. So, in
we start with lecture 23, we have covered in the previous lecture, how to start with
the plan?
First you need to convince yourself what shape you will take, yes we would not design alone
or design the acoustics of huge halls say; 2500, 2800 , but yes as an architect you will
have to come across, you may come across halls multipurpose halls of sizes 500, 700, 1000
capacities . That is that could be in schools, that could be any institutional complexes
that is quite often encountered . So, in this lecture, we will try to look into
how to take care of stage and the sections, how those are important in developing the
acoustical quality of the space or the auditorium which is under consideration?
. So, stage is the place where the performance is happening . And, we have already discussed
during class room, that if it is capacity is beyond 50, we need to raise the stage . And,
gradually we saw how much like it should make a certain angle with the back seat, back seat.
So, that around 5 to 7 degree angle is formed with the raise of the stage .
So, all these dimensions are already discussed and you can revisit those classroom slides,
and you can come back . So, coming to the stage floor height, it should be low enough
that a person sitting in the front row can see the actor's feet, that is of interest
. This interest is where our site goes and arrives . So, arrival point of site is the
lowest point of interest on stage . So, if you have a stage and then a flat totally flat
floor, the performer's feet is here should be visible by the person who is sitting here?
. So, this eye level and this level of the lowest
point of the stage that is the height of the stage should have a relationship . So, if
this height is known which is basic anthropometrics, we see for male and female it is like 48 inches
to 44 inches . So, this stage should be at a level lower to it so, that you can see what
is happening at the lowest point in that level.
So, stage heights are preferred to be around 40 inches to 42 inches . Now, once this arrival
point is determined which is almost at the center of the stage, the people behind the
first row should be capable of seeing that floor that foot level . So, it should be high
enough that it should be high enough that the arrival point of sight does not require
a high raking or high inclination.
So, that the last point last persons here at the end are capable of seeing what is happening
here?
. So, if this is there and if you start raking
; obviously, the person here should be capable of seeing the leg here . Now, we will come
to those calculations etcetera how you get?
So, if you have your seats one after the other, it is the eye level or the head level of all
these which should cross and give view of the last percent in this point . And, the
first point first viewer position has been set by setting the height of the stage . It
could have been further low, if it is further low the raking would be further high .
So, an optimum is 1 is to 9 rake, for the first 8 to 10 rows and thereafter 1 is to
8 slope, that can yield a good result for theater stage starting at a height of which
is at a height of 42 inches or 1.07 meters . So, the higher the apes aps the lower is
the sitting rake, lower the aps higher is the sitting rake, what I showed here, lower
is the stage height you have to rake it further.
So, that the last person can see higher the raking higher the floor stage lower is the
raking because you are already elevated at a point .
So, keeping these in mind we will go into other details, when we come to the audience
side , but for now we move to the other components of the stage the floor .
The floor of the stage is usually preferred to be wooden, vinyl floors, and you can see
the sockets are also there to take in power and that also shows, that over the structural
floor, gap is there and the wooden battens the wooden battens are seen . So, the battens
are fixed on top of it over the structural floor with a structural wooden framework that
absorbs sound and gives a good sound good quality of original sound or the performer
sound towards the audience . So, unwanted sound is actually absorbed here
. You can also see several other elements, which are the fins, which are the fins, you
can see the curtain, you can see the reflecting wall we will come to it next slide .
Now, coming to stage design, we I introduce you another term called ensemble, which means
musicians ability to hear each other . Say it is a concert by few performers, when one
is performing, some other also some others also are performing . And, they have to have
coordination amongst them otherwise the whole purpose will be lost.
So, it is also the audience, who should receive the sound as if the [muni/musicians] musicians
are playing in unison . So, to play in unison, the performers need to hear each other . And,
that can be measured and that is called ensemble, that can be measured by support factor . That
measures the [muni/musicians] musicians ability to hear to himself and others near him .
