Question about using 2 ports of unequal length. Why?

Brisso57 · 18th August 2008, 11:31 AM

I rescued a pair of Pioneer SCS13 2-ways from a footpath (/sidewalk /nature strip).
8" W + 2" T.
You can see in the attached photo that they have two ports of equal diameter, but unequal length.
Intriguing. Why did they do this?

Any ideas?

cheers

Doug

richie00boy · 18th August 2008, 11:39 AM

It's "supposed" to give two tuning frequencies to offer broader support to the driver, but I've read that it doesn't work that well. It would be fairly easy to tell by some manual visual sweeps and impedance curves.

sreten · 18th August 2008, 12:06 PM

Hi,

It does not work "well" at all. Close off the shorter port.

/sreten.

Brisso57 · 18th August 2008, 12:12 PM

Quote:

Originally posted by sreten
Hi,

It does not work "well" at all. Close off the shorter port.

/sreten.

That's easy enough to try. I'll have a go at it this weekend.

Cop ya later!

Doug

AndrewJ · 18th August 2008, 05:19 PM

It doesn't work full stop. The effect of two vents is to just combine the two vent masses into one single mass, and give one tuning frequency.
However if you block off the shorter vent you will change the overall vent tuning frequency. Whether you prefer the result is up to you, but you will have made a change to the sound.

Andrew

sreten · 18th August 2008, 05:58 PM

Quote:

Originally posted by AndrewJ

It doesn't work full stop.
The effect of two vents is to just combine the two vent masses into one single mass, and give one tuning frequency.

However if you block off the shorter vent you will change the overall vent tuning frequency.

Whether you prefer the result is up to you, but you will have made a change to the sound.

Andrew

Hi,

a) No. it does not work like that. The short one has more effect.

b) True. The asssumption being the lowered frequency still works.
Blocking off the longer port will not make much difference.

c) True. but b) should go deeper with less boom.

/sreten.

thoriated · 18th August 2008, 06:55 PM

I wonder what the effect would be on the BR impedance peaks with two ports of different lengths. Would the traditional two peaks broaden or even separate into more peaks?

Also, what if the 'shorter' port (tuned higher) also was of a slightly different diameter? Could this be used to counteract the tendency to favor that port as far as tuning goes?

Brisso57 · 18th August 2008, 09:01 PM

Quote:

Originally posted by thoriated
I wonder what the effect would be on the BR impedance peaks with two ports of different lengths. Would the traditional two peaks broaden or even separate into more peaks?

Also, what if the 'shorter' port (tuned higher) also was of a slightly different diameter? Could this be used to counteract the tendency to favor that port as far as tuning goes?

Visually they appear to be identical diameter, but I will measure them and report back.

Doug

crazyhub · 18th August 2008, 09:35 PM

Quote:

Originally posted by Brisso57
two ports of equal diameter, but unequal length.
Intriguing. Why did they do this?

Any ideas?

cheers

Doug

maybe lowering port self resonance?

crazyhub · 20th August 2008, 01:57 PM

Quote:

Originally posted by crazyhub
maybe lowering port self resonance?

does somebody agree with my statement?

18th August 2008, 11:39 AM	#2
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	It's "supposed" to give two tuning frequencies to offer broader support to the driver, but I've read that it doesn't work that well. It would be fairly easy to tell by some manual visual sweeps and impedance curves. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

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18th August 2008, 12:06 PM	#3
sreten diyAudio Member Join Date: Nov 2003 Location: Brighton UK	Hi, It does not work "well" at all. Close off the shorter port. /sreten.

18th August 2008, 05:19 PM	#5
AndrewJ diyAudio Member Join Date: Sep 2001	It doesn't work full stop. The effect of two vents is to just combine the two vent masses into one single mass, and give one tuning frequency. However if you block off the shorter vent you will change the overall vent tuning frequency. Whether you prefer the result is up to you, but you will have made a change to the sound. Andrew

18th August 2008, 06:55 PM	#7
thoriated diyAudio Member Join Date: Nov 2003 Location: flyover country	I wonder what the effect would be on the BR impedance peaks with two ports of different lengths. Would the traditional two peaks broaden or even separate into more peaks? Also, what if the 'shorter' port (tuned higher) also was of a slightly different diameter? Could this be used to counteract the tendency to favor that port as far as tuning goes?

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