# What does a spectrogram represent?

Hi,

I am wondering what does a spectrogram represent? I know that a spectrogram displays the amplitudes of specific frequencies, however, in the program Audacity, it plots a spectrogram of an entire sound (a few seconds). What does this spectrogram represent, and how is it different to at an individual time?

cheers.

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Funny you bring Audacity up. Thanks to you I've just realised I installed it by mistake instead of Audacious.

You have two readouts, one is left speaker the other is right speaker.

Now, each peak represents a sound (if it was a simple drum beat each second you would have a single peak each second on the graph).

I believe that the higher the peak, the louder / stronger the sound is.

Here's the Wiki on it: http://en.wikipedia.org/wiki/Spectrogram

Andy Resnick

Hi,

I am wondering what does a spectrogram represent? I know that a spectrogram displays the amplitudes of specific frequencies, however, in the program Audacity, it plots a spectrogram of an entire sound (a few seconds). What does this spectrogram represent, and how is it different to at an individual time?

cheers.
A spectrogram is basically the Fourier transform of a signal. For example, if you have a time-varying signal (like music), the spectrogram tells you the distribution of frequencies in the music.

There's a few subtleties to consider- a time-varying spectrogram means there is a lower limit on the frequency that can be represented (for example, a spectrogram that updates 30 times a second can't tell you anything about frequencies below about 60 Hz), and the spectrogram also 'bins' the frequencies like a histogram, but those are usually minor details.