Stereo was invented by audio pioneer Alan Blumlein in the 1930s. His work on capturing, storing and replaying two channel audio to reproduce a convincing stereo effect is still in use today. In spite of the technology being used having changed beyond all recognition the theory remains unchanged in that two channel audio, when captured and processed correctly can be replayed via two loudspeakers and if the listener sits in the sweet spot halfway between the two speakers a convincing soundstage can be reproduced between those speakers.
When Is Stereo Not Stereo?
The stereo effect relies on the brain’s ability to interpret directional cues. We experience these every day in the world and because we have 2 ears, one on each side of our heads, we can correctly identify the direction a sound is coming from. So what are these cues?
Level Difference - Sounds which are louder in the left ear, sound as if they are coming from the left (and vice versa). This is what makes pan pots work. Exactly why this happens is a combination of distance and the masking effect of the head - the head is in the way of the ear furthest from the sound. This is more significant than distance.
Time Of Arrival Difference This can be faked on mono sources as an alternative to level based panning. Your ears don’t occupy exactly the same point in space so sounds coming from the left or the right will arrive at each ear at slightly different times. Our brains are very good at interpreting these differences to determine the direction of the sound source.
Spectral Difference Our heads cast a significant acoustic shadow over our ears when sounds arrive from the other side of our heads. This together with the shape of our ears themselves contribute to frequency differences between the sound received by each ear.
What Does This Have To Do With Headphones?
When listening to music over stereo speakers we experience crosstalk between the left and right channels. This introduces time of arrival differences between the channels as well as spectral masking introduced by our heads. None of these are present when we listen on headphones. All we are left with is the level differences between the two channels of the stereo signal. What we hear over headphones is closer to binaural. This is why music sounds different on headphones and why mixing on headphones remains controversial.
So Are Headphones Better?
The delivery provided by headphones is different, not better, otherwise no-one would have speakers in the studio. Anyone who has ever listened to a good binaural recording will know how startlingly realistic they sound over headphones but the effect collapses completely when listened to over speakers. It’s very difficult to eliminate crosstalk when listening on speakers but it’s very easy to introduce it to headphones. Some SPL headphone amps have this feature built in. This is what these audio examples created using our free Pro Tools template session demonstrate.
Simulating the Crosstalk Of Speakers Over Headphones
These examples were created using a Pro Tools session, which seeks to simulate the effect of crosstalk between channels when listening on speakers. The session is very simple:
A send busses the audio to an Aux input labelled Crosstalk, which has its panning reversed, with left panner right and right panned left. This simulates the crosstalk (i.e. the fact that your left ear can hear both the left and the right speaker, and vice versa).
The Crosstalk Aux track has a very short delay on it, which approximates the difference in distance between a pair of nearfields and the ear.
The crosstalk track also has a low pass filter which simulates the effect of the acoustic shadow cast by the listeners head.
Mute the Crosstalk track to hear whatever you drop on the audio track without the processing. Unmute to hear the speaker simulation in the headphones
Download the free session which was used to create these examples. Dig in to the settings and see if you can improve on them. Share your settings in the comments below.