Acoustics

Arch – 402

Open Air Theatre Design

Introduction :

The purpose of theater is to provide recreation to people. The theatre is a branch of the
performing arts and it is concerned with the acting out stories in front of the audience.

Open Air Theatre is a space in which a performance may be given before an audience and
as the name suggests, it does not have any roof structure.

The word is from the Greek theatron, “a place of seeing.” A theatre usually has a stage area
where the performance itself takes place. Since ancient times the evolving design of theatres
has been determined largely by the spectators’ physical requirements for seeing and hearing
the performers and by the changing nature of the activity presented.
 Vondelpark Open Air Theatre, Amsterdam Regent's Park Open Air Theatre, London

Teatro Antico di Taormina, Taormina, Sicily

Toga Open Air Theatre, Toga Art Park, Greek Theatre of Syracuse, Sicily
Minack Theatre, Cornwall, UK Toyama Prefecture, Japan
How sound is affected by:
Meteorological Conditions and Topography
In order to design an open air theater one must recognize how sound is affected by
Meteorological Conditions and Topography.

When we talk about Meteorological Conditions, we are trying to see how the
atmospheric conditions, like, temperature, wind flow, and humidity, etc., affect sound.

Spaces where it is not covered, or it does not come under room acoustics, we have to
take care of the noise, which is created at some point, and is carried to another point
or location, where, some good acoustical quality is required, like open-air theater.

The influence of topographical conditions,

conditions particularly the slopes, undulations, then the
plantations and the vegetations, and how sound behaves in such environments.
Influences of Meteorological
Conditions
Sound Absorption by Air
Effect of Temperature on Sound
Effect of Humidity on Sound
Influence of Wind Speed on Sound
Sound Absorption by Air

Low frequency sound travels farther

The Air Absorption:
As per the curve, the frequency of 100 Hz is minimally absorbed in air (1 dB/km),

Whereas, a frequency of 10000 Hz is absorbed, up to 100 dB per kilometer of travel of sound.

So we see that, it is not necessarily true, that all frequencies get absorbed equally, while
passing through the air.

High-frequency wavelengths are more easily absorbed by the molecules of air.

That is why, there is a significant reduction of sound of higher frequencies; whereas, there is a
negligible reduction of sound, when it is of the lower order frequency, considering that, the
distance traveled is the same.

Sound absorption decreases with increasing humidity, whereas dry air has the least absorption.

When, a sound of high frequency band is moving through air, all of it will not be reaching the
listener at the same level, but they will be absorbed at different levels while passing through
the air.

# Low-frequency sound travels the farthest, whereas, higher frequency sounds do not move to
the desired extent.
 Effect of Temperature on Speed of Sound

-13 0 30 50
Temperature in deg C
How the temperature changes the speed of sound? Here you see a graph which has been
drawn while a sound of 1000 Hz is passing through the air. At 20°C, we see speed=343 m/s
precisely.

formula which has been derived:

V= 343 m/s + 0.6 × (T – 20);
where T is the temperature in degree centigrade, which is the gradient. So, at T=20°C, the
velocity of sound is 343 m/s.

And for every 1°C change of temperature, the change in speed is by 0.6 m/s.
0.6 meters per degree centigrade change of temperature is a big amount when we talk in
terms of the wavelength of sound. In feet, this comes around 2 ft/s.

Humidity, more or less up to 30°C, it is behaving in the same manner as temperature.

But the temperature and humidity do affect differently above this.
The red line (represents 100% humidity) has shifted from the black line (represents 0%
humidity) mostly above 30°C, till 50°C.
So, the gradient is higher when the humidity is more. We have to account for these changes
in speed when we are doing any outdoor space design.
Temperature gradient leads to bending up and down of the sound waves
DAY TIME

This is called a Temperature Lapse or Super Adiabetic

When it is day time, the ground is warm, and the atmosphere above it is cool.
The sound speed at a position much above the ground will be lower.
Whereas, the sound speed below (near the ground) will be higher.

At the lower portion (near the ground), the sound will be moving faster as compared to the
upper part.

If the direction is this (rightward), there will be a change

or the sound wave will move upward. This will lead to a no sound
zone in this particular area (rightwards of the source). If a receiver
is standing here, he will not get any sound. This area becomes a
“sound shadow” area.

Thus, even if the source is close, the people around may not be receiving the sound.
Whereas, the sound energy is drifting upward.

