Stereo imaging

Definition

Stereo imaging is the perception of the lateral position, width and depth of sources reproduced by two loudspeakers or on headphones. The apparent position depends on the differences between the channels:

• level difference (ILD between channels): with a 15–20 dB difference the source appears completely to one side;
• time difference (ITD between channels): with 1–1.5 ms the source appears completely to one side; between 1 and 30 ms the sound stays fused and is localized toward the channel that arrives first (precedence effect or Haas effect); beyond 30–40 ms an echo is perceived.

Constant-power amplitude panning uses g_L = cos θ, g_R = sin θ, with θ = (p + 1) · π/4 and p from −1 (left) to +1 (right): at the center each channel is at −3 dB and the total power stays constant. Other panning laws use −4.5 or −6 dB at the center.

The width of the image is described by the Side/Mid ratio, by the correlation between channels and by the L/R balance. For channels of equal power, the ratio between the Side and Mid energies is P_S / P_M = (1 − ρ) / (1 + ρ), where ρ is the correlation: ρ = 0.98 → −20 dB; ρ = 0.8 → −9.5 dB; ρ = 0.5 → −4.8 dB; ρ = 0 → 0 dB.

Parameters and units

• Pan: −100%/+100% (or L/R 64).
• Panning law: −3 dB (constant power), −4.5 dB, −6 dB at the center.
• Side/Mid: dB; about −20 dB or less is nearly mono; −10/−6 dB is a wide image; close to 0 dB is very wide, with a risk of phase problems.
• Correlation: from −1 to +1.
• L/R balance: energy difference between channels in dB; a centered mix stays within ±0.5 dB.
• Precedence delay: 1–30 ms.

Musical function

Stereo imaging separates the instruments, creates space and defines roles: in the center the load-bearing elements (kick, bass, snare, lead vocal), on the sides the supporting elements (doubled guitars, backing vocals, pads, percussion). Width in the low frequencies must be kept in check, because playback systems and club and car audiences receive the bass almost in mono and side energy below 120 Hz is lost or cancelled. Depth is built with reverbs, pre-delay and equalization: what is farther away has fewer high frequencies and more reverb.

Practical example

• Pan 50% to the right with a constant-power law: θ = 67.5°, g_L = 0.383 (−8.3 dB), g_R = 0.924 (−0.7 dB), level difference 7.7 dB.
• Doubled guitars (two takes) left and right: correlation 0.1–0.3, Side/Mid between −2 and −1 dB in the 500–4000 Hz band.
• Per-band width profile of a pop mix:

Common mistakes

• Widening every track: if everything is wide, nothing sounds wide and the center empties out.
• Leaving side energy in the bass (stereo synthesizers, unfiltered reverbs, widener on the bass).
• Using wideners based on delays or phase shifts without checking mono.
• Judging the image only on headphones, where side sources appear more extreme.
• Leaving the mix unbalanced by more than 1 dB toward one side.
• Mistaking the DAW's panning law for a level error when moving a track off the center.

How to check it

• App analysis: Side / Mid (total energy) for the overall image; the width of each band (per-band Side/Mid ratio) for the frequency profile; Stereo correlation; L/R balance; Mono sum drop.
• Goniometer and vectorscope: Wave Candy (FL Studio) in vectorscope mode, iZotope Insight, Ozone Imager: a vertical figure indicates a centered signal, a horizontal one a signal in antiphase.
• Listening: compare speakers and headphones and check the mix in mono.