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Audio controlled vertical scaling

I am trying to make the mouth of a character open and close in sync with the audio.
I have already linked the mouth scale to the audio amplitude, and it does open and close using the expression in the box.
I want to control the minimum scale and the maximum scale. Sometime the audio is quiet and so the mouth barely moves, and sometimes it is so loud the mouth goes ridiculously big. I have tried changing the number *18 in my expression to lower and higher, but the audio ranges are too big to get a nice consistency. I will also add I only want the mouth to open vertically as it does in this expression and not change scale horizontally.
I should imagine there is some use of min, max expressions, but I am too novice to work it out.
Thanks
[100, thisComp.layer("Audio Amplitude").effect("Both Channels")("Slider") * 18]