CRT Wobble from sound waves

[Phasor]

I've noticed that when I play the lowest note on my bass clarinet (D♭ ~69Hz), my computer monitor image (20" Mitsubishi CRT) wobbles quite strongly, on the vertical axis.

My girlfriend claims that she cannot see it, that it looks perfectly normal to her.

Can anyone put forward an explanation?

 

[dr pepper]

Thats some clarinet, 69hz, then I spose a contrabass sax can go near sub audio.

Assuming for a mo that your ladyfreind is correct and the picture stays normal then the situation is probably you, ie the vibration/sounds waves are affecting your vision, which is quite possible at reasonable volume.

If the picture does actually wobble then it could be the deflection coil assy either resonating or possibly a less than ideal connection or solder joint within the monitor is being affected.

Sound waves at least at that volume wont affect the electron beams within the tube as they are in a near vacuum.

 

[Phasor]

Assuming for a mo that your ladyfreind is correct and the picture stays normal then the situation is probably you, ie the vibration/sounds waves are affecting your vision, which is quite possible at reasonable volume.

I did consider that possibility that it was affecting my vision, but it only happens with that one monitor. It doesn't happen with the other monitors in the house (Laptops/LCD).

 

[ronsimpson]

If you head vibrates (moves) when you are looking at a object the picture appears to move.
You say it happens on a CRT but no on LCD (laptops).
A LCD has a picture 100% of the time.
A CRT has a flying spot. Once a spot is lit it stays on for a while then goes out. The entire picture is not present all the time. Because the eye is slow it thinks the entire CRT is lit.
The picture is being drawing on the CRT at 60 times a second, top to bottom. Your head is vibrating at 69 times a second. 69-60=9 These two effects have a difference of 9 times a second. Probably you should see some effect that happens 9 times a second.
I see movement when I eat at a CRT monitor. I believe you will see this when you are close to the monitor and not when you are back away.

 

[Phasor]

The picture is being drawing on the CRT at 60 times a second, top to bottom. Your head is vibrating at 69 times a second. 69-60=9 These two effects have a difference of 9 times a second. Probably you should see some effect that happens 9 times a second.
I see movement when I eat at a CRT monitor. I believe you will see this when you are close to the monitor and not when you are back away.

Interesting idea... I will try it when I get home tonight.
9Hz sounds about right for what I'm seeing, however, I think theoretically, that the beating of should be ~1.4Hz (ie. if you plot sin(60x) + sin(69x), you'll see the beating every 0.7 sec) [I could be talking out my a** here though]
I'll set up a video camera too, to see if that picks it up.

 

[ronsimpson]

We don't really know the vertical rate. 60hz is as slow as can be. Many times we push the vertical rate up to 65, 70 or some times 80hz to make eye strain less.
Your vertical could be 65 or 70hz.
70-69=1 69-65=4

 

[dr pepper]

I'd considered something similar, but then here refresh rate is 50hz.

 

[ronsimpson]

I looked at my computer and I have 60 or 75 as options. Another computer has options of 60 61 62.....74, 75. It appears to be dependent on video board and monitor. (and version of windows) 50 hz refresh hearts my head. Right click on the desk top and hit properties. Settings...advances....monitor see refresh rate. This depends on operating system.

 

[Mr RB]

It's your skull vibrating, or at least making your eyes vibrate. I have a neck massager thingy that does the same thing, it can be annoying watching TV and the screen goes wobbly. Like a Gilligan's Island flashback sequence.

 

[ronsimpson]

We have this happening when, eating, playing D♭, and when vibrating your skull.
We do not have this on a LCD monitor.
It appears to be a combination of eye vibration and persistence of the CRT phosphor.

 

[JimB]

I would also say that the effect is in your head.

Many years ago, at a place where I used to work, we had a long narrow computer room.
At the end of the computer room was a colour monitor facing along the length of the room.
One day as a was walking towards the monitor, I noticed that the picture was jumping up and down.
Damn! I thought, another problem to fix.
When I got to the monitor and looked at it, the picture was completely steady.
Wow! Strange optical effect I thought.

After that, everytime I walked along that room, I noticed the same effect.
I cannot remember if I have noticed the same effect elsewhere.

JimB

 

[ronsimpson]

We are all seeing the same effect.

I have done vibration tests on CRT monitors. Vibrating the monitor vertically at a frequency slightly off from the refresh rate causes strange effects. Horizontal vibration does not.

MrRB, I believe where you place the vibrator on your head will change the effect. (Horizontal/Vertical)
JimB, When you walk your head/eyes are sharply jarred vertically. I think if the CRT monitor was on it's side you would not see the effect. I have not tried that.

 

[Mr RB]

...
MrRB, I believe where you place the vibrator on your head will change the effect. (Horizontal/Vertical)...
...

Absolutely! The CRT monitor is not totally "lit up" like a LCD monitor. Only part of the screen is lit at any time, so if you are vibrating your point of view (mainly horizontal) it displaces the top and bottom parts of the picture as they are lit at different times.

Remember the old "flashback" effect from sitcoms? Where the screen goes all wavy? I think they did a parody of it in the movie Waynes World.