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Weird / stupid equalizer question - subjectively detectable changes in experiencing 20 hz sine test when "cut" starts from 18, why ?

Daltong

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So basically I have AeonRT and Qudelix (both reviewed by Amir and so far very okay experience)
For some reason I decided to check if reducing signal below 20hz (which shouldn't be audible or particularly relevant to headphones experience due to limited excursion magnitude, "bone bass" is not what headphones are reasonably expected to do... I mean Amir doesn't even measure that low haha ;)) would change anything about the experience.

So I've used Amir's EQ recipe and just added a couple of filters to cause a large signal drop below 20 Hz (Qudelix's EQ goes as low as 10 Hz), tweaked it so that the decrease would start at 18 Hz and not affect the shape of bass bump much (as per Qudelix's own curve visualization)

So I expected there to be no effect / subtle placeboish stuff. Like changing between two good cables stuff.

The change felt distinct, substantial.

So I thought that's weird.

So I took 20 Hz sine wave tests (one flac file and also online sine tone generator as additional reference) and listened to them at "detectable but not uncomfortable" loudness using Amir EQ as "baseline".

The tests sound differently based on whether the "below 20 Hz cut" is in or not.

This is true both for Syznalski's online tone generator and high quality 20 Hz flac file so it is unlikely to be an issue with the sine signal being somehow wrong.

What's going on and why is it going on ?

P.S.: I am so mystified I might get myself a semi-decent headphones recording mic just to take a look at what exactly is being played back with and without the "cut below 20" tweak
 

DonH56

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So basically I have AeonRT and Qudelix (both reviewed by Amir and so far very okay experience)
For some reason I decided to check if reducing signal below 20hz (which shouldn't be audible or particularly relevant to headphones experience due to limited excursion magnitude, "bone bass" is not what headphones are reasonably expected to do... I mean Amir doesn't even measure that low haha ;)) would change anything about the experience.

So I've used Amir's EQ recipe and just added a couple of filters to cause a large signal drop below 20 Hz (Qudelix's EQ goes as low as 10 Hz), tweaked it so that the decrease would start at 18 Hz and not affect the shape of bass bump much (as per Qudelix's own curve visualization)

So I expected there to be no effect / subtle placeboish stuff. Like changing between two good cables stuff.

The change felt distinct, substantial.

So I thought that's weird.

So I took 20 Hz sine wave tests (one flac file and also online sine tone generator as additional reference) and listened to them at "detectable but not uncomfortable" loudness using Amir EQ as "baseline".

The tests sound differently based on whether the "below 20 Hz cut" is in or not.

This is true both for Syznalski's online tone generator and high quality 20 Hz flac file so it is unlikely to be an issue with the sine signal being somehow wrong.

What's going on and why is it going on ?

P.S.: I am so mystified I might get myself a semi-decent headphones recording mic just to take a look at what exactly is being played back with and without the "cut below 20" tweak

Switching in a filter affects the response above and below the filter because it is not a brick wall -- the response rolls off gradually below the cut-off frequency, and begins rolling off (reducing) somewhat above the cut-off (corner) frequency. That means phase also changes, and that is particularly important for speaker interactions (e.g. mains and sub) and how they interact with the room. So inserting a filter at 18 Hz can change the response to perhaps 40 Hz or higher, and change the response due to phase changes from the filter that change how the su bis integrated with the mains and how it interacts with the room.

With headphones, you don't get room interaction, but the response still changes due to changes in amplitude and phase when you add the filter.

HTH - Don

Edit: This is a low-order crossover but shows the filter response above and below the corner frequency: https://www.audiosciencereview.com/forum/index.php?threads/bi-amping-101.22817/ There are several other articles in the link in my signature that may help.
 

egellings

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The filters in the EQ are not brick walls. There is some spread in the frequency range about the center value of attenuation when using any filter.
 

Cbdb2

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What your probably hearing is not the 20hz but the distortion, ie 40, 60, 80,100,120hz. And as already stated the filter will effect these freqs. From Fletcher Munson your 40db! more sensitive to 120hz than 20hz. So to hear the 20hz your probably overdriving the headphones making lots of these overtones. Set a comfortable level with a 1khz tone than listen to the 20hz at that level. What do you hear now.
 
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Daltong

Daltong

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I don't think I'm overdriving the headphones at all and Aeons have pretty low distortion. Tried to set the corner freq of the de-boost at 13 Hz (qudelix's equalizer reports maximum amplification at 20 Hz, 3.44 db with or without the ultra-low "cut") and the "odd effect" is still quite audible (even if different now)

I would not expect it to affect distortion products much when playing a strictly 20Hz sine (those are products of the headphone not the DAC/DSP/AMP in qudelix right? so why would it, unless Q is adding substantial distortion of its own when trying to play this kind of signal...)

I guess phase effects is is then, as Don says.

P.S. filters in higher range don't produce such weirdness, at least, I wasn't able to replicate by adding a similar cut under 1 or 4 Khz and playing respective tones
 

Cbdb2

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Did you listen to the 20hz at the same volume you listen to music? Most people can't hear 20hz. They hear the harmonics (distortion) or mechanical resonace. You feel 20hz, and that takes power. People are 50db more sensitive to 1khz than 20hz which means the 20hz takes 100,000 the power! Are you sure the 20hz is not distorting your headphones?
 
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Daltong

Daltong

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Feel/hear is a bit ambivalent here but yeah, I can "discern" it at fairly moderate (calibrated on 1 Khz or normal music) loudness levels, very distinctly if only a bit louder.

As for distortions, I think that would be a reasonable explanation except for that I would have expected distortion profile determined by the headphone to be pretty much same when "playing 20hz pure tone, no cut below 20hz in EQ" and "playing 20hz pure tone, a cut that doesn't start happening until 16hz or so as per EQ's curve UI and has a center frequency of 13 Hz".

I guess I can accept "it's phase, yeah" as explanation but still am very curious what does the change actually "look like" instrumentally, wish I had a mic that wasn't shite...
 
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Daltong

Daltong

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Oh, that makes sense, that makes sense a lot.

I've just tried to replicate "Amir's low shelf" bass boost shape so that I would have a "peak at 20hz, gentle slope down through 35-ish" bass boost with no specific cuts below 20hz (the peak's "inherent" shape at least makes sure that things like 10-25Hz don't get boosted much if present in the signal) and the difference between that and Amir's EQ settings in 20 Hz test is much smaller (possibly placebo when it comes specifically to 20Hz), not huge like I have when I add "cuts under the shelf"

Some-kind-of difference is still IMHO present when playing music per se (hard to describe, not worse, but also not somehow OMG better) which may be due to slightly different boost shape between 20 and 35 or due to not over-boosting whatever "below 20 ish" components are there thus reducing distortion... or maybe placebo lol ;):facepalm:)

In any case I am now inclined to ascribe the weird "cuts under 20 Hz change 20 Hz experience a huge lot" phenomenon to DSP bug.
 
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