This is called temperature lapse or “super-adiabatic”. So, temperature gradient leads to the
bending up and down of sound waves, and during the daytime, the ground being warmer,
the sound moves upwards, creating sound shadow zones in regions close to the ground.
Temperature inversion is when the temperature is coolest right next to the ground and
warmer as you increase in height above the ground

This downward refraction of sound helps hear the conversations even across
water bodies
The opposite phenomenon or an inversion happens during night time. During night time, the
ground surface is cooler, whereas, the surface or the atmosphere above is warmer (where the
sound tries to move faster), leading to an inversion.

You can see, here the sound is bending downwards or more towards the ground. And this is
called “temperature inversion”. This happens mostly during the night time when the
temperature is the coolest right next to the ground and warmer as you increase in height
above.

This downward refraction of sound helps people hear conversations even across water bodies
or across long distances, particularly at night time.

Hence, distant sounds are usually heard at night times rather than day times, particularly,
because of this temperature inversion.
This is what you see as a summary, that, hot air moves faster, and when it is night time, it bends
towards the ground, or towards the receiver from the source. And during the daytime, the hot
air drifts the sound upward which leaves the receiver with very less sound.
Influence of Wind Speed on Sound.
Influence of Wind Speed on Sound.
The effect of wind on sound outdoors is a complex phenomenon.
Downwind from the source, sound is normally bent toward the ground increasing its sound
level. Upwind, sound is bent upward causing a shadow zone where the sound level will be
reduced.

For example, at distances greater than 500 ft, as shown below, the upwind mid-frequency
attenuation can be about 10 dB for winds of 10 mi/h. However, a reversal of wind direction
can increase the sound level by about 10 dB at the same location.

Consequently, do not rely on attenuation from the wind when designing outdoor noise
control measures.
As a rule sound waves bend towards regions of lower sound speed
The influence of wind on the sound:

When the wind is flowing, it is found out that the wind speed varies logarithmically up to a
certain height (say around 30-100 meters) depending on the speed of the wind.

At the initial stage, it moves very rapidly. It is flowing, and after a certain height, it becomes
steady.

It is drifted upwards by the wind when the sound wave is traveling in a direction opposite to
that of the wind flow.

Since the source is producing sound in all directions, a portion of the sound which is in the
opposite direction to that of the wind gets drifted up. And on drifting up, some low sound
pressure zones are automatically created in the opposite direction, and the sound bends
down to fill up those spaces. So as a rule, sound wave bends towards regions of lower sound
speed.
 A

A sound wave propagating in direction of wind bends down

In upwind direction the sound speed decreases with altitude
In downwind direction the sound waves are bent towards the ground
We see that this drifting up and bending down causes a refractive effect of sound, particularly
when the wind is present and the sound is moving steadily, against the wind.

Thus in the direction of the wind, the wind speed gets summed up to the sound speed.

But when the sound is moving opposite to the wind direction, it gets subtracted.

So, when the source is at ‘A’ (see pic), and the wind is blowing from left to right direction. At
wind speed=3 m/s, say the sound is produced in the direction opposite to the wind. If we
consider the speed of sound =340 m/s, the wind speed (3 m/s) get subtracted from it.

So, here we are considering that the wind is steadily flowing, and hence we are just
subtracting the values of the speed at the particular unit. Here the unit is “meters per second”,
so 340 m/s -3 m/s= 337 m/s.
On the other hand, the sound which is moving in the same direction as that of the wind, that
particular sound energy will be moving faster (= 340+3 =343 m/s).
When we are trying to speak/ sing something in open space (in the open air),
we have to think of the combined effect from wind, humidity, temperature
changes, air absorption.

We have to account for each of them in our calculations and try to look into the
overall phenomena how the speed of sound gets affected; because, with
speed of sound, the distribution of it will be considered
Influence of Topographical Conditions
Slopes along hills
Plantation and Vegetation
hillock (or hill)

X
P

Y a plateau
Z

a plateau
Here is a hillock (or hill) as shown, and a sound is
produced at one end of the hill, which is not at all
reaching, or a very small amount is reaching, to the
house or the receiver at the other side of the hillock.

If we had a house here on this left side ‘P’, sufficient

amount of sound would have reached the receivers P
or the inmates in this house, but on the other slope
(right-side), no one can receive the sound. It is only
the diffracted sound which is reaching this particular
area. Z
Y
If you look back into history, the amphitheaters were usually located in regions along the hill slopes,
which were opposite to the sources of sound.

The noise which is created in ‘Y’ area does not travel into ‘Z’ area much, and ‘Z’ is considered as a
quieter area.

