Open Air Theatre

• The definition of the rows on the cavea
horizontal reflections between cavea and
proscenium. The sound will in this case
bounce back and forth between the vertical
seat rises and the proscenium wall. These
reflections will arrive at a listener position
with a large delay and attenuation but will
nevertheless contribute to a long
reverberation time.

• When the cavea of the theatre is modeled as sloped surfaces the
theatre will resemble the shape of an inverse cone. This shape will
tend to direct most of the reflections towards the open sky and
therefore the energy will dissipate quickly leaving few late
reflections.
• The diffraction and scattering effect from the empty seats is not
usually considered important in computer simulations of roofed
theatres.
• Many strong reflections from the roof and side walls mask the
much lower energy coming from scattering and diffraction.
• But in the case of the open-air theatres fewer strong reflections are
present and the gaps between strong reflections in the impulse
response have to be filled in with scattered energy in order to get a
smoother decay curve.
• An abrupt decay curve with a few strong reflections makes it very
difficult to estimate any acoustic parameter and can be misleading .

Design details :
•Selection of site is is a critical point. topographical, meteorological
and acoustical properties should be taken into consideration.
•Slope of seating should not be less than 12 degrees, for better
audibility n visibility
•Noise level should not exceed 40 decibel.;.
Seating arrangement
speaker
Sound reflection

Long term acoustic studies conducted in outdoor spaces
have shown that the basic principles of design include:
• minimisation of external noise (noise protection),
• harmonic development of the functional elements of
the theatrical space within the limits of human vocal
and acoustical scale,
• sufficient emergence of directly propagated sound and
its reinforcement through early positive sound
reflections (from the amphitheatre gradient and
natural loudspeaker response of the space),
• control of late sound reflections (limitations of the
reverberation time, eliminations of echoes)

• The design of the new open air theatres goes
back to the positive models of similar
monuments that have been preserved in the
contemporary urban plan. In order to optimise
advantages of acoustic design, the usual cases
involving adverse sound environments require
the combined exploitation of the advantagesof
open space plan and the limitation of the
boundaries of the theatre space based on the
model of Greco-roman theatre.

Historic Overview
Greek Theatre
• Open air
• Direct sound path
• No sound reinforcement
• Minimal reverberation
S: p. 785, F.18.17a

overhang
Sound reflecting surface
Acting arena
Orchestra area
seating

WHY THE GREEKS COULD HEAR FROM THE BACK ROW??????
• The wonderful acoustics for which the ancient
Greek theatre of Epidaurus is renowned may
come from exploiting complex acoustic
physics, new research shows.
• Its acoustics are extraordinary: a performer
standing on the open-air stage can be heard in
the back rows almost 60 metres away.
Architects and archaeologists have long
speculated about what makes the sound
transmit so well.

• The key is the arrangement of the stepped
rows of seats.
• Most of the noise produced in and around the
theatre was probably low-frequency noise, the
rustling trees and murmuring theatre-goers,
for instance. So filtering out the low
frequencies improves the audibility of the
performers' voices, which are rich in higher
frequencies, at the expense of the noise. The
cut-off frequency is right where you would
want it if you wanted to remove noise coming
from sources that were there in ancient times.

Architect’s Role
Source Path Receiver
slight major design primarily interest
influence

Acoustical Design Relationships
Site
Location
Orientation
Planning
Internal Layout

Site
Factory:
• Close to RR/Hwy
• Seismic

Location
Take advantage of distance/barriers
Distance

Location
Take advantage of distance/barriers
Natural or Man-made Berm

Location
Take advantage of distance/barriers
Building

Orientation
Orient Building for Acoustical Advantage
Parking Lot Factory
Office
Note: Sound is 3-dimensional,
check overhead for
flight paths

Mechanical vibration, physical wave or series of
pressure vibrations in an elastic medium
Described in Hertz (cycles per second)
Range of hearing: 20-20,000 hz
Sound

• An efficiently designed orchestral shell is mandatory, for uniform distribution
of average sound level and the shell enables on stage audibility of a performer .
• Sound amplification system should be provided for an audience over
600.

Sound Intensity
Sound power distributed over an area
I=P/A
I: sound (power) intensity, W/cm2
P: acoustic power, watts
A: area (cm2)

Reflector wall.

Reflection Paths of Sound
• The sound will in the case of the cavea horizontal
reflections between cavea proscenium bounce back
and forth between the vertical seat rises and the
proscenium wall. These reflections will arrive at a
listener position with a large delay and attenuation but
will nevertheless contribute to a long reverberation
time.
• When the cavea of the theatre is modeled as sloped
surfaces the theatre will resemble the shape of an
inverse cone. This shape will tend to direct most of the
reflections towards the open sky and therefore the
energy will dissipate quickly leaving few late reflections

• Many strong reflections from the roof and side
walls mask the much lower energy coming from
scattering and diffraction. But in the case of the
open-air theatres fewer strong reflections are
present and the gaps between strong reflections
in the impulse response have to be filled in with
scattered energy in order to get a smoother
decay curve. An abrupt decay curve with a few
strong reflections makes it very difficult to
estimate any acoustic parameter and can be
misleading.

• The function of an open air theatre must fulfil
the dual goal of maximising the advantages of
acoustic design and minimising the effects of
noise pollution on the environment, an
objective that can be described with the signal
to noise ratio S/N. on the basis of a practical
method for applying this acoustic evaluation
criterion, we shouldevaluate the spatial
distribution of the revelant derivative values
of ‘emergence’ and spectral density