Acoustics of enclosed space

BEHAVIOUR OF SOUND IN ENCLOSED SPACE*

PH0101 UNIT 1 LECTURE 7

Introduction to Building AcousticsChanges in the direction of sound travel Sound Absorption and Absorption CoefficientReverberation and Sabines FormulaFactors affecting Acoustics of Building


Building acoustics or architectural acoustics deals with sound in the built environment. Structures with acoustic implications:AirportsChurchesTheatres Concert and opera hallsEducational structures, including class rooms, lecture halls, libraries, music practice rooms etc.,


Sound waves change their direction of travel through four categories of phenomena : reflection, refraction, diffraction and diffusion. These phenomena can occur when changes occur in a sound waves medium of travel. These physical principles are the same as those that occur in the optical world with light.


When a sound wave encounters a sharp discontinuity in the density of a medium, some of its energy is reflected. Reflective surfaces are typically smooth and hard. A few common acoustic problems caused by reflections are echoes and room resonance.


Echoes are caused by the limitations of our hearing mechanisms in processing sounds. When the difference in arrival times between two sounds is less than 60 milliseconds, we hear the combination of the two sounds as a single sound. However, when this difference exceeds 60 milliseconds, we hear the two distinct sounds. When these two sounds are generated from the same source, this effect is known as ECHO ECHOES


When parallel surfaces are tall and fairly close to each other, a rapid succession of mid frequency echoes, known as flutter echo, can occur. FLUTTER ECHO


An anechoic chamber is a space in which there are no echoes or reverberations. The surfaces absorb all sound, and reflect none.


Room resonance occurs at specific frequencies in rooms where two reflective walls are parallel to each other. In these cases, whole number multiples of specific half wavelengths will fit between the two walls. Since their surfaces reflect the sound, their mirror images bounce off each wall to setup stationary pressure pattern in a room. This phenomenon is called a single dimensional (or axial) standing wave and it is the simplest form of room resonance. ROOM RESONANCE


(2) Refraction

Just as light bends through a prism, the direction of sound is altered when sound waves encounter changes in medium conditions that are not extreme enough to cause reflection, but are enough to change the speed of sound.In addition to the speed of sound changing for different materials or media, the speed of sound changes with changes in temperature within the same medium. This variation in sound travel direction, caused by variation in the speed of sound, is known as refraction.


The shape of a space determines the sound path within the space


(3) Diffraction

The principle of diffraction limits the sound reduction effectiveness of any open-plan office partition or outdoor noise barrier. Sound waves bend around and over these types of walls, independent of their material, to impose this limit. Sound waves are not always reflected or absorbed. When an obstacle is the same size as the wavelength or less, the sound can bend around obstacles or flow through small openings, and continue onward. This is called diffraction. This action is more likely for deeper sounds (of low frequency, and this with longer waveforms).


(3) Diffusion

When a sound wave reflects off a convex or un even surface, its energy is spread evenly rather than being limited to a discrete reflection. This phenomenon, known as diffusion.


Sound Absorption

The property of a surface by which sound energy is converted into other form of energy is known as absorption. In the process of absorption sound energy is converted into heat due to frictional resistance inside the pores of the material. The fibrous and porous materials absorb sound energy more, than other solid materials.


Sound Absorption Coefficient

The effectiveness of a surface in absorbing sound energy is expressed with the help of absorption coefficient. The coefficient of absorption ` of a materials is defined as the ratio of sound energy absorbed by its surface to that of the total sound energy incident on the surface. =


A unit area of open window is selected as the standard. All the sound incident on an open window is fully transmitted and none is reflected. Therefore, it is considered as an ideal absorber of sound. Thus the unit of absorption is the open window unit (O.W.U.), which is named a sabin after the scientist who established the unit. A 1m2 sabin is the amount of sound absorbed by one square metre area of fully open window.


The value of ` depends on the nature of the material as well as the frequency of sound. It is a common practice to use the value of ` at 500 Hz in acoustic designs.If a material has the value of as 0.5, it means that 50% of the incident sound energy will be absorbed per unit area. If the material has a surface area of S sq.m., then the absorption provided by that material is a = . S


If there are different materials in a hall, then the total sound absorption by the different materials is given byA = a1 + a2 + a3 + A = 1S1 + 2S2 + 3S3 +

or A = where 1, 2, 3 . are absorption coefficients of materials with areas S1, S2, S3, .


