|3rd June 2012, 08:30 PM||#12|
diyAudio Member RIP
Join Date: Nov 2003
Location: Brighton UK
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.
Last edited by sreten; 3rd June 2012 at 08:49 PM.
|3rd June 2012, 08:58 PM||#13|
diyAudio Moderator R.I.P.
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
|4th June 2012, 12:10 AM||#14|
methods. The math can for example to be found here:
* 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.
Last edited by bjorno; 4th June 2012 at 12:20 AM.
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