Woundproof wall

-Soundproof walls are built in a wide variety of locations and settings: churches, cinemas, assembly halls and the like. This is because sound travels very well through solids and liquids, but not so well through gases. An optimum headset provides you with complete isolation from your environment. It works on physical principles which have been known for many years. In fact the ear itself consists of a solid structure containing fluid into which is fitted a third device -the stirrup bone or stapes- that moves when sound impinges upon it.

-Sound has to travel from the outside world along two channels before it can reach our inner ear: an outer channel made up of the bones of the skull and cheekbones, an inner one consisting of a fine chain of bones called the ossicles -the incus, malleus and stapes- linking the fluid filled outer ear to the liquid filled inner one.

-The loudspeaker is very similar in principle to your headphone. The speaker cone’s motion is caused by forcing it back and forth through air or another gas. When sound impinges upon it, it moves forward until the restoring force of the spring brings it back again.

-If you are listening to music at a high volume you can feel this motion very clearly on your chest wall, because the loudspeaker itself is only about an inch from you, whereas when using headphones no physical connection with anything makes its presence felt. This movement drives associated parts of the sound system -the amplifier and the like- and these in turn drive a rigid membrane made from plastic, metal or paper depending on the design of your loudspeaker.

-If you take a pair of headphones apart you will find that there are two very thin membranes fastened to a light frame so as to be free to stretch and contract. In the centre of each membrane is a small square hole just large enough for a tiny bone called the stapes to pass through it when it moves under the influence of sound waves impinging on the ear drum. The ear drum itself is fixed, but this innermost stirrup-shaped bone is suspended by its long arm from various smaller bones situated in front of it inside your skull jar, which prevents its being displaced by pressure from the outside.

-These two membranes are linked at one end to the speaker cone inside the loudspeaker, and at their other ends to a support which in turn is fastened to the frame of your headphone set. When the speaker cone moves back and forth it causes these membranes to stretch or relax, thus setting up vibrations in them that travel across space by means of air molecules, just as sound waves do when they are produced by plucking strings on musical instruments -violins for example- instead of by forcing a column of compressed air backwards and forwards in an organ pipe.

-The membrane attached to your stapes bone vibrates so minutely in sympathy with its counterpart inside your headphones that this stirrup in turn causes soft tissue surrounding it to vibrate. T

-he ear drum, which is situated just behind the inner end of your nose bone (nasal bone) also moves minutely in sympathy with these vibrations. So does another set of bones called the columella that link the membrane inside your ear to the bony wall of your entrance canal; and finally, so do some small bones found inside your middle ear cavity.

-But although all these parts are connected together by means of minute bony levers or joints they are not hinged like a door or an arm, but instead swing back and forth like a pendulum driven by the stirrup-shaped bone at its base. All this happens many times every second for as long as you listen to through headphones.

-The speaker cone of your loudspeaker has to be driven back and forth more slowly than the one inside your headphone set, but its push-pull movements are nonetheless sufficient to move some parts of it -the outer edge of its moving part for example- backwards and forwards by an amount equal to about 10 percent of their own diameter. This makes any sound it emits audible.