Auditory Illusion
The main idea of auditory illusion that you must understand - if you wish to be an exceptional aural piano tuner and not be tricked by your ear - that your ear can trick you. Seriously. You can hear something, and think it's one way, when it is absolutely NOT!
If this is something you are not willing to explore - the possibility that maybe your ear isn't the God-given gift that you think it is - then there's no point in continuing to study this system.
Great! You're still here!
So, how come I am so sure that the ear can be fooled?
Because I am constantly fooled.
How do I know?
Because I use the objective proof of filtering intervals.
Example: I made two intervals, a M3 and a M10, to be the exact same size, which I confirmed by filtering and measuring the interval beat speeds. See Ocenaudio
When these are the same size, we say they produce a pure 4:2. (You do not need to understand what a 4:2 is or even a M3 or M10, just that when the beat rates are equal, the 4:2 is pure.)
I told a group of technicians that this was a wide 4:2, meaning the M10 beats faster than the M3.
I asked them, "How many can hear that the M10 is faster?"
Half of them put up their hands.
I then played the exact same intervals, told them they were different intervals, and that it was a narrow 4:2 which has the M10 slower than the M3.
I then asked them, "How many can hear that the M10 is slower than the M3?"
Half of them put up their hands.
The point?
"When things are close but wrong, and you try to hear them right, you will hear them right, and they will NOT get fixed!"
But
"When things are close but wrong, and you try to hear them WRONG, you WILL hear them wrong, and they WILL get fixed!"
In other words, to explain auditory illusion more simply,
"You hear what you want to hear"
How to Use Auditory Illusion to Our Advantage
#1 Realize it's true. Realize you can't trust your ear.
"That's fine, but that doesn't really leave us with much to go on, does it?"
Well, it's a start. If you enjoy logic, you could probably figure out your own way to deal with the phenomenon of auditory illusion, but for the rest of us, here are a few suggestions:
#2 Don't listen for beat rates to be correct, listen for them to be wrong.
When tuning intervals by ear, we are usually following a recipe - "Tune this so this sounds like that"
Using the pure 4:2 example above. "Tune the M10 so it is the same speed as the M3."
The problem is, what if these beat rates are close but wrong. If you are trying to make them the same speed, that is what you will hear; them the same speed when they are not.
The trick? Don't try to hear them right, try to hear them wrong.
I often use the idea of an internal dialogue to show people what I am thinking when I tune. Tuning a 4:2 goes something like this; a conversation between me and my ear.
Me: "OK. I need to make the M10 the same speed as the M3."
Me: Tuning.
My Ear: "OK. There it is."
Me: "Great! Time to move in to the next...wait a minute. They are the same speed?
My Ear: "Yep."
Me: "But, that can't be. I suck."
My Ear: "Nah. You're a great piano tuner. The best. None of your customers ever complain!"
Me: "That's because they are all old deaf people and little kids!"
My Ear: "True."
Me: "So they can't be the same speed. I'm going to listen again and this time hear them wrong. OK, Ear. Which one is faster?"
My ear: "Oh, this is hard. Um. Well, if I had to say..."
Me: "You do."
My ear: "...I'd say the M10 is faster."
And that is how I fixed the M3/M10 speeds and got a pure 4:2
There are other places where we ask our ear to confirm different pitch settings. Try to think how you could ask it to hear things wrong so you can fix them.