The dynamic spectrum analyzer takes that recorded signal and

performs a fast Fourier transform on it in order to get the frequency spectrum.

What you see plotted here is the magnitude in decibels versus frequency.

Frequency is on a liner scale, but once we put something in to decibels,

it is actually a log scale.

Because to compute the magnitude in decibels,

we take 20 times the log of the magnitude.

Now what we see here is that there's a little bit of a peak right there, and

I have not yet played a note.

So it's curious to see where that peak occurs.

I'm going to turn my cursor on and go ahead and

slide it across to see where the peak occurs.

It occurs right there.

And if I look at that frequency, that is at 60 hertz.

Well, it's very,

very common to get noise at 60 hertz because this is electromagnetic noise.

And it's induced by powerlines in the room, it could be induced by

vibrations from equipment, which is powered by 60 hertz powerlines.

So in this country, the line current is at 60 hertz, so

we see 60 hertz signals, in noise and signals.

In other countries you might have 50 hertz and then, therefore,

your noise would be at 50 hertz.

But that peak there has nothing to do with our experiment, so

we're going to ignore that 60 hertz peak in our experiment.

We're just going to be looking at the peaks due to plucking this string.

So if I pluck the string again.

[SOUND] What I see are all of these peaks.

This is the fundamental frequency that we saw in our times trace.

This is the second harmonic, the third harmonic.

And in this case the second harmonic is almost as strong as the first harmonic.

A little bit lower, and let's go ahead and measure what that frequency is.

I'm going to turn my cursor on.