The Haas effect refers to the physical phenomena of your ear deriving more directional information from TIME than from AMPLITUDE. Basically, what it means is that you can place an instrument more precisely in the stereo spectrum through the judicious use of delays than with the pan pot.
Most people can only really locate through amplitude panning (the pan pot) in about five locations; hard left, hard right, middle, and the halfway points between middle and hard left and right. Many amateurs can only locate three locations, and a very few professionals can locate as many as 7-9 locations. Additionally, in order for amplitude panning to be accurate, you must be in the "sweet" spot, or roughly equidistant between the stereo monitors. In fact, the closer to exactly in the middle, the better.
Haas panning, or delay panning, is far more precise. You can get an almost infinite number of locations, which will be perceptible from any location. Even cooler, with delay panning, you can get locations considerably OUTSIDE the spread of your speakers.
The reason this works is that, your ears are (approximately) 8 inches or so apart. Because of this, if a sound originates on the left side of your head, your left ear will hear it a little before your right ear. Your brain relies more on this time information for location than it does the amplitude difference, which is small over an 8 inch span.
To use delay panning, you pan your part hard to the side you want it coming from (let us pretend to the left). You then send the signal to a delay line (hardware, software, I don't care), and pan the returning delay hard to the other side. If both the original signal and the delay are set to the same level, and the delay is set to O ms, the sound is of course panned right down the middle. However, as you start to add delay, the signal will start to move towards the left. EVEN IF YOU LEAVE THE GAIN THE SAME.
It can be pretty shocking. I have vivid memories of my Mix Lab 261 (or what ever the hell the number was) at Berklee. My Teacher, the late, great Robin Coxe-Yelden (I am not sure on that spelling) set it up with two delay lines (Lexicon PCM-61's I believe), one on each side. She was able to move the sound from side to side with just the delays. I usually sat dead center in that class so I could hear everything, but that day I got there late. I am a bit of a skeptic, so I would frequently question her about the physics of things. She would always just setup an experiment (or have me do so), and show us. That day, sitting off to the side, directly in front of the left speaker, she started moving those delay knobs, and the sound was all of a sudden to my LEFT. It sounded like it was coming from the wall I was sitting next to.
She was kind of a witch at times, but Robin was a GREAT teacher, and incredibly knowledgeable. She taught me to understand MS micing, as well as many other phase issues. And to think of them as issues, not problems.
The Haas effect was at the heart of the whole "3D Sound" thing back in the early 1990's, though there was more going on there. The problem was always that, the 3D sound thing always worked better with headphones, as far as the depth perception aspect. I had another teacher, who owned one of the 3D sound companies back then, show us a demo he used for his company. They would sit you in a chair, and put some headphones on your head. There would be music going in the headphones, which was pretty cool in the depth thing, very three dimensional. Then you would hear someone knock the door directly behind you. You turn around to answer the door to find a solid wall. With headphones on, it was amazing. Through normal speakers, it just didn't work.
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