What happens when sound wave loses energy?

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I'm a high school student doing the project about sound absorbing material
When the sound is absorbed or loses its energy which of its value will change? pressure? wavelength? or frequency?
(at first I though that the sound wave will just lose its amplitude but after the result,It look like its frequency changes too)

This is the result from my experiment(measure the standing wave in the tube)
-500 Hz frequency : http://imgur.com/2Y2jCxD
(It looks weird on this frequency.my friend said this happens because of change in frequency)
-1000 Hz requency: http://imgur.com/oRRsRm8
Thank you very much for your answer
 

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fresh_42
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Have a look on this Wiki graphic:
https://de.wikipedia.org/wiki/Lautstärke#/media/File:Akustik_db2phon.jpg
(vertical axis: pressure, horizontal axis: frequency, graphic lines: iso-volume)

The formula for energy can be found here:
https://en.wikipedia.org/wiki/Sound_energy

Note: Volume is a human "psycho-acustic" term to describe amplitude or pressure in terms of perception.
so... from what i read in your link,It looks like the frequency is irrelevant from the sound energy.
That means the sound will have the same frequency if it loses the energy?
but Why that graph happen in 500 Hz?. It's like there's a 2 kind of node in it .
The graph should look like 1000 Hz graph?
 
fresh_42
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so... from what i read in your link,It looks like the frequency is irrelevant from the sound energy.
The kinetic part of the sound energy has particle velocity in it which again has the frequency as part of it. (I'm not saying that it is an essential part.)
That means the sound will have the same frequency if it loses the energy?
Sound is a longitudinal wave and its energy mainly determined by the pressure it puts on a surface, e.g. our eardrum.
but Why that graph happen in 500 Hz?. It's like there's a 2 kind of node in it .
The graph should look like 1000 Hz graph?
I don't find that the graphs are so much different. They both don't look optimal. Your measurement probably depends on many factors and some of them, like reflections in the room, might differ with frequency. It is impossible to tell without knowing everything about the experiment.
 

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