Flared ports and kitchen sinks.

The flow of air in a loudspeaker tuning port is a bit tricky to say the least. Here are some of my thoughts. Let’s first think about the flow of an incompressible fluid like water in a pipe, for example. Say you live in a multi-story apartment building and you have just finished washing the dishes one evening so you pull the plug in the kitchen sink and the water starts flowing out down a very long vertical pipe. Lets assume there is no “P” trap under the sink and there are no air spaces in the flowing water. At first you might think that as the water goes further down the pipe it flows faster and faster, compared with the water that is just leaving the sink and has not had time to accelerate. However this cannot happen if the pipe has a constant cross section because if the leading edge water starts going faster it exerts a negative pressure on the water behind it causing the water just entering the plug hole to be instantly accelerated to the leading edge water speed and violently sucked down the pipe. This is an inefficient way of emptying the sink because it puts a limit on how fast the leading edge water can flow because the kinetic energy it gains is continually diverted to the water just leaving the sink and giving it a big surprise.

A better system would be to have a tapered pipe that starts off very wide under the sink, and getting narrower as we go down. This way the leading edge water can indeed accelerate like it wants to, while the narrowing pipe diameter does not force it to transfer it’s kinetic energy to the water just leaving the sink and going down the plug hole that is now the same diameter as the whole sink. This latter water, because it is entering a very larger diameter pipe only has to move gradually at first, and then as it goes further down and the pipe narrows it picks up speed. This is all good stuff, but what about when the water falls out the bottom end of the pipe with a big splash because of all it’s kinetic energy because it is moving so fast? Can we recover some of that energy?

I think we can. If you simply start to expand the diameter of the pipe again as it is nearing its end, the water is forced to increase its flow diameter and therefore slow down. And if it slows down, it loses its kinetic energy so where does it go? I expect it would exert a great negative pressure on the column of water above it, causing the water just entering the plug hole to be sucked down with great enthusiasm. What’s more, the water exiting the bottom of the now large diameter pipe doesn’t roar and splash out because it has lost a lot of it’s energy.

Imagine if we applied this principle to a loudspeaker tuning port. Have a constantly varying diameter flared port that is wide at both ends and narrow in the middle. The air would enter and exit gracefully instead of “chuffing” at high power levels. The transformation of pressure to movement back to pressure (potential to kinetic back to potential) that occurs at the box to port interface should be rather more orderly. Maybe the tuning of the box would be improved somehow. What do you think?

GP.
 
GP, the following preprint might be of interest to you.

Maximizing Performance from Loudspeaker Ports
Preprint Number: 4855
Convention: 105 1998-09

and you may search for it from
http://www.aes.org/publications/preprints/search.html

The article presents an empirical equation based on numerous measurements of different size flares and shapes. It does seem that LspCAD's and
Adire Audio LspCAD version flared port calculator is based on the above-mentioned article. It was found that an elliptical port had the best fluid flow.

:)