# Effects of port area?



## jking29 (Jan 6, 2008)

I am trying to find out and understand the effects of port area on a ported enclosure, specifically on a trunk mounted sub. Is there a general rule of thumb for port area when designing a box? Does a low tuned box need a different port area than a higher tuned box? What affect does having too small of a port have on output? Too large? How high of port speed would cause objectionable noise on a trunk mounted sub? Thanks!


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## Thaid and Bound (May 15, 2010)

First, a few basic rules:

Larger port area, same tuning frequency = longer port length.
Larger port area, same length = higher tuning frequency.
Longer length, same port area = lower tuning.
Longer port = higher group delay.
Larger port area = reduced air velocity = lower port noise.
Lower port velocity = lower gain.

From this we can see that port area & length is a balance of velocity (not so little as to reduce gain, no too much as to cause 'chuffing'), group delay (lower is better for SQ), and length (workable within your physical constraints).

I'm sure someone will debate my numbers, but here's what I use as general guidelines:

Port velocity: 10m/s to 20m/s (meters/second)
Group Delay: < 50ms


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## Volenti (Feb 17, 2010)

> Longer port = higher group delay


Only if the enclosure volume is unchanged, take 2 enclosures, a 0.8cuft and one 2cuft both tuned to 30hz with identical diameter ports, the 0.8cuft enclosure has a port almost 3 times longer than the 2cuft one, yet the 2cuft enclosure has a group delay almost twice that of the 0.8cuft enclosure.

Another thing is that overly large and long ports (>0.4 cone area and >3' in length) begin to have 1/4 wave effects* that alter the response and normal enclosure sims ignore it.

*use horn design software to model this.


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## Patrick Bateman (Sep 11, 2006)

jking29 said:


> I am trying to find out and understand the effects of port area on a ported enclosure, specifically on a trunk mounted sub. Is there a general rule of thumb for port area when designing a box?


Use the biggest port that you can. If you can't afford the space for a really big port, flare the ends of the port as much as you possibly can. If you aren't able to flare both ends, flare the exit.

Have you ever seen those boom boxes where the port exits are nearly the size of the woofer? That isn't just for looks; it's the reason that those tiny cheap woofers can produce so much output.



jking29 said:


> Does a low tuned box need a different port area than a higher tuned box?


Yes. IIRC, it takes 4x as much air to produce 40hz than 80hz. You might want to double check that with boxmodel though  (It's free.)

*Keep in mind that you cannot rely on computer programs to tune your ports. YOU MUST MEASURE the impedance curve. This is especially important when you integrate a flare; it's effect is unpredictable. Even the location of the sub in the car itself will change the tuning frequency, due to boundary effects.*



jking29 said:


> What affect does having too small of a port have on output? Too large? How high of port speed would cause objectionable noise on a trunk mounted sub? Thanks!


As another poster noted, as the port size gets bigger and bigger, it begins to behave like a horn. Note that the whole thing is a continuum - as the port gets closer to the size and the shape of a horn, the output continues to rise and get flatter and flatter.

Ironically, you can get more low bass out of a pipe than a horn! Odd huh? That's why Bose can get bass out of a cheap pair of 5" woofers in their home speakers. (In a bandpass box, the front chamber physically filters out the high frequencies.)

Say what you will about Bose, these guys use well-designed ports:


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## mitchyz250f (May 14, 2005)

Is there a calulation for the benifits of flaring and port diameter?


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## TREETOP (Feb 11, 2009)

I've had good results with using 14-16 square inches of port area per cubic foot of net internal enclosure volume, and tuning via port length.


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