REDUCTION OF SOUND TRANSMISSION
"Fibreglass" quilting being laid at the South China Morning Post Staff Quarters, Bowen Road, Hong Kong.
When dealing with sound transmission in buildings, it is necessary to consider the problem from two aspects, viz: the transmission of airborne sounds and the transmission of impact sounds. Air-borne gounds include radio and conversation; typical impact sounds are footfalls, furniture movement, and sounds transmitted through the frame and support of a piano or other instrument, irrespective of the fact that such sounds finally reach the ear as air-borne sounds. As impact sounds are more often generated on floors than on walls, a floor construction needs special attention from that point of view. Concrete floors, although forming a fairly effective barrier to air-borne sour.ds. are notoriously poor as an insulation of impact sounds. It is often found necessary to improve concrete floors in respect of air-borne scunds but a greater improvement is needed in insulation for protection against sounds caused by impact.
There are three methods of improving the insulation of concrete floors: —
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(a) By use of a Resilient Surface: This has no influence on air-borne sound insulation but improves the impact
1. Insulating Handle
insulation to an extent depending on the softness of the material used. Materials such as Linoleum. Asphalt mastic give very little improvement, but soft materials such as carpets and underfelt are obviously useful.
(b) By using a Floating Floor: The floating floor is super-imposed on the structural floor but separated from it by a layer of resilient material. This gives a substantial improvement against impact sounds and a useful increase of insulation against air-borne sound, The floating floor is effective because the vibrations caused by impacts are confined to the surface which receives the impact and cannot easily pass into the surrounding wall and so down into the room below. (e) By providing a Suspended Ceiling: This screens tne underside of the flour receiving the impact, thus A reducing the sound reaching the room below. suspended ceiling improves the insulation against both air-borne and impact sounds by an amount depending on the degree of rigidity with which it is attached to the structural floor. A heavy ceiling not rigidly attached to the flear has the greatest value.
For a floating flour to be effective its natural resonant frequency should be as low as possible, preferably below the lower limit of sound frequency. The natural frequency depends on the weight of the floating part and the resiliency of the material. The weight of the floating portion is deter- mined largely by practical considerations, and this limits the choice of resilients. The suitability of a number of resilient materials has been tested, and the most satisfactory results have been cbtained with glass wool.
Referring to the flcating floor, in this type a layer of nominal 1′′ thick quilted or Bitumen Bonded glass wool blanket is laid on the bare concrete structural floor, the joints of the blanket being tightly butted, and the whole covered with a waterproof building paper with the edges overlapped about 3" to keep the concrete from getting through. A screed of 1" to 2" thick, lightly reinforced, e.g. with open mesh wire netting, is placed directly on the paper. The surface is then Anished as required.
Floating floors and suspended ceilings can be put into existing buildings, i.e. buildings that are several years old, and the cost of this treatment is reasonable and takes up only a small number of working hours.
Harry Wicking & Co., Ltd. are the local agents for "'Fibreglass".
HENLEY SLOW-BREAK SWITCH
FULL SIZE
2. Domed Silver Contact 3. Insulating Cover
4. Brass Cable Terminat 5. Silver Faced Copper Strip.
The Jardine Engineering Corporation Ltd. have sent us information concerning the new Insulated "Slow-Break" Switch. manufactured by W. T. Henley's Telegraph Works Co., Ltd. The following description of the principle of operation has been supplied to us.
The Henley Slow-Break Switch involves a principle which may be new to some of you.
We are all quite aware that, when a live electrical circuit is broken, an are is formed and the magnitude and length of duration of the arc depends upon the conditions of the electrical circuit. A switch is a mechanical device for making or breaking a circuit carrying a load current, and, therefore, when the switch is opened an arc will be formed. We are all familiar with the quick-break type of switch (designed for D.C. circuits) in which the switch-blades are withdrawn quickly from the contacts by means of a spring or some other device, and in this type of switch the gap between the fixed and the moving contacts is made long enough to prevent an arc being drawn and persisting when the switch is opened.
In designing the Henley Slow-Break Switch for A.C. circuits advantage has been taken of the fact that in a 50 cycle A.C. circuit the voltage is at zero potential every half cycle, ie, every hundredth of a second and, therefore, the currents in an are formed at a gap in the circuit will be interrupted at the instance the voltage passes through zero potential.
It can be demonstrated that If a pair of contacts carrying A.C. current in a substantially non-.nductive circuit are made to part slowly, the current in the arc will be interrupted even
though the gap between the contacts is only a few thousandths of an inch.
From this statement you will appreciate that the maximum time during which the are will be maintained can be at the most for only one half cycle, i.e. .01 seconds. The time for which the arc persists is of great importance in designing a switch, since the energy expended by the arc is made up of these three components: (a) the time during which the arc is maintained, (b) the current in the arc, and (c) the effec- tive value of the arc voltage.
Since the arc expends its energy by burning the metal of the contacts, it follows that as the time during which the are can be maintained cannot exceed .01 seconds, the energy expended is reduced to a minimum which ensures that the switch will have a satisfactory life.
Experiments which have been conducted show that with a switch breaking a load current of 60 amperes in a 250 volt non-inductive circuit, the maximum arc energy in a single gap is of the order of 7-watt seconds, a value which results in negligible contact burning.
As mentioned above, gaps of the order of a few thou- sandths of an inch are sufficient to ensure opening of the circuit, but in order to satisfy the voltage test clause of B.S. 861:1939, the final gap with the switch in the open position must be sufficiently great to withstand the 2,000 volt pressure test specified. In the Henley Slow-Break Switch this anal gap is of the order of 1 '16”.
ICE MAKING PLANT FOR INDIA
An engineering works at 's-Hertogenbosch (Bois-le-Duc). which is specializing in manufacturing industrial refrigerating machinery, has received an important order from India for supplying an ice-making machine which will be used in build- ing a large dam across the River Vaitarna.
This dam or barrage, which is made entirely of concrete, is 656 yards in length, 109 yards in width, and 87 yards in height.
Various difficulties have to be overcome in laying concrete for constructions in countries with a hot climate, inter alia. because as a result of the high temperature the concrete sets too quickly and may give rise to cracks or crumbling.
The ice-making machine, referred to above, will be used in India for making ice in-situ. The ice, after being crushed, is mixed with the concrete. Owing to the presence of ice in the concrete, the latter after being laid will set less quickly, so that the difficulties outlined above will be largely eliminated
(Netherlands Economic Bulletin)
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