Sound & Communications April 2008 issue

Reverberation: Is It A Good Or Bad Thing Part 2
Reverberation time and intelligibility.
By Peter Mapp, FASA
Last month, I noted that Reverberation Time
(RT) is not a particularly good indicator of speech
intelligibility, though there is a general trend
suggesting that the longer the RT, the worse the
potential intelligibility—unless specific steps are
taken to overcome this. I gave the example that a
one-second space should not really pose a problem
from an intelligibility point of view. (Indeed, it
takes considerable innate skill to screw up a sound
system in a one-second space, though, from my
workload, it appears that there are many out there
with this ability!).
The table on page 17 sets out a rough guide to
reverberation and sound system design when it
comes to achieving intelligibility. This is not a
theoretically based hypothesis, but one based on
the knowledge acquired from designing and
balancing more than 400 sound systems in all
types of acoustic environments, ranging from 11-
second cathedrals to one-second theaters and 0.25
to 0.5 second domestic rooms and home theaters.
Table 1 should be used only as a guide because
many other parameters/variables also play a part.
(The volume of the space is also a critical parameter,
for example). I should, perhaps, also explain my Low,
Medium and High Q categories. Roughly speaking, I
term Q values in the range 1 to 7 as Low Q, 10 to 20
as Medium Q and 20 to 50 as High Q. These days,
we could also, perhaps, introduce an Ultra or Super
Q range to deal with long line arrays, where Q
values of 60 to 100 can be achieved.
It might also be useful at this stage to have an idea
of some typical spaces and their reverberation times.
Again, these are purely guide numbers and can vary
greatly, depending on the amount of absorption or
acoustic treatment applied. Some of the values are
the generally regarded
ideal RTs, others are
what you get in
practice. See the table
at right. ■
Type of Space
Domestic Living
Rooms
Classrooms
Offices
Lecture Theaters
Theaters, Courts, Council
Chambers, Cinemas
Concert Halls
Churches (modern)
Churches (traditional/Catholic)
Cathedrals
Stadiums
Arenas
Ice Rinks
Railway Stations
(enclosed and
untreated)
Shopping Malls
Atria & Lobbies
Airport & Transportation
Concourses
Indoor Swimming
Pools
Aircraft Hangars
Typical Reverb
Time (Sec)
0.4-0.6
0.6-0.8
0.5-1.2
0.8 (max 1 sec)
0.8-1.2
1.7-2.2
0.8-1.5
1.4-4
3.5-10
1.8-3.5
2-8
2.5-6
2-9
1.5-5
2-5
1.5-2.5
2-5
4-8
Comment
Assumes reasonably well furnished. rooms with
polished floors and hard walls may have longer RT.
Normally should be 0.6 sec or lower. Larger
rooms, e.g., general teaching areas and open plan
areas, can be 0.8 sec.
Some offices can be surprisingly reverberant.
Intelligibility becomes volume dependent.
Small rooms < 50 people < 0.8
Size/volume dependent.
Concert halls can be remarkably difficult. High
intelligibility is required. Modern line arrays usually
are the best solution.
A range of options available. Not particularly
problematic. NB Feedback and gain requirements.
This is a large range and requires a range of
different approaches (see Table 1).
Great care required. Distributed line arrays usually
required.
Vary greatly, depending on size and roof shape/
structure. RT decreases markedly with occupancy.
Acoustic treatment essential. RT is occupancydependent.
Difficult spaces: Acoustic treatment essential for
good quality sound.
Acoustic treatment required for reasonable
intelligibility/VA standards in relatively small
spaces, dependent on volume and height.
Acoustic treatment generally required for good
intelligibility, and to meet VA standards.
Acoustic treatment or line arrays/High Q devices.
Distributed systems. Speaker type will depend on
volume/ceiling height of the space.
Acoustic treatment should be provided to reduce
RT/noise.
Extremely difficult to get good coverage/adequate
intelligibility.
United Kingdom-based Peter Mapp, FASA, is principal of Peter Mapp Associates. A chartered
engineer and physicist, he also is a Fellow of the Acoustical Society of America. Send
comments to him at pmapp@testa.com.
16 Sound & Communications
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