A tall planar speaker is a line source. Two things make it it's interaction within a room different:
- Sound comes from a line source as a CYLINDRICAL wavefront, whereas a regular speaker is more-or-less a point source which has a SPHERICAL wavefront. The surface area of a cylindrical wavefront expands by a factor of two for every doubling of the distance from the speaker to the wavefront ; the surface of a sphere increases by a factor of FOUR for every doubling of distance. Hence, sound level from a line source like a tall planar speaker drops by 3 db for every doubling of distance (out to a distance that is greater than most listening rooms,) where the sound level from a regular speaker drops by 6 db for every doubling. This means the NEAR FIELD of the line source extends out farther than a point source. The near field is that space where the direct sound from the speaker greatly predominates over sound reflected from the listening room walls. So, depending on placement, a line source can have a sound that is less influenced by the room. Note this does not apply to long wavelengths like bass, because often the wavelength of the sound is greater than the distance to the listener- bass with any speaker is subject to room coloration, although usually these are usually not narrow or complicated frequency response peaks or suckouts and careful placement and / or a little room EQ can keep them from dominating the sound. NOTE sound from the planar speaker CAN reflect off the wall BEHIND the listener, and if the speaker is a dipole, it will reflect off the wall behind the speakers in front of the listener, so these walls do have an effect on the sound.
- Sound coming from a line source like a tall planar speaker doesn't have very much vertical dispersion- especially at high frequencies line sources produce more or less a beam that shoots out at a right angle to their vertical surface, without much energy radiating up or down. This is especially true if the planar speaker is tall enough to be quite near the ceiling. At mid and treble frequencies, there really isn't much sound 'fired' at the ceiling or floor, so reflections from ceiling and floor are quite minimal. (Assuming the planar speaker is perpendicular to the floor and not tilted back.)
So, with a tall planar speaker (or a line array of regular cone drivers) reflections from side walls and floor / ceiling interact noticeably less than with a regular speaker.
Smaller electrostat panels like these little Stax, or a single pair of Quad ESL-57 etc are going to be somewhere between 'Line Source' and regular speaker behavior. The smaller the speaker the more it will act like a point source. Full line source properties accrue to the larger Magneplanars, the CLS type Martin-Logans, big Soundlab speakers, vintage KLH-9's, Acoustats, Final Design, Apogee, etc. Quads from the ESL-63 on are a point source at higher frequencies and a line source at lower frequencies, because of their construction.