Several ports of same diameter, different length.

Circlotron · 17th December 2003, 08:49 AM

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.

sreten · 17th December 2003, 09:25 AM

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.

michael · 17th December 2003, 06:37 PM

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

sreten · 17th December 2003, 07:25 PM

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.

Vigier · 17th December 2003, 08:14 PM

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

sreten · 17th December 2003, 08:20 PM

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.

Vigier · 17th December 2003, 08:22 PM

sreten,

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

Grtz, Joris

sreten · 17th December 2003, 09:38 PM

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.

KevinLee · 17th December 2003, 10:21 PM

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

michael · 17th December 2003, 10:58 PM

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.

17th December 2003, 08:49 AM	#1
Circlotron diyAudio Member Join Date: Jun 2002 Location: Melbourne, Australia	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. __________________ Best-ever T/S parameter spreadsheet. http://www.diyaudio.com/forums/multi...tml#post353269

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17th December 2003, 09:25 AM	#2
sreten diyAudio Member Join Date: Nov 2003 Location: Brighton UK	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.

17th December 2003, 06:37 PM	#3
michael diyAudio Member Join Date: Sep 2003 Location: Adelaide Hills	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

17th December 2003, 07:25 PM	#4
sreten diyAudio Member Join Date: Nov 2003 Location: Brighton UK	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.

17th December 2003, 08:14 PM	#5
Vigier diyAudio Member Join Date: Nov 2002 Location: Netherlands	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

17th December 2003, 08:22 PM	#7
Vigier diyAudio Member Join Date: Nov 2002 Location: Netherlands	sreten, I don't get you sentence... explain? Grtz, Joris

17th December 2003, 09:38 PM	#8
sreten diyAudio Member Join Date: Nov 2003 Location: Brighton UK	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.

17th December 2003, 10:21 PM	#9
KevinLee diyAudio Member Join Date: Oct 2002 Location: Winnipeg MB Canada	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

17th December 2003, 10:58 PM	#10
michael diyAudio Member Join Date: Sep 2003 Location: Adelaide Hills	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.