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Old 1st January 2007, 11:29 PM   #1
MadMutt is offline MadMutt  Australia
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Default Unequal port length = OK ??

Trying to work out a port system.

If the design/calculations call for 2 ports at say 60 dia x 200 long each, can I change this to a 60 x 100 and a 60 x 300

Or does this affect the final tuning freq ??
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Old 2nd January 2007, 06:34 AM   #2
Volenti is offline Volenti  Australia
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The ports need to be the same length, in your situation you would be tuning the box higher because of the shorter port, the longer port wouldn't really enter the equation much(it get's short circuited by the shorter port).
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Old 2nd January 2007, 01:23 PM   #3
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i guess, if the ports will be the same diameter, their lengths should be treated as resistors in parallel configuration.

so, 2 ports 60*200 are like 1 port 60*100
and, 1 port 60*100 and 1 port 60*300 are like 1 port 60*75
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Old 2nd January 2007, 01:47 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
just to give a third opinion.
2no. 60mm diameter ports 200mm long are equivalent to a single 85mm diameter port 200mm long.

Ports are usually the same length, but can be stagger tuned.

I would expect the stagger to be quite small +-20% giving a range of 160mm to 240mm.
I think each will come into resonance independant of each other but with an enormous overlap, so both will be working but at different levels of output.

Try to find the adjustable versions in your size, and after experimenting then replace them with fixed versions.
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Old 2nd January 2007, 02:29 PM   #5
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From the different dicussions we've had on this forum about ports, I would have to agree with Andrew. I would suspect that you would end up with a broader, flatter summed port output as each port will be working independently. Of course it is not clear what the resulting alignment will be. Here again, as Andrew says, you'll need to experiment.

The question I have is why would you want to use 2 different port lengths? Is there a space issue or are you just wanting to try different ideas?
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Old 2nd January 2007, 11:56 PM   #6
MadMutt is offline MadMutt  Australia
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Thanks for the replies.

Reason I'm asking is that I'm redesigning the sub system in my car. (sedan)
Originally I had a box with 2x8's (26.6Ltr effective) and now I'm trying to recover some space.

So instead of the box w 2x8, I'm building a box that will take over the space on the side where the jack WAS. (That's going in with the spare)

New box is 16Ltr for 1x8", Looks like a dog leg, and I'm trying to get a port with low volume (keeping effective vb high) while keeping port speed down.

Aiming for a tuning of about 42hz.
port options are;
2x 50x410 - a/s 16m/s 1.6ltr
1x 63x 306 - a/s 20 m/s .92ltr

Original box; (37hz - little bit low)
1x 85x400 0 a/s 16.8m/s 2.2ltr

Original had no port noise during music, not that I could tell anyway.
But running a sweep through it there was indeed noise.

So, being a dog leg box, I would have to stagger them if I was to use two.

Thinking I'll just use the 63 and be done with it.
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Old 3rd January 2007, 07:52 PM   #7
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I'm fairly certian using multiple ports of different lengths will still result in one single resonance frequency. The reason for this would be the mass-spring principle that ported enclosures work by. finding the resonance with two unequal ports would probably be more easily done by an impedence plot than using mathematical calculation however.

I would assume a 40liter Vb tuned to 42hz is roughly what you're after....

If you used a 40 liter enclosure with one 60x100mm vent then the tuning would be ~38hz

If you used a 40 liter enclosure with one 60x300mm vent then the tuning would be ~25hz

Both ports would contribute to some of the combined tuning resonance, but the shorter length port would definately put out much more of it than the longer one... You'd probably end up with a tuning around 40hz just guessing and the short port would seem to be doing all of the resonating...

Basically theres no free lunch here... If you want your tuning and no port chuffing then you can either use one larger diameter port or two smaller ones.... with the same cross section, the length of the ports will be very close... So as long as your going to keep port chuffing to a minimum its going to cost you a big port... or a passive radiator

IMO port chuffing at 40hz is probably going to be inaudible... First the sound at 40hz is audible so it will cover over the chuffing noise... Second, the cabin gain from an automotive environment will cover over cuffing even further... The chuffing you heard with a sine sweep was probably chuffing which occured below the box resonance
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Old 4th January 2007, 12:56 AM   #8
MadMutt is offline MadMutt  Australia
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Mutiple ports are now somewhat of a moot point as I've cut and installed a single 63mm one.

Yes the noise I was getting was at well below tuning, and only with a sweep, don't think much of what I listen to will have a ~20hz element.

End volume minus driver/port/bracing is 15.15 ltrs, port is 63x303 which according to isd pro should be 42hz.

I don't believe port noise will be an issue.
Freind has same driver in 15ltr with a 60x200 (length ??) for 47hz.
The amount of thump that generates scares people.
Then he shows them the single 8" and they think he's lying.
But anyway, he gives it a really hard work out and I have never heard port noise from his.
He should be getting around a/s 21
m/s, where mine should be around 20 m/s.
This is at 20hz with a linkwitz function for cabin gain included.

So all being equal, it should be good..

-------------------->>>>

Back to the theory of port length.

I can see both sides, that of it broadening the tuning and that of it making close to zero diff.

Only way I can see of finding out for sure is to do some tests !!!!

After this box is done and installed, I will have a spare 8" and an unused box, so I think when I get time I shall do some tests and find out what happens.
I shall post here to let all know...
Might be a week or two.
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Old 4th January 2007, 01:20 AM   #9
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Ask youself this....

What happens below the tuning frequency in a vented enclosure? If you answered that the port basically becomes a gaping hole in the enclosure and the system acts as if it were unloaded you would be correct....
Therefore a second "lower tuned" port in the enclosure would not re-enforce its resonant frequency because the higher tuned port would leak all of the sound pressure inside the box required to make the "lower tuned" port effective...

So in this instace the shorter port would provide the approximate resonant frequency and the longer one would do litte. Of course when the port lengths/tunings become further apart the effect will become more apparent.... But the moral is: you will only find one helmholtz resonance to a single chamber ported enclosure.
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Old 4th January 2007, 01:28 AM   #10
MadMutt is offline MadMutt  Australia
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Hmmm, yes I definatly need to think about this.
OOooooooo my head hurts.

got too many things happening at the moment.

finish box.
get sub in.
retrim boot.
fix another bose amp thats coming in.
fix luxman cd.
get car fixed (like thats gunna happen).

All while setting up new bussiness.....

list goes on...
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