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.htmlI 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
Sound Intensity
Started by ●August 21, 2007
Reply by ●August 22, 20072007-08-22
Reply by ●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 > > - RandySo if we have say 3dB/km absorption - is this as well as the inverse square losses?
Reply by ●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 ●August 23, 20072007-08-23
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