Because, in stage they are also getting reflected sound from the ceiling . And, also from the
sides and if the ratio of the energy which is returned between 20 100 milliseconds, you
remember clarity was c 80 80 milliseconds towards the audience . Here it is between
20 to 100 millisecond, the energy which is returning and it is compared with the initial
energy, between 0 to 10 milliseconds when the all the sounds were originated had originated
. So, this ratio is called the support factor
and that is also measured in decibels some minus 11 to minus 15 in a comparative scale
is required though you need not calculate all these, but need not get into all these
details , but you need to know that s ensemble that is getting the sound by the performers
within themselves to work in unison.
And, the audience feeling that yes everything is happening in the stage is in unison is
very much important . And, I give you the thumb rule like preferred
volume is 1000 to 2000 cubic meter . Now, I here give you a small example Sidney opera
house . The main auditorium designed by John Anderson, you must be all our of it . It is
a hall of to 6 some 7 9 audience capacity . It has a stage of 17 meters by 11.5 meters
and you are all our of the shell structure, the stage was below a shell, stage is below
a shell structure . And, that was of a very high volume.
And the performers actually raised complaints that they are getting much delayed sound and
if you think the shell was 25 meters . So, if you think of the volume of that stage
visa stage considering that height of 25 meters . So, 25 cross 11 cross 5 cross 11.5 cross
17 will give you something with the shell structure, you may not get exact same exact
cylindrical kind of volume on a rectangular volume . Even, if you cuttle down the volume
will be something around 5000 cubic meters , but other good preferred halls performing
halls good performing halls have a volume around 1000 to 2000 cubic meter .
So, that was the point when, the acoustician started experimenting how that could be brought
down and how that sound that will delayed reflection could be cuttled?
So, that the performers does not get confused.
There were clouds or reflected ceilings, which were hanged from the top, but yes a lot of
experiments are still dead going on . So, stage area ceiling height is important,
the volume I give you as a key or a thumb rule for your calculations.
Because, you will be calcu you will be knowing your seat stage dimension when you start and
then you try to achieve this volume, and then you can come up . See the stage [abov/above]
above the stage of the Kalidas auditorium, where you can see the ceilings the lights
the the folds which all help for acoustical quality gain .
In the stage area closely surrounded by reflective surfaces help to reinforce sound . We have
already discussed that in earlier lecture I think lecture 16, where we had talked of
precedence effect or space effect or hass effect . So, the back reflecting sound gives
directivity to the sound source and that gives the audience a feeling that all sound is coming
from the source . So, these reflecting surfaces along the stage
across the stage area and little beyond should be always angular and facing towards the audience,
it must be split and it must push the sound more towards the audience floor . Probable
materials, if you try to give plastered hard plastered surfaces, which are highly reflective
plaster of Paris surface, Gypsum board, Plywood, Plexiglas all these are being applied as very
high reflectors . Here is a picture of Kalidas auditorium, where
you can see that plywood has been applied on the side, on the splayed side, you can
see hard wall surface here also these are all splayed.
So, that the sound from this stage can actually move towards the audience, you can see this
play continues on the top.
So, that the balcony also receives reflected sound.
First reflections or the early reflections from the stage or the source where the sound
is you can also see these fiends in the earlier slide you could see, these also direct sound
towards the audience or towards the floor .
Now, coming to the ceiling, how should be the ceiling like?
You see in the first picture, if you cut across you see this is the transverse surface, if
it is concave that is you have a concave ceiling . Then, what will happen sound from the source
gets focused along the center of the audience, which is really not at all desired .
So, the at any point in the transverse section, the in the transverse section you see this
is a straight line and not a curved surface . So, concave enclosure, if it is continuous
should be in the direction of the length, in the transverse direction it should never
be concave . So, this is another shape another important point, while thinking of the shape
of the floor or the audience area . Yes, this can also lead to focusing of sound
, but look at the curvature it is quite flat.
So, even if there is sound focusing the focusing will happen below the sitting area.
This we had shown earlier, I think we have another slide to show that . This could be
at a height, which again, if here is the source the direct distance, we saw we the reflected
distance should not follow.
So, that echoes can happen . So, all these points are to be accounted when
you are thinking of the covering of your audience . Now, coming to diffused surfaces, while
you are doing your auditorium, you cannot forget thus columns the beams . For such column
free spaces, you need Crawford slab, you can keep them exposed and help them in diffusion,
if the scale is that big.