We, as architects also need to seek our open-air theatre locations in such areas where this kind of
a hillock or a barrier could be artificially created/ planned so that the sound source (noise) from
the other side of the slope does not affect our proposed location for the theatre.
Unfortunately, we may not always get a hillside for our
design, but a plateau situation (which the second
picture shows) is very common, where the sound (noise)
source is at point A. Some diffracted sound (may be X
little more than the previous case) is reaching this
particular area on the slope, which could be further
treated by landscape elements.

This house (where the receiver is) is not receiving

much amount of sound as desired, which it must
K have received if the house had been located
somewhere ‘X’.

This downhill is an appropriate location for seeking

areas, where we need to build our structure, which
needs acoustical quality. Here is another case, you
may have a sound source downhill (B), and the
source sound is moving upward but not affecting this
particular area on the plateau (K).
 K

This particular (above plateau) area, marked ‘K’ and beyond is also a good location,
provided the sound (noise) sources are in this particular downhill area ‘B’. Thus we can take
advantage of being at a raised level, so that sound from the roadside or industries can be
curtailed, and we can plan for an acoustical space at an elevated level.
The fact is that change of levels can change the amount of sound, whether desired or not;
depending on where you can choose your location. We should keep in mind that we need
not always adopt measures, but by taking advantage of the topography, we can plan our
entire set of buildings or set of requirements that have to be fitted into a given site.
 Pic : A Pic : B

Pic : C Pic : D
Plantations and vegetation;
Plantations and vegetation absorb sound. And, hence they must be taken into account when we
are doing some outdoor design, where noise (basically undesirable sound) has to be reduced.

Here is “Pic A”, the sound is coming from the source at the left direction, having a high sound
pressure level, and passes through multiple reflections between the leaves, the branches and
finally comes out from the other end with a low-pressure level.

Now, coming to questions about how much, how many, and what type of vegetation are to be
planted.

If you can plan a dense plantation with different types of plants/foliages, Pic : C and Pic : D, then
the amount of absorption will be better.
It has been proved that up to 5-10 dB (or even more) can be absorbed by dense foliages of
around 15 to 30 meters thick.

Grass or soft ground absorbs a lot of sound.

There may be areas where plantations are not present, but if the sound falls on to the grass, that is
the angle of incidence becomes such that it encounters the grass surface or soft ground covered
surface, then a significant amount of high decibel sound gets absorbed. Pic : B
Dense belts of trees 15 to 30 m absorbs sound 5 – 10 dB
(Cook and V. Haverbeke (1970), Leonard and Parr (1972), Reethof (1975))

Dense belts can reduce noise by 6dB (Huddurt 1990)

Different types of leaves provide better sound absorption

Rustling of leaves may help to mask noise

Noise reduction tends to increase with tree height up to 10-12m

Height of sound source and distance from source is also important

The rustling of leaves may help mask the noise; furthermore, dense belts, and different types of
leaves provide better sound absorption.

If we have trees planned, that is, landscaped, with different foliage like deciduous, evergreen,
or coniferous ones; then it would improve the acoustical situation.

Since we have the provision of growing plantations, we can create a belt of trees to check
noise or unwanted sound; and we know that rustling of the leaves helps mask the noise.

If the noises are of similar frequencies, then the masking of it can happen due to the rustling of
the leaves and the branches.

Noise reduction tends to increase with the height of the tree. Experimental researches say that
trees up to the height of up to 10-12 meters are effective in noise reduction. However,
canopies at higher levels, would not provide significant noise reduction effects.

Finally, the height of the sound source and the receiver’s distance from the source is also vital
 Historical Context : Early Greek

Greek Theaters were characterized by

Open Air
Direct Sound
Steep Slope
No Sound Reinforcement
Minimal Reverberation

Example: Epidaurus, 330BC

Seating plan : Segmented circle, more than 180deg, mostly on hill-sides facing the sea.
Steeply raked seats, low background noise, increased intelligibility.
With the understanding of the topographical and meteorological effects, we go back into the
early Greeks and the Romans and understand how did they plan.

We will see that they had planned everything in the open air, they took advantage of direct
sound reaching their audience, they did not have anything to reinforce sound, and there was
no concept of reverberation (neither at that time, nor it happens in open-air theatres as well).
So, there is no reverberation; we do not have any enclosure effect.

They had seating plans, which were formed like a segmented circle, more than 180°, mostly on
hillsides facing the sea.

On hillsides, the noise from the other side of the hill does not reach, and that the audience
must be facing the sea (speaker’s back towards the sea) – this shows the importance of the
wind direction.

