in this module we will be looking at details
of acoustic materials so far we have been talking about basics of acoustics and indoor
acoustic treatment you know the last module we were talking in elaborate about considerations
for acoustical design in buildings where material selection is a very crucial part unless you
select a right material your acoustics is not going to be alright so this module we
primarily focus on types of acoustic material observing materials insulating materials primarily
we will be starting with indices for measuring what parameter should be looked say if you
have to select from a list from a seller what parameter you should be keenly looking for
and then what are the types of materials available and their specific applications a quick you
know recap of what we saw last time this was a reverberation time equation this is formula
where you have reverberation time which is equal to point one six three volume by the
absorption a is alpha that is called absorption coefficient of specific material into the
surface area of that particular materials application
so if you have n number of materials spread over specific surface areas then you can have
a sigma that is specific absorption coefficient which into in the surface area in this particular
number a is referred in terms of absorption is referred in terms of sabin that is a unit
for absorption now we will look at more closely about what this alpha actually means alpha
is the absorption coefficient of a particular material it ranges from zero to one one is
hundred percent absorbing zero is hundred percent reflecting that is doesnt have any
specific absorption this particular absorption are alpha absorption coefficient varies with
respect to frequency so ideally you when you select a material if you want a real performance
based selection it is rather better to locate at least three specific absorption coefficient
that is absorption in the low frequency something around sixty three or one twenty five hertz
are sometime two fifty hertz you know better is to look at sixty three and one twenty five
hertz then in the mid frequency range that is around thousand hertz what is absorption
coefficient and in the high frequency range something after four thousands say eight thousand
hertz is a good indicator of high frequency absorption
there are two different types of tests through which absorption coefficient itself is determined
first is called reverberant chamber method we talked about two different types chambers
first was a reverberant chamber other is an anechoic chamber you know we had this references
in the last module reverberant chamber is a place or a testing room where the acoustic
absorption is almost zero it has infinite reflections or the reverberation time is very
large on the other hand anechoic chamber is something where there are almost no reflections
on the no absorption is very high the reverberation time is very low this is what makes the difference
so the first method you place that you know material to be tested you mount it inside
the reverberant chamber and you test the difference between the original reverberation time and
reverberation time after the material is placed inside the room typically test chambers have
their own reverberation time without any material present when the material is present there
is a second or the second reverberation time with this you can find out what is a reverberation
absorption coefficient of this particular material
this method of testing is more efficient for frequencies between hundred to five thousand
hertz and there are lot of other considerations like mounting on what surface you are mounting
edge diffraction lot of other things are coming we are looking into those specifications right
now just for information this is the first method and the second method is called impedance
tube method we studied about the principle standing waves this particular principle is
applied in this testing i will quickly show you the chambers and the testing devices this
is a reverberant chamber you place the material inside the chamber it has infinite reflections
or the reverberation time is very long without any material the chambers reverberation time
is taken and after the material is mounted then the absorption method you know measured
there are various advancements in these method of measurement this is a anechoic chamber
for your information it has a lot of each one is on absorbing surface
so the surface area is very high and the absorption is rarely high the reverberation time will
be very low negligible reverberation time this is second type of measurement which is
called standing wave method or the impedance tube method this a typical experimental setup
it’s available as a testing device or you can fabricate your own this diameter of the
tube and the length of the tube has a critical you know position to play in terms of the
testing efficiencies say this kind of thinner tubes you know thin diameter smaller radius
tubes are used for high frequency testing the larger ones used for typically low frequency
testing one side of it you mount the material there is a signal generator or a pure tone
generators specific frequency is set in which tones are produced a principle of standing
wave a standing waves are formed in you know inside this chamber then you have a microphone
where which is recording in the typical high and low that is a maxima and minima of the
wave length with which a specific calculation procedure will tell you what is absorption
coefficient so this is a second method of testing using
these methods typically you determined at specific frequencies what is a absorption
coefficient this is what we are referring as alpha so when we say a specific material
is there let us take for example the first the red line here this is a material x or
say for example we can say it is a cushion it’s a fifty m m cushion we are testing this
so at different frequency starting from one twenty five where your getting about negligible
absorption then you go all way up to four thousand where your getting about point eight
alpha or absorption coefficient the material is having a good absorption in the high frequency
verses it has a very negligible or low absorption in the low frequencies you take another material
y where you are mounting it say for example it could be glass behind a particular panel
or a fabric you are testing this particular thing again it’s a fifty to seventy five m
m which has a different type of alpha value at one twenty five hertz it has slightly better
