On Aug 23, 5:47 am, HardySpicer <gyansor...@gmail.com> wrote:
> On Aug 23, 1:55 am, Randy Poe <poespam-t...@yahoo.com> wrote:
>
>
>
> > On Aug 21, 10:57 pm, Sam Wormley <sworml...@mchsi.com> wrote:
>
> > > HardySpicer wrote:
> > > > On Aug 22, 11:51 am, Sam Wormley <sworml...@mchsi.com> wrote:
> > > >> HardySpicer wrote:
> > > >>> We are often told that sound intensity (I assume Power) goes down as
> > > >>> the inverse square of distance. However, I believe this is also
> > > >>> frequency dependent (as with e/m waves). What is the equation for a
> > > >>> sound source received at a distance d with frequency f say? I read
> > > >>> somewhere that low frequency sound (say 50 Hz or so) will travel vast
> > > >>> distances and is also humidity and temperature dependent. It mus be
> > > >>> something like
> > > >>> I=I0.exp(-alpha.d)
> > > >>> where I0 and I are the initial and final intensities, d is distance
> > > >>> and alpha is freq dependent. How is alpha found?
> > > >>> Thanks
> > > >>> Hardy
> > > >> Inverse Square Law, General
> > > >> http://hyperphysics.phy-astr.gsu.edu/hbase/forces/isq.html
>
> > > >> "Being strictly geometric in its origin, the inverse square law
> > > >> applies to diverse phenomena. Point sources of gravitational force,
> > > >> electric field, light, sound or radiation obey the inverse square
> > > >> law.
>
> > > >> Attenuation (scattering and absorption) may be factors in light,
> > > >> sound or radiation.
> > > >> http://en.wikipedia.org/wiki/Attenuation
>
> > > > Yes thanks, I saw that web page when I searched but it looked like a
> > > > basic Physics page - it had no info on frequency. If you look at it it
> > > > appears as if low and high frequencies behave the same - they don't.
> > > > eg microwaves may well follow an inverse square law but they won't
> > > > travel as far as an 100MHz signal.
>
> > > In space there is no frequency dependence. The pattern of radiations is
> > > the same until you start to take into consideration such things as
> > > absorption, scattering, refraction, reflection and wave-guide effects.
> > > Look what astronomers have to deal with for various frequencies (wavelengths)
> > > of electromagnetic radiation.
> > > http://www.mhhe.com/physsci/astronomy/fix/student/chapter6/06f28.html
>
> > I think absorption, i.e. frequency-dependent attenuation, may be what
> > OP is
> > referring to.
>
> > Using this calculator:http://www.csgnetwork.com/atmossndabsorbcalc.html
>
> > I get 0.078 db/km at 50 Hz, 2.73 db/km at 500 Hz, and
> > 44 dB/km at 5000 Hz. Very strong dependence on frequency.
> > (all other parameters set at default values).
>
> > Here's a table for 1-100 kHz:http://www.kayelaby.npl.co.uk/general_physics/2_4/2_4_1.html
>
> So if we have say 3dB/km absorption - is this as well as the inverse
> square losses?
Yes, it's an additional loss term. If you're doing things in dB, it's
just another item to subtract.
- Randy
Reply by Jerry Avins●August 23, 20072007-08-23
HardySpicer wrote:
...
> So if we have say 3dB/km absorption - is this as well as the inverse
> square losses?
The inverse-square law isn't related to absorption. In fact, it applies
strictly in the absence of absorption (and then only in the far field).
Imagine energy being radiated into some solid angle. Since the angle
doesn't matter, imagine an isotropic radiator, so that the intensity is
equal all over. The energy spreads in space as the distance from the
source increases, always lying on the surface of a sphere of radius R.
since the same amount of energy lies on every such surface, the
intensity (energy/unit area) decreases inversely with the area. Area is
proportional to distance squared.
Attenuation with distance is inverse square, as I've shown. Attenuation
due to uniform absorption is decaying exponential, quite different.
Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by HardySpicer●August 23, 20072007-08-23
On Aug 23, 1:55 am, Randy Poe <poespam-t...@yahoo.com> wrote:
> On Aug 21, 10:57 pm, Sam Wormley <sworml...@mchsi.com> wrote:
>
>
>
> > HardySpicer wrote:
> > > On Aug 22, 11:51 am, Sam Wormley <sworml...@mchsi.com> wrote:
> > >> HardySpicer wrote:
> > >>> We are often told that sound intensity (I assume Power) goes down as
> > >>> the inverse square of distance. However, I believe this is also
> > >>> frequency dependent (as with e/m waves). What is the equation for a
> > >>> sound source received at a distance d with frequency f say? I read
> > >>> somewhere that low frequency sound (say 50 Hz or so) will travel vast
> > >>> distances and is also humidity and temperature dependent. It mus be
> > >>> something like
> > >>> I=I0.exp(-alpha.d)
> > >>> where I0 and I are the initial and final intensities, d is distance
> > >>> and alpha is freq dependent. How is alpha found?
> > >>> Thanks
> > >>> Hardy
> > >> Inverse Square Law, General
> > >> http://hyperphysics.phy-astr.gsu.edu/hbase/forces/isq.html
>
> > >> "Being strictly geometric in its origin, the inverse square law
> > >> applies to diverse phenomena. Point sources of gravitational force,
> > >> electric field, light, sound or radiation obey the inverse square
> > >> law.
>
> > >> Attenuation (scattering and absorption) may be factors in light,
> > >> sound or radiation.
> > >> http://en.wikipedia.org/wiki/Attenuation
>
> > > Yes thanks, I saw that web page when I searched but it looked like a
> > > basic Physics page - it had no info on frequency. If you look at it it
> > > appears as if low and high frequencies behave the same - they don't.
> > > eg microwaves may well follow an inverse square law but they won't
> > > travel as far as an 100MHz signal.
>
> > In space there is no frequency dependence. The pattern of radiations is
> > the same until you start to take into consideration such things as
> > absorption, scattering, refraction, reflection and wave-guide effects.
> > Look what astronomers have to deal with for various frequencies (wavelengths)
> > of electromagnetic radiation.
> > http://www.mhhe.com/physsci/astronomy/fix/student/chapter6/06f28.html
>
> I think absorption, i.e. frequency-dependent attenuation, may be what
> OP is
> referring to.
>
> Using this calculator:http://www.csgnetwork.com/atmossndabsorbcalc.html
>
> I get 0.078 db/km at 50 Hz, 2.73 db/km at 500 Hz, and
> 44 dB/km at 5000 Hz. Very strong dependence on frequency.
> (all other parameters set at default values).
>
> Here's a table for 1-100 kHz:http://www.kayelaby.npl.co.uk/general_physics/2_4/2_4_1.html
>
> - Randy
So if we have say 3dB/km absorption - is this as well as the inverse
square losses?
Reply by Randy Poe●August 22, 20072007-08-22
On Aug 21, 10:57 pm, Sam Wormley <sworml...@mchsi.com> wrote:
> HardySpicer wrote:
> > On Aug 22, 11:51 am, Sam Wormley <sworml...@mchsi.com> wrote:
> >> HardySpicer wrote:
> >>> We are often told that sound intensity (I assume Power) goes down as
> >>> the inverse square of distance. However, I believe this is also
> >>> frequency dependent (as with e/m waves). What is the equation for a
> >>> sound source received at a distance d with frequency f say? I read
> >>> somewhere that low frequency sound (say 50 Hz or so) will travel vast
> >>> distances and is also humidity and temperature dependent. It mus be
> >>> something like
> >>> I=I0.exp(-alpha.d)
> >>> where I0 and I are the initial and final intensities, d is distance
> >>> and alpha is freq dependent. How is alpha found?
> >>> Thanks
> >>> Hardy
> >> Inverse Square Law, General
> >> http://hyperphysics.phy-astr.gsu.edu/hbase/forces/isq.html
>
> >> "Being strictly geometric in its origin, the inverse square law
> >> applies to diverse phenomena. Point sources of gravitational force,
> >> electric field, light, sound or radiation obey the inverse square
> >> law.
>
> >> Attenuation (scattering and absorption) may be factors in light,
> >> sound or radiation.
> >> http://en.wikipedia.org/wiki/Attenuation
>
> > Yes thanks, I saw that web page when I searched but it looked like a
> > basic Physics page - it had no info on frequency. If you look at it it
> > appears as if low and high frequencies behave the same - they don't.
> > eg microwaves may well follow an inverse square law but they won't
> > travel as far as an 100MHz signal.
