The Stereo Image

A convincing stereo image owes more to geometry than to equipment. Two loudspeakers create a phantom image between them only when the sound from each reaches you at matched levels and times. Anything that upsets that balance, such as an asymmetric room, a loudspeaker closer to one wall or a reflection on one side only, blurs the picture.

Start with symmetry. Place the loudspeakers at equal distances from the side walls, and sit on the centre line between them. A common starting point is an equilateral triangle, with the distance between the loudspeakers equal to the distance from each to your head. Set the tweeters at roughly ear height when you are seated, unless the manufacturer advises otherwise.

Toe-in, turning the loudspeakers towards the listener, adjusts the balance between direct and reflected sound. Pointed straight ahead, loudspeakers give a broad, diffuse stage. Aimed directly at your ears, they give sharper focus, sometimes at the cost of width. Many rooms suit a setting somewhere in between. Small changes of a few degrees are audible, so adjust one loudspeaker and then match the other precisely.

Early reflections are the next concern. Sound from each loudspeaker bounces off the side walls, the floor and the ceiling and arrives just after the direct sound. Strong side-wall reflections are the usual cause of a vague centre image. A mirror held flat against the side wall shows the reflection point: wherever you can see the tweeter from your seat, absorption, diffusion or a bookcase will help. A rug between the loudspeakers and the listener deals with the floor.

Keep the space between the loudspeakers as clear as you can. A large rack or a television between them reflects sound and flattens depth. If you have to put equipment there, keep it low and set back.

Two quick checks catch common faults. Play a mono recording or a well-recorded solo voice: it should appear as a small, stable image exactly between the loudspeakers. If it is vague, diffuse or difficult to place, check that both loudspeakers are wired with the same polarity, because one reversed connection produces a hollow sound and thin bass. Then check that the channel balance is centred and that both loudspeakers are the same model and in the same condition.

Stereo is designed for one listening position, and the image will always be sharpest there. To broaden the area where it holds, angle the loudspeakers so that their axes cross a little in front of the listening seat. This evens out the levels for listeners who sit slightly to one side. Small adjustments, carefully made, move the stage more than most upgrades.