Sound Is Produced When Part Of
Sound is produced when part of the atmosphere is compressed suddenly. It is transmitted in the form of waves. The waves are a series of compression and rarefaction created in the air medium.
The average sound travels at a speed of 340 m per second at ordinary temperature. It depends on the medium through which it travels.
Effects of Audible Sound
Sound can travel through some medium like air. It cannot travel in vacuum. Thus, for the sound to be audible to the ears, the sound source and ear must be connected by an elastic medium like air. Following are the characteristics of audible sound:
1. Frequency of Sound
Frequency of pitch of sound is defined as the number of cycles or vibrations per second.
The highest audible sound (e.g., whistle) has a frequency of 20000 cps (cycles or vibrations per second).
The lowest audible sound has a frequency of 20 cps (e.g., whispering). The frequency is a measure of the quality of sound.
2. Intensity of Sound
The intensity of sound is defined as the flow of sound energy per second through unit area. The intensity of sound is the strength of the sensation received by the human ear.
Intensity of sound is a purely physical quantity. But loudness of sound depends on the characteristics of ear.
3. Measurement of Sound
The range of intensity of sound is very large. The loudest sound is about 1013 times the sound which is just audible by the human ear. There is a wide range of sound levels, it is realised that a scale has to be adopted as a guidance.
The intensity of sound is measured on a logarithmic scale due to wide range of variation of the intensity of sound. Bel is the measure of intensity of sound named after Graham Bell, the inventor of telephone.
As the unit of bel is comparatively large, hence a shorter practical unit decibel (db) equal to 1/10 of a bel isused.
The range of audible sound to painful noise varies from 1 to 1013 which is covered on logarithmic scale between 1 to 130 db units.
One db unit is approximately the smallest change of sound intensity which the human ear can hear.
Principles of Acoustics
The behaviour of sound plays an effective role in the acoustical design of different type of buildings and in the sound insulation process.
A sound originating from a source, such as music or operation of machine, is transmitted through the medium in all directions. The transmitted sound strikes on some surface, like wall, ceiling, floor or any other barrier.
Depending on the type of surface part of it is reflected back, and a part being absorbed by the surface. If the sound is not absorbed by the material, it will be transmitted in part to another side of barrier.
If the ultimately reflected sound is not properly controlled the reflection may result in acoustical defects, viz., echoes and reverberation. This reflected sound is important in the acoustical design of buildings.
The part of sound absorbed by the surface is represented by an absorption coefficient. This coefficient is the ratio of the energy absorbed by the area of the surface to the energy striking the area. This coefficient is a function of the frequency of sound.
The reduction of intensity of sound of a transmitted sound through a barrier is called as transmission loss. This transmission loss is a measure of the effectiveness of a surface as an insulating material.
Thus, transmitted and absorbed sound have important bearing on the acoustical condition of a building. However, both transmitted and absorbed sounds are inter related and influence the acoustic and sound insulation.
Highly porous materials have the quality to dissipate considerable energy and the absorption will be relatively high.
The acoustical design of an enclosed space is basically depend on the behaviour of the reflected sound. Due to the reflection of sound two main defects are developed, viz., echoes and reverberation.
Echo is said to be produced when a reflected sound wave reaches the ear, just when the original sound from the same source has already been reached.
The sensation of sound persists for one-tenth of a second after the source has ceased. Thus, an echo forms when the time lag between the two sounds is about 1/17th of the second.
Further considering the velocity of sound in the atmosphere air as 34.3 m/sec., it is shown that when the distance of the reflecting surface is between 8 m and 17 m, echoes are formed.
The defect of echoes also occurs when the shape of the reflected surface is curved with smooth character. Echoes are unpleasant to hear and cause disturbance in hearing.
The methods of reducing this defect are choosing proper shape of the surface, a rough and porous materials to disperse the energy of echoes.
When the surfaces of an indoor place are hard and smooth, very small energy is lost at each impact of sound and many reflections take place before the sound dies down.
This repeated reflection of sound is called prolongation or reverberation. Thus, if sound exists too long, then successive words of a speech will overlap and confuse.
The remedy for this defect is to select a correct time of reverberation known as optimum time of reverberation. This is achieved by suitably selecting a proper absorbent or acoustical materials for different reflecting surfaces.
Acoustic level of a room and reverberation time are shown in Table .
Common building materials are absorbents of different levels. Such materials are called as absorbent materials.
Some of the acoustical materials are as follows:
1. Acoustic Plaster
2. Acoustic Tiles
3. Porous Boards
4. Perforated Boards
5. Quilt and Mats
When some materials get ignited, the material catches fire and spreads. If there are openings in walls and floors, the fire spreads to more areas.
If there are no openings, the temperature of the structure is increased by fire. In buildings, staircases and lift shafts act as flues for fire and increase the possibility of spreading of fire.
There are natural and man-made causes for fire to occur. They may be caused due to faulty workmanship in electrical wiring, leakages in heating and cooking equipment, flammable liquids, careless throwing of cigarette bits and matches, lightening, spontaneous combustion, etc.
The fire spreads over different materials and produces different gases of which some are poisonous.
