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Mysterious 50Hz cancellation by left and right

skyfly

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mysterious_50Hz_cancellation.jpg


Any idea why I get cancellation at 50 Hz when I turn on both left and right channels?

The measurement was not done at my home. So I cannot take additional measurement at my convenient time.

The mic is one marketed for people doing voice over. I do not know the accuracy for acoustic measurement.

The speakers are Bose 901 VI.

I suspected that the 901 EQ was defective and was sending out-of-phase signals. So I played a 50 Hz sinusoidal tone, and measured the voltage with a multi-meter: L+ and ground 1.121 V; R+ and ground 1.145; L+ and R+ 0.036 V.

P.S.:
I performed the measurement to see how better it is to put PSI AVAA at the corner on the listeners' wall than at the corner of the speakers' wall. However, I found an unexpected 50 Hz cancellation. Even with bare ears, I could feel that the listening position was a null position for 50 Hz.

By the way, PSI AVAA remained at the corner of speakers' wall. The reason was the distance from the window. In winter, they open the windows for some moment everyday for air circulation. If PSI AVAA is at the he corner on the listeners' wall, it will be right below the window, and get temperature shock every day during freezing winter season. PSI AVAA is more expensive than their main speaker system, and they do not want to break it. PSI AVVA's main purpose is to reduce 37 Hz standing wave that is very noticeable at the listening position.

Listening near wall is not desirable, but I understand them because they are not audiophiles. They have a little bit of feeling against hi-end audio market.
 

RayDunzl

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skyfly

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AVAA on and off comparison. 1/48 oct smoothing on REW. 4M length sweep signal from 15Hz to 20kHz.
AVAA_on_off_1_ov_48oct_smoothingl.jpg

AVAA_off_waterfall.jpg

AVAA_on_waterfall.jpg
 

RayDunzl

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The measurement was not done at my home.

Then, what you show above may or (more probably) may not be representative of what happens at your place.
 

QMuse

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I don't know how to diplay unwrapped frequency. Anyway, the phase difference at 50 Hz is not close to 180 degrees.
View attachment 61599
View attachment 61600

You don't need to unwrap phase but you should apply FDW of app 6 cycles (under Tools, IR Windows). Show both phase responses on one graph in Overlays window.

It doesn't need to be 180 for the cancellation to occur, it happens continuously. How much cancellation you will get depends how close the difference in phases is close to 180. You have app 117 deg of difference which is quite large. This usually comes from differences in positions of your speakers so you should consider moving one (or both) of them so their position is more similar in terms of corner placement, front and side wall reflections.
 
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QMuse

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Then, what you show above may or (more probably) may not be representative of what happens at your place.

True. Most probably it will be quite different as phase cancellation is placement dependent.
 

RayDunzl

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I don't know how to diplay unwrapped frequency. Anyway, the phase difference at 50 Hz is not close to 180 degrees.

There's a button for unwrapping and another for wrapping in the controls for the main SPL&Phase and the overlay Phase windows.

As for not close to 180 degrees--

The difference between +125 and -39 is 164 degrees, which is kinda close.
 

A800

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And glass :/
 

QMuse

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There's a button for unwrapping and another for wrapping in the controls for the main SPL&Phase and the overlay Phase windows.

As for not close to 180 degrees--

The difference between +125 and -39 is 164 degrees, which is kinda close.

Deg scale is on the right side so phase difference is 118 which makes cancellation factor of sin(118/2)=0,857 and that is quite large.
 

RayDunzl

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Deg scale is on the right side

Oh, duh,

This usually comes from differences in positions of your speakers

My room is something like this:

1588490500287.png


I figure the left speaker has more opportunity for a long bounce, and the right sees more of a short bounce, and that combination makes a hole at 48Hz at the sofa.
 
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skyfly

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At 50 Hz, where the cancellation is most dramatic, the phase difference is not closest to 180 degrees. Quite complicated things seem to be happening.
phase_L_R.jpg
 

QMuse

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At 50 Hz, where the cancellation is most dramatic, the phase difference is not closest to 180 degrees. Quite complicated things seem to be happening.View attachment 61605

Wrap the phase and keep reducing FDW until you aren't geting phase rotations (dotted vertical lines) on both lines.
 

QMuse

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Or post your mdat file with extension changed to .txt so I can have a look.
 
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skyfly

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The files.
 

Attachments

  • May2_PSIoff_2ML.txt
    161.2 KB · Views: 90
  • May2_PSIoff_2MR.txt
    161.2 KB · Views: 61
  • May2_PSIoff_2MLR.txt
    161.2 KB · Views: 61

QMuse

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The files.

There is something wrong with your measurement, impulse response looks very weird and thus the phase cannot be displayed correctly.

Capture.JPG


What I'm pretty sure is there are multiple phase cancellations here, not only at 50Hz but at 80Hz as well.
 
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skyfly

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There is something wrong with your measurement, impulse response looks very weird and thus the phase cannot be displayed correctly.

View attachment 61617

What I'm pretty sure is there are multiple phase cancellations here, not only at 50Hz but at 80Hz as well.

The cancellation at 50 Hz is real. I can play sinusoidal tones. I can move myself toward the listening position. The sound disappears to my ears.

By the way this is the impuse on my REW.

2MLR_impulse.jpg
 
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