# Intensity, frequency and amplitude

1. Feb 3, 2014

1. The problem statement, all variables and given/known data

A wave has an amplitude a1, intensity I, and frequency f. A second wave of the same type has twice the intensity and double the frequency, find the amplitude of the second wave in terms of a1.

2. Relevant equations

Using the relationship I=k a2f2, where k is a constant:

3. The attempt at a solution
$\frac{I}{a12f2}$ = $\frac{2I}{a22(0.5f)2}$
So a2 = √8 a1

But according to my answer key, the answer is √2 a1

2. Feb 3, 2014

### BvU

Well, what did you do with "double the frequency" ?

TeX tips: within $[$ itex $]$ ... $[$ /itex $]$ a subscript is done using _ and a superscript is done using ^
so $[$ itex $]$ a^2_\epsilon $[$ /itex $]$ gives $a^2_\epsilon$

Last edited: Feb 3, 2014
3. Feb 4, 2014

I'm so sorry! I meant the frequency is halved.

4. Feb 4, 2014

I meant to say that the frequency is halved, can you look into it again?

5. Feb 4, 2014

### BvU

Well, if the intensity isn't also halved, I agree with your a2 = √8 a1. Can't find a way to dismiss the relationship you're using. So, I'm puzzled.

6. Feb 5, 2014

What I mean to say is, is the relationship between I a and f valid? Or is intensity just proportional to the square of the amplitude?

7. Feb 6, 2014

### BvU

The latter could be quite right. I've seen it come by while googling around. But your orginal formula as well. Is there something in the context that can help you on your way (except for the answer that indeed points to I $\propto$ amplitude2) ? What is the subject of the chapter at hand ?

8. Feb 6, 2014

It is on the basic properties of waves in general, and the question did not specify the type of wave.

9. Feb 6, 2014

### BvU

Well, then the caveat is we've done it for e.g. electromagnetic waves. Sound or springs have I $\propto \omega^2$ amplitude2 as you wrote.

Anyone else have an idea here ? I'm at a loss.