The gases produced are carbon-monoxide, carbon-dioxide, hydrogen sulphide, nitrogen dioxide, etc. Thus to protect the goods and activities within a building or structure and of adjacent buildings fire-protection has to be resorted to.
Fire resistance of a material is the time during which a structure fulfills its function with reference to safety when a fire presents with a particular intensity.
Fire-resisting Properties of Building Materials
With reference to fire, materials may be classified as combustible materials and non-combustible materials.
Combustible materials are the materials which combine exo-thermally with oxygen and give rise to flame at a particular range of temperature.
Examples of such materials are wood, wooden products, animal products, and man-made products like fibre board, straw-board, etc.
Non-combustible materials are those which when decomposed by heat will do so endo-thermically. These materials do not catch fire by one decomposed at a particular range of temperature.
Examples of such materials are metal, stone, glass, concrete, clay products, gypsum products, asbestos products, etc.
The building materials have varying fire-resistant properties which are discussed below.
Bricks in general have good fire-resistant property. Particularly first class bricks are fire-proof and can withstand heat for a considerable length of time.
As bricks are made out of clay, which is a poor conductor, can withstand heat as high as 1300Â°C. Special type of bricks called fire-bricks are best for use in fire-resistance constructions.
Terra-cotta is also a clay product which has better fire-resisting properties than bricks. As the cost is high it is used only in restricted places.
Although stone may resist high temperature but deteriorates due to sudden cooling. Thus, stone should be used only for a limited use in buildings with reference to fire-resistance. Granite although very strong crumbles or cracks when subjected to heat.
As concrete is a bad conductor of heat, it has high fire-resistance capability. The extent of fire resistance depends on the aggregate, density and position of reinforcement in RCC. Use of foamed slag, blast furnace slag, crushed brick, cinder, crushed limestone, etc, form the best aggregate for fire-resisting concrete.
Mortar is a cheap and best incombustible material. Cement mortar is better fire-resistant than lime mortar as lime plaster is susceptable by calcination.
To increase the fire-resistant property, the thickness may be increased.
Cement mortar with surki or pozzolana shows very high fire-resistance capability.
The amount of heat liberated in combustion of any content or part of a building of a floor area is referred to as fire-load. It is represented in kilo joules per square metre (kj/m2).
The fire-load is the ratio of the weight of all combustable materials to the floor area under consideration.
For example, let a floor area of 120 m2 contain 18 Ã— 103 N of combustible material having caloric value of 1.5 Ã— 103 j/N, then the
The fire-load is used as a measure of grading of occupancies by Bureau of Indian Standards (BIS 1641-1960).
Accordingly the classification are given as
Table: Grading of occupancies by fire-load (BIS 1641-1960)
General Safety Requirements Against Fire
All buildings should satisfy certain safety requirements against fire, smoke and fumes.
The height of a building is restricted depending on the number of storeys, the number of occupancy and the type of construction.
Further all the above factors in turn depends on the width of the road in front of the building, floor-area ratio and the local fire fighting facility available.
In general, every room for use by human beings should have an interior or exterior open space or on an open verandas.
The open spaces inside or outside should be able to provide sufficient lighting and ventilation. Further, the open space in adjoining a road should be well inside giving scope for widening of the road.
When a building is used for more than one type of occupancy, e.g., residential, godown, shops, etc then it should conform to the requirements for the most hazardous of the occupancies. Such mixed occupancy should be avoided as there is more risk for life of occupants.
If mixed occupancy is separated by walls of 4-hour fire resistance, then the occupancy can be treated individually and safety measures can be taken.
4.Openings in Separating Walls and Floors
The openings in separating walls and floors should be designed in such a way that necessary protection is guaranteed to all such factors which may spread fire.
For Types 1 to 3 construction a door way or opening in a separating wall may be limited to about 6 m2 (i.e., height 2.75 m and width 2.1 m). Such wall openings should be provided with fire-resisting doors or steel rolling shutters.
All openings in the floors shall be protected by vertical enclosures. In Type 4 construction, openings in the separating walls or floors should be fitted with 2 hour fire-resisting assemblies.
5.Enclosure on Openings
Wherever openings are permitted, they should not exceed three-fourths the area of the wall in the case of external wall and should be protected with fire-resisting assembles or enclosures. Such assembles and enclosures shall also be capable of preventing the spread of fumes or smokes.
Electrical power installations and gas connections for kitchen, if any, should be done as per norms and requirements from the point of view of fire safety.
7.Materials of Construction
The structural elements of the building such as floors, partitions, roofs, walls, etc should be invariably constructed with fire-resisting materials.
In general, non-combustible materials like stones, bricks, concrete, metal, glass, clay products, etc should be used in construction.
Combustible materials such as wood and wood-products, fibre boards, straw boards, etc should be avoided or used only for the most essential places.
Emergency Fire-Safety Measures
Apart from the steps taken in construction of buildings, the following general measures of fire-safety have to be adopted. It includes the following:
1. Alarm Systems
2. Fire-extinguishing Arrangements
Various types of extinguishing arrangements are provided to extinguish the fire depending on the importance of the building.They are as follows:
(iii)Automatic Sprinkler System