Why Does a Single Sine Wave Show Multiple Frequencies on a dB Graph?

[Teachme]

So I am just working with a synth and I am having it create a single sine wave. I am then looking at the output on a db vs frequency graph and I would expect the db to only be reading at the frequency of that sine wave, but there are readings from 20 Hz all the way to 2k Hz sometimes(althought the higher hz are much lower db leves). Why is this?

 

[anorlunda]

A sin wave which extends in time from minus infinity to infinity has a single frequency. One with an abrupt start and end has other frequency components. Is that your case?

 

[berkeman]

Something is not right. The "sine wave" is not very good/pure, apparently. Can you post screenshots of the time-domain "sine wave" and the frequency domain spectrum?

 

[Teachme]

Ahh my sine has an abrubt start and end as it is a quarter note. So it has to be some sum of sines that make the start and end go to zero then?

 

[anorlunda]

You got it.

Are you familiar with Fourier Analysis? Any shape can be expressed as the sum of sin waves of different frequencies. An abrupt step has lots of high frequencies.

 

[Teachme]

Yeah, I'm a bit familiar, but I guess I should read up on it more. Thanks a bunch. Once more thing though. When I press and hold a high note I seem to get a much narrower spectrum as to when i press and hold a low note I get a broader spectrum. Here is an image of what I am talking about

(high note left low note right). What is the reason for a low note having bigger spectrum?

 

[anorlunda]

Can't see your image. Use the UPLOAD button.

 

[Teachme]

Sorry. Thanks

 

[anorlunda]

A sample with more cycles gives a narrower spectrum. How many cycles were in included in your high and low samples?

 

[Teachme]

Hmm I didn't count the cycles, but i just tested this by holding down the lower note for a longer period of time and the area taken up on the db vs frequency graph doesn't get lower over time. Would I expect it to?

 

[DrGreg]

When I press and hold a high note I seem to get a much narrower spectrum as to when i press and hold a low note I get a broader spectrum.

Look at the frequency scale. It's not uniform (it's logarithmic). The low frequency note is shown as being about 200 Hz wide. At the high frequency note, the resolution ("bar width") is wider than that.

Note that if the display updates many times a second, it means it isn't analysing an infinitely long sine wave, it is analysing a wave that effectively lasts for only a fraction of a second. Does the device or software that generates the graph have some adjustable settings? Is there a "windowing" feature that can treat the signal as fading-in and fading-out instead of abruptly starting and stopping?

 

[Teachme]

Look at the frequency scale. It's not uniform (it's logarithmic). The low frequency note is shown as being about 200 Hz wide. At the high frequency note, the resolution ("bar width") is wider than that.

Note that if the display updates many times a second, it means it isn't analysing an infinitely long sine wave, it is analysing a wave that effectively lasts for only a fraction of a second. Does the device or software that generates the graph have some adjustable settings? Is there a "windowing" feature that can treat the signal as fading-in and fading-out instead of abruptly starting and stopping?

Oh I completely didn't think about that. That was totally confusing me. Thanks. There is no option to do that with the analyzer I am using, but I could probably just find another one that has that option.

thanks again

 

[anorlunda]

Oh I completely didn't think about that. That was totally confusing me. Thanks. There is no option to do that with the analyzer I am using, but I could probably just find another one that has that option.

thanks again

The devil is in the details. You should carefully think through all the details before asking the kind of question in your OP. Why? Because you may know the answer already if you think it through.

 

[Teachme]

The devil is in the details. You should carefully think through all the details before asking the kind of question in your OP. Why? Because you may know the answer already if you think it through.

Yeah you are right. Thanks for your help though and making me realize the minus infinity to infinity thing. Don't think I would have thought about that even if I tried for a long time.