Of the three dimensions of the soundstage — width, height, and depth — depth is the one that most directly reveals the resolving power of the playback system and the quality of the recording. Width can be exaggerated by room acoustics and speaker placement; depth cannot be manufactured. It is either retrieved from the recording or it is absent.
Soundstage depth — the perception that some instruments are further away than others, that the acoustic space of the recording venue extends behind the speaker plane — depends on the playback system’s ability to preserve and accurately reproduce very subtle level, timing, and spectral cues embedded in the recording. These include the ratio of direct to reverberant sound from each instrument, the high-frequency air loss that occurs with distance, and the microphone perspective captured during recording.
A system with limited resolution — whether due to driver quality, crossover design, amplifier noise floor, or room acoustic interference — compresses these cues. All instruments appear to exist at roughly the same distance, producing a flat, two-dimensional image that lacks the sense of the space in which the recording was made.
Room acoustics interact critically with depth perception. A room with strong, uncontrolled early reflections — particularly from the front wall behind the speakers — introduces competing spatial information that confuses the auditory system’s depth processing. Treating the front wall and managing reflections from the speaker’s rear radiation can significantly improve the system’s ability to project a convincing depth perspective.
Speaker placement relative to the front wall also matters. Pulling speakers away from the front wall reduces boundary interference at low frequencies and allows the speaker’s rear radiation — which carries some of the reverberant information contributing to depth — to develop before reaching the listening position.
Depth is what separates a competent system from a truly convincing one. It is the dimension worth optimising for.