>
> In space there is no frequency dependence. The pattern of radiations is
> the same until you start to take into consideration such things as
> absorption, scattering, refraction, reflection and wave-guide effects.
> Look what astronomers have to deal with for various frequencies (wavelengths)
> of electromagnetic radiation.
> http://www.mhhe.com/physsci/astronomy/fix/student/chapter6/06f28.html
> We are often told that sound intensity (I assume Power) goes down as
> the inverse square of distance. However, I believe this is also
> frequency dependent (as with e/m waves). What is the equation for a
> sound source received at a distance d with frequency f say? I read
> somewhere that low frequency sound (say 50 Hz or so) will travel vast
> distances and is also humidity and temperature dependent. It mus be
> something like
>
> I=I0.exp(-alpha.d)
>
> where I0 and I are the initial and final intensities, d is distance
> and alpha is freq dependent. How is alpha found?
The equation you have is for weakly-scattering situations. The
attentuation 'alpha' (in optics the equation is Beer's law and alpha is
the optical density) is generally found by direct measurement.
--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University
Reply by Rune Allnor●August 22, 20072007-08-22
On 21 Aug, 23:57, HardySpicer <gyansor...@gmail.com> wrote:
> We are often told that sound intensity (I assume Power) goes down as
> the inverse square of distance. However, I believe this is also
> frequency dependent (as with e/m waves). What is the equation for a
> sound source received at a distance d with frequency f say? I read
> somewhere that low frequency sound (say 50 Hz or so) will travel vast
> distances and is also humidity and temperature dependent. It mus be
> something like
>
> I=I0.exp(-alpha.d)
>
> where I0 and I are the initial and final intensities, d is distance
> and alpha is freq dependent. How is alpha found?
You aren't completely right but you aren't completely wrong
either.
Acoustics theory separate the attenuation according to
the main cause. Purely geometric factors are modeled
as a 1/R^2 term for spherical attenuation. But that
does not account for internal friction to the material.
The internal friction of the material shows up as a
exp(alpha*R/lambda) where lambda is the wavelength.
The friction term is frequency dependent in that
lambda varies with frequancey, but also because
different attenuation mechanisms dominate at
different scales.
Rune
Reply by Jerry Avins●August 22, 20072007-08-22
HardySpicer wrote:
> On Aug 22, 11:51 am, Sam Wormley <sworml...@mchsi.com> wrote:
>> HardySpicer wrote:
>>> We are often told that sound intensity (I assume Power) goes down as
>>> the inverse square of distance. However, I believe this is also
>>> frequency dependent (as with e/m waves). What is the equation for a
>>> sound source received at a distance d with frequency f say? I read
>>> somewhere that low frequency sound (say 50 Hz or so) will travel vast
>>> distances and is also humidity and temperature dependent. It mus be
>>> something like
>>> I=I0.exp(-alpha.d)
>>> where I0 and I are the initial and final intensities, d is distance
>>> and alpha is freq dependent. How is alpha found?
>>> Thanks
>>> Hardy
>> Inverse Square Law, General
>> http://hyperphysics.phy-astr.gsu.edu/hbase/forces/isq.html
>>
>> "Being strictly geometric in its origin, the inverse square law
>> applies to diverse phenomena. Point sources of gravitational force,
>> electric field, light, sound or radiation obey the inverse square
>> law.
>>
>> Attenuation (scattering and absorption) may be factors in light,
>> sound or radiation.
>> http://en.wikipedia.org/wiki/Attenuation
>
> Yes thanks, I saw that web page when I searched but it looked like a
> basic Physics page - it had no info on frequency. If you look at it it
> appears as if low and high frequencies behave the same - they don't.
> eg microwaves may well follow an inverse square law but they won't
> travel as far as an 100MHz signal.
Search for "ionospheric propagation", "e layer", and "skip" to see why.
Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by Vladimir Vassilevsky●August 22, 20072007-08-22
"HardySpicer" <gyansorova@gmail.com> wrote in message
news:1187733452.719370.65750@l22g2000prc.googlegroups.com...