absorption point four point five mid frequencies close to five hundred it is ok after mid frequency
it has very low absorption commonly when you buy a material you have
a value called n r c or noise reduction coefficient which is nothing but an average absorption
coefficient of four different frequencies two fifty five hundred thousand and two thousand
any material you chose the first index or the number you get to see or refer plus a
supplier gives you will be the noise reduction coefficient then say the n r c value is point
eight point nine four it is a good absorbing material yes it indicates the materials absorbing
performance but one short note you have to remember it is an average value so take the
case of two materials that we were comparing material x and material y both of them the
absorbing pattern is different first material has good absorption in the high frequency
low frequency it is not performing well where as the other material the material y has the
totally reverse type of performance in the low frequency range it is better as it gets
higher the absorption is coming down but as you get see the n r c both have around point
four n r c so in this way n r c as such simply referring
to n r c little misleading so it is rather a good idea or a better strategy to look at
what is the specific absorption coefficient at different frequencies if you take a closure
look if you want to control reflections or if you want more absorption in the low frequency
if your reverberation time calculations shows that your r t at the low frequency needs to
be reduced then you need to go for material y in place of material x on the other hand
if you find that mid and high frequency you need more absorption or at specific location
you need to arrest mid and high frequency sounds then better go for material x so n
r c as such is a good indicator for absorption absorption coefficient of a material but since
it is an average it sometimes is misleading because it averages out both high mid and
all the three high mid and low frequencies another consideration you have to be careful
about while you chose a material for instance i have given an example you are wanting to
buy a carpet and the you know material suppliers says the carpet has a n r c of point eight
which means it is extremely good absorbing material so typically you will prefer going
for this particular carpet but if you closely take a look at it the mounting condition in
which the material was tested was at only the carpet or was there any backing given
to the carpet in this case for example if the carpet where you know was installed over
a block of fiber glass or a panel of fiber glass then this n r c particular n r c of
point eight actually refers to a better absorption not just of the carpet but it is carpet as
well as the backing and the fiber glass you know glass will backing which is resulting
in this point eight it was not the sole performance of the carpet alone
so better you also ask the supplier or check for the data base in the data sheets what
was the mounting condition if you take a closer look then your material selection is more
economical or more trustable there are three types primary types of acoustic absorbers
first is porous absorbing material second is panel absorber and the last is cavity resonators
each one has a specific band width or frequencies spectrum in which they have a very good performance
it can be stretched out to other frequencies but their primary performance area are different
take a porous material any type of cushion glass will you have boards there are few varieties
i will show you now simple example is a cushion or a foam acoustic foam which will have very
good absorption in the mid and high frequency range is a cushion if you take look at microstructure
it has lack of air porous in it so typically the acoustic signal or the sound energy is
getting converted into heat energy inside these porous because of which you get very
good absorption in the mid and high frequency ranges
as i said mid and high frequency is good but if you take simple fifty m m or twenty five
m m thick cushion it you know it’s absorption or alpha value absorption coefficient is rather
week or low in low frequency it is not very well performing in the low frequency if at
all you are left with only ah option of cushion you dont have any other option you have to
go for a foam then if you still want low frequency absorption then you can increase the thickness
or the spacing between the solid backing solid backing can be your wall so probably you should
go for a fifty m m or hundred m m space air gap then mount instead of a twenty five or
fifty m m foam you go for hundred or one fifty m m foam with the air gap then it can be made
to or stretched to improve the performance in the low frequency also but if you are simply
mounting it on the wall surface a thin fifty m m cushion standard panels boards are available
if you are simply going to mount it then you can only expect a very good performance in
the high frequency low frequency you cannot expect much
there are different types perforated fissured boards textured materials different types
of patterns textures are available they are also available in the form of acoustic boards
hangers geo acoustic tiles lot of you know panels nice looking you know architecturally
good looking interior you know suited to different themes of interiors panels are available commercially
today other variance include acoustic plasters sprayed fibrous materials blankets foam boards
carpets fabrics are available so there are lot of varieties and this is one of the most
predominantly used acoustic absorber as far architectural applications are concerned the
second type is panel absorber imagine a thin plywood panel mounted on a frame there is
a air gap you have the wall section here this also absorbs at a specific particular frequency
band width the absorption is restricted to one or two specific band widths which depends
on the thickness of the panel the area of absorption and the air gap which is available
behind that is the spacing depending on these three variables the density of the parallel
as well depending in these three crucial things density thickness as well as the mounting
distance the absorption specifically varies i show you a specific examples of how the
absorption is it typically resonates at a particular frequency i am i am going to show
you some examples the third type is called cavity resonators
these are typically box shaped absorbers these are also called helmholtz resonators working
