Sorry newbie questions that has probably been asked many times before but search did not throw up anything -
How important is port shape ?
Example - Can I just ensure the port area and length are the same irrespective of the shape - Is a circular port 2" diameter (approx 3.14 sq.in. area) and 8" long the same as a slot port 9"x0.35" and 8" long, i.e. have the same tuned frequency ?
The reason is to keep the box small for a desktop monitor with a front firing port.
How important is port shape ?
Example - Can I just ensure the port area and length are the same irrespective of the shape - Is a circular port 2" diameter (approx 3.14 sq.in. area) and 8" long the same as a slot port 9"x0.35" and 8" long, i.e. have the same tuned frequency ?
The reason is to keep the box small for a desktop monitor with a front firing port.
Yes you can. The port forms part of a Helmholtz resonator. The port can take any shape, assuming any dimension is much less than the half wavelength of the frequency in question (which it will be!).
You have a given mass of air in the port, this is proportional to port volume (area x length), and this mass forms a resonant system in conjunction with the compliance of the air in the enclosure. That's how it works.
A flared circular port is usually favoured because it will be less prone to "chuffing" at high sound levels, but there have been many bass-reflex speakers with rectangular ports, over the years.
You have a given mass of air in the port, this is proportional to port volume (area x length), and this mass forms a resonant system in conjunction with the compliance of the air in the enclosure. That's how it works.
A flared circular port is usually favoured because it will be less prone to "chuffing" at high sound levels, but there have been many bass-reflex speakers with rectangular ports, over the years.
Thanks both. Found this calculator after a bit more searching
Port Length Calculator
I am looking to base a design on the Jim Holtz Statement Monitor 3 way. The above calculator gives the following -
2" circular port with Vol 0.85 cu ft (24l) and Resonant Freq of 32hz - 8.26" - not far from that quoted (area = 3.14 sq in)
Rectangular port with 9" width with the same length (8.2"), volume and resonant frequency give a height of 0.52" (area 4.68 sq in)
I suspect there is something to do with surface area (as well as volume) and boundary layer dynamics in a slotted port rather than the circular i.e. slot has more surface area, more boundary layer and so has effectively less free flow in the pipe therefore bigger slot is required to push the same volume of air. .
So looks like there is a difference but not really significant one in terms of design. I think I can live with a 0.5x9" slot in the bottom front of the cabinet rather than a rear port helping close to wall use. This looks a similar size to that on the Focal Brio6 monitors which are a similar size/volume to the Statement Monitors.
Port Length Calculator
I am looking to base a design on the Jim Holtz Statement Monitor 3 way. The above calculator gives the following -
2" circular port with Vol 0.85 cu ft (24l) and Resonant Freq of 32hz - 8.26" - not far from that quoted (area = 3.14 sq in)
Rectangular port with 9" width with the same length (8.2"), volume and resonant frequency give a height of 0.52" (area 4.68 sq in)
I suspect there is something to do with surface area (as well as volume) and boundary layer dynamics in a slotted port rather than the circular i.e. slot has more surface area, more boundary layer and so has effectively less free flow in the pipe therefore bigger slot is required to push the same volume of air. .
So looks like there is a difference but not really significant one in terms of design. I think I can live with a 0.5x9" slot in the bottom front of the cabinet rather than a rear port helping close to wall use. This looks a similar size to that on the Focal Brio6 monitors which are a similar size/volume to the Statement Monitors.
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