loudspeaker is usually modeled in software to
keep its projected sound off the rear and distant walls.
This reduces echo and improves clarity. Good loudspeaker integration is a priority for acoustical design
and for designers who understand acoustics with professional audio is an improved trend for the A/E industry.

Room " Voice "

All rooms have a transfer function. This term means
how the room translates sound as a composite acoustic
signature that we hear and process in our human hearing mechanism. This refers to our term " sense of place "
discussed in last month's column. That sense of place
can be understood and heard intuitively by humans due
to small aural cues embedded in what we hear and perceive. For example, a blind person can perceive when
they approach a wall as their sense of place is naturally
enabled. Room acoustics are full of these small aural
cues.
We can see this when a child enters a long tunnel and
naturally likes to make noise and listen excitedly to the
tunnel's noise signature. He is not told about this, but
he hears and reacts intuitively. These aural cues are
very important for remote conferencing and clarity for
conference rooms, classrooms, and intelligibility in

occupied spaces.
The room's aural translation to support good clarity depends ﬁrst on the important direct, line of sight
shortest signal, then its ratio to everything else in the
signal chain, such as reverberance (RT60), to name but
one. The direct sound of a talker or a loudspeaker must
be unimpeded to be clear. This clear sound information must preserve speech consonants. Think of words
you can abbreviate-you can leave out the vowels. Then
timing and strength of all other room reﬂections are
important. These short room reﬂections can be positive and constructive if their reﬂection time is short
(less than approximately 30 milliseconds). This time
also converts to distance-approximately 30+ ft (9 m) as
a general dimension marker. So, a total path of 60 ft
(18 m) (30 ft [9 m] from podium to rear wall and 30 ft
[9 m] back to a front row can be a late-arriving destructive inﬂuence.
These longer reﬂections are destructive due to long
physical distances that distract listeners-the brain
interprets them as echoes instead of part of the original
speech signal. The existence of these late signals can
determine when to acoustically treat a near surface for
positive support for speech clarity, or when to treat for
late and destructive echo. Your local acoustician will be
glad to help.

Conclusion

Acoustical design is a basic process with fundamental metrics assembled in a thoughtful sequence.
Absorption has been equated with " acoustics " in the
last century, but absorption is often misunderstood
and overused. While absorption remains useful,
recent design has advanced to new practices that
are integrated and more important for architectural
styles found today. This includes using diffusion and
controlled reﬂection for speech support as well as
designing HVAC with predictable quietness. While soft
absorptive panels and new acoustic felts are exciting
visual geometrics, they should only be used in appropriate or limited amounts because the room composite
signature is a balance of hard diffusion and the amount
of quietness needed ﬁrst. When a room
is properly designed it may not need to
be treated with soft absorptive panels,
and the goal of appropriate acoustics
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