THE GAP - Down Firing Ports (& Subs - more or less) - diyAudio
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Old 23rd July 2013, 10:10 PM   #1
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Default THE GAP - Down Firing Ports (& Subs - more or less)

I know this has been discussed briefly before, but I want (hopefully) to address a new aspect of it.

It is becoming more common for speakers, bookshelf and floorstanding, to have Down Firing Ports, though certainly still in a minority.

Here is an example of a modern speaker with a Down Firing Port -

Canton Reference 3.2 DC loudspeaker | Stereophile.com

Also this concept has been applied to the new Wharfedale Diamond 100 series, not yet released in the USA. Even the bookshelf have Down Firing Ports.

Determining the Port size is easy enough, but what concerns me is the Gap between the Port and the Floor or the Plinth. If the Gap is too small, then the Gap becomes part of the Port and throws off the tuning. I'm not sure what happens if the Gap is too large, perhaps nothing of relevance.

But that is precisely the question, how much is too little Gap and how much is too much Gap?

Has anyone worked out the underlying science for determining the size of that Gap? Or at least someone might have some guidelines or rules of thumb.

The concept seems like a good idea, but other than Gap Size, what are the pitfall of using Down Firing?

Also, if you were building a Down Firing Box, would you build a plinth or stand, or let it fire directly down onto the floor? And if into the floor, what effect would different floors have? For example, would Down Firing into carpet be an advantage or disadvantage?

Applying the same concept but in a different way, Polk Audio has a Rear Firing Port that has a re-director panel on the back that redirects the port output to the sides. Centered on the Port is a teardrop shaped area to smooth the transition (one assumes) from rear-directed to side-directed port output.

But even in the above Polk example, the issue of the Gap arises.

I suppose the same question could be applied to the Down Firing Sub. That Gap between the Port and the Floor has to have some effect on things. But what?

Which brings up an addition issue "The Tear" or dome/cone/whatever just under the center of the port to smooth (one assumes) the re-direction. Is this valid or is it just cosmetic hype?

But the Primary issue is -

How does changing the size of that gap effect the final result?

I'm assuming, and perhaps this is where I am wrong, that the ideal Gap is neutral and has no effect on the loading or resonance of the Port.

I'm open to hearing any discussion on the idea of Down Firing Ports, whether general discussion or directly addressing the Gap issue.

Thanks to any help people can give.

Steve/bluewizard

Last edited by BlueWizard; 23rd July 2013 at 10:24 PM.
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Old 23rd July 2013, 10:33 PM   #2
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Well, my answer to your main question is just get a mic and measure the tuning yourself because it's too hard to try to figure out . I do have a few maybe more useful comments, though:

- There is no too much gap, as far as the port functioning normally.

- These are long wavelengths. The Polk thing is not redirecting anything, it's just part of the port. Kind of an unusual way to make a port with a large flaring exit, that's all. If you made your "gap" that close to the floor, the floor would become part of your port walls in the same way that does. That is fine, except for when you've got thick carpet down there, etc.
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Old 23rd July 2013, 10:49 PM   #3
Jay1111 is offline Jay1111  United States
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The rule of thumb for not affecting the port tune, is to have a gap that is at least equal to the port diameter. 3" port needs 3"+ gap. If gaps are smaller then it is very likely the designer intentionally is increasing the port length without having a very long port inside the box. I'm not good enough at math to figure all of that out on my own, but simply cutting some squares out of 1/4" thick wood, tweaking the height of your speaker in 1/4" increments and measuring the results as you go would work.
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Old 23rd July 2013, 11:38 PM   #4
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Quote:
Originally Posted by BlueWizard View Post
Also, if you were building a Down Firing Box, would you build a plinth or stand, or let it fire directly down onto the floor? And if into the floor, what effect would different floors have? For example, would Down Firing into carpet be an advantage or disadvantage?

Which brings up an addition issue "The Tear" or dome/cone/whatever just under the center of the port to smooth (one assumes) the re-direction. Is this valid or is it just cosmetic hype?
Steve,

Posts #2 & 3 answer most of your questions.

Organic carpeting would absorb some high frequency content reflected through the port, which would tend to reduce harmonic distortion somewhat.

