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Help understanding graph from a sound measurement
Help understanding graph from a sound measurement
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Old 5th June 2019, 12:37 PM   #1
seb123audio is offline seb123audio
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Post Help understanding graph from a sound measurement

Hi, I have a graph that has been created from a "Distortion Product Ratio H2" test.
The parameters for the test are output volume and the frequency sweep range (20Hz-1kHz) and a sweeptime of 5 seconds. 3 channels have been recorded but just ignore all but one. I am trying to understand how to interpret a single channel's result.

Here is the graph:
Click the image to open in full size.

Link to image: Imgur: The magic of the Internet

If I am not mistaken, the test does the following:


- A)It sweeps through the frequency range for a duration of the sweeptime.

- B)For each and every frequency in that range: the dBSPL of the 2nd harmonic frequency of the currently played frequency, is recorded as a value.

- C)The result is a graph were each value in the graph shows the difference between the 2nd harmonic and its fundamental in dBSPL. The x-value is the
fundamental frequency. The y-value is the difference between the dBSPL of the fundamental frequency and its 2nd harmonic expressed in dB.

- D)Formula: <Fundamental frequency in dBSPL> - <2nd harmonic in dBSPL> = <distortion product ratio for that fundamental frequency in dB>.



Am I correct in my interpretation?
If I am correct, were can I find a source for this? I have looked and looked but can't find any site explaining it.

Last edited by seb123audio; 7th June 2019 at 12:20 PM.
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Old 5th June 2019, 06:00 PM   #2
adason is offline adason  United States
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i see no image
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Old 7th June 2019, 12:18 PM   #3
seb123audio is offline seb123audio
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Quote:
Originally Posted by adason View Post
i see no image
Fixed it.
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Old 7th June 2019, 06:37 PM   #4
DF96 is offline DF96  England
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On the Audio Precision website it says
Quote:
In the Distortion Product Level view, the graph displays rms level. In the Distortion Product Ratio view, the graph displays the ratio of the rms level of the distortion product to the rms level of the fundamental tone.
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Old 7th June 2019, 09:09 PM   #5
Elvee is offline Elvee  Belgium
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I do not see why the graph seems to be centered about ~-5dB.
In fact, for H2 (or any H), the values should all be largely negative, unless there is something really wrong with the test setup or the DUT (can happen, but should be fixed before trying to interpret the results)
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Old 7th June 2019, 10:11 PM   #6
cbdb is offline cbdb  Canada
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According to the graph the distortion varies 80db! Isn't +40db 10,000% or more distortion? It might be normalized +60db or something but that's confusing. Any way very bad distortion if it's from an audio device.
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Old 8th June 2019, 02:56 PM   #7
DF96 is offline DF96  England
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I am going to guess that this is a plot from a speaker test, done by someone who is still learning about dB and reference levels.
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Old 8th June 2019, 10:49 PM   #8
PRR is offline PRR  United States
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It might help if we knew what the driver is.

If the cone is smaller than your palm, it is very reasonable that above 150Hz the THD at high level is near 10% (-20dB); and below 150Hz the speaker is not making much fundamental and the distortion and stray wheeze reads higher than the fundamental (positive dB).
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