# FFT amplitude or magnitude

Can I use the word amplitude instead of magnitude when I describe FFT bins? I dont see any similar word in my language.

I've always considered them to be somewhat related, but different:

• Amplitude is the peak value of a sinusoid in the time domain
• Magnitude is the absolute value of any value, as opposed to its phase.

With these meanings, you would not use amplitude for FFT bins, you would use magnitude, since you are describing a single value. The link would be that for a pure sinusoid, the signal amplitude would be the same as the magnitude of the appropriate FFT bin ('same as' depending on what scaling etc is used in the FFT implementation, but at the very least will be 'proportional to').

In saying all that, if you were to tell me about the amplitude of an FFT bin, I would know exactly what you were talking about.

Even i had confusion differentiating between these two terms at the beginning , have a look at this explanation from one of the Award winning DSP books.

Amplitudes,

Magnitudes,

• Just because one book defines it that way doesn't mean it's standard among engineers. – endolith Mar 28 '13 at 19:52
• Which book is this from? – M. Dudley Mar 23 '15 at 13:50
• @M.Dudley // Not sure, but probably it's from Oppenheim's book I learnt. powells.com/book/digital-signal-processing-9780132146357 – Youngjae Sep 7 '16 at 1:11
• @M.Dudley I just happened to come across this post while reading this exact book. It's from Chapter 1 (section 1.2) in "Understanding Digital Signal Processing" by Richard G. Lyons. – tjwrona1992 Jun 29 at 17:30

It appears there are technically two types of amplitude.

This would explain the discrepancy between the answers posted here.

The term peak amplitude, often shortened to amplitude, is the nonnegative value of the waveform's peak (either positive or negative).

The instantaneous amplitude of $x$ is the value of $x(t)$ (either positive or negative) at time $t$.

Source:

"CMPT 318: Lecture 3, Sinusoids" by Tamara Smyth, Computing Science, Simon Fraser University. https://www.cs.sfu.ca/~tamaras/sinusoids318/Amplitude_Magnitude.html