principle is they actually trap the acoustic signal it can be specific boxes like this
or it can be panels with perforated in which each of these perforated are going to act
us is going to act us an acoustic cavity inside which the signals or the incoming signals
can be trapped this signal can be have simple white or this can be lined for improved absorption
and these are typically well performing in the mid and low frequency range of course
it depends on the width of the cavity that is the diameter or the cross section of this
whole thing as well the resonate frequency for example at which frequency this cavity
is going to be effective or if you have a specific frequency in which you have to address
say for example your hall or your room is not performing well or the reverberation time
is excessively high at one twenty five hertz or sixty three hertz in this case you have
a formula you know the frequency now velocity is fixed now you can determine the dimension
and the cross section of the cavity as well as the volume of the cavity which will be
able to arrest frequencies at sixty three hertz frequencies of one sixty three hertz
or one twenty five hertz there are different types individual cavity
resonators or perforated panel absorbers like i said it can be perforated boards or it can
be slit resonators some examples you can have glass backing behind this you can foam backing
behind this there are different types and the performance where as i show you some examples
apart from this they can also be separately suspended in the form of specific boxes which
act as resonating cavities are like i said it can be individual cavities which can arrest
ah you know very low frequency sounds typically used in recording studios where low frequency
creates a lot of problem because of the smaller volume of the room as well as a type of sound
signals which are generated a few examples first let us start with cavity you know panel
resonators panel absorbers let us take an example you have a wall solid wall you are
mounting twenty five m m thick glass then you have a panel plywood panel which is eight
point four kg per meter square typically what you get you get a sharp absorption around
hundred hertz are close to one twenty five hertz you have a sharp absorption of around
point seven alpha or you know absorbing coefficient of point seven close to one twenty five hertz
but after that the performances almost all the alpha values are almost zero
you get very good absorption in this part after that it is dropping down but if you
take a look at the n r c it will show zero because it is only taking into account frequencies
after this particular frame this lower frequency of sixty three and hundred hertz is not covered
here if at all your requiring this particular frequency absorption sometimes thin panel
absorbers would be really of good help the absorption also varies for the thickness as
well as the presence are absence or type of material used for the backing now i am increasing
this you know glass from twenty five mm two fifty mm the material is same wall everything
is same just the thickness has increased now you take a look at this the absorption sound
absorption the frequency is shifted further lower now it is getting much closure to sixty
three hertz and the absorption coefficient is also increasing you also get a very high
absorption it is shifting towards a low frequency it is a good way of you know good strategy
to use in case your requiring very specific frequency in which there is a problem in the
room or your hall now instead of a solid panel if you have a
slated panel you have certain slats here twenty five mm thick glass backing same wall material
plus you have a six mm thin panel and you have slats here look at the absorption coefficient
low frequency like you saw in the previous one you dont get much in the low frequency
range a proper you know ah perceivable absorption coefficient you will get somewhere around
two fifty hertz where it is above point five then you get a better absorption in the mid
frequency and in the high frequency range it is eventually dropping down this is with
the twenty five mm backing now if i increase the backing from twenty five to fifty mm then
you take a note the absorption slightly gets a shift you also get realizable values somewhere
around one twenty five hertz here itself it is crossing point five absorption alpha value
and then you get a good absorption in the slightly low frequency around two fifty five
hundred you get good absorption after that two thousand hertz it is eventually dropping
down just the variation in this thickness apart from this the type of slats the thickness
of slats is there you know thin backing behind this a cushion backing behind this are what
is a dimension instead of slats if you have porous or perforations the kind of absorption
your going to get is going to be different take a look at this now instead of a slat
you have a perforated facing now your n r c value is very good the absorption coefficient
pattern you also get a good absorption in the mid and high frequency range here the
critical thing is what is a diameter this perforation and what is a distance between
the perforation so first i will be defining here it is three mm dia and the spacing between
center to center spacing is nine mm and what is backing here i have given fifty mm now
now moment i am adjusting you know decreasing it increasing it or i am varying this perforation
type as well as in a spacing and the diameter or dimension surface then the absorption coefficient
or the performance of the material itself is going to vary
for example in this case i have a open area of around close to nine percentage i can increase
this if i increase this the material is going to give better absorption and the low frequencies
it is kind variable you have to actually get an understanding of how the material is going
to behave with three different things in this case spacing diameter and the backing now
we so far we have been looking at acoustic absorption so inside a hall you need an reverberation
time you need specific type of signaled noise ratio and acoustic absorption for this you
are using the factor called noise reduction coefficient and alpha or the absorption coefficient
now let us talk about sound insulation two things we are talking about first was acoustic
absorption inside a room now we are talking about sound you know partitioning and insulating
sound between one room to the other room say if source room to the receiver room it can