Reverberation

Sound produced in an enclosure does not die out immediately after the source has ceased to produce it. A sound produced in a hall undergoes multiple reflections from the walls, floor and ceiling before it becomes inaudible. A person in the hall continues to receive successive reflections of progressively diminishing intensity. This prolongation of sound before it decays to a negligible intensity is called reverberation.


Reverberation TimeThe time taken by the sound in a room to fall from its average intensity to inaudibility level is called the reverberation time of the room. Reverberation time is defined as the time during which the sound energy density falls from its steady state value to its one-millionth (10-6) value after the source is shut off. Reverberation time must match room functionPure speech requires short reverberation timeSymphony blends notes with long reverberation timeReverberation time (in seconds) =.05 x volume of room sabins


Reverberation time in seconds Speech Small offices 0.50 to 0.75 Classrooms/lecture rooms 0.75 to 1.00 Work rooms 1.00 to 2.00 Music Rehearsal rooms 0.80 to 1.00 Chamber music 1.00 to 1.50 Orchestral/Choral/ Average church music1.50 to 2.00 Large organ/liturgical choir 2.00 to 2.25


Sabines Formula for Reverberation Time

Prof.Wallace C.Sabine (1868-1919) determined the reverberation times of empty halls and furnished halls of different sizes and arrived at the following conclusions.Sabin The amount of sound absorbed is measured in sabins. One sabin is equal to the sound absorption of one square foot of perfectly absorptive surface. The sound absorption equivalent to an open window of one square foot. (theoretical, since no such surface exists). The reverberation time depends on the reflecting properties of the walls, floor and ceiling of the hall. The reverberation time depends directly upon the physical volume V of the hall.The reverberation time depends on the absorption coefficient of various surfaces such as carpets, cushions, curtains etc present in the hall.The reverberation time depends on the frequency of the sound wave because absorption coefficient of most of the materials increases with frequency.


Prof. Sabine summarized his results in the form of the following equation.Reverberation Time, T or T =

where K is a proportionality constant. It is found to have a value of 0.161 when the dimensions are measured in metric units. Thus, T =

This Equation is known as Sabines formula for reverberation time.


It may be rewritten as

T = or T =


Optimum Reverberation Time

Sabine determined the time of reverberation for halls of various sizes. And from the results, he deduced the reverberation time that is likely to be most satisfactory for the purpose for which a hall is built. Such satisfactory value is known as the optimum reverberation time.


PH0101 UNIT 1 LECTURE 7 *

Activity in HallOptimum Reverberation Time (Sec) Conference hallsCinema theatre Assembly hallsPublic lecture hallsMusic concert halls Churches 1 to 1.51.31 to 1.51.5 to 21.5 to 21.8 to 3


Factors Affecting Acoustics of Buildings Reverberation Time

If a hall is to be acoustically satisfactory, it is essential that it should have the right reverberation time. The reverberation time should be neither too long nor too short. A very short reverberation time makes a room `dead. On the other hand, a long reverberation time renders speech unintelligible. The optimum value for reverberation time depends on the purpose for which a hall is designed.


RemediesThe reverberation time can be controlled by the suitable choice of building materials and furnishing materials. Since open windows allow the sound energy to flow out of the hall, there should be a limited number of windows. They may be opened or closed to obtain optimum reverberation time.


RemediesCarboard sheets, perforated sheets, felt, heavy curtains, thick carpets etc are used to increase wall and floor surface absorption. Therefore, the walls are to be provided with absorptive materials to the required extent and at suitable places. Heavy fold curtains may be used to increase the absorption. Covering the floor with carpet also increase the absorption.


RemediesAudience also contribute to absorption of sound. The absorption coefficient of an individual is about 0.45 sabins. In order to compensate for an increase in the reverberation time due to an unexpected decrease in audience strength, upholstered seats are to be provided in the hall. Absorption due to an upholstered chair is equivalent to that of an individual.


(2) Loudness

Sufficient loudness at every point in the hall is an important factor for satisfactory hearing. Excessive absorption in the hall or lack of reflecting surfaces near the sound source may lead to decrease in the loudness of the sound.


RemediesA hard reflecting surface positioned near the sound source improve the loudness. Low ceilings are also of help in reflecting the sound energy towards the audience. Adjusting the absorptive material in the hall will improve the situation.When the hall is large and audience more, loud speakers are to be installed to obtain the desired level of loudness.


(3) Focussing

Reflecting concave surfaces cause concentration of reflected sound, creating a sound of larger intensity at the focal point. These spots are known as sound foci. Such concentrations of sound intensity at some points lead to deficiency of reflected sound at other points. The spots of sound deficiency are known as dead spots. The sound intensity will be low at dead spots and inadequate hearing. Further, if there are highly reflecting parallel surfaces in the hall, the reflected and direct sound waves may form standing waves which leads to uneven distribution of sound in the hall.