You, see how diffusive surfaces could be of 4 inches or greater beams, which come intermittently
interred in the entire ceiling.
These are for small sized auditoriums.
If, you have broken surfaces with shallow recesses say 2 inches or less deep structural
system, those can act as medium diffusive surfaces . So, surface diffusivity index which
is 1 considered by Haan and Fricke in their study in 1997.
They told this Crawford ceiling has a high diffusivity of 1, medium diffusivity are 2
inches or lesser surfaces projected surfaces and low diffusivity which are actually absorbers
or the reflectors.
So, that is 0 . So, you can think of your structural system
acting as a diffuser, because we need uniform sound in the audience . And, we cannot avoid
the ceiling; beams columns, the edge beam can beams columns .
So, if you have projections of this nature . You can actually treat them and really create
diffused surfaces on the ceiling as well as the side walls . So, these projected structural
elements columns enclosures can actually act as diffusers and help in achieving uniform
sound in the system . So, we have talked of the ceiling once it is curved we have talked
of the ceiling if it is Crawford.
Here again we come to a previously previously discussed slide, where we see that direct
distance and the reflective distance is of still at most importance, because this is
a big space . It is not room acoustics that is not in a scale of classroom .
Here, the space can lead to eco can lead to concentration of sound and we have to look
into this condition of R 1 plus R 2 minus D, that is the difference in the reflected
path and the direct path considering 300 millimeter per second speed of sound should be less than
9 meters, earlier it was worked out with 340 meters per second and some 11 meters, who
are shown . So, we have to keep in mind this condition.
You see here the reflected ceiling shown shown here is very flat.
The curved ceiling shown here is really very flat . And, below this is the and the height
of it that can lead to echo condition is first drawn . So, any curve below it below that
particular height will really not lead to any problem caused due to echo.
. So, if you draw the boundary condition of echo and plan your curvature, if you keep
in mind that the radius of curvature does not convert within the floor area, then you
can have a shell structure . Now, coming to the floor slope we had talked
about it while we were talking of stage . Now, if you have regular floor slope the first
that is the a which is showing that is a constant slope, you can gradually what is happening
till the top you can see the distance is gradually diminishing . Towards the aps or the source,
where here it is little raised , but in the second case you see the slope has changed.
A constant slope level is less favorable and an increased accent is preferred, because
here the heights if you can compare here it is getting closer and closer , but here it
is following a regular pattern , but this has another advantage .
We will see that in a few minutes.
So, reasons for changing slope floor slope of the floor is audience can see the lowest
point which I had already discussed, which is the point of interest, lowest point of
interest that is the aid arrival point of sight . And, low frequency sound decays, as
we had discussed while talking grazing attenuation . If your sound level is moving parallel to
your seats just changing by within 5 degrees, that is grazing attenuation, then sound gets
lost . Sound seed tip effect takes place low frequency sound actually enters as if the
seeds are resonators.
We have discussed this earlier . So, grazing attenuation to cut down grazing attenuation
to cut down seed tip effect to cut down, low frequency sound getting lost, we need to rake
the slope, we need to rake the floor . So, attenuation of sound wave parallel to the
audience sound of particular frequency diffracts and gets absorbed by audience or the seeds,
which is vertical resonance and which happens in 125 to 200 hertz .
If, you remember warmth which was discussed in the first introduction to auditoriums,
you will actually lose the low frequency sound that does not give warmth . So, if you raise
it, you really cut down all these raising problems or all these defects arising due
to grazing attenuation . So, by raising the talker height and sloping
the floor you can check or control grazing attenuation another is overhead reflections
are always preferred.
So, that it can help in beneficial reflection reinforcement of this low frequency sound
in particular, because if the direct sound is lost the reflected sound should actually
fill in the gap . So, this changing floor can break this grazing attenuation.
Even the raising of the slope raising of the floor can check grazing attenuation .
So, apart from viewing acoustically these are the purposes for raking the floor . Now,
we move to the position of the reflective surfaces on ceiling, if it is a flat surface.
