Auditorium Acoustics


ok so let’s start so last class we have discussed
about that the different acoustics parameters of the room clarity blendingness all those
things we have discussed then we have discussed the how to calculate the reverberation time
mean free path what is initial time gap initial signal delay and ah what should be the value
then percentage halcyon all those things we are defined now we go for a practical implementation
which is auditorium acoustics suppose you want to design an auditorium or you want to
design an lecture hall that who who who are doing that architectural business the design
that lecture hall how should we design that lecture hall so that acoustic is take and
care what are the parameter who should look for in case of acoustic treatment so there
is a two kinds of thing one is that the realism architecture parameter for which we have to
ah we have to said that if i want a good acoustic those architecture parameter should be like
this and then we have to provide a wide range of solution or you can you have to provide
a solution for acoustic treatment of the auditorium for better sound arrangement ok
so now we discuss one by one let’s auditorium it’s called auditorium acoustics so auditorium
design has three three part one is the room acoustics how do you the treat the acoustics
of the inside auditorium sound system what kind of sound system i should use auditorium
so that that that ultimately aim is that in the auditorium every every seat of the auditorium
should receive that equal amount of sound that is the my ultimate objective ok how to
design that what kind of sound system i should use how it should placed all those things
and then noise control how these my auditorium is free from external noise if the external
noise is entered in the auditorium it empowered that auditorium acoustics because environmental
noise of the auditorium will be increased so which which is ah distract that sound system
was the auditorium so we have to pre prompt that external sound source so that is called
noise control we have to control the noise of the auditorium so that by external noise
should not entered in the auditorium so one by one we discuss all that issue and then
we will aim we go for a design class then architectural contributions to auditorium
design what architectural do room shape is define by the architecture volume and dimensions
is define by the architecture layout of boundary surfaces is also define by the architecture
surface treatment audience capacity seating arrangement all at the civil design parameter
when the auditorium is designed initially they are consider those of the parameters
now as an acoustic engineer what kind of feedback you should provide to those to those architectural
engineer that he has look design an auditorium due to the acoustics point of view room shape
volume of the room this should be look like this so those guideline is called the acoustical
guideline to the architectural design ok so let’s one by one discuss room shape what is
the acoustical requirement ok before come to room the shape let’s start what is the
acoustical requirement in auditorium design there should be adequate lies loudness in
every part of the auditorium particularly the remote seats so what is my requirement
is that at the end seat is some some audience is seating is loudness of the of that sound
on that seat should be same as the loudness of the sound at the front seat so that then
the design very good so i want there should be adequate loudness is that the last seat
cannot heard if the sound level is reduce that should not be happen so adequate loudness
should be provide at in the last seat also so that last seat people can also heard that
sound so and the goal is that should not waste the sound energy so reducing sound energy
loss how can each analyse the sound or how can distributed in the sound inside the whole
auditorium is our goal so the auditorium should be shaped so that the audience is as close
to the sound source as possible as thereby the reducing the distance from so[und] the
distance the sound must travel what is the meaning is that i want to design
an auditorium so that everybody has a feeling that he is seating close to the stage because
performance is happening on the stage so everybody should have a feeling that yes i am close
to the stage what is the what is the acoustical effect suppose the there is only loud speaker
is placed in the front front of the stage then the setting on the large distance the
sound will be less setting in the front seat sound will be high so i am to minimize the
detect sound travel time to the last seat that means i have to minimize the distance
from the of the stage to the each of the seat how do i minimize it suppose if the design
auditorium room that the room shape if that is if the design auditorium that is the second
case in this region so that the centroid the average distance between the stage and the
seat lets calculated is this d two is the centroid is [vocalised-noise] d two is the
last compact of d one if i place the audience like this way because that if this the wide
angle so that the distance from the stage to the person is less so average distance
will reduce ok so everybody has a feeling that yes i am close to the stage
now if you required large capacity let’s you want to required a design length is very coming
is very high then use the volcanic to reduce the length use the volcanic to reduce the
distance from the stage ok so there is some guideline that then another one is that parallelism
between the opposite sound reflective boundary surfaces should be avoided to eliminate the
that means it said that any auditorium must must be have any parallel reflector ok suppose
i make this auditorium or make this auditorium without this curve without this curved surface
inside there is a seat the inside this curved surface if the curved surface is not there
and this wall and this wall is parallel which effect which effect the sound quality ok which
undesirable reflection will come and spend can generated so it to reduce that what i
do yes this is the concrete wall no problem but i should make a inside arrangements such
