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Vaughan 17th January 2009 07:00 AM

Ported operation and mid-range leakage
Hi there,

Just wanted to ask a few questions. Port operation is under a narrow band of frequencies and coupling between driver and port goes down as frequency increases (above Fb).

Now I had a discussion the other day where a guy said that the rear wave of the driver leaks through the port. We were discussing subwoofers. He explained that with a ported sub, the rear wave of the driver always escapes through the vent.

Now I explained port operation as best I could and that above Fb the port isn't really active (it is, but the strength of coupling goes down) and well above Fb the driver does most of the work.

Then he said that in a full range speaker that mid-range leaks through the port where does mean that sound in a ported speaker (the rear wave of the driver) does leak through the vent.

On one hand the resonating frequency of the vent won't actively do anything unless frequencies start to approach fB. What about the mid-range leak ?

Calling the experts. . .


Vaughan 17th January 2009 01:45 PM

Sorry about the poor sentence structure but I was really in a rush when I typed the above.

Basically, it has been said that the cones rear wave energy will leak through the port but this is a misconception because the vent only operates under a limited bandwidth.

So if the vent isn't used well above Fb then how can mid-range info leak through the ports ?


Ron E 17th January 2009 02:55 PM

It comes right through the cone as well. You can talk through a piece of paper can't you?

Vaughan 17th January 2009 03:06 PM

So are you saying that mid-range doesn't leak through the port (if tuning is low) but through the driver only ? Some claim to hear mid-range leaking through the port if they put their heads at the port but how does that gel with the fact that port operation (given that it's range is bandwidth limited by definition) shouldn't be operating much higher than Fb ?

What else would be causing the mid-range to be leaking through ?

Please explain. Thanks.


HK26147 17th January 2009 03:22 PM

The port is tuned to a frequency, but it doesn't mean the out of band frequencies don't get through. It a hole - That let's anything that comes off the back of the cone to escape.
Suppression of the mid & high radiation is part of the function of the internal damping applied.
In Stereophile mag J. Atkinson places a mic to measure the port output. Many times a listening test correlates to what is contributed by the port's upper freq output.

Vaughan 17th January 2009 03:27 PM

So are you saying that the mid-range leak is almost completely explained by the lack of damping in the box (like fiberglass lining) ?

It's it's reflective and not mechanical ?


HK26147 18th January 2009 01:28 AM

I'd say primarily reflective:
Diffraction of sound by an aperture is many wavelengths wide.

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