The first one third part of the ceiling always helps in reflection, helps in strengthening
or reinforcing sound and these are useful reflection , but if it is a bigger size hall
we can add or hang reflectors in an angular fashion.
So, that it throws the first reflected sound to a further distance to the maximum distance
it can pose, it can possibly do . You see if you can actually keep on adding
one by one reflectors then sound from the source directly hitting those reflective pieces
or the reflective surfaces can direct sound to the entire hall.
So, you see here this picture also shows, how the maximum sound is received at the central
part of the auditorium, apart from the side reflections apart from other phenomena . So,
this direct sound first reflected can from the ceiling can reach the central part at
maximum maximum . So, if you go back to the seating plan, you
can get a justification why the viewing viewing as well as the listening part best is the
central part, because ceiling is undisturbed . So, once you have got the entire floor being
being supported with sound, you need to put absorbing surfaces at the rest of the ceiling,
the back of the audience The lateral reflections do take place from
these side walls.
So, these lateral reflections can actually happen can reach this audience from the source
and laterally moving, from the source laterally moving . And from the above they are really
they may lead to delayed reflections to this audience.
But, yes if there is no more audience at the upper level . If, there are audiences at the
upper level they do help in better listening to the balcony locations . So, to create the
spatial impression, the useful lateral reflections and the ceiling reflections can create that
intimacy and that envelopment of sound, which will make them feel, that they are very close
to the source . So, back and ceiling of the hall requires
absorbing or scattering surfaces, they will be they will be checking the delayed reflections,
and we are encouraging ceiling reflection.
As well as lateral reflection in the first part and ceiling reflection up to a distance
which is really required . Now, coming to another important effect, which is the echelon
effect or the eco effect you can say . So, these when you have this rise or the treads
with which you are going up at a very regular way, which we discussed in the if below before
last to last slide . Here, you see that this regular hitting and going back of the sound
at a very regular interval of time and at a regular interval of distance creates a regular
phase difference and echo while the sound is going back . This actually is heard along
with the direct sound and this is called echelon effect .
If, you actually have different slopes or different row widths at least broken at some
points this echelon effect can be reduced . So, you can have varying widths of the trades
or you can have varying heights and varying varying widths of the trades at varying heights
. So, this echelon effect can be cuttled.
You see here the picture, where you where they have taken care of the echelon effect
you see these are these are wider trades, these are wider, these are wider and later
on some are some are of same same height and again some are wider .
So, the echelon effect cannot have happened or is taken care in this particular case . Where
else can it happen, it can happen in ceiling patterns, that is the palings . Various reflectors
or surfaces can be put as you have seen in the last last slide, that the ceiling reflectors
are put . Various shapes are taken , but if it is a repetitive shape of a particular concave
surface or a flats even a flat surface the same effect can happen considering the
angle in which it is falling, it will come back and again hit, again come back, again
hit and create a additional sound that will actually disturb the basic or the original
sound or the desired sound . So, that is called echelon effect and that is how you can take
care.
So, no continuous repetitive form is encouraged within the auditorium . And, again of particular
wavelength of particular dimension which corresponds to the wavelength which is being performed
in that particular space . So, if it is less than lambda or one-fourth lambda for 500 hertz
it is 2 fit for 1000 hertz it is 1 fit for 2000 hertz, it is half it is half of it
So, you have to keep those wavelengths in mind, what frequency is being performed?
What would be the wavelength?
How it could take?
How it could affect the sound quality?
And, how it could affect or create this f fill echelon effect or not you have to keep
this in mind while you are designing?
. So, with all these I request you again to
revisit the earlier slides, where the basic principles were told these dimensions the
wavelength, the the frequencies, what kind of frequencies do take or do happen when it
is speech, what is it is 500?
When, it is a performance it is from 125 to 2200 4000?
. So, you have to know what is the range dimension
of the wave that is the wave lengths also, and then how to plan it?
So, it is not very difficult if you keep this in mind you can single handedly take care
of small auditoriums . So, we will move to the balconies which I have not addressed it
in the next lecture and hopefully you will get much more interest and will surely take
up one such design in your entire architectural program .