be a nice source or it can be a sound source which you want to really arrest for this similar
to n r c we use a term called s t c or sound transmission coefficient
any particular material say it can be a door it can be a window or it can be a wall system
typically there is a standard which asks you to go for and s t c rating of say thirty five
forty fifty there will be specific s t c ratings say for example your designing a board room
which is next one open plan office there will be a specific demand which says that s t c
rating of the partition used between these two should be say forty five which means if
there is you know the board room requires around thirty or thirty five d b as a background
noise even if the source room that is a next room that is open office if the sound signal
goes to say around eighty d b which means a fifty d b reduction will be required between
these two in that case the sound insulation or the sound transmission class which we call
is now should be carefully chosen such that it is higher it is again a single number rating
of a particular material or an assemblys ability to resist airborne sound transfer
here we have talking about airborne sound transfer there are different modes airborne
structure borne sound now we are specifically focusing on airborne sound that is air to
air sound transmission at specific frequencies between one twenty five to four thousand hertz
there are different types insulation like rigid panels you can haves you know something
called s p f or spray polyurethane foam then you have s i p structural insulated panels
different varieties variants are available then you have blankets batt insulation specific
applications demand different type of sound insulating materials we refer to a simple
term called transmission loss you have sound transmission by inserting you have certain
thing called insertion loss or a transmission loss without this particular material there
was this much amount of sound transmission happening between the source room and the
receiver room after i insulate you know insert the particular material there is some amount
of transmission loss which is happening this is determining the efficiency of a particular
material similar to noise reduction coefficient this
is also determined that different frequencies and then totally you represented in terms
of something called sound transmission class one important thing you have to notice say
for example there are two rooms separated by a partition simply the partition itself
might have a very good sound transmission class see you have a gypsum board two sets
of gypsum board with some glass insulation material inside it may have a very good sound
insulation say there is source room and there is a receiver room you have a gypsum you know
partition plus you have a glass then you have a second gypsum panel this has a simple partition
is available now source room to the receiver room you may have this particular thing can
have an s t c of for example forty five a good s t c value sound transmission class
is available moment you insert a particular window or a door the s t c of a door this
is s t c of partition now you insert a door and a window this not
openable just a see through window for example the door which you chose has a s t c of thirty
five the window which you chose has an s t c of thirty then you have to tally or account
for all the three things together which is referred as composite sound transmission class
when you have three four or n number of different elements across the wall or a partition then
each of it’s sound transmission properties or sound insulating properties have to be
accounted which we commonly refer as sound transmission class or composite sound transmission
class of a particular partition now this is a weaker link if you have to really improve
the sound transmission of the whole partition system then you have to really address this
particular window say instead of a single glass you go for a double glass unit or you
go for a laminated window panel which will be slightly more sound insulative
then you address the door you check whether it is a single door double door doors doors
typically give you a better sound insulation in the low frequencies you can go for acoustic
gasketing sound you know insulating gaskets then you can improve the acoustic performance
of the door there is another component here say particular supplier might give you a s
t c of the door as thirty five this is something which you find in the materials catalog this
is a laboratory tested value you also call have something called field s t c this is
a laboratory tested value so you have to actually give certain concessions for it’s performance
in the field moment you have certain installation related things you cannot do such a fine installation
like they do a testing in the laboratory so accounting for all that if you are doing it
practical design go for a slightly higher s t c rating instead of thirty five it is
suggested or advisable to go for thirty eight or forty s t c so that you can actually achieve
an s t c of thirty five it is a general design consideration which
any acoustic designer typically does simply putting it a sound transmission class between
say forty to fifty is considered to be very good higher it is say higher it above forty
it is considered to be typically good between the rooms that is even if you have above say
eighty d b sound in the other side you are going to get thirty five to forty decibels
if you have a s t c in this range below that is fair and it is very poor if the s t c ratings
are twenty to twenty five i will give you i i am going to give you some specific examples
of this as well if you have a s t c between twenty to twenty five it also means that the
speech between these two rooms ah source room and the receiver room is going to be audible
with each other typically s t c of the panel say you have gypsum board you have a plywood
panel there are different types you know ranges or regions in which the sound reduction is
more effective there is a specific range called mass law region where the density of the material
increases so the higher density material and thickness you go for in the insulation properties
increase but there are two ends to it in the lower end you have something called resonant
region where the materials start resonating to low frequencies typically it happens below
hundred hertz again it depends on the material and it’s dimensions typically in the lower
side it happens in the resonance region this is not proportional