They accounted for the meteorological conditions as well as the topographical condition,
way back.
They had steeply raked the seats and obviously, had low background sound because they
were on the other side of the hill (opposite the noise source). And, all these increased the
intelligibility, and you can see the source sound is directly reaching the audience.
Epidaurus
This is Epidaurus, which was
unearthed, it had 34 rows initially
which you see here, and later 21
more rows were added, and it had
a radius of 60 meters (186 feet). The
sound reached at the farthest
point, which has been explored
further by researchers of
Georgia Institute of Technology in
2007.
Sectional view
Limestone seats filter out low
frequency sounds

Amplifies high frequency sounds

from the stage

Seating formed corrugated

surfaces acted as filters to
emphasize certain frequencies

Prevailing direction of the wind

blows mainly from the stage to the
audience
The researchers from the Georgia Institute of Technology found that limestone seats filter out
low-frequency sounds (which were not desired) and they allowed or amplified the high-
frequency sounds from the stage.

These limestone seats are considered as corrugated surfaces, which acted as filters to
emphasize certain frequencies.

The designers at that time specifically used the limestone material and this particular
corrugated or folded structure to get the advantage of cutting down low frequency sound,
due to footfalls of the crowds, etc., and to amplify the high-frequency sound, which was
desired.

The prevailing wind direction was also considered, and that was flowing from the direction of
the stage towards the audience.

That also helped in getting better sound quality. The steep slope helped the sound to move to
the farthest point, which was around 60 meters away.
The dense foliages at the back, which did not allow sound to reflect back;
Rather, sound was absorbed within the foliages.

Although this is a present view, it is believed that during those times, too, such
kind of foliage existed and the trees could absorb the unwanted or delayed
reflection, which would have otherwise disturbed
the sound quality in this amphitheater
Sectional View
Sectional View
 Historical Context : Early Roman
• Seating arc limited to 180O
• Used arch features instead of hill slopes
• Added a stagehouse (skien) behind the actors, a raised seating area
(proskenion), hung awnings (valeria) to shade the patrons
• Aspendos Roman theatre, Turkey
The Romans also followed the seating arc limited to 180°, and used arch features instead of hill
slopes.

They had a high guard wall which prevented the sound (noise) outside from entering the
amphitheater area. This was adopted by the Greeks along the hillslope; the Romans created
an arched facade to cover or enclose the sound (noise) that could disturb the performance
(the desired sound from the performers).

Romans had a high wall at the back to cut down the noise from outside. They added the
stagehouse, and the raised seating area in front of the stage, and had huge awnings to
shade the patrons and these helped in early reflections of sound.

Roman theatres, like the Colosseum, or the Aspendous, has these features.

The colonnades at the back which helped in cutting down the noise. At the front (on the
stage area), there was a high wall, which also helped to cut down sound, and the seating
area is similarly planned like the Greeks.
All these helped in getting a better acoustical quality for these spaces, but these were done
several centuries back. The understanding which we have formed from the meteorological
conditions and the topographical conditions were adopted.
Sectional View
Conclusions

Minimization of external noise

– selected sites along hill slopes

Sufficient directly propagated sound

– consider wind direction

Sound from first reflections – made steps along hill slopes

– used materials like limestone

Control of late reflections and elimination of echoes

– trees and vegetation

Orientation for acoustical advantage

– facing the sea, wind direction
We can conclude that minimization of external noise is required and that could be achieved
by selecting sites along hill slopes or at an elevated level, or by shielding the site with barriers/
walls which could check the external noise.

Sufficient directly propagated sound has to be encouraged since there are neither much
reflections nor diffusion/ diffraction, or anything playing a role here in propagating the sound.
and it is the direct sound which needs to reach, and for that, we can take into account the
wind direction.

The orientation is crucial, which should account for the wind direction. To utilize sound from the
first reflections, we can plan a canopy over the stage, as the Romans did.

Thus, when we look into the open-air designs, we must keep in mind that first reflections can be
utilized by providing some sort of a canopy over the stage so that the performers can receive
back their own sounds (as feedback), and also to spread the sound more towards the
audience.
Moreover, the steps along the hill-slopes also helped in sound reflection, as seen in the
limestone steps of the Epidaurus. Materials like limestone can be used in corrugated fashion so
that they can help in first reflections. Control of late reflections and elimination of echoes by
having proper plantations at the back can help in checking late reflections.

You can also propose or build a wall at the back to allow the sound to move towards the
trees and vegetation, and it can check the external noise.

Alignment for acoustical advantages, such as orientation towards the wind direction
considering the noise source location, are also to be taken into account when we think of
planning for an open-air theatre.
 Open-air theatre (OAT) design of the present day

We have not made a miraculous difference in design concepts, from what the Greeks or the
Romans did. We are trying to follow them with few additions/alterations to their concepts.