> We are often told that sound intensity (I assume Power) goes down as
> the inverse square of distance. However, I believe this is also
> frequency dependent (as with e/m waves). What is the equation for a
> sound source received at a distance d with frequency f say? I read
> somewhere that low frequency sound (say 50 Hz or so) will travel vast
> distances and is also humidity and temperature dependent. It mus be
> something like
>
> I=I0.exp(-alpha.d)
>
> where I0 and I are the initial and final intensities, d is distance
> and alpha is freq dependent. How is alpha found?
For the small intensities, the propagation can be approximated as the
inverse square law times exp(-distance/decay_const). The decay_const can be
found from the thermodynamic considerations; it is at the order of 10m at
1kHz in the dry air at normal conditions. The decay_const is inverse
proportional to the square root of the frequency.
It is a lot more complicated for the high intensities when there is a
noticeable macroscopic movement of the air. The nonlinear effects come into
play.
Vladimir Vassilevsky
DSP and Mixed Signal Consultant
www.abvolt.com
Reply by Sam Wormley●August 21, 20072007-08-21
HardySpicer wrote:
> On Aug 22, 11:51 am, Sam Wormley <sworml...@mchsi.com> wrote:
>> HardySpicer wrote:
>>> We are often told that sound intensity (I assume Power) goes down as
>>> the inverse square of distance. However, I believe this is also
>>> frequency dependent (as with e/m waves). What is the equation for a
>>> sound source received at a distance d with frequency f say? I read
>>> somewhere that low frequency sound (say 50 Hz or so) will travel vast
>>> distances and is also humidity and temperature dependent. It mus be
>>> something like
>>> I=I0.exp(-alpha.d)
>>> where I0 and I are the initial and final intensities, d is distance
>>> and alpha is freq dependent. How is alpha found?
>>> Thanks
>>> Hardy
>> Inverse Square Law, General
>> http://hyperphysics.phy-astr.gsu.edu/hbase/forces/isq.html
>>
>> "Being strictly geometric in its origin, the inverse square law
>> applies to diverse phenomena. Point sources of gravitational force,
>> electric field, light, sound or radiation obey the inverse square
>> law.
>>
>> Attenuation (scattering and absorption) may be factors in light,
>> sound or radiation.
>> http://en.wikipedia.org/wiki/Attenuation
>
> Yes thanks, I saw that web page when I searched but it looked like a
> basic Physics page - it had no info on frequency. If you look at it it
> appears as if low and high frequencies behave the same - they don't.
> eg microwaves may well follow an inverse square law but they won't
> travel as far as an 100MHz signal.
>
In space there is no frequency dependence. The pattern of radiations is
the same until you start to take into consideration such things as
absorption, scattering, refraction, reflection and wave-guide effects.
Look what astronomers have to deal with for various frequencies (wavelengths)
of electromagnetic radiation.
http://www.mhhe.com/physsci/astronomy/fix/student/chapter6/06f28.html
Reply by HardySpicer●August 21, 20072007-08-21
On Aug 22, 11:30 am, Jerry Avins <j...@ieee.org> wrote:
> HardySpicer wrote:
> > We are often told that sound intensity (I assume Power) goes down as
> > the inverse square of distance. However, I believe this is also
> > frequency dependent (as with e/m waves). What is the equation for a
> > sound source received at a distance d with frequency f say? I read
> > somewhere that low frequency sound (say 50 Hz or so) will travel vast
> > distances and is also humidity and temperature dependent. It mus be
> > something like
>
> > I=3DI0.exp(-alpha.d)
>
> > where I0 and I are the initial and final intensities, d is distance
> > and alpha is freq dependent. How is alpha found?
>
> The attenuation of sound with distance *in the earth's atmosphere*
> depends on more than the free-space propagation. Refraction and
> diffraction play a large part. Low frequencies that originate at the
> surface tend to hug the surface. Their long wavelength make the terrain
> acoustically smooth, while at higher frequencies, surface irregularities
> scatter more effectively.
>
> Refraction is also important. One can often see lightning over a flat
> surface like a large lake or extended grassland, yet hear no thunder.
> The thunder is there, but refraction due to different air density as a
> function of height bends the sound so that it passes overhead. An
> observer on a tower or rooftop can hear thunder from much further away.
>
> Jerry
> --
> Engineering is the art of making what you want from things you can get.
> =AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=
=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=
=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF
This is the paper I read the info from - the non-labeled equation
after equation 1 (not eq 2!)
http://www.sandv.com/downloads/0410vino.pdf