When a speaker or other sound source is placed in a free field, the sound it produces is able to radiate in all directions (depending, of course, on the design of the speaker enclosure). When a sound source is placed against a solid barrier, such as a wall, that same amount of energy is radiated into the space on one side of the barrier only, or into “half space.” This has the effect of doubling the amount of sound energy into that half space environment, yielding a 3 dB increase in sound power level. The phenomenon can be particularly noticeable at lower Frequency. Place a stereo speaker up against a wall and you will usually find it puts more bass energy into the listening space. The highs aren’t effected as much because they are already pretty directional, and since the tweeter is mounted to the front surface of the cabinet it is already operating in a half space environment. The low frequencies, on the other hand, may be able to pass right through the thin cabinet behind the speaker, but when they encounter the wall (even a standard household wall) more of the energy is reflected back into the room. Many speakers are pre-tuned at the factory to account for this phenomenon.