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In the Forum: Audio Discussions
In the Thread: Macondo’s MiniMe or about Pilot Acoustic Systems
Post Subject: Port PredicamentsPosted by Lbjefferies7 on: 9/18/2008
 Romy the Cat wrote:


...but I was wondering how the length of the port and the most important the proximity of it to boundary affects sound.



A general rule is to keep the end of the port at least twice its diameter away from walls.  I have placed ports right up next to a parallel wall and had no problems at all.

Now, as for your drawing of the exaggerated port flaires, it is excessive.  A port is not a horn and, from an engineering standpoint, I treat them very differently.  A port is a resonator.  They opperate almost exactly as do passive radiators.  Nothing very fancy is needed to keep a port from "chuffing."  The first step is to design a port that is not too small.  Small ports mean more compression and greater velocity.  I have just used a simple .75" round-over and have been perfectly happy with it.  Having said that, Petar's port design looks very good, but when using eight or ten ports, adjustability will be a must.  The greater surface area of the mulitple ports will cause more friction against the moving air and will screw up the Fb.

Using ports as anti-compression devices is just plain bogus, in my estimation.  Above the port's tuning frequency (Fb), the enclosure acts just as a large sealed enclosure.  Below the Fb, it acts as a quasi-infinite baffle system.  When the driver reproduces the Fb, the port "opens up" at the Helmholtz frequency...where the air masses in the box and in the port reach a "balance" where the output of the port is 365 degrees out of phase with the driver.  The responces of each resonator combine and create a greater overall response.  By tuning the port, the port's output can extend the sealed response of the acoustic system.  This is how I understand it, at least.

Now, Petar is spot on when he says that the Fb must not be below the resonant frequency (Fs) of the driver.  If it is, you will certainly get a horribly disgusting cone-flapping sound and may just fry your drivers.  The vision of murder will be dancing in your head...like sugarplums on steroids!

LBJ

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