# Spectrogram ?



## Nodrog (Dec 29, 2011)

I've searched but I still can't find a good explanation of what the spectrogram is telling me. I know it shows energy, time and frequency and even negative time. Can someone give me a clue?


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## fotto (Jan 17, 2010)

In my opinion (I'm no expert) the spectogram is basically just another way to look at intensity, decay over time like the waterfall graph.

Following are my subwoofer waterfall and corresponding spectograms:



















You'll notice on the waterfall that there is a spike in spl (84dB) at around 18hz, with a long decay time associated with it. That combination normally indicates a room mode at that frequency.

If you compare that with the spectrogram, you'll notice the same peak in SPL at 18hz with intensity pictured in Red area, with decay level shown on Y axis. The higher you go on the decay time, the "cooler" the temperature (corresponds to decay time) gets.

Ideally, you'd have something similar to the spectrogram graph starting at 25Hz and higher..steady color level (SPL) with low decay time (Y axis).

That's how I interpret it.


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## jtalden (Mar 12, 2009)

Nodrog said:


> I've searched but I still can't find a good explanation of what the spectrogram is telling me. I know it shows energy, time and frequency and even negative time. Can someone give me a clue?


Fotto beat me to it with a very good explanation, but I will still post this anyway since it is ready and maybe other wording will be helpful. Like Fotto, I am a hobbyist and may not have the full understanding. If it is not good someone will jump in and clarify it for all of us. 

If you understand the waterfall plot it is similar information from another perspective. The waterfall shows the decay of the SPL with time starting from time zero (zero representing the peak SPL). Now understand that the SPL cannot rise to this peak instantaneously, but the rise of the SPL cannot be seen because it is hidden behind the waterfall peak. 

The spectrogram shows us both the rise and the fall of the SPL by looking down from above the waterfall graph. It can now show the rise of SPL (negative time) and the fall of SPL (positive time). Since the magnitude of the SPL is coming straight up towards us the magnitude has to be displayed in a little differently. That is why the magnitude of the SPL is shown like elevations on a topographic map with a contour line at each 5 dB SPL change (or 1, 2 or 10 dB depending on the setting of the “Draw Contours” control). The colors are used to help you follow where the various 5 dB levels extend.

Between Fotto’s and my explanation I’m hoping they will trigger your initial “light bulb” moment.


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## fotto (Jan 17, 2010)

I'm glad you posted that....I certainly learned something. Didn't understand the negative portion of the spectrogram, and concept of "looking down" on the waterfall is a great analogy.


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## SAC (Dec 3, 2009)

Think of it as simply a topographical map viewed from above, where color is used to indicate the various levels of intensity.


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## Nodrog (Dec 29, 2011)

That seems to make sense but I'm still having trouble conceptualizing the rise of SPL before time =0. Also the rise in SPL would certainly be faster than the fall of the SPL and on the spectrogram they look pretty close the the same. I mean looking at Fotto's post, it can't take 100ms for a 60hz tone to reach max SPL can it? And why would the tone start before Time=0? Maybe I'ts my limited linear, causal thinking. Still, I think there may be something really usefull there if I can just get an understanding - other than just a really cool looking graph.


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## jtalden (Mar 12, 2009)

Nodrog said:


> That seems to make sense but I'm still having trouble conceptualizing the rise of SPL before time =0. Also the rise in SPL would certainly be faster than the fall of the SPL and on the spectrogram they look pretty close the the same. I mean looking at Fotto's post, it can't take 100ms for a 60hz tone to reach max SPL can it? And why would the tone start before Time=0? Maybe I'ts my limited linear, causal thinking. Still, I think there may be something really usefull there if I can just get an understanding - other than just a really cool looking graph.


I'm not qualified to say why the test signals are constructed the way they are, but I still think I can help you understand how to interpreting the results of the spectrogram plot. 
I suspect you will find the attached charts helpful. I measured my soundcard with a loopback cable attached. There is no speaker or mic involved, just the soundcard input measuring the soundcard output. This way so we can see how the REW test signal measures.

SPL and Phase:








Waterfall:








Spectrogram:








So you can see the rise and fall of the test signal. It rises above the 45 dB floor at about -200 ms and reaches the 75 dB peak at 0 ms then falls below the floor again at about 200 ms. The 75 dB peak test level was set by me before this measurement and the 45 db floor in the plot was set in the control panel for this chart. It can be adjusted as needed. Now you should see why the speaker measurement will not rise immediately to the peak SPL. The signal provided does not rise immediately to the peak. 

I hope this helps.


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## SAC (Dec 3, 2009)

Nodrog said:


> That seems to make sense but I'm still having trouble conceptualizing the rise of SPL before time =0. Also the rise in SPL would certainly be faster than the fall of the SPL and on the spectrogram they look pretty close the the same. I mean looking at Fotto's post, it can't take 100ms for a 60hz tone to reach max SPL can it? And why would the tone start before Time=0? Maybe I'ts my limited linear, causal thinking. Still, I think there may be something really usefull there if I can just get an understanding - other than just a really cool looking graph.


Again, problems arise from imposed assumptions rather than from the actual behavior.

A waterfall is not a true 'time domain' measurement, thus the assumptions regarding the behavior of the response in time are not valid.

Without descending into the realm of math and measurement processes, if you want to know more about the waterfall/cumulative spectral decay, you might want to take a look at this paper that explains it in more detail.

And at the very least, adjust your assumptions, as I will suggest that it is easier to do that than it is to change the 'rules' of physics...


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## Nodrog (Dec 29, 2011)

jtalden said:


> I'm not qualified to say why the test signals are constructed the way they are, but I still think I can help you understand how to interpreting the results of the spectrogram plot.
> I suspect you will find the attached charts helpful. I measured my soundcard with a loopback cable attached. There is no speaker or mic involved, just the soundcard input measuring the soundcard output. This way so we can see how the REW test signal measures.
> 
> SPL and Phase:
> ...


Aha, I was assuming the spectrogram was computed from the impulse response much the same way as a waterfall plot but I guess I was wrong. Very interesting.


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