RemediesThe sound foci and dead spots may be eliminated if curvilinear interiors are avoided. If such surfaces are present, they should be covered by highly absorptive materials.Suitable sound diffusers are to be installed in the hall to cause even distribution of sound in the hall. A paraboloidal reflecting surface arranged with the speaker at its focus is helpful in directing a uniform reflected beam of sound in the hall.


(4) EchoesWhen the walls of the hall are parallel, hard and separated by about 34m distance, echoes are formed. Curved smooth surfaces of walls also produce echoes. RemediesThis defect is avoided by selecting proper shape for the auditorium. Use of splayed side walls instead of parallel walls greatly reduces the problem and enhance the acoustical quality of the hall.Echoes may be avoided by covering the opposite walls and high ceiling with absorptive material.


(5) Echelon effectIf a hall has a flight of steps, with equal width, the sound waves reflected from them will consist of echoes with regular phase difference. These echoes combine to produce a musical note which will be heard along with the direct sound. This is called echelon effect. It makes the original sound unintelligible or confusing. RemediesIt may be remedied by having steps of unequal width.The steps may be covered with proper sound absorbing materials, for example with a carpet.


(6) ResonanceSound waves are capable of setting physical vibration in surrounding objects, such as window panes, walls, enclosed air etc. The vibrating objects in turn produce sound waves. The frequency of the forced vibration may match some frequency of the sound produced and hence result in resonance phenomenon. Due to the resonance, certain tones of the original music may get reinforced that may result in distortion of the original sound. RemediesThe vibrations of bodies may be suitably damped to eliminate resonance due to them by proper maintenance and selection.


(7) Noise

Noise is unwanted sound which masks the satisfactory hearing of speech and music. There are mainly three types of noises that are to be minimized. They are (i) air-borne noise, (ii) structure-borne noise and (iii) internal noise.


The noise that comes into building through air from distant sources is called air-borne noise. A part of it directly enters the hall through the open windows, doors or other openings while another part enters by transmission through walls and floors. RemediesThe building may be located on quite sites away from heavy traffic, market places, railway stations, airports etc. They may be shaded from noise by interposing a buffer zone of trees, gardens etc.(i) Air-Borne Noise


The noise which comes from impact sources on the structural extents of the building is known- as the structure-borne noise. It is directly transmitted to the building by vibrations in the structure. The common sources of this type of noise are foot-steps, moving of furniture, operating machinery etc. Remedies The problem due to machinery and domestic appliances can be overcome by placing vibration isolators between machines and their supports.Cavity walls, compound walls may be used to increase the noise transmission loss.(ii) Structure-Borne Noise


Internal noise is the noise produced in the hall or office etc. They are produced by air conditioners, movement of people etc. RemediesThe walls, floors and ceilings may be provided with enough sound absorbing materials. The gadgets or machinery should be placed on sound absorbent material.(iii) Internal Noise


Absorbing MaterialsCarpetSoft ceiling tileRigid foampeople Reflecting MaterialsMasonryWood smooth panelsSmooth concreteGlass


Live Auditoriums, theaters (for music) Obtain proper reverberation time to enhance musical quality. Provide reflective surfaces near source to reinforce sound; absorptive surfaces toward rear. Medium Live Conference and board rooms Normal speech must be heard over distances up to about 35 ft. Allow middle section of ceiling to act as a reinforcing sound-reflector. Apply absorbent to periphery of ceiling or to wall surfaces (not both). Additional treatment will contribute little to noise reduction. Medium Cafeterias (school or office) Reduce overall noise level. Use highly sound-absorptive ceiling; also use quiet equipment such as rubberized dish trays. Gymnasiums Instructor must be heard over background noise Use acoustical material over entire ceiling to reduce noise; walls remain untreated to permit some reflected sound.


Medium Cafeterias (school or office) Reduce overall noise level. Use highly sound-absorptive ceiling; also use quiet equipment such as rubberized dish trays. Gymnasiums Instructor must be heard over background noise Use acoustical material over entire ceiling to reduce noise; walls remain untreated to permit some reflected sound. Dead Kindergarten Maximum noise reduction. Maximum acoustical treatment on ceiling; space units on available wall surfaces. Vocational classrooms and shops Maximum noise reduction. Acoustical tile or lay-in panel ceiling, plus acoustical treatment of available upper wall areas; locate away from normal use rooms.


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Acoustics of enclosed space