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Science of sound dealing with wanted andunwanted sounds, assures optimum
conditions for producing and listing to
speech , music..
Hearing/Listeningcommunication channel
Next to vision
Unwanted scenes Vs Unwanted sounds
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Need To Study Acoustics
Optimum speech intelligibility Sound quality
Uniformity Loudness Richness
Clarity Minimum background noise Freedom from echoes
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lHistory of AcousticsAcoustics, one of the oldest branches of physics,originated with Pythagoras's studies of music over
2,500 years ago. Scientific milestones abound inthis field:
Galileo Galilei (1564-1642) discovered thegeneral principles of sympathetic vibrations,
or resonance, and the correspondencebetween the frequency of vibrations and thelength of a pendulum.
Leonhard Euler and Daniel Bernoulli's studiesof vibrating cords in the 18th centuryeventually led to the development of Fourieranalysis, one of the most important tools of
mathematics and mathematical physics.
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Sir Isaac Newton and Gottfried Wilhelm
Leibniz independently developed the theoryof calculus, which in turn allowed thederivation of the general wave equation bythe French mathematician and scientist Jean
Le Rond d'Alembert in the 1740s.Hermann von Helmholtz's On the
Sensations of Tone As a Physiological Basisfor the Theory of Music (1863) madesubstantial contributions to understanding themechanisms of hearing and to thepsychophysics of sound and music.
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John William Strutt's The Theory ofSound (1877/78), a monument of acousticalliterature, was the first treatise to examinequestions of vibrations, the resonance ofelastic solids and gases, and acoustical
propagation in material media.
Jean Baptiste-Joseph Fourier, a 19th-century French mathematician, establishedhis theory about the analysis of a complex
periodic wave into its spectral components.
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German physicist Georg Simon Ohmhypothesized that the human ear is sensitive
to these spectral components. His Law ofHearing stated that the ear is sensitive to theamplitudes, but not the phases, of theharmonics of a complex tone.
20th-century American physicist WallaceSabine initiated the science of modernarchitectural acoustics by finding ways tocorrect the acoustics of noisy rooms.
Hungarian-born American physicist Georg vonBksy validated Helmholtz's theory ofhearing with his Experiments in Hearing(1960), the classic of the modern theory of
the ear.
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Generation, Propagation,Transmission of sound
Sound results from vibrations in the mediumgas, liquid and solid.
e.g :tunnning fork, musical instruments etc
Speech is produced-complicated interaction ofthe lungs, vocal cords and passages in thethroat
The number of vibrations which occur in onesecond is called the frequency of the sound andis given the name Hertz
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Normal speech contains frequencies rangingfrom 20 Hz to 20,000 Hz, which is also therange of human hearing.
When the vibrations occur at frequencies less
than 20 Hz they cannot be heard and arecalled infrasonic
while ones above 20,000 Hz, which are alsoinaudible, are referred to as ultrasonic.
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Audible ranges of frequencies /intensity
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Acoustics involves three aspects
Production of the sound or vibration
Transmission of the sound through somemedium
Reception or detection of the sound
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Different levels of sound and theirsubjective effects
65 db Annoyance (psychological) 90 db Permanent hear loss(long
duration) 100 db Aural acuity (short duration)and
damage to Auditory Organs (long
duration) 120 db Pain 150 db instant loss of hearing
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Ears SensitivityAverage persons ears capacity - 20 20,000
Hz
(decreasing with age and subjective factors)
Sound Emitted- rate of energy flow(power) in watts
Lowest intensityThreshold of Audibility(10^-12w/sq.m)
Highest intensityThreshold of Pain(1w/sq.m)
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Decibel scale
Ear - Self defense mechanism Ears sensitivity decreases high intensity
sound Proportionate logarithm of intensity
N(db) = 20 Log I (measured intensity)
Io(threshold of audibility 10 w/sq.m)
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Behaviour of sound Enclosedspace
Reflected (r)
Absorbed(a)
Transmitted(t)If source I = 1,
r + a + t = 1
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Absorption Co-efficient
Absorption Co-efficient = a + t
(all thats not reflected : 1-r )
Absorption (A) = a x s
a = absorption co-efficient
s = Area of given surface
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Total Sound components: Direct component
Reverberant component
Sound types: Airborne
Structure borne
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Reverberation Time Reverberation time is defined as the time for
the sound to die away to a level 60 decibelsbelow its original level
Reverberation Time = RT 60 = time to drop60db
below originallevel
Sabines Formula:
RT=(0.16)V/Se in metres=(0.049)V/Se infeet's
V = Volume , Se = effective absorbingarea
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Reverberation for different
uses
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Acoustical Requirements :
Floors : Carpets,wooden,elevated floors
Ceiling : Wooden, Acoustical tiles(reflective)
Side Walls : Wooden (Reflective ),acousticalboards
Rear walls :Perforated acousticalboards(Absorptive ), soft boards with
upholstery
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Effects of Geometry andshapeSound is more pleasing if it is evenly dispersed, with no
prominent echoes, no significant "dead spots" or "livespots" in the auditorium. This even dispersion isusually achieved by avoiding any focusing surfacesand avoiding large flat areas which reflect sound intothe listing area. Sometimes it is desirable to add some
anti-focusing surfaces
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Focusing Surfaces
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Acoustical Properties ofBuilding Materials
Wood Reflective Gypsum Reflective , Perforated Gypsum Reflective cum
Absorption Concrete Reflective Glass Reflective Metal Reflective
Perforated gypsum board Absorptive Acoustical Tile, Glass wool Absorptive Fabric (carpet, seat upholstery, draperies)-
Absorptive
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Shilpa kala Vedika, Madhapur GREAT EMPHASIS IS LAID
ON ACOUSTICS TO CREATEA FOOLPROOF SOUNDSYSTEM FOR THEAUDITORIUM.