whatever thick whatever density it is when resonates the acoustic insulation is very
poor you cannot interpolate or extrapolate in this region again in the higher end you
have something called coincidence region where different planes of acoustic signals coincide
with each other and again the insulation property drastically drops down getting certain specific
examples let us take a single layer of gypsum board a thin gypsum panel say around thirty
in mm or fifty in mm gypsum panel this is a absorption you know sound transmission class
or the sound insulation property at different frequency the red one this is what your going
to get now if you add a cavity at two specific gypsum boards you are trying to increase this
particular number the s t c value from twenty seven your able to achieve thirty four
now you have a glass associated with it you can raise it to forty you can increase the
width of it from say seventy five mm you are getting it to one twenty five mm now from
forty you have a slight increase you have a air gap that is you know you have a air
gap here air gap here the same fifty mm insulation glass is put up here you are getting a marginal
increase in s t c we can take a note of the curve here now further to increase this is
what we had in the previous graph we had forty one s t c you can actually replace this wooden
studs with specific metal studs with gasketing you will have a very good increase in sound
transmission class in this example instead of this particular wood if i am going to introduce
a metal stud with certain gasketing in both sides and kind of isolating both the things
i am actually arresting the sound transmission path with which i am able to get from forty
one i am trying to improve i am able to from forty one i am trying to improve i able to
improve it to forty seven s t c further to it there are specific types of studs metal
studs which are further good at isolating the sound transmission path that is airborne
sound transmission path it would be able to arrest further
so we will be able to achieve for example with two lines of gypsum in both sides with
the specific type of acoustical you know stud with the insulation of fifty mm and air gap
on both side we can raise it to up to fifty nine s t c which is really a good s t c to
achieve between the room so typically you have to understand as the type of s t c requirement
increases the amount of s t c is essential higher s t cs are essential there are various
strategies like going in for thickener panels adding another board two three boards or introducing
an air air cavity introducing insulation material changing the stud types or adding panels on
both the sides increasing the total air cavity depending on these things you can actually
attain an improvement in terms of s t c common mistake which people do in the industry when
you have fall ceiling typically people stop this particular panel at the fall ceiling
level ideally when you need a perfect insulation between both the rooms it is suggested that
you raise this partition up to the ceiling clear ceiling height dont stop it at the fall
ceiling level these are certain common things mistakes which are done
so you have certain thing called you know to the ceiling ceiling at a class where through
the ceiling fall ceiling it gets in passes through this and goes to the receiver room
so it is a good practice to get it further other common you know considerations are like
so you have two panels there is one board here there is one more board here this is
a source room this is a receiver room if your placings a switch boxes on both these things
certain common things like you know dont place it side by side it is a better you know advisable
thing to place it in a staggered manner anything like you know staggering these some principles
common principles logical things can be apply so that an effective s t c can be attained
how do you calculate the level difference between these rooms is a simple example see
you have a room a room b so source room and receiver room typical example in hotels or
you know apartment buildings typically in hotels the problem is more because some of
the rooms have connecting doors sometimes they make use of is as suit’s they can open
up this double doors so they become suit’s together this whole room is rented out but
when they close it there is a typical problem of sound transmission from one room to the
other room so when you have to calculate this for example
you have the transmission loss of this particular partition here you have different types of
wall section there is wardrobe here there is a door double door here then there is a
simple wall section thin wall section there is a thicker wall section here accounting
for all this you take the composite transmission loss then you take into account the absorption
which is present in the second room divided by the surface area of the partition together
if you take then we will be able to determine what is a level difference ideally what happens
you will not in practice you will not be required to calculate the difference but you will know
what difference you need to achieve you know that the source room is going to produce something
like ninety db where as here you want to ensure thirty five db so you know what is a delta
t required the possible options are you increase the transmission loss you take a look at each
of these elements and increase the transmission loss or you increase the absorption in the
receiver room you make the receiver room more absorbing then you will also find an associated
improvement in terms of the overall sound transmission or the sound reduction which
is attained closing this section so far we looked at different
indices of measurement we looked at two key things one is acoustic absorption where we
started with reverberation time and talked about noise reduction coefficient and specific
absorption at different frequencies how it is important other thing we looked at the
different type of materials porous absorbers panel absorbers and cavity resonators and
the second thing we looked at is sound insulating material where we talked about transmission
loss and sound transmission coefficient or s t c how do we calculate and how do we calculate
the transmission loss between two different rooms we also looked at few examples of how
do we increase the s t c of a particular partition system
thank you


  1. thank you sir, really informative…good research done at lab…will help in making good sound absorbing materials…

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