A dome/cone/whatever just under the center of the port could reduce turbulence, a good attribute. For it to be effective, the port would need approximately the same flare as the trumpet shaped "cone".

Having measured Fb in boxes with the port in various configurations, I agree with Jay1111's "rule of thumb".
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Old 23rd July 2013, 11:42 PM   #5
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Originally Posted by Jay1111 View Post
The rule of thumb for not affecting the port tune, is to have a gap that is at least equal to the port diameter. 3" port needs 3"+ gap. If gaps are smaller then it is very likely the designer intentionally is increasing the port length without having a very long port inside the box. I'm not good enough at math to figure all of that out on my own, but simply cutting some squares out of 1/4" thick wood, tweaking the height of your speaker in 1/4" increments and measuring the results as you go would work.
But a 3" diameter port has a surface or opening area of 7.1 inČ. If we have a cabinet say 10" W x 12" D, and we have a Gap of 1" then the surface area of the opening is 44 inČ. Assuming a 3" diameter port, we have 7.1 inČ feeding into a space that has an opening of 44 inČ more than 6 times bigger.

And of course, if the gap is 2", then the opening of the floor gap would be 88 inČ. That pretty big. That would be an opening the equivalent of a 9.4" x 9.4" square (10.6" circular Dia).

In the first case, could we treat the two ports independently? One port in internal 3" in diameter and 8" long (picked at random) tuned to a specific frequency.

Then as a separate entity, we have port with a average length of 6.27 inches and an opening size of 44 inČ (6.6" x 6.6" equivalent, or 7.49"dia). I'm not sure about the tuning frequency of this, but I could use a port calculator to work it out.

I do agree that with a single down firing port of 3" diameter, a 3" gap would certainly be sufficient. But the gaps on all the speakers I see seems much smaller than that. For the records, for a 10" x 12" speaker, a 3" gap would be 132 inČ or 11.5" x 11.5" or 13" circular diameter. That more that 18 times the 3" port opening area.

Though the idea of incrementally increasing the gap and testing the speaker, is a good idea.

I think from a practical perspective, that is about the best you can do.

However, it would still be nice to have some sense of the theory behind it. We can all cut-n-try on very aspect, but it is nice to at least start out in the ball park.

Steve/bluewizard

Last edited by BlueWizard; 24th July 2013 at 12:02 AM.
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Old 24th July 2013, 01:47 AM   #6
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Just out of curiosity, let me use the Linear Team Port Calculator on a hypothetical cabinet, and determine the standard port tuning compared to the "Gap" (external) port tuning, and see if that gives us any helpful information.

LinearTeam - Vent Calculator

Using my original example cabinet of 10" W x 12" D, let's convert that to a circle to make the math easier. With a 44" perimeter, the equivalent circle would be 14" in diameter or 7" radius.

Let's start with a 1" GAP.

Using the port calculator, let's assume a 3 ftł cabinet with one 3" port, 30hz tuning frequency, and determine the port length -

Pl = 4.85 inches

So, we have a 3 cu.ft. Cabinet, with a 3" down firing port, 4.58 inches long, tuned to 30hz.

Now, let's model the external port. The Length is 7" (Radius of the base of the speaker cabinet), the port opening is 44 inČ. What is the equivalent tube port Diameter?

Pd = SqRt(44 / pi) x 2 = 7.48 inches

So, the external port is the equivalent of a 7" long port of 7.48" in diameter.

Now we determine what frequency that tube resonates at using the Port Calculator -

Fr = 56hz (7.087" port)

Now let's expand the external port to 2" or 88 inČ.

The length is still 7", what is the functional diameter of the port -

Pd = SqRt(88 / pi) x 2 = 10.59 inches

So what frequency is the external port tuned to -

Fr = 73hz (7.066" port length)

Now 3" Gap, 132 inČ, or

Pd = SqRt(132 / pi) x 2 = 12.96 inches

External Port Tuning -

Fr = 84hz to 85hz (84hz = 7.274"; 85hz = 6.882")

Fr = 84.5hz (7.076" port length)

So, what does this tell us? How does that help us? Does that help us?

Remember the range of frequencies feeding the second external port is going to be very narrow when you consider the internal port is tuned to 30hz.

