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How Good Is Your Port?

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I think you've mistaken 'generally accepted' for 'universal'.

At any rate - care to enlighten us? How do 'you' determine how much port for a given woofer?

I'm all ears...

4.2 before or after displacement?

I believe the generally accepted ratio is 12"-16" of port area per cubic foot of box...

You currently have 6.72" of port per cube - about half what you probably should (if that 4.2 is after displacement)...

Okay, so what I'm picking up here, my single flared 6" aero in my 4.2 cube net box, is too small. Basically, if I play a lower note (I'm tuned to 33hz) ~35hz and lower, it blows a tremendous amount of wind out. My burp frequency is 38 hz and I raise from .7 to 4.5 ohms. This all has to do with my port being to small and it acting more like a severely leaky sealed box correct?

the "generally accepted rule" is bs, it depends on the driver used.

It depends heavily on the amount of air the driver displaces. Minimum port area can be calculated using the Sd and Xmax parameters of the driver along with the number of ports used.

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Okay, so let's use a single SSA Icon 12... (Sd=480 cm^2, Xmax=22mm) and decide to use a single port...

How do you calculate how much port area you would need?

I think you've mistaken 'generally accepted' for 'universal'.

At any rate - care to enlighten us? How do 'you' determine how much port for a given woofer?

I'm all ears...

4.2 before or after displacement?

I believe the generally accepted ratio is 12"-16" of port area per cubic foot of box...

You currently have 6.72" of port per cube - about half what you probably should (if that 4.2 is after displacement)...

Okay, so what I'm picking up here, my single flared 6" aero in my 4.2 cube net box, is too small. Basically, if I play a lower note (I'm tuned to 33hz) ~35hz and lower, it blows a tremendous amount of wind out. My burp frequency is 38 hz and I raise from .7 to 4.5 ohms. This all has to do with my port being to small and it acting more like a severely leaky sealed box correct?

the "generally accepted rule" is bs, it depends on the driver used.

It depends heavily on the amount of air the driver displaces. Minimum port area can be calculated using the Sd and Xmax parameters of the driver along with the number of ports used.

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Okay, so let's use a single SSA Icon 12... (Sd=480 cm^2, Xmax=22mm) and decide to use a single port...

How do you calculate how much port area you would need?

I think you've mistaken 'generally accepted' for 'universal'.

At any rate - care to enlighten us? How do 'you' determine how much port for a given woofer?

I'm all ears...

4.2 before or after displacement?

I believe the generally accepted ratio is 12"-16" of port area per cubic foot of box...

You currently have 6.72" of port per cube - about half what you probably should (if that 4.2 is after displacement)...

Okay, so what I'm picking up here, my single flared 6" aero in my 4.2 cube net box, is too small. Basically, if I play a lower note (I'm tuned to 33hz) ~35hz and lower, it blows a tremendous amount of wind out. My burp frequency is 38 hz and I raise from .7 to 4.5 ohms. This all has to do with my port being to small and it acting more like a severely leaky sealed box correct?

the "generally accepted rule" is bs, it depends on the driver used.

It depends heavily on the amount of air the driver displaces. Minimum port area can be calculated using the Sd and Xmax parameters of the driver along with the number of ports used.

Heres a good calculator

My link

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Okay, so let's use a single SSA Icon 12... (Sd=480 cm^2, Xmax=22mm) and decide to use a single port...

How do you calculate how much port area you would need?

I think you've mistaken 'generally accepted' for 'universal'.

At any rate - care to enlighten us? How do 'you' determine how much port for a given woofer?

I'm all ears...

4.2 before or after displacement?

I believe the generally accepted ratio is 12"-16" of port area per cubic foot of box...

You currently have 6.72" of port per cube - about half what you probably should (if that 4.2 is after displacement)...