Site Location

Orientation

Seating Plan and Section

Acoustical Plan
 Site Location

As we had already learnt the meteorological and topographical conditions, we will try to see
where to locate and how to locate an open-air theatre in a given site.

Unlike earlier times, we do not have an abundance of land or hill slopes, where we can freely
locate these open-air theatres as per desire, in the middle of the city or elsewhere.

We also have to look into the orientation, as Vitruvius Pollio had already said that if the
audience faces the sun, they will find it difficult to observe the performances.

The seating arrangements, which were initially started by the Greeks and Romans, are to be
decided.
We need to acknowledge their thoughts and would mostly follow their principles.

Coming to the acoustical plan, we have added certain features which were possibly
unavailable during those times, and we have tried to improve the listening conditions while
building our open-air theatres in cities or urban fields.
 Site Location

Minimization of External Noise

Noise Level should be bellow 40 dB

Topographical Aspects

Quite zone in flat site

Downhill slope against noise

Raised location within site

 Site Location

The site location, our first objective is to minimize the external

noise.
Because, if we are in the middle of a city, we can expect a
lot of noise from the roadsides or from other buildings, which
can add to the existing noise.

To keep away the noise, we have to use buffers.

The noise level should be mandatorily below 40 dB,

we also need to know the topographical aspects. We have

to find a quiet zone in a flat site or somewhere downhill of
the slope if the site has a terrain, or raised locations within
the site (that fall in the sound shadow area considering
where the source is).

Capacity up to 600 for unreinforced sound

(without electronic speaker system) Example >>
 Site Location
When we think of locating an open-air theatre in an urbanscape, if
the site is like rectangular plot (as seen in the picture) and having
adjacent roads, we can see that a quiet area forms at this
particular zone (away from the road).

But if you check out the other activities in the adjoining plots, you
will understand that these may be also creating some noise. So,
though maintaining a setback from the road can protect our
proposed location from the roadside noise, we may end up with
more noise from the activities on the adjoining sites.

Firstly, we need to understand where the site is located and the

adjacent conditions. That may probably have some influence or
can add to the prevailing external noise for that particular site.

In that case, we may consider the central part of the site as being
sufficiently distant as far as noise sources from outside are
concerned. This decision has to be taken first before we start
designing our open-air theatre.
Site Location
 Site Location

Now assume, we have the adjoining road and other parameters, and it is quiet on the
adjacent plots, then we can plan to cut down the noise from the road by considering making
a row of buildings, that would remain functionally unaffected when placed along the noisy
road. These buildings act as a reflector to the sound which is created in the road and does
not allow the sound to enter into the open-air theatre zone.

Furthermore, there is an addition of a backstage (a wall behind the stage), which can also
prevent the sound (external noise) from entering into this particular area. Thus, providing a
noise-free zone within the site by creating a line of buildings and/or a back wall behind the
stage to protect the OAT from the external noise.

Similarly, by planting a few rows of trees of different sizes of foliages as discussed previously
(on plantations and vegetations), we can get a reduction in noise level. By maintaining
shrubbery heights of up to 10-12 meters, we can prevent the incoming noise and allow the
OAT to be a noise-free zone.

For the main theatre, usually, with capacities of up to 600, we do not require any
amplification of the sound and can carry on with the performers’ own voice.
 Orientation

Time of performance and Sun path

Performer and Sun path – Use of awnings, shadings

Audience should not face sun

Effect of Temperature

Should be accounted if it is day time gathering under

open sky ex. Public (youth, devotees) gatherings
addressed by political or religious leaders

Control of late reflections and elimination of echoes avoiding

buildings in backdrops
 Orientation

Coming to the orientation; the time of the performance is very much important, along with it,
the study of the sun path is also crucial.
The concepts of awnings and shadings, which the Romans had utilized around 2000 years ago
could be adopted. In case the performances are scheduled during day time, it is imperative
that the audience must not be facing the sun.

Suppose we have some religious or youth gatherings, where there is no open-air theatre, but
just an open field where the ground level is flat all along, we have to address the effect of
temperature.
Recall, that the temperature on the ground surface is higher and that helps bend the sound
on the upper side creating sound shadow areas at the back.

It may happen that the audience members seated at the backside are not getting any sound.
This occurs not because the actor on the stage is speaking with a low (unamplified) voice, but
because of the effect of temperature. We must also consider preventing late reflections
because those are the causes of echoes.
We must avoid buildings at the backdrops, that is, at the back of the audience. There should
not exist any hard surfaces (like tall buildings) at the end, which will, otherwise, reflect the
sound and generate unwanted echoes.
 Historically; the Greek theatres were
characterized by open air, with direct
sound moving to the top (up to 60 meters).

