Open Air Theatre

• The definition of the rows on the caveahorizontal 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 advantagesofopen 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 forflight 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