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jmechie1 21st July 2011 08:40 PM

Thoughts on Ports...
 
I was just thinking about ports for a vented enclosure.

I'm curious as to what would happen if you were to use two ports per box, but to have them tuned to different frequencies, for example, one tuned to 30hz and one tuned to 45hz (per box)?

Anyone have any idea/experience in this?

James

revboden 21st July 2011 08:55 PM

essentially they average, although it dosent really work because one (45hz) blows out before the other (30hz) really gets going.

vacuphile 21st July 2011 09:20 PM

You will lower the Q of the enclosure untill it will stop working at all as a BR.

Another thing, if it can be avoided, work with one port only. Two ports generate more resistance and huffing and puffing than a single one with the same surface area as the two combined.

DBMandrake 21st July 2011 10:21 PM

Quote:

Originally Posted by jmechie1 (Post 2646171)
I was just thinking about ports for a vented enclosure.

I'm curious as to what would happen if you were to use two ports per box, but to have them tuned to different frequencies, for example, one tuned to 30hz and one tuned to 45hz (per box)?

Anyone have any idea/experience in this?

James

Quote:

Originally Posted by revboden (Post 2646186)
essentially they average, although it dosent really work because one (45hz) blows out before the other (30hz) really gets going.

You can't tune two ports to two different frequencies in the same enclosure by using two different lengths - there is only ever one box resonance frequency.

Note that I said box resonance not port resonance, as a port by itself doesn't form a resonator. (At least at bass frequencies... there is an undesirable tube resonance in the midrange of most cylinder shaped ports typically between 500-1000Hz depending on tube length)

It's the combination of the compliance of the air in the box acting as a spring, and the mass of air within the port(s) acting as an attached mass that together form a mechanical resonant device, very much like a mass hung from the end of a coil spring and bounced up and down.

If you were to add two unequal mass weights to the end of the same spring there will still only be one resonant frequency, which will be determined by the sum of the two masses. (And the spring stiffness, which is equivalent to box volume)

Likewise using two unequal length ports will still resonate at only one frequency, and it will be higher in frequency than if both ports were the same length as the longer port, but lower in frequency than if both ports were the same length as the shorter port. (A similar effect applies if you used two ports of different diameters but the same length)

As far as the bass tuning goes, if you use multiple ports you want them to all be the same diameter and length, so that their air velocities are balanced. If you make one port smaller in diameter than the other, the smaller one will have a higher air velocity than the larger one, increasing turbulence.

Likewise, if you use two ports of the same diameter but different lengths, the air flow will favour the "easier" path of the shorter port, resulting in higher air velocity and turbulence in the shorter port, and more net turbulence than if both ports were the same length for the same tuned frequency.

The only reason you might ever want to make the ports slightly different lengths, is to stagger the high Q "tube" resonances in the midrange apart so they don't both occur at exactly the same frequency - by doing so you could reduce the peak in the midrange by up to 6dB, but I wouldn't make them more than 20% different in length, and a far better solution would be to just put the ports on the back of the cabinet facing away from the listener, which will essentially eliminate the port induced midrange resonance/leakage at the listening position.

chlorofille 21st July 2011 10:41 PM

Awesome post!!

vacuphile 21st July 2011 11:48 PM

Quote:

Originally Posted by DBMandrake (Post 2646252)
.

If you were to add two unequal mass weights to the end of the same spring there will still only be one resonant frequency, which will be determined by the sum of the two masses. (And the spring stiffness, which is equivalent to box volume)

Likewise using two unequal length ports will still resonate at only one frequency, and it will be higher in frequency than if both ports were the same length as the longer port, but lower in frequency than if both ports were the same length as the shorter port. (A similar effect applies if you used two ports of different diameters but the same length)

Hi DBmandrake,

I agree with your post, but I have my doubts about the part I quoted. I do not think that the analogy with two different weights on top of one spring is correct. The reason is that two weights on top of one spring would always have to move in unison, whereas the air mass in two tubes of different lengths would not.

It is therefore a much more complicated situation, and I would expect a system like that to go through motions that are impossible to model with the sort of mathematics you use to calculate a Helmholz resonator. It is much like the sort of executive desk toy you might know, a sort of cardanically suspended inbalance. A system like that displays erratic behaviour, which can be modelled with a mathematical equivalent, but that cannot be calculated in the classical sense.

Vac

DBMandrake 24th July 2011 04:45 PM

Quote:

Originally Posted by vacuphile (Post 2646310)
I agree with your post, but I have my doubts about the part I quoted. I do not think that the analogy with two different weights on top of one spring is correct. The reason is that two weights on top of one spring would always have to move in unison, whereas the air mass in two tubes of different lengths would not.

It is therefore a much more complicated situation, and I would expect a system like that to go through motions that are impossible to model with the sort of mathematics you use to calculate a Helmholz resonator. It is much like the sort of executive desk toy you might know, a sort of cardanically suspended inbalance. A system like that displays erratic behaviour, which can be modelled with a mathematical equivalent, but that cannot be calculated in the classical sense.

I'm not sure that I see where the "erratic behaviour" is coming from - yes the air velocity will be different when two ports are not the same dimensions, but that doesn't constitute erratic behaviour. It's the total momentum of the air in the ports that controls the resonance.

It's easy enough to test experimentally. Do a nearfield measurement of the output of each port on a cabinet when the two ports are significantly different lengths.

You should find the resonance peak occurs at exactly the same frequency for both, but the amplitude will measure higher on the shorter port due to a higher velocity.

I'm not advocating unequal size ports by the way, anything other than equally sized ports is non optimal in a number of ways, but it does still work as a single Helmholtz resonator...


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