You got me curious about answers also!
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You might check out the North Creek Music website. www.northcreekmusic.com
Here is an exerpt from the Rhythm speaker building manual courtesy of North Creek's Web Site:
2) Cut the straws into 5" lengths. Insert the straws into the end of the tube with one inch sticking out.
The fit should be so tight that the straws begin to go out of round (65 straws for a 2" port, 125 straws for a
3) Wrap the straw bunch sticking out of the port tube with tape. This secures the straws and makes it
easy to adjust the length emerging from the port.
One can adjust the length of straws emerging from the port to adjust the tuning frequency by ear. We
have found the optimum to be between 1" and 2½".
Once the "perfect" tuning is reached, the port can be fixed by dipping the entire assembly in clear shellac
or black paint.
For those that are curious about the origin of this procedure, to the best of my knowledge it was first
discussed in writing by Neville Theile himself. The earliest reference I know of was in an article by Dr.
Theile in an ASA journal from the mid '60's, on non-linear port behavior. Which article and year it was
precisely is now unknown to me. -GS
The method also involves inserting a straw sized wooden dowel into the mass of taped straws, dipping the taped straws and dowel into shellac and letting it dry for a couple days. You can use the inserted dowel (which should be longer than the straws) to tune your port. The dowel, now glued to the straw mass will move the straw in or out of the port effectively increasing or decreasing the length of the port and its output in hz and db. As the port gets longer the tuning reaches lower frequencies but the output in db's drops.
A better way than do resistive damping like putting straws is to go for proven modern aeroport, which has flares in both sides of the lenght of the port . Port noise or chaffing is reduced significantly. A lot of companies have been using these. Notice those curved opening on the ports? exactly like that but on both sides. PNR has the patent to this , i am aware the computation for port length over the conventional one has a slight variation. Check PNR aeroport.
Port noise or "chuffing" comes from increased turbulence within the port. The best solution is to either avoid using a port or to use a port that maintains a more consistent flow velocity and level of turbulence across a wider spl range.
As Goroi mentions and i've discussed in many of my previous posts, the use of a port with radiused ( flares ) on both the inner and outer edges can drastically improve the performance of a conventionally ported speaker. As a general rule, going from a straight port to a port with dual radiuses typically requires additional port length. From what i've seen using identical diameter ports, the "aeroports" ended up being appr 1" longer than the orignal straight port in order to obtain the same tuning. The end result was more more consistent performance from the speaker at low frequencies, regardless of the spl range covered.
While adding straws to a port typically increases low frequency output at reduced spl's, it also reduces maximum port flow too. As such, the tuning of the cabinet actually varies with SPL, which isn't what we want. That is, if one is listening over a wide spl range. If listening at strictly lower levels, adding straws can be somewhat beneficial with the aforementioned caveats being noted. Sean