as the surface is curve see that inside surface is curve to avoid the parallelism between
the two reflecting surface ok similarly i can do like this change that change the shape
parallelism is avoided or i can do like this way design the auditorium like this way ok
here see that the no parallel wall or if it is rectangular then inside the auditorium
keep the wall such that it create the curve surface which is required
then what should be the shape of the auditorium volume and dimension of the auditorium shape
of the auditorium is discuss the shape of their should two parallel wall parallel reflecting
surface and the distance between the audience and stage must be minimize so to do that what
kind of arrangement is required you have to do mostly this fan shape is use to with balcony
if have to design a large capacity auditorium ok now volume and dimension the floor area
and the volume of the auditorium should be kept at as reasonable reasonable minimum because
unnecessary creating volume is not required it is architectural it is civil construction
cost also reduce the sound energy so minimize and the volume starting the distance starting
the distance stage and people so that distance should be minimize of two parameter optimum
volume and optimum distance from the stage to the person is required for that i have
to design the shape and then i have to design the volume and dimension of the auditorium
what is the length width breath all kind of things so it some rule is that in audience
size floor area should be six to eight square feet per seat if the hundred seat then six
hundred square feet to eight hundred square feet the floor area ok
then recommended volume per seat it dependence on the type of the auditorium it is for speech
there it is said minimum like the lecture theatre it is two point three maximum three
point one maximum four pint three square feet per seat ok then six point two seven point
eight ten point eight square feet per seat for the concert hall for the picture theatre
like cinema hall it is two pint eight three point five is optimum minimum is two pint
eight and maximum is five point one is sufficient for the lecture hall for the picture theatre
means cinema hall so you know the volume and also the volume come from the large room acoustics
the volume must be support the large room acoustics so that suppose you can say ok i
am to design the room for ten people then the volume may be very small so that connect
support the large room acoustics then it i cannot design that large room acoustics design
which is followed by the auditorium design for that kind of room then we have to go for
the small room acoustics design ok so this is minimum volume per seat that some rule
is that that volume should support the large room acoustics then ceiling height volume
by floor area generally it is twenty into multiplication of the reverberation time so
it is volume i once i get the volume then divided by the floor area so floor area i
get per seat i get the volume per seat i calculate the volume and floor area then i get the ceiling
height then typically the dimension will come in
the l length width and height is two h length should be twice then the height width should
be one point five times of the height and height in edge so if the height is ten feet
let’s ten feets then it should be twenty feet and one point ah [vocalised-noise] and two
times one point five times that means fifteen feet and ten feet but the volume should support
the large room acoustics so it may not be support the large room acoustics so it cannot
be large room acoustics auditorium so minimum volume we have to say i have to design for
two hundred plus i have to design for two hundred capacity auditorium so two hundred
people if i say the picture hall that means cinema hall then let’s optimum values three
point five so into three point five is the volume minimum volume is required which is
nothing but a three point ah ah seventy into ah seven into ten to the power so seven hundred
square feet feet square feet cube ok or meter cube whatever is required so this the minimum
feet q is required meter is not meter q so this is the minimum volume is required ok
so you know that then how to design that then i get the ceiling height once i get the height
i have to design the floor area that will come in this almost come now suppose in this
formula if you learn this very long suppose length is come one twenty getter than one
twenty feet then what will happen it is it is increasing the direct sound distance so
i have to reduce it how can i reduce it use the volcanic if i use volcanic the length
will be reduce so i can use the volcanic as stage opening should be this side so those
of the basic parameter which is available from the literature and i have given you then
layout of the surface how we are design the surface this is see that there is no parallel
curve and there is a ramping here and balcony also ramped so why the ramp is required the
sound source must be raised as much as feasible in order to secure the free flow of the direct
sound so this is the one one criteria that if i want to put a loudspeaker in here you
should not be put in below the stage it should be as raise as possible so that the direct
sound has a direct link with that last seat of the volcanic i have to see that the if
i put the box in here let’s i design an auditorium like this and this is the balcony and i put
the sound loudspeaker in here what will happen there is no direct sound to this seat so this
will effect so raise the loudspeaker in here so that sound is come here and here here also
ok as as much as possible so the direct sound reach the last seat of the balcony then another
criteria if i if i place that all seat in flat so human being is a observe observer
of the sound so this man may foul to the direct sound to the next man so direct sound cannot
reach there so there is a ramping is required this is also required for a hearing in viewing
side also so there is a ramping is required but this ramping has an limitation i cannot
ramped would not like this ok so i ramp it one is to eight in ratio so if
