With two speakers, or two earbuds, you can create the illusion of sounds coming from different points in space. This illusion is called the stereo image, and it is built with panning and with the phase relationships between channels. Understanding these concepts prevents a very concrete problem: a mix that sounds wide on headphones and then, played from a smartphone or a smart speaker with a single driver, loses instruments or voices.
How we perceive direction
The brain determines where a sound comes from by comparing what reaches the two ears:
- level differences: a sound on the right reaches the right ear louder;
- time differences: it reaches the nearer ear first, with delays of fractions of a millisecond.
DAW panning almost always works on level differences, while some stereo widening tools use time differences.
Panning
The pan of a mono track decides how much of the signal goes to the left channel and how much to the right. In the center the sound is split equally and appears between the two speakers, the so-called phantom image.
The pan law
If a centered signal were sent at full power to both channels, it would sound louder than when it is entirely on one side. That is why DAWs apply a pan law: in the center the signal is attenuated, typically by 3 dB, so that perceived volume stays similar as you move it. The exact value varies between DAWs and is often configurable.
Placing the elements
A few practical principles:
- Center: lead vocal, bass, kick. These are the elements that hold the mix up and must stay strong in mono too.
- Sides: guitars, keyboards, backing vocals, percussion, often in pairs for balance.
- In podcasts: participants' voices almost always sit in the center, or with very slight offsets. Voices hard left or right tire people listening on headphones.
Phase and polarity
Two concepts that are often confused:
- Polarity: inverting polarity means flipping the waveform upside down, turning positive values into negative ones. It is an instantaneous operation, the same at all frequencies.
- Phase: describes the time offset of a periodic signal relative to another, and it depends on frequency. A 1-millisecond delay corresponds to half a cycle at 500 Hz, but a full cycle at 1,000 Hz.
When two similar signals are summed with a slight delay between them, some frequencies reinforce and others cancel. The result is a "combed", hollow, metallic sound: comb filtering.
Where it shows up in practice
- Two microphones on the same source at different distances, as when recording an acoustic guitar.
- Two people close together in a podcast: each person's microphone also picks up the other voice, delayed. When the tracks are summed, the voice gets colored.
- Cables or devices with inverted polarity.
Remedies: move each microphone closer to its own source, move the microphones further apart, use a gate or expander on voice tracks when they are not speaking, and check polarity with the DAW's dedicated button.
Mono compatibility
Many devices play in mono: single speakers, some smartphones, systems in public spaces. When a stereo mix is folded down to mono, the channels are summed. Whatever was in opposite phase between left and right cancels out.
The most aggressive stereo widening effects work precisely by creating phase differences between channels: on headphones they sound wide, in mono they can make an element disappear.
How to check
- Put a plug-in on the master that lets you listen in mono with a button.
- Switch regularly between stereo and mono while mixing: if the voice or bass changes level a lot, there is a problem.
- Use a phase correlation meter: an indicator ranging from +1 (identical channels, perfect mono) to -1 (opposite channels). Consistently negative values mean part of the signal will be lost in mono.
In my work I check mono above all on spoken content: a voice that gets colored or quieter in mono is almost always a sign of microphones placed too close together.
In short
- The stereo image comes from level and time differences between channels.
- The pan law attenuates the center to keep perceived volume constant.
- Voice, bass and kick in the center; in podcasts voices stay central.
- Summing delayed signals creates comb filtering: move microphones closer to their sources.
- Always check the mix in mono and with a phase correlation meter.