They had steep slopes of 2:1 gradient, there

was no sound reinforcement during those
times, and there was no concept of
reverberation (which were very minimal in
that kind of structures).

The same features are adopted for our

Greek Theaters were characterized by open-air theatres too.
Open Air
Direct Sound
Steep Slope
No Sound Reinforcement
Minimal Reverberation
 Orientation

# Accounting wind direction for acoustical advantage

# Sufficient directly propagated sound

# A hard surface between stage and first row seating

# Wind helps carrying sound to the back seat
# Stepped seating helps in early reflections to reinforce sound
 Orientation

The Greeks and Romans also accounted for the wind direction, which we had seen that if the
speech is along the wind direction, it helps the sound to reach to the farthest audience.

Accounting the wind direction for acoustical advantage has to be kept in mind, sufficient
directly propagated sound has to reach the audience, and we do not consider a very steep
slope. However, this particular slope should be at least 12° (minimum).

A hard surface at the back of the stage can help reflected sound to move towards the
audience. If the source on stage, then reflected sound from that area can move towards the
audience, with help from the wall at the back of the stage.

Some amount of sound also can move towards the floor, because in an open-air theatre,
there are very less chances of getting early reflections or helpful reflections to reinforce the
source sound.

We have already seen that the limestone steps at Epidaurus (which had corrugated or folded
surfaces) helped in multiple reflections of the sound; the same concept can be adopted in
our design.
 Seating

# Stepped seating allows reflection from proscenium (stage)

# To get sufficient directly propagated sound

– need for compact planning
– welcome sound from first reflections

# Sound bounces back and fourth from vertical risers and stage wall

# Delayed reflection may happen from back wall to be checked

# Scattered energy is required to fill in the gaps between fewer reflected sound
unlike closed space design

# Diffraction and scattering from seating areas help to achieve a smooth sound
decay curve.
 Seating

Seating;
Stepped seating allows sound reflection from the proscenium side (or the stage side).

To get sufficient directly propagated sound, we need compact planning.

The sound from the first reflections is always welcome.

Only the direct sound will not be sufficient to reach the last row of seats, we need the first
reflections also, such that they can create better sound quality within the space.

Sound bounces back and forth from the vertical risers and the stage wall. Hence, we can
take advantage of these as creating first reflections. These help in creating a blended sound
within the open-air theatre.

The major problem here is that we are in the open air, we cannot follow the principles of room
acoustics; hence concepts of reflection from the “ceiling” or the “side walls” are no more
prevalent.

While trying to take advantage of whatever reflecting areas that exist, to enable first
reflections for better hearing or better blending of the sound.
 Seating

Delayed reflections may happen from the back wall; so if we plan for a back wall at the end
of the OAT, then we have to check the delayed reflections or the echoes. If the back wall is
present, it should be limited to a particular height.

Scattered energy is required to fill in the gaps between the few first reflections happening.
And, these scattered energies from the reflected sounds coming from the treads and the
risers (in audience seating area) can be utilized.

The gaps, where there are no audiences, help in accumulating this scattered energy for
better hearing or better quality of sound.

Diffraction and scattering from seating areas help to achieve a smooth decay curve. The
decay curve and the reverberation time is the summation of many early reflections, this does
not happen to create a blended sound. (Info about the decay curve page 108 & 109
Architectural Acoustics, M. David Egan)

This diffracted and scattered sound adds on to the first reflections to achieve a smooth sound
decay curve, creating a blended sound within the audience area.
 Seating

The back wall and the risers (of seats), Provides early reflections which
reinforces the source sound.

The corrugated seats help to get blended sound of better quality.

Seating

<<<<
The “inverted cone” effect on flat slope directs
the sound upwards,
# Loss of energy,
# Fewer early / first reflections,
# Undesired design.

<<<<
If audience is sitting, they might get some
scattered sound. Hence, undulated surfaces are
better than flat slopes
 Seating

Here, we see how the sound is moving. The direct lines are shown in pink, and the reflected
sound is shown in blue. By taking advantage of the back wall and the risers (of seats), some
amount of early reflections occur, to reinforce the source sound.

Additionally, these corrugated seats help to get blended sound of better quality.

However, if we have a flat slope, these multiple reflections does not happen; it is like an
“inverted cone” situation. In this case, whatever sound that strikes the sloped surface, gets
reflected up towards the open sky.

So, if we think of a flat slope, then you would be ending up with bad quality of sound.

