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

Click For Summary

Discussion Overview

The discussion revolves around the observation of multiple frequency readings on a dB vs frequency graph when generating a single sine wave using a synthesizer. Participants explore the implications of waveform characteristics, such as abrupt starts and ends, and how these affect the frequency spectrum displayed.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant notes that a sine wave extending from minus infinity to infinity has a single frequency, while one with an abrupt start and end contains additional frequency components.
  • Another participant suggests that the sine wave may not be pure, prompting a request for screenshots of the time-domain and frequency-domain representations.
  • It is proposed that an abrupt waveform, such as a quarter note, can be represented as a sum of sine waves, which introduces higher frequencies.
  • A participant observes that holding a high note results in a narrower spectrum compared to a low note, leading to questions about the reasons behind this phenomenon.
  • Discussion includes the idea that a sample with more cycles yields a narrower spectrum, and participants consider the implications of the duration of held notes on the frequency graph.
  • One participant points out that the frequency scale is logarithmic, which affects the representation of low and high frequencies on the graph.
  • There is mention of the possibility that the analyzer being used does not have a windowing feature, which could treat signals as fading in and out rather than abruptly starting and stopping.
  • Another participant emphasizes the importance of considering details in the analysis and suggests that the original poster may have already known some of the answers through careful thought.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the effects of waveform characteristics on frequency representation. There is no consensus on the specific reasons for the observed differences in spectrum width between high and low notes, and the discussion remains open to interpretation and further exploration.

Contextual Notes

Participants note that the display updates frequently, indicating that the analysis is not based on an infinitely long sine wave, which may contribute to the observed frequency readings. The lack of a windowing feature in the analyzer is also highlighted as a potential limitation.

Teachme
Messages
72
Reaction score
0
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?
 
Physics news on Phys.org
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?
 
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?
 
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?
 
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.
 
  • Like
Likes   Reactions: berkeman
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
4tLf5Xy
(high note left low note right). What is the reason for a low note having bigger spectrum?
 
Can't see your image. Use the UPLOAD button.
 
Sorry. Thanks
 

Attachments

  • notes.png
    notes.png
    18.3 KB · Views: 560
A sample with more cycles gives a narrower spectrum. How many cycles were in included in your high and low samples?
 
  • #10
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?
 
  • #11
Teachme said:
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?
 
  • #12
DrGreg said:
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
 
  • #13
Teachme said:
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.
 
  • #14
anorlunda said:
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.
 

Similar threads

Undergrad Sound power, amplitude, frequency, and decibels
  • · Replies 5 ·
Replies
5
Views
2K
Undergrad Why do sine waves define pure frequency?
  • · Replies 19 ·
Replies
19
Views
6K
Undergrad Why is Photon Energy Quantized in Terms of Sine Wave Frequency?
  • · Replies 78 ·
3
Replies
78
Views
6K
Graduate Why do sound waves with slightly different frequencies create beats?
  • · Replies 6 ·
Replies
6
Views
3K
Undergrad Relationship between frequency and power for sound?
  • · Replies 11 ·
Replies
11
Views
6K
Graduate Why didn't I see a peak at half the frequency in my FFT analysis of two waves?
  • · Replies 4 ·
Replies
4
Views
2K
Why does a series of pulses generate a pitch?
  • · Replies 27 ·
Replies
27
Views
5K
High School Why does the same frequency sounds differ?
  • · Replies 36 ·
2
Replies
36
Views
12K
Graduate Understanding propagation loss: What does this output mean?
  • · Replies 18 ·
Replies
18
Views
6K
Undergrad Wavelength to Frequency Relationship in Musical Notes
  • · Replies 11 ·
Replies
11
Views
6K