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Author Topic: Do we see the lips move before we hear the sound?  (Read 774 times)

Offline Thebox

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The speed of light is faster than the speed of sound, light is reflected into our eyes from the observed at the speed of light that allows us to see, the sound is slower than the light, but the sound from my partners lips seemingly is in time with her lips and arrives in time.
If the light arrives first because it is faster, then shouldn't I observe her lips move then a delay in the sound?

added-

added- the speed of sound 340.29 m / s

the speed of light 299 792 458 m / s

Even at close ranges I can not ''see'' how this huge difference in speed could not form out of sync speech?

We see the lips move 879414.661191 times faster than we hear the sound ,huh?
« Last Edit: 14/08/2016 09:38:20 by Thebox »


 

Offline alancalverd

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Re: Do we see the lips move before we hear the sound?
« Reply #1 on: 14/08/2016 09:38:14 »
Quote
If the light arrives first because it is faster, then shouldn't I observe her lips move then a delay in the sound?

Yes, but you have to be very observant. The speed of sound in air is around 1100 ft/sec, so you probably won't notice the difference at less than 100 ft separation, and nice ladies don't shout. She could however clap her hands at the end of a long garden. I was going to suggest firing a gun, but given your exasperating nature, you might find the outcome unacceptable.
 

Offline Thebox

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Re: Do we see the lips move before we hear the sound?
« Reply #2 on: 14/08/2016 09:42:41 »
Quote
If the light arrives first because it is faster, then shouldn't I observe her lips move then a delay in the sound?

Yes, but you have to be very observant. The speed of sound in air is around 1100 ft/sec, so you probably won't notice the difference at less than 100 ft separation, and nice ladies don't shout. She could however clap her hands at the end of a long garden. I was going to suggest firing a gun, but given your exasperating nature, you might find the outcome unacceptable.

I added to my last post, while you was typing I think sorry Alan.

Light is  879414.661191 times faster than sound?  is that correct at sea level?

I am trying to understand the distance factor, I do ''see'' your point.

added- ok , so far I have this diagram, is this what we are saying?  (the only possible 100% synchronisation of light and sound by receiver is if there is no distance to travel for the light or the sound, i.e r=0?

added- what is the speed of energy transfer from two adjoining bodies r=0?

« Last Edit: 14/08/2016 10:00:03 by Thebox »
 

Offline Bored chemist

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Re: Do we see the lips move before we hear the sound?
« Reply #3 on: 14/08/2016 10:24:00 »
It's a commonplace observation that, if the action is at the other end of the pitch, you see a football kicked, before you hear the kick.

Re "Light is  879414.661191 "
do you understand what "spurious accuracy" is?
Re.
"what is the speed of energy transfer from two adjoining bodies r=0?"
That only has a meaning for objects of zero size, i.e. objects that don't exist.

 

Offline evan_au

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Re: Do we see the lips move before we hear the sound?
« Reply #4 on: 14/08/2016 22:10:35 »
Quote from: TheBox
If the light arrives first because it is faster, then shouldn't I observe her lips move then a delay in the sound?
You see the flash of lightning before you hear the thunder. It's really scary when they arrive at the same time (it happened to me once...).

Our brains unconsciously "know" that light is faster than sound, so our brains are quite tolerant of the image arriving before the sound. But whenever the sound arrives before the image, we get a feeling that something is subtly "wrong" (the "lipsynch" problem at the movies). We are much more sensitive to sound arriving early than late.

In reality, it takes your brain about 100ms to process the sound, and 300ms to process the image, so in integrating the two, your brain slows down the speech so it is in sync with the image.
 

Offline Thebox

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Re: Do we see the lips move before we hear the sound?
« Reply #5 on: 15/08/2016 09:17:01 »
Quote from: TheBox
If the light arrives first because it is faster, then shouldn't I observe her lips move then a delay in the sound?
You see the flash of lightning before you hear the thunder. It's really scary when they arrive at the same time (it happened to me once...).

Our brains unconsciously "know" that light is faster than sound, so our brains are quite tolerant of the image arriving before the sound. But whenever the sound arrives before the image, we get a feeling that something is subtly "wrong" (the "lipsynch" problem at the movies). We are much more sensitive to sound arriving early than late.

In reality, it takes your brain about 100ms to process the sound, and 300ms to process the image, so in integrating the two, your brain slows down the speech so it is in sync with the image.

Thank you Evan, the flash of lightning is a good example.  When you see the flash of lightning, what are you seeing the flash of lightning ''compared'' too?

Meaning you are already seeing when you see the flash and the flash is observable compared to its surrounding of ''free space'', where sound is not seen and a product of ''vibrating'' air.

When seeing the lightning, are you saying that an observer closer to the lightning on Earth see's the lightning before an observer on the space station? 

 

Offline evan_au

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Re: Do we see the lips move before we hear the sound?
« Reply #6 on: 15/08/2016 10:53:36 »
Quote from: TheBox
When you see the flash of lightning, what are you seeing the flash of lightning ''compared'' too?
You are comparing it to your previous state of not seeing a flash.

However, a scientist could set up an accurate clock, and an optical sensor that is triggered by lightning, and a circuit that recorded the time of the flash. By providing several observers with equipment like this, you could record the exact time that different observers see the lightning.

In fact, a system like this is already in place in the UK to record the location and time of lightning strikes. Lightning emits a burst of radio waves, which are picked up by receivers around the country. By comparing the timing of these radio bursts from 3 nearby stations, it is possible to determine the location of the lightning strike (it's like GPS in reverse).
See: http://www.netweather.tv/index.cgi?action=lightning;sess=

Quote from: TheBox
When seeing the lightning, are you saying that an observer closer to the lightning on Earth see's the lightning before an observer on the space station?
Yes; an observer 1km away on Earth will see it about 3us after the flash (plus the 300ms to process it through the visual cortex).
- An observer 300km away on the ISS will see it about 300us after the flash (plus the 300ms to process it through the visual cortex).
- If an accurate clock were sent up with the ISS, the clock on the ISS wall will show a different time than the accurate clock of an observer on the ground, because clocks on the ground tick at a different rate than clocks in the ISS (due to orbital velocity+gravitational time dilation). 
 

Offline nicco

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Re: Do we see the lips move before we hear the sound?
« Reply #7 on: 05/09/2016 22:54:14 »
This is more a question about consciousness and brain process than physics, as Evan alludes.  firstly, at a conversation distance, the processing time the brain takes dwarfs the travel time of the signals, so you would reasonably expect there to be no discernible difference.  but our consciousness is tricked by our brain in a time shifting way.  if you were to hold your hands out in front of you and clap them, there is at least a quarter second difference, up to half a second, in the time your brain takes to process the audio, visual and sensation signals.  this should be obvious, but it is presented to the consciousness as a simultaneous event.  of course this is a useful trick in managing all the information we receive into a package that can be coped with on a daily basis.  imagine a world that has to be interpreted with all the senses out of sync.
i will offer a prediction.  there is obviously a distance where our brains are happy with a visual signal and audio signals being out of sync, a ball kicked on a pitch has been offered as an example.  i would hazard a guess that our brains will jam two signals together and present them to our conscious selves as synchronised at a distance beyond that which they should readily be separated, and that this effect would be more pronounced with speech (imagine a drive-in cinema screen as an experimental apparatus)
 

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Re: Do we see the lips move before we hear the sound?
« Reply #7 on: 05/09/2016 22:54:14 »

 

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