Imaging in audio: how a system places each sound

How two channels place a voice

Your brain works out where a sound is from small differences between what your two ears hear: which ear it reaches first, and how much louder it is on one side. Stereo borrows those differences. A voice mixed equally into both channels, played through two matched speakers at the same distance from you, is heard midway between them, where there is no speaker at all. That phantom image is what imaging describes.

Move the balance and the image moves. Give one channel more level and the voice slides toward that side, and give it a head start in time and it slides the same way. A mix places every instrument like this, mostly with level, so the positions you hear are only as accurate as the two channels are equal everywhere else.

That's why imaging depends on symmetry. EBU Tech 3276, the European Broadcasting Union's standard for critical listening rooms, wants the speakers placed symmetrically about the center line, the two channels within 1 dB of each other, and a room that is acoustically symmetric about the listening direction.

What pulls the center off

These are the usual reasons a centered voice drifts away from the middle, and they're worth ruling out before you blame a component.

• If you sit nearer one speaker, its sound arrives first, and hearing gives the first arrival priority, so the whole picture leans toward that side.
• A small level imbalance between the channels drags every image toward the louder side. An analog volume knob near the bottom of its travel is a common cause, which the gain explainer covers.
• A room with a wall close on one side and open space on the other reflects the two channels differently, so they no longer match by the time they reach you.
• With one channel wired backwards, a centered voice turns vague and hard to place, and over speakers the bass thins out because the two cabinets partly cancel each other.

The Ortvik Parametric Tower review describes the room case in the reviewer's own room. With a wall on the left and open space on the right, vocals that should have sat dead center drifted, as if the band always stood slightly to one side. A tower placed on the open side gave the right something to work against, the two sides started to behave alike, and the vocals locked between the speakers. Early reflections blur images in a different way, which the comb filtering explainer covers.

Imaging on headphones

On headphones each channel reaches one ear only, so the same level and timing differences spread the picture along a line through your head, with hard-panned parts right at one ear. The soundstage explainer covers why that picture usually sits inside the head. With no room involved, positions don't shift with where you sit, which keeps headphone images steady.

Matching between the two sides counts for more. A difference between the two drivers, or a pad that seals better on one side, moves the center, and if the two sides differ in one frequency range only, a centered voice can shift sideways as it moves into that range.

Width and precision still vary independently. The HIFIMAN Arya Stealth review describes a stage that isn't very wide, with imaging it calls unbeatable and lots of distinct places for sounds to sit. The Simgot EM6L is the reverse: a wide stage with great channel separation, but without pinpoint precision in where the instruments are placed.

Checking your system with test tones

The online tone generator has the two tests worth running first. Route a tone to the left channel only, then to the right: each should come only from its own side, at the same loudness. The polarity check plays a short click through both channels at once, and correctly wired speakers give one pop with some bass in it, while reversed polarity sounds thin and short of bass.

Then play a recording you know with a solo voice in the middle. It should sit in one fixed spot and stay there as the singer moves up and down the scale. On speakers, lean a little to each side and listen for how quickly the voice follows you to the nearer cabinet; the sooner it goes, the narrower your good listening position is.

Imaging, stage and separation in reviews

Three words get mixed up. Soundstage is the size of the space, imaging is where each source sits within it and how firmly, and separation is how distinct the parts stay when many play at once. They often travel together and don't have to.

Pinpoint imaging isn't the only way to get it right. The Diora Acoustics Chors 5 review finds its stage believable, with each source diffused a little and imaging far from razor-sharp, and argues that real sources don't sound as if they come from a single point in space. Whether you want an image fixed in place or slightly blurred is a matter of taste.

The recording sets a limit on all of it. A mono recording puts everything in the middle, and most studio mixes place each part with a pan control, which works by level alone. Recordings made with a coincident pair of microphones, capsules together and angled apart, capture level differences that image sharply, while spaced pairs add timing differences and tend to sound more diffuse.

When you compare picks on the best open-back headphones guide, read each review's lines on stage and on imaging as two separate findings, since a headphone can do well on one and less well on the other. The Arya Stealth, one of its picks, and the EM6L in-ear above lean opposite ways.