*** 0VU woz 'ere! ***
Hugh Robjohns wrote: I'm open to persuasion here, but empiricle experience suggests to me that the LF energy passing through a simple window can be significant.
Physics here is not a matter of being persuaded, but of acoustic math.
The question is not what's out there or what phenomena happen there, but what's the outside escaped energy ratio versus the energy inside in order to rate the influence on the sound inside.
Ethan Winer wrote:
Ido wrote: How does the window’s “LF energy passing” as you call it, change this?
Any sound that passes through the window to the outside is not reflected back into the room. I don't know any way to state it more simply so you'll understand.
Ido clearly refers to this ratio.
Ido implies that this energy RATIO outside varies from low to high frequencies from about negligible to very negligible.
As such his question: "How does .... change this."
YES IT IS
Ethan Winer wrote:
Ido wrote: The “passing” is directly related to the “absorption”
No it's not! ....
, and in more than one sense.
This are NOT independent properties.
I told how TL works here.
http://www.soundonsound.com/forum/showf ... rt=1#67207
And TL is not only defined by mass, but VERY dependent on internal damping. And internal damping = dissipation of energy which equals absorption or transformation into heat.
The sound does not go through the object but brings it in vibration, making it a source in itself.
And this vibration is either dissipated in the single window pane and/or connected structure or radiated in both directions (in + out).
The strength and physical properties of this vibration is what causes the single window pane to act as a low quality absorber. And this vibration of this window pane is very related with this internal damping
Ido gave a careful correct summary.
Playing with words shouldn't substitute the core of things.
And this core is:
Loud outside noise should substantiate the statement that most or lots of the bass should have left the room and therefore prevent or strongly diminish internal low frequency problems.
THIS STATEMENT IS WRONG, PLAIN WRONG, in function of normal TL values to be found for standard building applications.
True is that low TL panels as drywall in double leaf systems will increase absorption in the room, increasingly towards lower frequencies, but limited by the MSM (mass-spring-mass) resonance were this phenomenon reverses again. In fact such a wall is a huge panel trap (more or less damped depending on construction).
The absorption difference with a concrete room is enhanced by the huge quantity (read surface) of such an application.
In itself it are low efficiency absorbers, it's the quantity compensating for that.
But even then the advice of diminishing insulation by using 1 layer instead of 2 to diminish low frequency problems is plain dangerous, when not executed very controlled.
This relates to the MSM (mass-spring-mass) resonance of the double leaf system.
Below this MSM the TL of the wall increases again towards the lower frequencies (read diminishing absorption) and such an application makes a more or less damped panel trap/resonator of the whole room, which can be tuned to a not desirable frequency or tuned to a useful disturbing frequency, but simultaneously creating an acoustic leak at this frequency caused by the amplification in this MSM range.
This kind of headliner acoustics
, without knowing or understanding the consequences or basic underlying concepts are dangerous and can lead to wrong applications by people blindly trusting such [incomplete and over-generalised] (edited 0VU) advice.
They almost guaranteed have trouble with too low insulation, this MSM resonance can really be disturbing (+ increases in frequency with lowering panel mass) while the internal acoustics can as well be influenced in a questionable and uncontrolled manner.