A loudspeaker is said to be in “free-field” when it is placed in a space where there are no reflections or boundaries to restrict the sound radiation. These conditions can be found high in the sky (not so practical for listening or measurements) and in anechoic chambers. In practice, a loudspeaker placed on a stand far way from a room’s walls can be considered to be in free space.|
A loudspeaker is said to be in “half-space” when it is placed next to a large solid surface such as a wall or on the floor. The surface (acoustical boundary) limits the sound radiation to half of what it is in the free field. This has the effect of boosting the frequencies that are radiated omni-directionally, i.e. the low frequencies. The amount of boost depends on the solidity of the surface (theoretically 6 dB, in practice about 4 dB), and the frequency range depends on the size of the loudspeaker. This can be corrected with a suitably shaped filter, called “Bass” on Neumann loudspeakers. In practice, half-space is experienced when a loudspeaker is placed next to wall or flush mounted into a wall.
Similarly, a loudspeaker is said to be in “quarter-space” when it is placed next to two large solid surfaces such as a wall and a floor, or a front wall and a side wall. The surfaces limit the sound radiation to quarter of what it was in free-space. This has the effect of boosting the low frequencies twice as much as seen in half-space. This can be corrected with a suitably shaped filter (“Bass” and possible some “Mid” too), or in the case of subwoofers, attenuating the output.
“One-eighth space” is seen in the corner of rooms, and a very large boost is seen at low frequencies. This is a good location for subwoofers as the entire passband is boosted. This can be corrected by simply attenuating the output. For the same reason, loudspeakers generally sound very bassy in corners and so it is positioning that is not recommended for critical listening.