Start the expression by first defining the variable by typing 't = ' and then using the pickwhip to point to the Both Channels parameter. It is a good idea to do as little typing as possible writing expressions because it eliminates the possibility of typo's. I want my new values to start as soon as the audio starts to rise from the average low point and end when I get about 80% of the way to the loudest values so I make note of the high and low values. In this example, the audio ranges from a high of 2.5 units: It is easy to see what values you have to work with by simply hovering over a keyframe. This is the graph editor of a sample audio file. That's how you decide what values to use for the variables.

These are very easy to see if you just open up the Graph Editor and you look at the value curve. 'tMin' and 'tMax' are the minimum and maximum values of the keyframes. 't' would be the value you are getting from the Both Channels keyframes that were generated when you used the keyframe assistant to analyze the audio. If the Operator was set to ease or easeIn or easeOut the graph of the function would be a curve. The Operator is linear and that means that there is a direct ratio established. Here's the linear method as written automatically from the expression language menu: Inside the method, there is the operator (linear), a variable (tMin and tMax) and the output (value1 and value2). The method defines what you are going to do. You should explore all of the various methods in the menu and become familiar with how they work and the way they are written. Let me help you understand the Interpolation methods from the Expression Language menu.

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