What Makes Good Stereo?

Stereo requires two loudspeakers. But two loudspeakers do not necessarily produce good stereo. The purpose of stereophonic reproduction is not merely to put different sounds into the left and right channels. Its purpose is to create a convincing auditory illusion.

Close your eyes in front of a good stereo system, and the loudspeakers themselves should become surprisingly difficult to locate. A singer may appear directly between them. A piano may occupy a believable space to one side. A drum kit may extend across part of the room. Some sounds may appear closer while others seem farther away. None of those sounds actually originate there. Two loudspeakers and the room have persuaded your hearing that they do.

The Phantom Center

The simplest stereo illusion is a sound directly between the loudspeakers. There is no loudspeaker in the center. When the same sound reaches both ears with the appropriate relationships, the brain interprets it as coming from a position between the two actual speakers. This is called a phantom image.

Change those relationships, and the apparent sound can move left or right. A recording engineer uses this phenomenon to construct a stereo panorama containing apparent sound sources distributed between, and sometimes beyond, the loudspeakers. Reproducing those relationships accurately is fundamental to good stereo.

Loudspeakers Must Disappear

A loudspeaker can have excellent tonal balance yet reproduce stereo poorly. One reason is that a multiway loudspeaker contains several physical sound sources. If the woofer and tweeter do not integrate properly, the ear may begin identifying them separately. Instead of hearing one trumpet occupying one position in space, portions of its sound may seem to originate from slightly different locations. The illusion weakens.

For convincing stereo, each loudspeaker should behave as nearly as possible like a single coherent source. The listener should hear the recorded event, not the individual drivers producing it.

Position Matters

Stereo depends upon differences between what reaches the two ears. Consequently, loudspeaker placement matters. The two speakers should normally form a reasonably symmetrical relationship with the listener. Large differences in distance alter arrival times and sound levels. Nearby walls and furniture can also change what each speaker contributes. Perfect symmetry is neither always possible nor always necessary, but gross asymmetry makes the stereo illusion more difficult to preserve.

Speaker spacing matters too. Move the speakers too close together and the stereo panorama becomes narrow. Move them too far apart and the center may weaken, producing the familiar impression of sound coming from the left speaker, the right speaker, and nowhere in between. The proper spacing is the one that creates a continuous, stable panorama.

The Room Is Part of the System

We do not listen only to the direct sound from the loudspeakers. We also hear reflections. Sound reaches the floor, walls, ceiling, furniture, and other surfaces, then arrives at our ears slightly later. These reflections contribute to our perception of tonal balance, spaciousness, and image location.

That means a loudspeaker cannot be completely separated from the room in which it operates. A good stereo system does not require an acoustically dead room. In fact, an excessively dead room can sound unnatural. What matters is that reflections do not overwhelm or seriously contradict the spatial information contained in the recording.

Dispersion Matters

A loudspeaker does not radiate every frequency equally in every direction. At some frequencies, its radiation may be broad. At others, it may become narrow. If this change occurs abruptly, reflected sound can have a substantially different tonal character from the direct sound reaching the listener. That affects more than tone. It can also affect spatial stability. For this reason, smooth dispersion is an important part of stereo reproduction. The loudspeaker should behave sensibly not only directly in front of it but throughout the useful listening area.

Stereo Is More Than Left and Right

A convincing stereo recording can contain much more than horizontal position. It can suggest width, depth, distance, scale, and the acoustic character of the recording space. A small jazz ensemble recorded in an intimate room should not occupy the same apparent space as a symphony orchestra in a concert hall.

A close microphone should sound close. A distant microphone should sound distant. When a playback system preserves these relationships, the listener begins perceiving the recording as a space rather than merely a collection of sounds.

That is when stereo becomes fascinating.

The Recording Matters

No playback system can reproduce spatial information that the recording does not contain. Some recordings deliberately create artificial spatial effects. Others capture a natural acoustic environment. Some contain wonderful stereo information. Others contain very little. A good stereo system should not impose the same enormous soundstage on every recording. Quite the opposite. It should reveal the differences.

A mono recording should remain essentially mono. An intimate recording should remain intimate. A spacious recording should be allowed to become spacious. When every recording produces the same spectacular stereo effect, the system may be entertaining, but it is adding something of its own.

The Objective Is the Illusion

Good stereo is therefore not measured by how far sounds appear from the loudspeakers or by how enormous the soundstage seems. The real question is simpler: Does the reproduced acoustic event make sense? Are positions stable? Are instruments believable in size? Does the center remain convincing? Can you perceive relationships among performers rather than merely left and right? Does the apparent space change when the recording changes? And, most importantly, can you stop thinking about the two loudspeakers sitting in front of you? When that happens, stereo has accomplished its purpose.

The equipment disappears, and in its place is a convincing illusion of a musical event.