Architectural Acoustics in Relation to Music and
Hearing
Bill Gastmeier
WCA, 2016
Good Acoustics?
 The design of interior spaces for “good acoustics” has been defined
since the early 1900’s in terms of reverberation and background
sound. Both of these are highly integrated quantities which relate
primarily to the overall acoustical characteristics of a space.
 High levels of both reverberation and background sound are well
understood to interfere with intelligibility of speech in normal hearing
persons. Lower levels of reverberation and background sound
interfere with speech intelligibility in individuals using hearing aids.
The first portion of this presentation deals with these effects
What is Good Acoustics?
 What is good acoustics?
 Careful attention to the
physical details
 Depends on the activity.
Music? Speech?
Classrooms?
 ANSI S12.60-2010 –
“Acoustical Performance
Criteria, Design
Requirements and
Guidelines for Schools
Reverberation …….
 Reverberation is
“left over” sound
energy. It is like
the mythical giant
whose voice was
so loud you still
hear it a year
later!
Reverberation ………..
 Looking at it another way, the
Reverberation Time (RT) is the time
sound decays by 60 decibels.
 Wallace Sabine understood that some
materials absorb sound and some
reflect it and that you can determine
the amount of absorption of finishes
and furnishings.
 Sound travels at 344 m/s, so you can
measure the dimensions of a room,
find the mean free path of sound and
determine how long sound will linger.
 RT=.161 V/A, V = Physical Volume
 A= Total Absorption (Sabines)
A New Way to Look at Reverberation
 Now we can delve into how we “hear”
reverberation.
 The important cues are the early reflections
(less than 40 – 80 ms) as they enable us to
locate ourselves in our surroundings. Lateral
reflections are most important.
 Our hearing systems have adapted to
strongly listening to these reflections and
suppression of later arriving reverberation.
 This is illustrated by time reversed
recordings. For Beatles fans, Paul is not dead
and we still don’t know who is the Walrus.
Background Sound Levels
 ANSI S12.60-2010 recommends a background sound level of less than 35 dBA in
classrooms.
 From the Standard …” Young children are also more susceptible to temporary conductive
hearing impairment caused by ear infection (otitis media) than adults. Demographic
research has identified otitis media as the most common medical disorder in young children,
with an estimated incidence as high as 25% to 30% among kindergarten and first graders.
 Other research has found an incidence greater than 10% of mild high-frequency
sensorineural hearing impairment among children 6 to 19 years of age. Signal-to-noise ratio
improvements of 3 dB to 5 dB together with increases in absolute speech sound levels of 10
dB to 30 dB are necessary for children with these impairments to achieve the same level of
speech intelligibility in classrooms with high background noise.
Reverberation Criteria
Criteria for reverberation have been developed based on Sabine’s original work and are given
in the following table compiled from many sources.
Performance
RT (seconds)
Traditional Organ Music
2.5 – 5.0
Symphonic Repertoire
1.8 – 2.1
Chamber Music
1.6 – 1.8
Opera
1.3 – 1.6
Modern Music
1.1 – 1.7
Live Theatre
0.9 – 1.4
Lecture or Conference
0.6 - 1.1
OOOPS
The tricky bit is that they don’t work very well as illustrated in the
following table.
Different quality Concert Halls but Similar Reverberation Times (RT)
Adapted from Hakida et. Al., 1995
Concert Hall
Concert Hall
Quality
Quality
Symphony
Hall, Boston
Symphony
Hall,
Superior
Superior
1.85
1.85
Good
Good
1.85
1.85
Fair to Good
Fair-to-good
1.7
1.7
Boston
Davies
Hall, San
Francisco
Davies
Hall,
San Fransisco
Barbican
Hall,
Barbican
Large Large
Hall, London
London
RT
RT(sec)
New Acoustical Measures
 In recent decades, quantities related to the fine structure of sound
arriving at the ear and most importantly early arriving lateral sound
have been found to be important to the musical experience.
 Terms such as Initial Time Delay Gap, Lateral Fraction and Binaural
Quality Index, Interaural Cross Correlation Coefficient and C80 are
coming into more common usage.
Early Time Decay (EDT)
 The Early Decay Time (EDT) is based on the initial decay
(10 decibels) which is the only part of the decay process
which remains audible between rapidly played notes. As
such, it tends to correlate better with acoustical quality for
symphonic music than RT.
C 80
 The C80 ratio of early to late sound energy is determined by
the energy summed in the first 80 ms as compared to the
energy in the remainder of the reverberant sound. (Bradley,
1990)
 This ratio correlates highly with “Clarity” and “Definition”.
 One reason for the success and lasting usefulness of
rectangular music halls is that strong early reflections from
side walls and ceiling enhance clarity.
Interaural Cross Correlation Coefficient
 The Interaural cross-correlation coefficient (IACC) is a
measure of the difference in sound between the two
ears with the listener facing the source (Hidaka et al.,
1995).
 It correlates well with spaciousness since the sensation
of spaciousness depends strongly on early lateral
(horizontal) reflections.
Thank-You!