The “inverted cone” effect directs the sound upwards, causing loss of energy, and fewer
early/first reflections occur, and thus, is an undesired design.

But if you plan a mound for the audience area, keeping it very green, the undulations there
will help in scattering sound towards the audience.
 Seating

Sound reflection from the tires of benches, if at a distance, produces a sustained echo.

Pitch can be determined by the distance separating the risers.

This causes distortion in some frequencies and affects sound quality

In OAT the frequency dependent reflections generally can pass overhead but they can
converge to the stage disturbing the performers

Provision for covering stage – Performer audibility and directing sound

 Seating

Moreover, the sound reflected from the tiers of benches (if at a distance) can produce
sustained echoes. So, these early reflections are required to happen in the front part of
the open-air theatre only. We can even determine the pitch of those particular
frequencies that will be echoed (as learnt during auditorium design). This distorts some
particular frequencies and may affect sound quality.

The echelon effect of auditorium design can happen for this particular kind of
corrugated seating too. In OAT, the frequency-dependent reflections, generally, can
pass overhead but can converge to the stage, disturbing the performers. Since the seats
are circular, they can create a concave effect and can focus sound back to the stage,
and the performers can feel this disturbing sound coming from the audience side.

It could be helpful if the performer’s side has a covering over the stage to cut down the
late reflections, which are coming back from the concave surfaces of the seating area.
 Seating

<< Lets say this is the stage area, and these are the
concentric seating, the sound may get reflected back
towards the stage area from the back row seats. This can
create an echo condition, that is, the source can get back
the sound at a later time, as a late reflection from the
backseat curvature.

As you are gradually moving towards the back, your height is

gaining. Hence, an awning or support over the stage can cut
down the reflected sound coming towards the stage at a
certain height. Thus you can cut down the late reflections
happening from the last row of the seating.

This convergence of sound towards the stage can be cut

down by the help of this awning or the support over the
stage. Their dimensions must be calculated as specific to site.
Seating : Conditions for stopping echo
 Seating : Conditions for stopping echo

Let us now see the conditions for stopping echoes coming towards the audience.

A back wall can be created, but if the source sound is hitting directly to the features at the
back (maybe a building, a tree, or a line of foliages), they can partially echo it back towards
the audience.

The building, if made of brick or concrete, will absorb a very small amount of this sound and
the major part of the sound energy will be reflected back, in the form of an echo.
In case of trees, it will absorb a portion and will reflect back the remaining portion,
depending on the density of foliage over there.

Picture on the previous slide : the back wall has been raised, and that stops the reflection/
return sound. Thus, creating a guard wall at the rear end is very important and has to be built
accordingly.

If the back wall is too high, it will completely stop echoes. In this case, the sound that is
coming from the source, will completely move out from the seating zone, which is also not
very desirable.
 Provision of Back wall / side wall

Creation of a closed space plan with -

# Limitation of the lateral openings at the side entrances
# Lateral walls may be diffusive / convex surface to help scatter sound to floor
 Provision of Back wall / side wall

It is important to increase the lateral reflections for an open-air theatre because on the top
we are not allowed to provide any cover. There is no concept of “ceiling” for an open-air
theatre.

In the figure, a side wall is planned. These side walls can also help in reflecting sound towards
the audience, which can be considered as lateral reflections.

# Nowadays, these side walls are made such that the sound is pushed more towards the
audience side.
# A percentage of the sound energy moves back towards the audience, helping in early
reflections or better blending of sound. Hence, this creates a “closed space” plan.
# Also, provide door openings in these side walls for people to enter the OAT.

# Minimal entry through these walls, or indirect entry plan, is a good choice.

The entry should be located in such a manner that it does not cut down the lateral
reflections in the process. You can also plan for convex or corrugated surfaces along the
lateral wall.
<< Here is the open-air theatre at IIT Kharagpur.
You can see trees at the back, which helps in
absorbing some amount of sound as well as
reflecting the rest of the sound. You can see the
raised stage here, the circular compact seating
plan with a gradient, and the lateral walls of
varying shapes at the side.

<< This area, you see here, the front part has a
folded surface for sound diffusion. This is a
curved surface to disperse the sound towards
the middle portion of the theatre.

The sound which is reaching here will disperse in

this fashion (blue arrows); these corrugated
features will help in diffusing sound towards the
audience. Another surface here with a different
radius of curvature. Entries are planned at
different points.
 Case study
Ansal Plaza (HUDCO Place)
A part of HUDCO Place built on 35 acres of land,
Ansal Plaza is a shopping complex situated near
South Extension.

