Several ports of same diameter, different length. - diyAudio
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Old 17th December 2003, 08:49 AM   #1
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Default Several ports of same diameter, different length.

If we made a single-chamber box with two or more ports of same diameter but different length, would it broaden the tuning like a bandpass effect?

What if we had only one port and put some biggish holes in the side of it like the finger holes on a woodwind instrument?

It's the water, really.
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Old 17th December 2003, 09:25 AM   #2
sreten is offline sreten  United Kingdom
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Its been done, and it doesn't work, or if it does only slightly.

If the ports have an appreciable difference, the longer one becomes
less and less effective, as its 'shorted' by the shorter port.

Below port resonance the shorter port progressively prevents the
box from pressurizing, giving the longer port nothing to work with.

Your woodwind analogy would be applicable to a transmission
line box, its not to the lumped mass of the air in a port.

sreten.
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Old 17th December 2003, 06:37 PM   #3
michael is offline michael  Australia
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i love what ifs, cos chances are that i haave asked myself the same question before, but not been game to ask it here.
i think that sreten is right about the shorter port negating the llonger port, but if the was not much difference i think it would work but even so a port legth needs to be increased ALOT to get lower tuning over the other port and then we have the problem outlined earlier.
This would be really cool if only physics could be pushed in our favour.....damn physics, always giving us the short end of the stick
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Old 17th December 2003, 07:25 PM   #4
sreten is offline sreten  United Kingdom
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If the port tunings are close you do get an effect, a broadening
and a lower port Q, but adding damping to the port / box air
volume has the same effect moreorless.

The reason for this is the large phase change as you cross the
port resonance, even when relatively closely tuned due to to
phase they work against each other giving the damping effect.

sreten.
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Old 17th December 2003, 08:14 PM   #5
Vigier is offline Vigier  Netherlands
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Hi,

I only know one design where this idea has been used:
Klang&Ton MSW 1 aktiv; it's in the Klang&Ton of january 2003.

MSW means "Manger Schall-Wandler". It uses a Manger unit and a GIA 32 woofer (12 inch), in a 81 liter bruto box (airvolume, when you leave the unit and the reflexports out, I'm to lazy to calculate it exactly).
The ports are 46mm diameter each, resp. 228, 278, 328 and 378 mm long.
What's the use of this? Easy: the effect of the port will be less than only one port, but the "band" will larger (in other words, the Q wil be lower).

Grtz, Joris
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Old 17th December 2003, 08:20 PM   #6
sreten is offline sreten  United Kingdom
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Quote:
What's the use of this? Easy: the effect of the port will be less than only one port, but the "band" will larger (in other words, the Q wil be lower).
True, but not for the reasoning you seem to think.

sreten.
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Old 17th December 2003, 08:22 PM   #7
Vigier is offline Vigier  Netherlands
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sreten,

I don't get you sentence... explain?

Grtz, Joris
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Old 17th December 2003, 09:38 PM   #8
sreten is offline sreten  United Kingdom
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You imply spread tuning, simply broadening the effect.

If your not implying this, then sorry for misinterpreting you.

But I'd suggest the longest port, taking up the most volume of
the four is essentially useless. The second probably is as well.

See my other posts.

sreten.
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Old 17th December 2003, 10:21 PM   #9
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Hello,

The driver will see an average of the ports.

Example: You have (1) 3 inch round port 8 inches long and (1) 3 inch port 6 inches long.

The box/driver will respond as if there is (2) 7" ports.

This is my experience using Term-Pro etc.

The only time I could see this being usefull is if your box was a weird shape and you had to compromise port length.

Cheers
Kevinlee
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Old 17th December 2003, 10:58 PM   #10
michael is offline michael  Australia
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Kevin lee:
i do not agree with you here, you say the speaker see's an average length of the ports, by this same reasoning we could say that by adding additional ports of the same length the tuning remains the same, but it would infact raise the tuning as the driver would see larger port diameter, and not longer length.
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