Why isn there a sound equivalent of the one-way mirror?
Sound is the result of vibration of some sort — vibration of materials (strings, drum skins, etc) turning into vibrating waves of air molecules. This energy is transmitted from material to material (hit something sitting on a table, and the entire table will vibrate). In other words, anything you use as an encasement around a sound-creating device will vibrate … and sound emitting from outside the encasement will vibrate the encasement, too. You can dampen vibrations (with foam, for example), or shape the acoustics with material structures. But you cannot prevent it from happening, and you cannot prevent it from transmitting to all materials in all directions. There is no ability to “go one way and not the other” in terms of dampening vibration.
That one-way sound Google link above has lots of results for dampening materials and structures. That’s useful for dampening sound from one location generating sound to another — say, preventing the guy located next to the recording studio from being bothered by the racket. But it will also dampen sound being generated in the opposite direction, as well, but less effectively. Even the very best recording studios have occasional trouble with trucks driving past the building, planes flying overhead, etc.
There are a couple reasons for this, touched on in the above answers. Part of it has to do with how large the range is in terms of things you can perceive. The “dynamic range,” if you will, of vision is extremely high. Your healthy human retina, properly dark-adapted, can reliably detect a single photon. However, your brain doesn’t tell you how many photons you’re seeing. The single photon emerging out of darkness looks, to you, like a brief flash; a firework detonating in the sky giving off bazillions of photons also looks like a brief flash. In other words, your visual apparatus is not set up to judge the absolute intensity of a visual stimulus. So when you’re sitting in the dark room behind the half-silvered mirror, receiving half the amount of light you normally would, your brain compensates. You don’t realize that you’re only seeing half the number of photons as you would if the mirror were not there. In contrast, if half the sound energy of an audible stimulus is attenuated, you’
