Logarithmic curves

This is practically a brain teaser in it's complexity but I strongly feel like the solution isn't that difficult. I have data based on Frequency (X-axis) and Acceleration (Y-axis). There are 3 sets of data based on 2%, 3% and 4% "damping curves" (knowledge of these curves aren't neccesary). The curves themselves have a logarithmic relationship.

I need to create a 7% curve.

The obvious solution is not really feasible due to the shear amount of data I need to parse:

Take the Acceleration value (Y) at a frequency (X) for 2,3, and 4%. Plot them with % on the X-axis and acceleration on the Y-axis. Fit a logarithmic curve, find the value at X=7. I then have my value at a specific frequency for 7%.

What I need is a way to get excel to do this for me without actually manually plotting all my data since I have over 2000 data points.

In my head, I'm looking for some sort of function that can take 3 cells of data (2%,3%,4%) and find a 7% value based on a logarithmic relationship. Any help or thoughts are greatly appreciated.

Welcome to the forum.

You can do this with LOGEST, which will give you the parameters b and m for y = b * m^x

Post a workbook with an example if you need help.

The LOGEST worked GREAT (GROWTH was the actual function I used, since I am solving for Y values at 7%), but unfortunately after some testing with R^2 values, the relationship isn't log like I thought, it's power. So I am actually solving for an equation in the form y = a*x^-b.

Is there a command for that one as well?

[e] I've read that I can use:

=LINEST(LN(y-values), LN(x-values))

Gives Ln(a) and b

But I am not trying to get the function of the curve, I am trying to solve for when X = 7. Is there any chance I can get this to work?

[e2] Okay I think I got it but I would love someone to look over my thought process.

I used http://www.tushar-mehta.com/publish_...nalysis/16.htm to help me through. So I have an equation in the form of y = a * x ^ b. I transformed this into:

ln (y) = ln (a) +b * ln (x)

I then used the TREND command which solves linear functions:

TREND(LN(y1:y3),LN({2,3,4},LN(7))

This value gives me a LN (Y) value when X = 7. So to get my answer I'm looking for I just use the EXP command:

EXP(TREND(LN(y1:y3),LN({2,3,4},LN(7)))

Please review and thanks for the help!