acoustics (design)

27
ACOUSTICS

Upload: mrinal

Post on 06-Nov-2015

31 views

Category:

Documents


4 download

DESCRIPTION

ccc

TRANSCRIPT

Slide 1

ACOUSTICSWhat is acoustics ?Acoustics is the science of sound, which deals with origin , propogation and auditory sensation of sound, and also with design and construction of different building units to set optimum conditions for producing and listening speech,music,etc.The knowledge of this science is important for proper functional design of theatres,cinema halls,auditoriums,conference halls etc.Sound behaves as follows in an enclosure:- Reflection: when a sound wave is incident on a surface at normal it is reflected back such that angle of incidence is equal to angle of reflection.Absorption: when a sound wave strikes a surface,a part of its energy is absorbed and is not reflected back.

GENERAL PRINCIPLES AND FACTORS IN ACOUSTICAL DESIGN.Site selection and planning.Dimensions.Shape.Seats and seating arrangement.Treatment of interior surfaces.Reverberation and sound absorption.Site selection and planning: A noise survey should be made, site selected should be in quietest surroundingDimension: It should be decided according to the number of audience to be seated and in proportion of the intensity of sound to be generated, slight increase in height towards the centre of hall may be provided, average height may vary from 6m to 7.5 m in small and big halls respectively.Suitable volumes for different types of halls:

Public lecture halls: 3.5 4.5 cubic metre/personCinemas or theatres: 4.0 5.0 cubic metre/personMusical halls or concert halls: 4.0 5.5 cubic metre/person

Shape: It is a governing factor in acoustical defects, shape of hall should be geometrically arranged for better audibility. The farthest seat from the curtain should not exceed more than 23 m.

Seats and seating arrangement: Seats should be arranged in concentric arcs.First row distance from stage should be about 3.6 m for drama and 4.5 m for cinema purposes.Upholstered chairs/seats should be provided.Successive rows of seats have to be raised over preceding ones by 8cm 12cm and angle of elevation should be minimum 8 degrees.Line of sight shouldn't be inclined more than 30 degrees to horizontal.

Treatment of interior surfacesCeilings and wall should provide favourable reflection to reinforce the sound.Rear wall of an auditorium should be either flat or convex.REQUIREMENT OF GOOD ACOUSTIC MATERIALShould be efficient over a wide range of frequenciesShould be relatively cheap and easily available.Should give pleasant appearance after fixing.Should be self supporting and should afford easy fixing.It should be fire resistant.Should have sufficient structural strength.Should be durable and should not be liable to attack by insects, termites etc.Acoustical DefectsDue to reflected sound:EchoesReverberation

Common defects:Sound fociDead spots Insufficient loudnessEchoes An echo is produced when the reflected sound wave reaches the ear just when original sound from the same source has already heard. Thus there is a repetition of sound. It also happens when reflecting surfaces are situated at a distance greater than 17 m and when shape of room is curved. This defect can be removed by selecting proper shape of hall and by providing rough and porous surface

ReverberationThe persistence of sound in an enclosed space even after the source of sound has stopped is called reverberation. It is due to multiple reflection.

Reverberation time is the time taken by sound waves to reflect back and forth between reflective surfacesInstalling appropriate absorbent material can be used to rectify this defect.Optimum reverberation time TYPE OF BUILDINGOPTIMUM REVERBERATION TIME (sec)Cinema theaters1.3Churches 1.8 to 3Law courts, committee rooms1 to 1.5Large halls2 to 3Music concert hall1.6 to 2Parliament house, assembly hall1 to 1.5Public lecture hall1.5 to 2SOUND FOCIreflecting concave surface cause concentration of reflected sound wave at certain spot, creating a sound of large intensity. These spots are called sound foci.These defects can be removed by-Geometrical designed shapes of interior faces , including ceilingsProviding highly absorbent materials on focusing areas.

DEAD SPOTSThis defect is an outcome of formation of sound focibecause of high concentration of reflected sound wave at sound foci, there is a deficiency of reflected sound at some other point.This defect can be removed by installation of suitable diffuser so that there is even distribution of sound in hall. INSUFFICIENT LOUDNESS: This defect is caused due to lack of sound reflecting flat surface near the sound source. It can be treated by providing hard reflective surface near source and by adjusting absorption of hall

EXTERNAL NOISE: external noise from vehicles, traffic engine, factories, cooling plants etc.This defect can be removed by proper planning with respect to its surrounding.AbsorbantsSpecial materials used on boundary surfaces to increase absorption are known as absorbants.Porous materialsResonant panelsCavity resonatorsComposite typesPorous material: Absorption in porous materials is mainly due to sound waves causing to and fro movement of the air contained in the material. They are especially useful in high frequencies. Their efficiency is dependent on porosity, the resistance to air flow and thickness. For ex:- Wood wool, curtains, glass silk etc.

Resonant panels: These absorb the sound waves of lower frequencies (50-200 cycles) by vibrating.Depth of air space is left behind it.

Cavity resonator: it is a container with a small opening,and it functions by resonance of air in it. It can be used for any frequency.

Composite absorbers: It combines function of all of the three materials. It consists of a perforated panel fixed over an air space containing porous absorbent. The perforations should form atleast 10% of total area to allow porous materials to absorb sound at higher frequencies.Ex: Acoustic plaster(granulated insulation material with cement),wood particle board,compressed wood wool, rock wool etc.