Okay, so what I'm picking up here, my single flared 6" aero in my 4.2 cube net box, is too small. Basically, if I play a lower note (I'm tuned to 33hz) ~35hz and lower, it blows a tremendous amount of wind out. My burp frequency is 38 hz and I raise from .7 to 4.5 ohms. This all has to do with my port being to small and it acting more like a severely leaky sealed box correct?

the "generally accepted rule" is bs, it depends on the driver used.

It depends heavily on the amount of air the driver displaces. Minimum port area can be calculated using the Sd and Xmax parameters of the driver along with the number of ports used.

Let me draw some stuff out for you.

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Okay, so let's use a single SSA Icon 12... (Sd=480 cm^2, Xmax=22mm) and decide to use a single port...

How do you calculate how much port area you would need?

I think you've mistaken 'generally accepted' for 'universal'.

At any rate - care to enlighten us? How do 'you' determine how much port for a given woofer?

I'm all ears...

4.2 before or after displacement?

I believe the generally accepted ratio is 12"-16" of port area per cubic foot of box...

You currently have 6.72" of port per cube - about half what you probably should (if that 4.2 is after displacement)...

Okay, so what I'm picking up here, my single flared 6" aero in my 4.2 cube net box, is too small. Basically, if I play a lower note (I'm tuned to 33hz) ~35hz and lower, it blows a tremendous amount of wind out. My burp frequency is 38 hz and I raise from .7 to 4.5 ohms. This all has to do with my port being to small and it acting more like a severely leaky sealed box correct?

the "generally accepted rule" is bs, it depends on the driver used.

It depends heavily on the amount of air the driver displaces. Minimum port area can be calculated using the Sd and Xmax parameters of the driver along with the number of ports used.

Let me draw some stuff out for you.

Ive seen several formulas, using vent velocity and such

Just posted a general statement regarding displacement

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vmh.jpg

Take note in the ref eff formula, part of it changes if the vas isn't listed in liters, although generally I believe it is.

Also note radius of a circular port is being used for "port area", if your using a slot port, multiply the W*H, divide by pi, then square root, this will give you the equivalency to plug in for r if you would like to use the formula.

example: 5"x5" = 25 in^2

25/pi = 7.9577... sqrt(7.9577...) = 2.8209" for r.

You copy all the T/S specs for the 12" icon, drop them in winISD, by default it'll use the driver's rms when finding W, go to vents tab, plug in 8" for diameter of one circular port, and the vent mach shown should be ~0.04

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But since we can't see the formula that particular online calculator is using - we don't know how they are coming up with their 'ideal' port area - (and it doesn't specify if they are using an end correction factor for flared ports or not...) Let alone the results... According to that calculator - you'd need one 7" round port for the single SSA-12 (to tune to say 35hz). I'm gonna go out on a limb and say thats way too big to be practical. If I used SSA's recommend a 2.25 cube box the port would have to be 28" long! The links to SSA's box plans are broken at the moment, so can't compare against what they recommend for port - but I'd wager it's a lot less than this calculator is coming up with...

Thoughts?

Okay, so let's use a single SSA Icon 12... (Sd=480 cm^2, Xmax=22mm) and decide to use a single port...

How do you calculate how much port area you would need?

I think you've mistaken 'generally accepted' for 'universal'.

At any rate - care to enlighten us? How do 'you' determine how much port for a given woofer?

I'm all ears...

4.2 before or after displacement?

I believe the generally accepted ratio is 12"-16" of port area per cubic foot of box...

You currently have 6.72" of port per cube - about half what you probably should (if that 4.2 is after displacement)...

Okay, so what I'm picking up here, my single flared 6" aero in my 4.2 cube net box, is too small. Basically, if I play a lower note (I'm tuned to 33hz) ~35hz and lower, it blows a tremendous amount of wind out. My burp frequency is 38 hz and I raise from .7 to 4.5 ohms. This all has to do with my port being to small and it acting more like a severely leaky sealed box correct?

the "generally accepted rule" is bs, it depends on the driver used.

