# The Naked Scientists Forum

### Author Topic: How do we percieve waves, of either sound of light?  (Read 3260 times)

#### James Kroffinger

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##### How do we percieve waves, of either sound of light?
« on: 28/10/2008 03:12:11 »

1. When you see the model of a sine wave sound, you see a line with an increasing amplitude, that hits a certain point, then the amplitude decreases. Why don't we hear it that way? Why don't we hear something slowly getting louder, hitting a point, then slowly getting quieter - why do we hear a flat out tone?

2. When materials absorb/reflect light, how do they know what the wavelength will be? I.e. you have a red pigment. You put white light on it, and it absorbs the blue/green and reflects red. But, how does it know what wavelength that photon that hits it will have? Like if a photon hits it, then another photon hits it with less energy, then another one with less energy, that defines a wave. But for one specific photon, how will that material know at the moment of the photon hitting it that it will have a certain wavelength?

3. This kind of joins #1 and #2: How do your ears know what frequency a sound will have?  Some energy hits your ear drum and vibrates it. Then, later more hits and vibrates it, but this time more weakly. But at the first moment of vibration, how does it know that the next "phonon" will have a higher or lower frequency?

4. Are sound, wind (air movement), and heat related? Sound is the oscillation of atoms. Wind is the movement of atoms. Heat is the vibration of atoms. Can low enough heat create sound? Can high enough frequency sound create heat? Can wind move sound?

Mod edit - formatted the subject as a question - please do this to help keep the forum tidy and easy to navigate - thanks.  If I haven't quite got the jist of your question, feel free to change it!
« Last Edit: 28/10/2008 10:38:17 by BenV »

#### lyner

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##### Re: How do we percieve waves, of either sound of light?
« Reply #1 on: 28/10/2008 09:11:07 »
I don't know what your first question means. The normal way of 'drawing' a sinewave is to have several cycles, all of the same amplitude (loudness). The ups and downs occur at the rate / frequency /pitch of the sound you hear or the light that enters your eyes. A one second burst of 1kHz tone will have 1000 complete cycles of sound in it, for instance. A short flash of red light will have hundreds of millions of cycles of light in it.

A train of these waves may have an abrupt beginning and end or it may grow and decay slowly.

How do your ears / eyes know what the frequency 'will be'? They don't. You have to wait for several or even many cycles of oscillation before you know the frequency. There will always be a finite delay between the wave starting to arrive and your awareness of its frequency.

Quote
Can low enough heat create sound?
That's the best of your questions. The answer is, definitely, YES. The movement (vibration) of air molecules due to heat (and there's always some) causes them to hit your eardrum all the time. This causes a constant low level 'white' noise which limits the quietest sound that you could hear because it interferes with what you want to hear.

#### RD

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##### Waves (sound, light).
« Reply #2 on: 28/10/2008 10:23:02 »
1. When you see the model of a sine wave sound, you see a line with an increasing amplitude, that hits a certain point, then the amplitude decreases. Why don't we hear it that way? Why don't we hear something slowly getting louder, hitting a point, then slowly getting quieter - why do we hear a flat out tone?

If you listen to a low frequency 20Hz sine wave tone it is possible to hear the individual peaks and troughs.
(Allegedly exposure to low frequency vibrations can cause unpleasant symptoms).

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Can high enough frequency sound create heat?

Yes, e.g. ultrasonic welding (typically of plastics).
« Last Edit: 28/10/2008 10:44:18 by RD »

#### lyner

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##### How do we percieve waves, of either sound of light?
« Reply #3 on: 28/10/2008 10:52:37 »
I was thinking further about the "how does it  know the frequency" question. The way your ears work and the way a substance absorbs or reflects light are both based on resonance.
If you whistle next to a guitar you can choose a pitch at which one of the strings will resonate. Other frequencies have no effect. It takes time for the resonance to build up; several hundred oscillations, perhaps. Your cochlea has many of these little resonators and you 'hear' a note because only certain ones respond to a given sound. And they all take a finite time to respond.

The same happens when light hits a substance (only here we have the complication that the energy of the light arrives in quantised packets, which we refer to as photons) the atoms take a while to resonate with appropriate frequencies of light. If they do resonate, the light energy will be absorbed, if not, it may pass straight through

#### The Naked Scientists Forum

##### How do we percieve waves, of either sound of light?
« Reply #3 on: 28/10/2008 10:52:37 »