How to differentiate dampened cycling data and constant cycling data in Matlab?

I have a group of data, which I want to see if it gradually dampens.

Here are four vector examples to explain what I mean:

a=[4,5,3,4,5,3,4,5,3];
b=[5,4,3,4,3,2,3,2,1];
c=[5,5,5,4,4,4,3,3,3];
d=[5,4,3,2,1,0,-1,-2];

a) is periodic data with cycles at the same size, while all others (b,c,d) gradually dampened.

My question here, how can I differentiate the dampened data?

Here is what I tried but still did not get right answer:

d=diff(a);
s=sign(d);

I am not sure what is the next step to do? I could be in a wrong direction.