Unexpected Bass and Treble Action

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kozikowski
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Posts: 69285
Joined: Thu Aug 02, 2007 5:57 pm
Operating System: macOS 10.13 High Sierra

Unexpected Bass and Treble Action

Post by kozikowski » Fri May 14, 2021 5:52 pm

I'm not sure I can explain this one. When I reduce the Bass in Effect > Bass and Treble, the bass tones do go down, but both the RMS and waveform peaks go up. I don't have any automatic tools selected.

BassAndTreble.png
BassAndTreble.png (59.24 KiB) Viewed 66 times

One of the poster/producers is having odd experiences with the Bass and Treble effect.

Note in these tests, reducing treble has normal effect, but reducing bass doesn't.




Audacity 3.0.2, High Sierra Mac.


Generate White Noise, 0.5 amplitude.

Amplify > Measure Peaks = -6.021
Analyze > Contrast > Measure Selection = -10.79dB.
Analyze > Plot Spectrum = Flat with peaks about -48dB.


-- Effect > Bass and Treble > Treble -12dB > OK.

Amplify > Measure Peaks = -7.413dB
Analyze > Contrast > Measure Selection = -18.2dB.
Analyze > Plot Spectrum = Droop to the right with remaining peaks about -48dB.


EDIT > UNDO (back to flat noise).


-- Effect > Bass and Treble > Bass -12dB > OK.

Amplify > Measure Peaks = -3.665dB -- ???
Analyze > Contrast > Measure Selection = -10.96dB. -- ???
Analyze > Plot Spectrum = Droop to the left with remaining peaks about -48dB.



Koz

steve
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Joined: Sat Dec 01, 2007 11:43 am
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Re: Unexpected Bass and Treble Action

Post by steve » Fri May 14, 2021 6:47 pm

kozikowski wrote:
Fri May 14, 2021 5:52 pm
-- Effect > Bass and Treble > Bass -12dB > OK.

Amplify > Measure Peaks = -3.665dB -- ???
Analyze > Contrast > Measure Selection = -10.96dB. -- ???
Analyze > Plot Spectrum = Droop to the left with remaining peaks about -48dB.
My results:

Starting with white noise generated with amplitude 0.5

Before B&T:
Peak: -6.021 dB
RMS: -10.80 dB
Analyze > Plot Spectrum (Size: 2048): Flat, approx -36 dB


B&T settings:
Bass: -12 dB
Treble: 0 dB
Volume: 0 dB
Link volume: Off


After B&T:
Peak: -3.435 dB
RMS: -11.22 dB
Analyze > Plot Spectrum (Size: 2048): Curved, approx -48 dB on left, -36 dB on right.


There's a reason why the peak level increases. It's not a bug.

White noise is random numbers where each sample value "x" is a random number between a max and min value.
For white noise amplitude 0.5, each sample value is a random number: -0.5 > x < 0.5,
with equal probability for any value within that range.

In other words, the 0.5 peak white noise is random number constrained to the range -0.5 to + 0.5.

The -12 dB bass (shelf) filter has little effect on the overall (RMS) level, because white noise has relatively little energy in the bass frequencies (hence it sounds like "sssss" rather than "urrrrr").

However, the filter does jiggle around all of the sample values, and they are NOT constrained to the range -0.5 to +0.5, so some of the sample values end up outside of that range. Thus, given that "peak" level is measuring the outliers (max absolute sample values), the peak measurement goes up.


Does that make sense?

If you try the same test with Pink noise, you will see more "expected" results, because Pink noise has more energy in the bass frequencies and is less tightly constrained.


I note that in this post: viewtopic.php?p=425760#p425760
they are boosting both bass and treble, with a small reduction in the overall volume. That will push up the low and high frequencies, and pull down the middle. The overall effect on peak level cannot be predicted, but it is likely to be slightly increased with the settings they are using.
9/10 questions are answered in the FREQUENTLY ASKED QUESTIONS (FAQ)

kozikowski
Forum Staff
Posts: 69285
Joined: Thu Aug 02, 2007 5:57 pm
Operating System: macOS 10.13 High Sierra

Re: Unexpected Bass and Treble Action

Post by kozikowski » Fri May 14, 2021 7:04 pm

viewtopic.php?p=425760#p425760
I think we're juggling more than one problem.

Koz

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