THE PERFORATEDALUMINUM PANELS ONTHE SIDES ARE IMPORTEDFROM ITALY.
ACOUSTICAL MATERIALGLASS WOOL BEING USEDIN THE CHECKERED
FALSE CEILING OFENTRANCE LOBBY
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Characteristics of sound
Frequency-no.of vibrations per second,measured in cycles per sec or Hertz
Intensity or Loudness-measurement of quantityof sound energy, measured as below(intensityphysical quantity, loudness depends on humanear)
Measurement of sound- very sophisticated someasured as logarithm of intensity of sound i.edecibels.
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Outdoor Noise Levels
Air Traffic 100-110 db
Rail traffic 90-110 db Heavy road traffic 80-90 db Medium road traffic 70 80 db
Light road traffic 60 70 db
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Acceptable indoor Noise levels
Radio and T.V studios 25- 30db Music room 30- 35 db
Hospitals 35-50db Apartments, hotels, homes35-40db Conference rooms, small offices, libraries35-
40db Class rooms40-45 db Public offices, banks45-50db
Restaurants 50-55db
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Optimum reverberation time and audience factorsfor acoustical design
Type of building Reverberationtime
Audience factor
Cinema theatres 1.3-1.5s 2/3
Churches 1.8-3s 2/3
Conference rooms 1-1.5s 1/3
Music concert Hall 1.6-2s Full
Assembly hall 1-1.5s Quorum(probable
size)Lecture hall 1.5-2s 1/3
Large halls 2-3s full
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General principles in acoustical design
Site selection and planningVolume, i.e size and height
Shape Treatment of interior surfaces Reverberation time
Seat, Seating arrangements, audience Sound absorption
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Auditorium AcousticsAcoustic separation is necessary at the entrance of theauditorium and also between projection room and
auditoria. At entrances, this is achieved with lobbies andsound reducing door sets.
Sound Propagation in an Auditorium
Sound waves travel at about 345 meters/second, so thatthe sound coming directly from a source within anauditorium will generally reach a listener after a time ofanywhere from 0.01 to 0.2 seconds.
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Shortly after the arrival of the direct sound, aseries of semi-distinct reflections from various
reflecting surfaces (walls and ceiling) will reach
the listener. These early reflections typically will
occur within about 50 - 80 milliseconds.
The reflections which reach the listener afterthe early reflections are typically of lower
amplitude and very closely spaced in time.
These reflections merge into what is called thereverberant sound or late reflections.
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If the source emits a continuous sound, thereverberant sound builds up until it reaches an
equilibrium level. When the sound stops, the
sound level decreases at a more or less constant
rate until it reaches inaudibility.
For impulsive sounds, the reverberant sound
begins to decay immediately.
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Direct sound and earlyReflections Early reflections are the single
most overlooked opportunityfor developing good soundingspeech in todays auditoriumsand sanctuaries, live theaters,lecture halls and evenclassrooms.
These most desirablereflections can be mechanicallyinduced by appropriatepositioning and shaping ofsound reflecting surfaces. Theycan be electronically emulatedusing a time delayed
distributed sound system.
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Direct , Early Reflections , late
reflections
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Good sounding auditoriums dont misuse theirsound systems to create overly loud sounds.
They use sound systems to generate acomfortable loudness for the direct sound andthen compliment this with a bevy of earlyreflections, immediately followed by a distinct
absence of late reflections and finally backfilledwith a groundswell of distant soundingreverberation.
Early reflections cant be distinguished from thedirect sound and thats why they are the onlyreflections that actually add to clarity of speech.
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Echoes
Echoes can be fun but they also ruin theunderstanding of speech Early reflections are those that bounce off nearby objects.
But when the object is located some distance away, thesituation changes, you can hear the reflection off of it and we
call this acoustic event an echo. An echo can be great fun at times but when its time to pay
attention to someone talking, it also makes listening verydifficult. The echo is a good example of a late reflection
because it is a reflection that can be distinguished as beingseparate from the direct signal.
A good, clear and bright sounding auditorium providesample opportunity for many early reflections to reach eachseat in the hall.
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Types of noises/ methods tocontrol
External noises Distance Screening
Planning Positioning of opening Noise insulating building
envelope
Internal noises Reduction at source Enclosing and isolating source by
absorbent screens
Planning :separating noisy spacesfrom quiet ones Placing of noise equipment
massive part of the building. Covering surfaces resilient
materials Noise in space- by absorbent
surfaces Airborne-airtight and insulating
construction, Structure bornediscontinuity.
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Kala Akademi, panaji,Goa.Ar.Charlea Correa
Visual construction of Acoustically
transparent materials- Auditorium
Interior