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Old 3rd June 2012, 09:15 PM   #11
bjorno is offline bjorno  Sweden
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FYI:

The difference of using a 14 cm long port versus a 7 cm, both damped proportionally.

b
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Old 3rd June 2012, 09:30 PM   #12
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by bjorno View Post
FYI:

The difference of using a 14 cm long port versus a 7 cm, both damped proportionally.

b
Hi,

Your damping criteria is very obscure and hard to relate to practice.
Whatever number your playing with there seems no related practise.
I can't see the system the two cases are applicable to.

Lining a 70mm port with 6mm foam will reduce nominal diameter
to about 60mm and damp port modes. I can't see how you can
model this, I do it and it works well quite often. 1 cm foam in a
70mm port ~ detunes by half an octave, but can't be modeled
well as far as I know, its generally suck it and see, or measure.

rgds, sreten.
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Last edited by sreten; 3rd June 2012 at 09:49 PM.
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Old 3rd June 2012, 09:58 PM   #13
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by sreten View Post
reducing its length
increases the tuning frequency and is generally a very bad idea.
you say generally a bad idea ?

what is your reason for saying that ?

I have often found a shorter port to sound better
cant say the same about making it longer

but Im only talking about very smaller changes

putting foam in a port is way too unpredictatble
unless its hard foam
but sometimes light stuffing with acoustic material works well
but then its really not ported any longer

but actually I have had positive result with placing smaller pieces in some areas of the port
I have no idea what happens, but it works well, sometimes
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Old 4th June 2012, 01:10 AM   #14
bjorno is offline bjorno  Sweden
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[QUOTE=sreten;3047122]Hi,

Quote:
Your damping criteria is very obscure and hard to relate to practice.
Whatever number your playing with there seems no related practise.
I can't see the system the two cases are applicable to.
Obscure See*

Quote:
Lining a 70mm port with 6mm foam will reduce nominal diameter
to about 60mm and damp port modes.
What Port modes?. I see only quarter-wave modes related to the entire length of the airways where the port contribution is minimal.

Quote:
I can't see how you can
model this,
It can easily be modeled by using suitable acoustical engineering
methods. The math can for example to be found here:

http://grputland.com/files/thes.pdf


Quote:
I do it and it works well quite often. 1 cm foam in a
70mm port ~ detunes by half an octave, but can't be modeled
well as far as I know, its generally suck it and see, or measure.
I have access to the appropriate tools for this but showing these here is way above my commitment to help DIY:ers.


* Rather the opposite, I submitted all the numbers needed for an successful damped box/damped port design, here is more:
Select (dedicated for speaker use) a lump of modern ordinary fibrous damping material with the given weight of~ 1gram for a 7 cm long port.

Compress or tear the fibers apart in order to fill the entire volume of the port.
By doing so: This would correspond to a ~ density of ~0.2 kg/m^3.
Secure both sides by using an insect screen or similar.

The fiber tangle will stay and act as a flow resistance much like the 'Scan-Speak Flow Resistance Vent'.

The wider the Port diameter is compared to the port length, the better the linearity is expected to be for this arrangement.14 cm===> 7 cm is a good way to start with.

The airflow in the port is IMO/IME very small and already as a precondition, (pre-)filtered through the entire enclosure= a fill of denser stuffing in the box, thus only a near laminar flow is expected in the port.
Look at the magnitude of the port air velocity.

Considering this case(design): Your suggestion using port wall drag coefficients is IMO only a first step to consider if much higher port velocity exists and if the box were only lined at the walls for defeating internal reflexes.

b

Last edited by bjorno; 4th June 2012 at 01:20 AM.
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