At this point it is deemed best to describe the more common materials used for deafening, ie., for absorbing and dissipating the sound waves. Most of these materials are of the nature of paper felts, while some resemble carpet lining. All of these are put up in rolls or bales, usually 3 feet wide and containing 500 square feet. Cabot's Sheathing Quilt. - This consists of a felted matting of eel-grass held in place between two layers of strong manilla paper by quilting, its appearance being as shown in Fig. 179. " The long, flat fibres of eel-grass cross each other at every angle and form within each layer of quilt innumerable minute dead-air spaces, making a soft, elastic cushion, which gives the most perfect conditions for non-conduction." Eel-grass was chosen for the filling because of its long, flat fibre, which makes it especially adapted for felting, and also because of its great durability,* its resistance to fire, and because, owing to the large percentage of iodine which it contains, it is repellent to rats and vermin.
This quilt is made in single and double ply, each being put up in bales of 500 square feet, and costing in Boston $4.50 and $5.50 per bale, respectively. It is also now made with a covering of asbestos, thus rendering it thoroughly fireproof. To obtain the best results in floor deafening the double-ply should be used, and in the manner shown in Fig. 178, the floor floating, as it were, upon the quilt. The material is also very efficient for heat insulation, and when used for this purpose there is no objection to the nails passing through it.
* A sample of eel-grass 250 years old, and in a perfect state of preservation, may be seen at Mr. Cabot's office.
Felt Papers. - There are a great many felt papers made for lining floors, and a few are made fireproof by means of chemicals. As a rule these felts are cheaper than Cabot's "Quilt " (although the saving in an ordinary residence would be but little), and even among the felts there is quite a difference in cost. In choosing a felt paper for lining the architect should select one that is soft and elastic, so as to-form a cushion, and the thicker the felt, provided it has the above qualities, the greater will be its non-conduction. The conducting property of a hard vibrant substance, however, is but little affected by its thickness. Some felts are made waterproof by having an asphalt centre, which is an advantage in case of fire or leaks, but it is doubtful if such felts obstruct the passage of sound as well as felts without the asphalt centre.
Bird's Florian Fireproof Paper. - This is a deafening felt made by the manufacturers of the "Neponset" waterproof sheathing papers, which the author believes to be a very good article.
The material itself is rather hard and thin, but it is pressed in such a way as to form small indentations or air cells, as shown in Fig. 180, which make it elastic and break up the sound waves. The fact that is also non-combustible makes it of especial value where absolute non-conduction of sound is not necessary. The cost of this material in Boston is about $2.28 per 500 square feet.
Asbestos Sheathing. - Sheathing papers or building felts made of asbestos are used to some extent for floor lining and for covering the outside walls of wooden buildings, principally on account of their fireproof quality. The best known of asbestos building papers are those made by the H. W. Johns Manufacturing Co. This company makes three thicknesses of asbestos building felt - thin, medium and heavy - and for floor lining only the heavy or two layers of the medium should be used, two layers probably being better than one heavy layer. Asbestos felt would appear to have about the same effect in retarding the passage of sound waves as other felt papers of the same thickness, while in dwellings its fireproof quality is a decided advantage; it is also waterproof, and rats and vermin do not attack it. Asbestos felt naturally costs more than wood or paper felts, the cost of John's heavy felt being about 42 cents per 100 square feet, and of the medium 30 cents.