Hi,
Im unsure where to put this, but have viewed the subwoofer section many times, and know there are lots of smart people on here.
Im trying to calculate SPL attenuation over distance. I understand that for every doubling of distance, you'll have a 6dB SPL loss.
Therefore if i have 88dB @ 1meter, i can calculate what the distance will be when the SPL is reduced to 76dB. Using the attached equation, I am finding "r2".
This is a simple equation that calculates distance at which you have attenuation to a known / desired SPL level.
However, i also want to factor in another 1dB loss for every 1 meter (due to humidity)
This is where my maths skills come to an end.
Would anyone know how to convert the equation, where im still finding "r2", but calculates it with a 6db loss for every doubling of distance and a 1dB loss for every 1 meter of distance.
I can do it by jotting lots of numbers down, but need it in equation form so i can plug it into a spreadsheet, and compare lots of different input variables.
Any help from any mathematicians /Acoustic Engineers woukd be appreciated.
Many thank, Martin
Im unsure where to put this, but have viewed the subwoofer section many times, and know there are lots of smart people on here.
Im trying to calculate SPL attenuation over distance. I understand that for every doubling of distance, you'll have a 6dB SPL loss.
Therefore if i have 88dB @ 1meter, i can calculate what the distance will be when the SPL is reduced to 76dB. Using the attached equation, I am finding "r2".
This is a simple equation that calculates distance at which you have attenuation to a known / desired SPL level.
However, i also want to factor in another 1dB loss for every 1 meter (due to humidity)
This is where my maths skills come to an end.
Would anyone know how to convert the equation, where im still finding "r2", but calculates it with a 6db loss for every doubling of distance and a 1dB loss for every 1 meter of distance.
I can do it by jotting lots of numbers down, but need it in equation form so i can plug it into a spreadsheet, and compare lots of different input variables.
Any help from any mathematicians /Acoustic Engineers woukd be appreciated.
Many thank, Martin
Attachments
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there are also SPL, loss frequency dependence with source, room size, distance that don't follow 1/r^2 spherical or other, like plane wave/transmission line based approximation
particularly in "nearfield" less than a few wavelengths from source or boundaries and with resonances - which is few 10s of feet for subwoofer frequencies
particularly in "nearfield" less than a few wavelengths from source or boundaries and with resonances - which is few 10s of feet for subwoofer frequencies
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Not sure why you think humidity adds 1 db of loss per meter. The loss varies with wavelength. See Calculation method of absorption of sound by atmosphere air damping dissipation absorbtion high frequencies attenuation sound during propagation outdoors outdoor - sengpielaudio Sengpiel Berlin
Thanks. I was using 1dB as an example. Im yet to determine the actual figure for my calcs.
The link you have provided is intersting, but its an air (and humidity) damping calculation only.
Im looking for this included within a distance attenuation calculation, all in one.
so, free space distance attenuation and air damping attenuation is calculated in one formula. Where im looking for "r2".
For example, if i know the SPL is 88dB @ 1meter, i want to calculate the distance (r2) at which the SPL decreases to 76dB.
Really appreciate your response, thanks again
Mart,
Believe me, living in the high desert I'm well aware of needing as much as 20 dB more HF due to air absorption due to low humidity, but humidity has no effect on low frequencies, other than soggy cones frequency and impulse response can change.
At subwoofer frequencies (below 100 Hz), humidity causes only so little change (.001 dB per meter at 100 Hz at 10% RH) that simply using the inverse distance law (6.02 dB per doubling of distance) will suffice.
The frontal area of the subwoofer (array) is needed to calculate at what distance the measurement is out of the near field and will drop according to the inverse distance law, the first measurement distance must be around the same as the radiating surface area diameter to be out of the near field.
Art
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Thanks Art, very interesting. In my particular application, I will take into account the frequency, and the dB attenuation factored in will take frequency into account.
However, my difficulty is not determining the dB loss per meter of distance, it is difficulty in formulating the distance at which the dB drops to a pre-determined level, with the 6dB loss per doubling of distance and an amount of dB loss per meter.
I have attached two equations. The first is the simple 6dB loss of SPL per doubling of Distance. The second is 6dB loss of SPL per doubling of Distance, and dB per meter loss included. The dB number is "f". But now I have the problem of having r2 on both sides of the equation. I'm not sure if it's possible to solve this.
Attachments
Thanks but it doesn't have the equations to consolidate both the doubling of distance attenuation, and attenuation by humidity (dB per meter).
As per the attachment in response to Art above, I have formulated an equation, but have a feeling its impossible to solve, without getting into complex maths.
I can calculate it in tables on a spreadsheet, but I'm looking for one equation to wrap it up in one hit, with varying inputs.
Many thanks
Mart
Hi Mart,
A deep dive into different atmospheric related Losses:
http://www.mne.psu.edu/lamancusa/me458/10_osp.pdf
b🙂
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