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XXX

Posts: 4400
Joined: Apr. 14 2005

Interested in science? please read, ...

One question is bugging me since years.

By definition, the energy of a wave is proportional to its frequency, the higher the frequency, the higher quantum of energy the wave is transporting, right?

BUT, if your neighbor is listening to music for example, it is always the low frequencies you will hear, ie the basses that permeate the wall. So, why do walls absorb high frequencies better than low ones, although latter ones transport less energy?

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REPORT THIS POST AS INAPPROPRIATE | Date Mar. 6 2011 16:06:29

NormanKliman

Posts: 1143
Joined: Sep. 1 2007

RE: Interested in science? please re... (in reply to XXX)

Not sure but maybe you'll find information here.

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REPORT THIS POST AS INAPPROPRIATE | Date Mar. 6 2011 17:40:46

XXX

Posts: 4400
Joined: Apr. 14 2005

RE: Interested in science? please re... (in reply to NormanKliman)

Thx i didnt expect an answer so soon! :)

quote:

Figure 6: The higher frequencies (top) are absorbed well because their velocity peaks fall within the material thickness. The lower frequency at the bottom does not achieve as much velocity so it's absorbed less.

This seems to be the answer! I still have to read everything in detail but i have a feeling this is it.

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REPORT THIS POST AS INAPPROPRIATE | Date Mar. 6 2011 19:15:08

elgreco

Posts: 247
Joined: Nov. 24 2010
From: San Francisco CA

RE: Interested in science? please re... (in reply to XXX)

Hi Deniz,

I think it is the same reason that you get a better signal from FM than AM. The bigger the wave the easier it is to get through matter. It can find more free space and large parts of it penetrate. Higher frequency means shorter wavelength, hence if it hits an obstacle it gets stuck.

D.

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Captain Esteban: Caballeros! I believe you all know each other?
Don Diego from San Fernando.
Don Francisco from San Jose.
Don Fernando from San Diego.
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REPORT THIS POST AS INAPPROPRIATE | Date Mar. 6 2011 21:40:38

hamia

Posts: 403
Joined: Jun. 25 2004

RE: Interested in science? please re... (in reply to elgreco)

quote:

ORIGINAL: elgreco

Hi Deniz,

I think it is the same reason that you get a better signal from FM than AM. The bigger the wave the easier it is to get through matter. It can find more free space and large parts of it penetrate. Higher frequency means shorter wavelength, hence if it hits an obstacle it gets stuck.

D.

FM=frequency modulation, AM=amplitude modulation. So in AM the amplitude of the signal is being varied (in order to carry the signal information) whereas in FM the frequency is varied. This is why AM is not so good as FM.

Penetration of an acoustic wave through a medium depends on various things. The proportion of energy transmitted/reflected at a boundary is dependent on the acoustic impedance change between one type of material and another (also the angle of incidence). At an interface with a high AI change most of the energy will be reflected and not much will pass through. Eg a sound going from water to air will be almost 100% reflected back due to the high AI contrast between water and air. Same for air to concrete. Materials with similar AI will allow more energy to propagate from one material to the other.

Energy loss (which is separate from the loss due to reflection) depends on the number of cycles / unit distance. So for example, 100 cycles per 100m will lose much more energy than 1 cycle per unit energy. This energy is lost as heat.

REPORT THIS POST AS INAPPROPRIATE | Date Mar. 7 2011 10:00:20

elgreco

Posts: 247
Joined: Nov. 24 2010
From: San Francisco CA

RE: Interested in science? please re... (in reply to hamia)

Thanx Hamia,

You brought back physics flashbacks from my undergrad years of so looong ago...!

:)
Dinos

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Captain Esteban: Caballeros! I believe you all know each other?
Don Diego from San Fernando.
Don Francisco from San Jose.
Don Fernando from San Diego.
Don Jose from San Bernardino.
Luis Obispo from Bakersfield.

REPORT THIS POST AS INAPPROPRIATE | Date Mar. 7 2011 16:59:55