A perfect hangout destination, it is built in a circular

fashion around an amphitheatre with a centre stage.
Different cultural functions are organized here from
time to time, such as fashion shows, live band
performances and performing arts to promote the
retail area.
 The circular entity of the building has a special
significance - it reflects the individuality of a shopping Amphitheatre
mall, indicating to the customer that each and every Area:3000sqm
part of the building has something of quality to offer.
Leading off from the blue bowl and circular minar
here the central axis path at the stepped level,
interacting spaces and communicative linkage with
nature, generates soothing effect.
# The semi-circular design
clearly indicates a
fundamental understanding
of the spherical propagation
of sound, and a desire that
every member of an
audience should hear
equally well what is being # The acoustic advantages of a
intoned on stage . stepped dish are only optimized
however when the speaker is standing
at the exact centre stage; only then
will the voice propagate
symmetrically, and reflect back from
unoccupied areas to the same point
# The surrounding building blocks and
the landscaped area in the back of
the seating acts the embankment to
the amphitheatre from the noise
coming from the surrounding area
 Seating area

View from inside the

ansal’s # As such no backstage is
provided, the sound appears
to spread out when no
audience is sitting and no The stage with
extra measures have been marble finish-
taken to enhance the sound reflecting
propagation. surface
# The small green areas are
provided around the View towards
amphitheatre which act as a the stage
absorbers along the
colonnades
The colonnaded The reverberation sound produced
feature around the by the reflecting surface of the
seating area, sound glass is absorbed by the porous
will in this case finish of the colonnades
bounces back and
forth between the
vertical seat risers ,
the colonnaded This OAT lies in
feature and the the depression
Source of the (bowl) of the
surrounding built
sound ansal’s
forms
Reflected sound Propagated
Sound sound Sound
absorbed reflecting from
by the the glass
colonnades surface of the
building

Sound is absorbed by the heavy landscaped area (buffer zone)

The amphitheatre is situated in a sound shadow area casted by the surrounding
shopping complex and also act as sound barriers
Sound barriers work on the
principle of casting a “acoustic
shadow” away from the
source. Clearly if there is line-of-
sight between source and the
listener, there is no “acoustic
shadow” and no noise
reduction.

# The glass provided around the amphitheatre(on the

building façade) acts as the reflectors
# The shape of the theatre is such that most of the
audience is drawn close to the stage.
 The colonnades surrounding the amphitheatre are used
for three purposes:
(a)sound absorbers and reflectors (due to surface finish
material-porous red sandstone)
(b) placing of speakers and lights
(c) decoration at the time of events

The back of the

# The slope of the floor is speaker has to be
towards the stage and it is covered at the
nearly about 12 to 15 degrees time of show
to the horizontal. because of the
surrounding
# Feels almost horizontal parking
 Varying
lights heights to
break the
Placement of path of
speakers sound as
required

The sound
prorogating from the
back is absorbed at
Propagation of sound the ends by the plants
towards the audience
 Open Air Stadium

Open Air Stadium require cut down on

reverberations that can interfere with
announcements and other speech.

In Stadiums designers often use fiberglass and

other porous materials to absorb sound. Roof
should direct sound to field.

Crowd noise in stadiums during moments of

high excitement in games is typically between
95 to 105 dB and even 110 dB.

Steeper grandstands make more noise since

the fans are all closer to the playing field. A roof
or dome holds noise in and directs it towards
the field.

Giant open air bowl configuration dissipates

noise
Example of an open-air stadium:

Nowadays these are covered with fiberglass, ETFE (Ethylene tetrafluoroethylene) or PTFE
(Polytetrafluoroethylene) sheets. And when the crowds cheer or applaud the
sportspersons/players, it creates a sound of around 90-95 dB, which may go up to 105 dB, and
occasionally even 110 dB. Those sounds are all directed towards the field.

Its better to have a curvature or a covering which will direct the sound towards the field, then
it is fine. Otherwise, this amount of noise will not allow other spectators to listen to the
announcements being delivered or on-field discussions made during the game.

The ongoing sports performance is intended to be watched by the spectators, rather than
being shouted at, but such noise that is produced by the crowd reaches the field.

Steeper grandstands make more noise since the fans (spectators) are located closer towards
the playing field, and the roof or dome holds the noise as it directs the sound towards the field.

So, these stadium-covering concepts are to be kept in mind while planning for covered open-
air stadiums.
This presentation is only a compilation of all the relevant
information available on the internet and books.

https://www.archdaily.com/900318/cloud-tower-tne-architects
https://www.researchgate.net/publication/317754965

https://nptel.ac.in/courses/124105004/

Architectural Acoustics, M. David Egan

Thanks!!