It depends heavily on the amount of air the driver displaces. Minimum port area can be calculated using the Sd and Xmax parameters of the driver along with the number of ports used.

Let me draw some stuff out for you.

Ive seen several formulas, using vent velocity and such

Just posted a general statement regarding displacement

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At the end of all that math - what is the desirable range for vent mach?

vmh.jpg

Take note in the ref eff formula, part of it changes if the vas isn't listed in liters, although generally I believe it is.

Also note radius of a circular port is being used for "port area", if your using a slot port, multiply the W*H, divide by pi, then square root, this will give you the equivalency to plug in for r if you would like to use the formula.

example: 5"x5" = 25 in^2

25/pi = 7.9577... sqrt(7.9577...) = 2.8209" for r.

You copy all the T/S specs for the 12" icon, drop them in winISD, by default it'll use the driver's rms when finding W, go to vents tab, plug in 8" for diameter of one circular port, and the vent mach shown should be ~0.04

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I believe most people shoot for: 0.045 (50.8 ft/sec) or less.

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I don't claim to know the math.

I've always modeled, and looked at the port velocity (I use BBP6).

I try to balance velocity with the volume available.

I tend to like smaller net volumes, which result in long(er) ports.

I quickly read through the link.

Great explanation, but I found the graphs to be not as helpful as they could be.

Different color plots would have simplified the explanations/results, I found myself assuming which curve was being referred to.

Thanks for the link, I'll reread it again later.

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I don't claim to know the math.

I've always modeled, and looked at the port velocity (I use BBP6).

I try to balance velocity with the volume available.

I tend to like smaller net volumes, which result in long(er) ports.

I quickly read through the link.

Great explanation, but I found the graphs to be not as helpful as they could be.

Different color plots would have simplified the explanations/results, I found myself assuming which curve was being referred to.

Thanks for the link, I'll reread it again later.

Each plot is labeled, 0, 1, and 2. To the right of the graph lists which was the impedance, amplitude, and vent air velocity.

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I don't claim to know the math.

I've always modeled, and looked at the port velocity (I use BBP6).

I try to balance velocity with the volume available.

I tend to like smaller net volumes, which result in long(er) ports.

I quickly read through the link.

Great explanation, but I found the graphs to be not as helpful as they could be.

Different color plots would have simplified the explanations/results, I found myself assuming which curve was being referred to.

Thanks for the link, I'll reread it again later.

Each plot is labeled, 0, 1, and 2. To the right of the graph lists which was the impedance, amplitude, and vent air velocity.

You're right.

I see the plot numbered at the bottom of the curve on the second look.

Thanks Steph!

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On 1/5/2012 at 5:50 PM, n8skow said:

But since we can't see the formula that particular online calculator is using - we don't know how they are coming up with their 'ideal' port area - (and it doesn't specify if they are using an end correction factor for flared ports or not...) Let alone the results... According to that calculator - you'd need one 7" round port for the single SSA-12 (to tune to say 35hz). I'm gonna go out on a limb and say thats way too big to be practical. If I used SSA's recommend a 2.25 cube box the port would have to be 28" long! The links to SSA's box plans are broken at the moment, so can't compare against what they recommend for port - but I'd wager it's a lot less than this calculator is coming up with...

Thoughts?

28" isn't anywhere near too long to be practical. That would mean no tlines are practical. Too small of a port creates several issues, what does too big of a port do? I always go as big as possible for my design (up to 16in/cube)

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5 minutes ago, Randal Johnson said:

Way to bring back an old thread.

and with fodder

3 hours ago, Smanleyman said:

That would mean no tlines are practical. 

T-lines aren't practical in a car.

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Not to mention another "rule of thumb" way of thinking. The only number that should matter is keeping the vent Mach as low as possible, or at least reconcile the amount of it a person is willing to deal with. 

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LOL I knew something was coming with that T Line remark. Thanks Sean for not letting me down !!!

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I am only running 84 sq inches of port area for my two 12" JBL W12GTI's.............:phatyo:

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