the length is eight feet then the ramp last seat ramp should be one feet if length is
hundred feet one is to eight ratio i have to ramp it so i ramp it then how do how how
do arranging the seating seating pattern how do you arrange the seat if i arrange the seat
suppose i put a seat in here and next seat in here then what will happen the direct this
man foul the direct sound to the next man so instead of doing that you can make the
seat arrangement like this picture this man setting in the gap between this two seats
so there is a possibility the direct sound will be go there ok so this kind of seating
pattern must be maintain maintained then the sound source should be closely and up close
so ok let this one don’t want i don’t want read the slide what is say that suppose i
put a extra sound sound source in the wall or in the stage wherever i put the sound source
it should be covered by a reflector or it sound source should be put in such area square
the sound is reflecting so that it reinforce the sound reflection sound in the room ok
so if i i put a acoustics blanket in backside and then put a sound source what will happen
the if i put an acoustics blanket in here so let’s hundred percent absorbed if i put
a sound source here so the source sound which suppose to come this side and reflected back
to this side to increase the directivity will be absorb so directivity cannot be two it
come one because the backside sound is absorb instead of reflected into the front side so
that it reflected sound can travel to the audience it is absorb so if i put a loudspeaker
in backside if i required then loudspeaker should be surrounded by a sound reflector
so that the reflection of the sound should reach to the audience ok
next time delay this is very important we have to think about that at least initial
reflections with the audience should we reach within the thirty milliseconds which is nothing
but direct sound is d distance then r sound is reflected sound is the r one plus r two
so r one plus r two minus d divided by zero point three five so this that distance time
time gap between the direct sound and positive reflection it should be within the thirty
milliseconds ok so how they are do it so the surface of the auditorium if i say that or
the or the or the top of the auditorium the top surface of the auditorium must be not
like a just plain ceiling if i put a plain ceiling what will happen if i in top of the
auditorium is just plain ceiling then it is not it will not create that equal reflection
in the all part of the auditorium instead of top ceiling if i make a curve ceiling i
put a reflector in curve surface or suppose my ceiling is flat but i can put a reflector
to make a curve so that the reflection can happen in every one ok so unless what will
happen these reflection does not have a meaning because if i want that that is again reflected
by the back surface and come to here it may harder so this reflection because backside
will be treated as a acoustics blanket so this will not reinforce the sound system so
what i will do i will make a curve reflector in the top ceiling so that it reflect the
sound ok unless the ceiling height is very high always put reflector in the ceiling if
the ceiling height is very high then what will happen from the ceiling reflection may
create an eco so to avoid that eco in that case i required a treatment of the light wave
total absorptive surface in the ceiling so that there is no reflection come from the
ceiling if it is too high if it is not that high like that it it there is possibility
to the harden eco i can put a sound reflector there so that it reinforce the reverence sound
ok so that kind of treatment i have to make now
there is a thumb rule of the surface treatment of the auditorium so near the stage it should
be reflecting so that maximum energy channelize to the audience so stage should be reflective
or near the stage and stage should be reflective next is middle portion should be diffusing
means there should be a irregular res reflection from the surface not regular reflection if
it is regular reflection then what kind what will happen the some area will be focused
and some area will be defocused so i want diffusive treatment after the stage after
the front part of the stage let’s that up to may be few and few fit that there will
be a reflective surface then next portion is diffusive surface and last portion should
be absorptive why because if the length is long enough then i there may be a chance that
i can harden eco to avoid that the last portion of the auditorium must be as much as absorptive
so that the reflection sound intensity is very low so that the eco is avoided so why
know we design an auditorium the acoustics treatment of the auditorium is reflective
in the stage and front part of the auditorium middle part of the auditorium will be diffusive
and rare part of the auditorium or the last part rare part of the auditorium should be
as possible as absorptive so that no sound should be reflected from that area ok so that
is the thumb rule for acoustic treatment in any auditorium if you any good auditorium
you found that front stage is stage front part of the stage or made up a polish wood
and some channel so the channel create an diffusive reflection and it is a reflective
glace so glace means reflection co efficient is very high there is no acoustics tiles is
placed on the front part of the stage or few after stage there is a few few metres or look
in a one or three metres there will be in all reflective surface
then next part is diffusive so there is a acoustic diffuser is placed as you discuss
in the last class the acoustic diffuser how do how do know that you know that acoustics
diffuser is a irregular surface and you know what kind of acoustic diffuser is available
in the market you can go to the market and see that what kind of acoustic diffuser so
that diffusing surface not the regular surface if i make a wall of the auditorium painted
glace painted in the surface then what will happen it create a regular reflection so that’s
why auditori inside the auditorium you don’t find that except stage part that there is
a glace surface which can produce a regular reflection there is a reflector but it is