If the External Port is tuned well above the frequencies available to it, is it, in a sense, unloaded, or functionally not there?

How narrow is the bandwidth of the internal port?

How high above the internal port tuning frequency do we have to go? One Octave? Two Octaves?

Is there a mistake in my math?

We have information ... but do we have knowledge?

Steve/bluewizard

Last edited by BlueWizard; 24th July 2013 at 01:56 AM.
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Old 24th July 2013, 02:32 AM   #7
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Until you start restricting the air, it will lower the tuning frequency, not raise it, so that isn't right. It's not conceptually right either - if you model the "second" port as acting on the output of the first, you don't get to use the internal box volume to calculate the tuning.

I think if you're trying to calculate it like this (with a "gap" small enough that the floor is part of the port), you need to do something more along the lines of considering the two spaces as one port, and then figure out the average cross section. The effective length would be somewhat longer than it looks (maybe a lot) because the air mass will extend out onto the floor and up the cabinet walls a little (that's a change in "end correction" factor). I suppose it would make things simpler if you made the gap such that the port cross section stays constant. edit: heh, oh wait, that would be a ridiculous shape... nevermind that, better stick to averages.

Last edited by dumptruck; 24th July 2013 at 02:39 AM.
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Old 24th July 2013, 02:47 AM   #8
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Originally Posted by dumptruck View Post
Until you start restricting the air, it will lower the tuning frequency, not raise it, so that isn't right.

....
That doesn't quite add up. The external port is a self-contained entity being driven by the Internal Port. The external port as a limited range of frequencies feeding it.

So, in this case, at least from my current perspective, the internal port is the sound driver feeding the external port.

But is does get complicated which is why I'm asking questions. All responses welcome.

From another perspective, it is like you have a 4.58" long tube 3" in diameter with a 10.59" diameter tube 7" long appended on to it. But does such a large second Tube with more than THREE TIMES the Diameter really effect the first internal tube?

From an even different perspective, what if both tubes were internal? Say we had a 4.58"x3" exiting the box, but internal to the box, the 3" tube had an additional 10.6" diameter tube 7" long attached to it. Is that the same thing Will the Internal+Internal system act the same as an Internal+External system?

Is the second 10.6"Wx7"L tube so large that it doesn't matter?

Can we simply average to two together and come up with a fixed consistent diameter tube with the same total length that acts the same way? And what is the equivalent model, do we average the diameters in proportion to their lengths and come up with an average proportional diameter?

I'm not sure, what the equivalent model is. I'll let other clarify that.

Inquiring minds want to know.

Steve/bluewizard

Last edited by BlueWizard; 24th July 2013 at 02:51 AM.
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Old 24th July 2013, 03:11 AM   #9
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Originally Posted by BlueWizard View Post
That doesn't quite add up. The external port is a self-contained entity being driven by the Internal Port. The external port as a limited range of frequencies feeding it.
I don't think it possibly can be?

Quote:
Originally Posted by BlueWizard View Post
Now, let's model the external port. The Length is 7" (Radius of the base of the speaker cabinet), the port opening is 44 inČ. What is the equivalent tube port Diameter?

Pd = SqRt(44 / pi) x 2 = 7.48 inches

So, the external port is the equivalent of a 7" long port of 7.48" in diameter.
This isn't right. Be warned I make a lot of math errors, but this should explain how it isn't right even if I do. The cross section leaving the end of the port after the right angle turn is:
inner port diameter * pi * 1 = 9.425 inČ.

The cross section leaving the cabinet edge is:
cabinet circumference * 1 = 14*4 = 56 inČ

The cross section halfway to the cabinet edge is:
7*4*1 = 28 inČ

This is a flare, and will still be a flare if you convert to equivalent diameters.

Last edited by dumptruck; 24th July 2013 at 03:15 AM.
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Old 24th July 2013, 03:22 AM   #10
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Let me restate that to see if I understand what you are getting at. If we look at the external port in isolation, it has a 3" diameter entrance, and a 10.6" diameter exit. Consequently it is not a straight tube port.

Is that the point you are trying to make? I think in a context you might be right, though I can't quite wrap my head around it.

It is complicated, which is probably why so few people do it.

Steve/bluewizard
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