PLASTER OF PARIS
5000 years ago Egyptians burnt the gypsum and they obtained plaster of paris. They used this powder for constructing monuments. The powder required for pro- ducing gypsum is largely available in paris. Hence it is called plaster of paris. Initially it was used in construction but now it is also used in medical fields and for manufac turing sports material.
USES OF PLASTER OF PARIS
i. Used for manufacturing chalk pieces, toys and deco- rative items
ii. Used to prepare smooth finishing surfaces, false ceil- ing and to prepare smooth surfaces for painting.
iii. Used to manufacture sculptures.
iv. Used in orthopaedic and dental hospitals.
v. Used in laboratories to close small openings in the equipments.
ACOUSTICS
Acoustics is the science that deals with the origin, propagation and auditory sensation of sound.
ACOUSTICAL INSULATION MATERIALS
1 ACOUSTIC PLASTER: It is a granulated insula- tion material mixed with cement. It is called as fibrous plaster. The quantity of cement used in such a material is decided so as to provide enough strength and have sufficient pores. Increase in quan- tity of cement will affect the acoustic property. Decrease in quantity of cement will reduce the strength of the plaster.
2. ACOUSTIC TILES: These are commercially avail- able materials manufactured in factories. It can be easily fixed in the walls. The absorption of sound is uniform. It is most suitable for small rooms. But it is costly compared to other acoustical materials.
3. PLYWOOD: Three to four thin sheets which are cut from the wooden planks are (veneer) placed such that the grains in each sheet are perpendicular to each other. Then they are pasted by vajram and pressed together in the machines.
4. STRAW BOARD: Straw boards are manufactured in hydraulic pressing machines by using chopped straw by chemical methods. These are best acoustic absorbent materials.
5. FIBRE BOARD: This material may be perforated or unperforated. These are available in 3 mm to 12 mm in thickness. It is manufactured in pressing machines by using wooden pulp.
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