Room acoustics and means of control 103 which Laplace found it necessary to multiply Newton's original calculation in order to correct his calculated speed of sound in air to the observed speed. The reason for the difference between the two specific heats is that in the second instance (C p ) extra heat is consumed in the work of expanding the gas. 4.5.2 Other properties of fibrous materials Still on the subject of the absorbent properties of fibrous materials, there are other forces at work besides the ability to convert the sound propagation in air from adiabatic to isothermal. There is a factor known as tortuosity, which describes the obstruction placed in the way of the air particles in forcing them to negotiate their way round the medium. The tortuosity, in increasing the path length of the sound which travels through the fibres, also increases the viscous losses which the air encounters as the sound waves try to find their way through the small passageways available for their propagation. In certain conditions, air can be quite a sticky fluid. There are also internal losses as the vibrations of the air cause the fibres to vibrate, and in order to bend, they must consume energy. Frictional losses are also present as vibrating fibres rub against each other. Energy is required for all of this motion of the fibres, and by these means, acoustic energy becomes transformed into heat energy. As the above losses are proportional to the speed with which a particle of vibrating air tries to pass through the material, their absorption is greater when the particle velocity is higher. If we consider a sound wave arriving at a wall, the wall will stop its progress and reflect it back. At this point of