# How can you get a negative value from the Magnitude formula

## Recommended Posts

Somebody on this site (I won't name drop) said that he was surprised that the magnitudes of my spectrum were not negative. Do you "scienceforum" people not understand the formula for magnitude = sqrt(amp.re^2+amp.im^2)? Exactly how do you get a negative value out of that?

##### Share on other sites

!

Moderator Note

That's not going to work, because I don't see where anyone said this. That's one of the problems of paraphrasing instead of providing a quote, with context. Even if the quote is unattributed.

A second problem is the extrapolation of one purported statement as being representative of thousands of people on the site, as if this were The Borg.

I'll leave this open you you can copy/paste the quote

##### Share on other sites

31 minutes ago, Dan Bullard said:

Somebody on this site (I won't name drop) said that he was surprised that the magnitudes of my spectrum were not negative. Do you "scienceforum" people not understand the formula for magnitude = sqrt(amp.re^2+amp.im^2)? Exactly how do you get a negative value out of that?

That would be me.

You are quite at liberty to quote what others have been saying, moreso , you are even encouraged so to do.

We call that discussion at ScienceFroums.

##### Share on other sites

1 hour ago, Dan Bullard said:

Somebody on this site (I won't name drop) said that he was surprised that the magnitudes of my spectrum were not negative. Do you "scienceforum" people not understand the formula for magnitude = sqrt(amp.re^2+amp.im^2)? Exactly how do you get a negative value out of that?

Surely if it is a square root there should be both +ve and -ve solutions, shouldn't there?

##### Share on other sites

No, there shouldn't.  While, for any positive number, a, there exist two values of x, one positive and one negative, such that $x^2= a$,  $$\sqrt{a}$$ is, by definition, the positive one only.

That is why, when writing the solutions to

##### Share on other sites

I rather feel that dicussion is proceeding at cross purposes here.

I am also trying not to emulate the OP's rude way of addressing forum members but wait until he posts what exactly what he means.

In particular what 'formula' is he using for the 'magnitude' of a partly clipped sine wave ?

And what, exactly is meant by 'magnitude' anyway.

The average value of any alternating current must be exactly zero.

##### Share on other sites

2 hours ago, studiot said:

I rather feel that dicussion is proceeding at cross purposes here.

I am also trying not to emulate the OP's rude way of addressing forum members but wait until he posts what exactly what he means.

In particular what 'formula' is he using for the 'magnitude' of a partly clipped sine wave ?

And what, exactly is meant by 'magnitude' anyway.

The average value of any alternating current must be exactly zero.

Magnitude is defined as the unsigned value of any quantity. When an FFT is done, the magnitude (aka Amplitude) is extracted with the formula given, and given the double (actually triple dose) of sign removal (any value squared is positive and the square root of any number is a positive value), there is no way the magnitude could be a negative value.

##### Share on other sites

13 minutes ago, Dan Bullard said:

Magnitude is defined as the unsigned value of any quantity. When an FFT is done, the magnitude (aka Amplitude) is extracted with the formula given, and given the double (actually triple dose) of sign removal (any value squared is positive and the square root of any number is a positive value), there is no way the magnitude could be a negative value.

There is a difference between a spectral analysis of a waveform and the waveform itself.

You have claimed to be talking about the spectral analysis of a clipped sine wave.

A function that has the form f(t)  Asin(wt) + C is not a sine wave

##### Share on other sites

18 minutes ago, Dan Bullard said:

Magnitude is defined as the unsigned value of any quantity. When an FFT is done, the magnitude (aka Amplitude) is extracted with the formula given, and given the double (actually triple dose) of sign removal (any value squared is positive and the square root of any number is a positive value), there is no way the magnitude could be a negative value.

!

Moderator Note

You were asked to copy/paste the passage in question

## Create an account

Register a new account