diffuse reflector that means it create an irregular reflection so instead of plain painted
wall we use a rough wall kind of things to provide a diffuse reflection ok so there is
a material provide a surface irregularities random alternative so i can see there is a
some moral if you see digital auditorium some time is the front part of the stage there
is a some moral since moral has an ordinary surface it can access diffuse reflection and
moral is painted then the absolutely of the moral is very less so front part of the stage
is reflective so i can do some moral or wooden work in the front part of the stage if you
visit in our kalidas auditorium you see the front part is made painted wooden design so
it is a irregular depression diffuse design but it is a reflection coefficient is much
higher so that it reflect the maximum energy but after that we put some acoustics tiles
which is a acoustic diffuser so that it create a diffuse reflection
but in the rear part of the auditorium we should did total absorptive that means the
material is absorption coefficient is very high should be put in the rear part of the
auditorium now the room must maintain optimum reverberation characteristics the reverberation
time must allow favourable reception and efficient presentation so i cannot say that ok i make
a auditorium whose reverberation time is very less and whose reverberation time is very
high both are problematic so i have already said you give you a slides were i say that
different kind of purpose of the auditorium if this purpose is peace theatre if it is
purpose is concentrate all what should be the optimum reverberation side time that there
is a chart available from the architectural design side ok then the room should be free
from acoustical defects such an echo long delayed reflection sound concentration and
coupled space that means what is the meaning when i get an echo if the delay between the
direct sound and reverberation sound is much high then there is a possibility that i hard
an echo so suppose my room has an long distance long long end room if the end reflection come
to me then i am a hard is an echo then what ki then what i do at the end part of the auditorium
i must put absorptive material so that the maximum sound energy is observed by the surface
so the reflection sound intensity is very low then i can avoid that echo the echo will
be not there or i can change the shape if you see the shape of the rear part if i design
a auditorium ceiling such that there no sound energy can receive here and or the minimum
sound energy will be reach in the last portion of the auditorium then i can tape that second
part so the volume of the rear part is reduce to reduce the number of reflection ok
then i can make a rear wall acoustic blanket kind of arrangement so that even if a reflection
is come totally absorbed every direct sound is fall on that wall is totally absorbed so
if you visit any auditorium you should see the end wall or the rear wall of the auditorium
will be treated with an acoustic blanket or a material which has high absorptive ok if
the primary sound source which is normally located at the front part of the audit so
suppose i want to put you can read the slide i not reading the slide so suppose you want
to put an extra loudspeaker in the in the auditorium so as i mentioned while i put the
extra loudspeaker i should put with the reflective surface so that the sound energy of the loudspeaker
is reflected back to the auditorium ok so i put a reflective surface
noise external noise is very important suppose we are creating a a c dock in the auditorium
you should very causes that acoustic with through the acoustic dock external sound should
not enter in the auditorium so there is within the dock also you required an acoustic so
that external noise is avoided to the a c dock also and also we have to see suppose
i have build an auditorium beside highway what will happen the sound highway sound will
be there is a vibration conduct sound will be there conduct sound means say let’s you
put that wall thick so that the external sound not entered in the auditorium then you put
a acoustic blanket after the the then you put the acoustic reflector so the inside sound
is reflected back but outside sound cannot be enter because there is a acoustic blanket
and there is a brick wall you have hard ever the intensity transmitted intensity is inversely
proportional to the l square so i can if the length of that wall is increases then the
rear sound intensity will be enter in the auditorium so i can say that yes i can make
that arrangement so that highway noise is not entered
but if it is totally concrete to the highway what will happen the vibration conduct vibration
conduct conduction will come out so to avoid that that i should make an arrangement so
that the auditorium is separated from that conduction so i can make a channel outside
the auditorium i put a stand there so it actual absorption and direct sound will not come
similarly in the floor how to treat a floor of the auditorium you you found the generally
the auditorium floor is corporate why because we we want that there is two kind of things
if it is not glacis surface there will be a reflection from the surface ok so that audience
also create a noise that noise also reflected ok so i want the auditorium that diffuse see
see the i required diffuse diffuser in the middle portion so instead of providing a middle
portion only blanket i whole auditorium is a stop acoustic corporate so that that that
reflection sound does not come but the stage i required a reflective surface so i can make
a concrete stage with the high reflection high reflection coefficient or the front of
the stage also i can make a high reflection coefficient so i can make a concrete concrete
on that that area which is high reflective coefficient
but in the rear part in the floor also has to be treated as a either diffusive and the
sorry in the rear part the the i have to treat that floor also whose absorption two percent
is very high so that is nothing but corporate ok so there is an example ok forgot about
