how does a port work?

Okay, so I know what a port is, and I've built multiple ported enclosures and I know how to tune an enclosure and what frequency is and all of that...but I really don't get a how port works.  I understand that it utilizes the backwave of the subwoofer but I don't really understand what's going on.  Like, say for a particular airspace and port diameter, a length of 15" tunes the box to 30hz (just using 30hz as an example).  What is actually happening?   I guess what I'm wondering is: what is it about that particular length/diameter that causes only frequencies at/near the tuning frequency to come out of the port...why does 30hz "like" that length but 80hz (for example) doesn't?

http://en.wikipedia.org/wiki/Helmholtz_resonance It has little to do with the "back wave" (as sound) of the woofer. The woofer is the exciter of the air mass, it's not necessarily the back wave. The woofer has a similar function as the air compressor in a pipe organ - it's the pump. It's mass. Air is springy, and has weight. The air in the vent acts as both the mass and the spring. The more air there is (in both the port and the enclosure), the more it weighs (thus a lower resonant frequency) and the more compliant the "spring", i.e. "softer", thus a lower resonance. They work together. The peak resonance of the port is where these two effects (spring compliance and air mass) land on one another (acoustically speaking) to make the port make noise.It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

Huh...interesting; thank you for such a thorough/technical answer.  So I wonder if the sound the port produces at say, sea level would be different than at 10,000ft because of the air density change.  Or likewise listening to it in 30F in winter vs 110F in summer.  Seems like anything that affected air density (or maybe even humidity) could have an impact on the sound. 

douglas79 wrote: Huh...interesting; thank you for such a thorough/technical answer.  So I wonder if the sound the port produces at say, sea level would be different than at 10,000ft because of the air density change.  Or likewise listening to it in 30F in winter vs 110F in summer.  Seems like anything that affected air density (or maybe even humidity) could have an impact on the sound.

Absolutely.  It changes with air temperature, density and humidity.  Actually any loudspeaker's performance will change due to those factors, not just ported enclosures.

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