[Les Cargill[_4_]]

(Don Pearce) wrote:
On Sat, 17 Jan 2015 14:18:36 -0600, Les Cargill
wrote:


Suppose you are designing a ported speaker enclosure. Suppose this
design wants two 2 inch ports @ 0.80 inches.

Suppose also you are using 3/4" plywood.

Must the port extend 0.80 into the internal volume of the speaker box
or would the slightly less than .75 inches of the box material suffice?

The length of the port is used to determine the mass of air that is
"bouncing" against the spring of the air inside the cabinet, so yes,
that length is the total length of the port as calculated.

Now it gets tricky. The moving mass of port air extends beyond the
physical port for some distance - so called end effect - so you need
to shorten the physical port to allow for it. In your design, that
probably means a physical port length around zero.


That sounds like a feature, not a bug. This then becomes just a hole in
the outer wall of the box. No tube required. I 'd expect a couple
millimeters to not have much effect. If it does, cut a ring out of
something to make it up.

The "chuff" factor - "vent mach" in WinISD parlance - is 0.10; not
bad at all.

You should be able to refine the design parameters you feed into the
programme to end up with a port length of at least a couple of inches
in order to make it physically possible to build. The programme should
be able to give you the end effect lengths that you subtract from the
tube when building it.


The program is WinISD and it just calculates a port length. I
deliberately chose 2X2" because it has calculated a small - 0.5 cu. ft.
box - that will need to be doubled in volume to actually be able to
hold the speaker in question.

The ports could be in the back, but 2x2" is about all you could afford
for front-mounted ports.

d

--
Les Cargill