quote:Originally posted by Death By Bass: but you said without using the formula...
me confused
I mean how did the scientists come up with that formula for calculating the tuning of a helmholtz resonator/resonance?
The first person to see a working system (sub in box, etc) probably had no idea what the tuning was. How did he firgure it out, and how did he come up with the Fb formula?
And is it any relation to the reactive impeadance of the subwoofer system?
I have about 2.1cubic foot enclosure with 2 4" PNR's 12" from flange to flange. Box proggies and the formula come out to about 49-50hz tuning. This box is also...kind of....wierd, hehe. The reactive impeadance spikes at 55hz at 12.2ohm vs the dcr of 2ohms
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posted
interesting, im not getting a double bumped VAC nor reactive impeadance. Instead I'm getting something that looks like a bell curve whose peak is at 59hz and my reactive impeadance is around 12ohms from 1.4 dcr...
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posted
I'll skip the derivation of the following equation, but it comes from LC-resonator resonant frequency:
Port tuning frequency: fb = c/(2*pi)*sqrt(Sv/(lt*Vb)), where c is speed of sound (m/s), pi = ratio of circle circumference to diameter (about 3.142), Sv = port area (m^2), Vb = effective Box Volume (m^3), lt = acoustic length of the port (m) (you must add the end corrections).
Regards, Janne
-------------------- For box/port calculations, try these:
quote:Originally posted by Bumpin' Yota: interesting, im not getting a double bumped VAC nor reactive impeadance. Instead I'm getting something that looks like a bell curve whose peak is at 59hz and my reactive impeadance is around 12ohms from 1.4 dcr...
Either your measuring wrong,or your ported box isnt ported
posted
radiator excursion is at minimum per watt input, so you are looking for the highest current/voltage ratio (lowest measured Z) between the two box Z peaks.
ShadowStar
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quote:Originally posted by Bumpin' Yota: interesting, im not getting a double bumped VAC nor reactive impeadance. Instead I'm getting something that looks like a bell curve whose peak is at 59hz and my reactive impeadance is around 12ohms from 1.4 dcr...
Either your measuring wrong,or your ported box isnt ported
I measured in 2hz incriments from 35hz to 75hz...could the two humps have been overlooked? Are they not that pronounced?
The input vac to the subs remained relatively constant at 5vac with only the current changing with the freq played. All done at low volume on my spl setup.
quote:Originally posted by ShadowStar: radiator excursion is at minimum per watt input, so you are looking for the highest current/voltage ratio (lowest measured Z) between the two box Z peaks.
ShadowStar
So that would be where the current is the highest and the voltage is, in my case, constant.
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posted
the impedance will have nothing to do with the box response or the port tuning, (to the box) it is effectively nothing more than a piston in an engine and it's effective displacement per stroke does not care that it's made of aluminum or cast iron or a hypereutectic alloy by keith black. but for enclosure info, ask finlands' Mikko. his screename is splwarrior on termpro. get ready to write down 14 pages of info and calculation material and spend the subsequent following 14 days studying it. good luck!
Posts: 320 | From: all over the place | Registered: Oct 2001
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quote:Originally posted by Truth: the impedance will have nothing to do with the box response or the port tuning, (to the box) it is effectively nothing more than a piston in an engine and it's effective displacement per stroke does not care that it's made of aluminum or cast iron or a hypereutectic alloy by keith black. but for enclosure info, ask finlands' Mikko. his screename is splwarrior on termpro. get ready to write down 14 pages of info and calculation material and spend the subsequent following 14 days studying it. good luck!
The impedence does reveal the port tuning but not the frequency response.
No need for 14pages of derivation or acoustic theory.
There will be a double hump Z plot.You just must be making an error.
posted
everything changes when it's stuck in a given volume (vehicle) or within a specific proximity to the neares surface, this is how i meant that your computed impedance cannot determine the actual tuning of the box since it is completely determined by the vehicle it resides in. but for this guys basic question, he needs to know how to figure it out BEFORE the vehicle is introduced, that is where i pointed out that the tuning frequency cannot be determined by impedance until it's in it's final resting place. hopefully obviously not stating that a Z measurement does not indicate Fb. my simple recommendation is most definitely to chat with Mikko, he is bound to learn a load.
Posts: 320 | From: all over the place | Registered: Oct 2001
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quote:Originally posted by Bumpin' Yota: interesting, im not getting a double bumped VAC nor reactive impeadance. Instead I'm getting something that looks like a bell curve whose peak is at 59hz and my reactive impeadance is around 12ohms from 1.4 dcr...
Either your measuring wrong,or your ported box isnt ported
I measured in 2hz incriments from 35hz to 75hz...could the two humps have been overlooked? Are they not that pronounced?
The input vac to the subs remained relatively constant at 5vac with only the current changing with the freq played. All done at low volume on my spl setup.
quote:Originally posted by ShadowStar: radiator excursion is at minimum per watt input, so you are looking for the highest current/voltage ratio (lowest measured Z) between the two box Z peaks.
ShadowStar
So that would be where the current is the highest and the voltage is, in my case, constant.
The only possibility I see is if you get a plateau style reading, in which case your two Z peaks are unrealistically close together. I would expect at least a half octave of separation between even the closest peaks of the ported Z plot. Try measuring again?
ShadowStar
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posted
IMO Steve has tweaked his spl enclosure to get it as loud as possible. (peaky) In spl apps I would imagine you would want the "bumps" to be as close as possible and the overall best scenario for spl would be to have them at the same frequency, if possible. In Steve's case his first peak could be at 58.5hz and the second at 59.5hz and seeing as how our test discs only play in 1hz increments it appears as though he only has one peak.
-------------------- Scottie Johnson Sound Pressure Technologies
posted
I feel that you should go back and test again. This time don't skip any frequencies, becuase your peak could be brief and very sharp. Also are you trying to find the peak of your box relative to your application or just the tuning of the box it's self? In your application the reactive load will tell you in general where your box is tuned. The voltage should remain the same. When your reactive load is at it's highest you are basically at your tuning. If you want to get exact you will have to use the formula.
posted
Normal tunings IE for flat response give the mentioned double bump impedence response.
With large box volumes and high port tunings,the result can be one peak down low,and only a hint of the upper peak. In one case in the program,with 600L,75hz tuning and one S15L74,the upper peak occurs at 75hz-the port tuning!
quote:Originally posted by Bumpin' Yota: ....without using the formula or a box program?
The only other way I know, without using any formulaes, or programs, is to speak into the box..,add insulation..,try again.. Try it out, let us know what happens.
-------------------- E.C. Wuz here Posts: 1057 | From: Vancouver, British Columbia, Canada 'eh | Registered: May 1999
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