that there is an example i have given in here say that i think it is not visible so let’s
i make it large what i said that an auditorium has on the following specification design
the acoustic treatment what is that i given a table people on upholstered quantity i have
done vacant seat so there is a all acoustic coefficient is given in table one absorption
coefficient vacant seat and absorption coefficient vacant seat is point three and if it is people
will occupy point four then wall in diffusive module and space between the modules and ceiling
the surface area is given area floor without seat rear interior wall rear ceiling surface
area is given absorptive ceiling on side wall ceiling reflector stage front wall diffusive
side wall stage floor stage side wall then i said write the material name which kind
of material you should acoustic material you should use to treat this area
so what i say thumb rule is that stage area must be reflective so when i design when i
said the stage front wall i should use the material which absorption coefficient as low
as possible suppose absorption said the those of the material i want to use in that auditorium
so i can say that see that fiber cement board absorption coefficient is very low so i can
use that in stage front wall stage floor is there anything floor floor hard wood i can
use in stage floor painted concrete i can use in the stage side wall so all those things
so reflective part i design then if you see the rear ceiling and rear interior wall rear
part of the auditorium must be as much as possible absorptive so i have to see which
material has a large material absorption coefficient acoustic blanket wood fiber board so i can
say acoustic blanket i can use in the rear interior wall and rear ceiling i can use wood
fiber board so depending on the plus aesthetic i can choose the material from the market
and treat that area but thumb rule is that stage area must be as reflective as possible
middle area must be as diffusive as possible and rear part must be as depth of absorptive
as possible so you put that material name in here then
you know that absorption coefficient multiply with the quant surface area you get the absorption
in sabin then add them to get the total sabin then you can calculate the reverberation time
if the volume is given number of seat is given so acoustic volume so it is not meter square
so it is meter cube sorry it is a meter cube not square ok so volume is given a is given
so reverberation time is equal to zero point one six one into v by a v known a known then
i estimated reverberation time suppose the acoustics says that and my auditorium should
be reverberation time should be one point two to next one point five second then after
calculation i found it is coming one point one so i have to realist the material and
decrease a so that reverberation time is increase so front wall part front part of the stage
i can use other material which has absorption coefficient is much lower if it is much lower
then the a will be suppose point zero zero one s into point zero one s into point zero
zero one then a will be less in here ok so i much reflective surface i will be used so
then i just changing the a i find out the then my design is dock
so why know you do a design of an auditorium two things one is that concentrate is give
a consultant to the acoustic architectural engineer that i required this kind of shape
because if it shape is not describe then i cannot auditorium acoustics will not good
so i required this kind of seat arrangement i required this kind of ceiling volume this
minimum volume should be this i required this kind of ceiling arrangement this kind of ramping
and this kind of seat arrangement all things you can con give that suggestion these are
the desired for a good acoustics then you know the surface area then design is done
you know the surface area then the design then the user said i required reverberation
time one point two to one point five then you to choose the material which portion you
want to treat by which kind of material acoustic material so there is the thumb rule is that
reflective diffusive absorptive so that is thumb rule it cannot put a glace style on
the rear wall because then the echo will high in the auditorium so rear side has to be absorptive
now you adjust the material depending on the aesthetic aesthetic part is also very important
so depending on the aesthetic and reverberation time you adjust the material and design the
auditorium and also there is a another one work work
is there you should also give an advice that what kind of sound isolation is required inside
the auditorium outside noise sound should not enter in the auditorium so there should
be suppose i make an auditorium in a glass window can it be possible no if it is the
glass window then outside noise should come enter in the auditorium i cannot avoid that
ambient noise so ambient noise will be increases so that is avoided now once want to design
a door i just simpler glass door if the door is glass door then outside noise is come inside
so i have treat the door also in acoustic treatment so that the outside noise should
not enter when the door is closed so i have to design the door also i have to choose the
material when where the i have to design the door such that that outside intensity sound
intensity should not entered in the room you know that how the sound intensity is travel
through the through the material if the two boundary then you know that the it depends
on the l thickness suppose there is no thickness thickness is poss thickness percentage is
not possible i cannot make the door so thick so what i will do i make the air gap inside
the door or i make a i put a acoustic blanket in middle middle of the door i make a two
two kind of wooden wooden plate and then make a acoustic blanket here then the sound transmission
will be reduce so that kind of consultancy you have to provide
to the engineer so this is the auditorium design let’s try a design of this kind of
auditorium kind of things real part then if you have any query then you ask me then if
i know i will answer it ok thank you

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