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post #1 of 44 Old 01-30-2016, 12:07 PM - Thread Starter
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Subwoofer Size Versus Port Size and Chuffing

I know there are programs out there that will figure port velocity and whether you can have chuffing issues on your subwoofer with certain port sizes, but has anyone actually tested the numbers?

There's a commercial subwoofer that uses something 'similar' to an Eminence Lab-15 with a 4" Precision Port. It was tested by DataBass and the end result showed that chuffing wasn't really an issue until it was driven really hard, and if I recall, even then didn't seem like a big issue.

Quickly simplifying things here: the surface area of a 15" is about 825 and a 4" port area is 12.56.....65.68 per 1" of port area.

The surface area of an 18" is around 1175 and a 6" port is 28.26......41.58 per 1" of port area......less than the ported 15" above. Even dual 4" ports on an 18" subwoofer work out better than the 15" above.

Maybe my math/understanding is completely wrong and I'll be embarrassed here, but why aren't we seeing 18" ported subs in a 6-7cuft enclosure with a 6" Precision Port or even two 4" Precision Ports?

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post #2 of 44 Old 01-30-2016, 01:00 PM
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Couple reasons.

People are graph crazy and don't realize how hard it is to make a port actually chuff audibly.

Everyone has had a bad chuffing experience so they way over design the port size to make sure that won't happen.

They don't notice that most of the port velocity is below tune and when a HPF is applied it is significantly knocked down.



Personally leaving a bit of port velocity on the table isn't a bad thing. If you hear that chuffing you are on the ragged edge and it's time to turn it down. As the port compresses it saves your driver. Chuffing is and on/off thing. Like it's loud and clean and then bam, farts sounds coming out. It usually is so subtle it's missed and the louder you go the progressively louder the chuffing is. By the time it's gross, you're into major driver distortion anyways. That's my take on it anyways, but I'm not big on ports
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post #3 of 44 Old 01-30-2016, 01:26 PM
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there are several issues. if i am thinking of what you are thinking, that sub only has a 500w amp and the low end got compressed out (by either the amp, the port, or both).



audibility of port chuffing is tested here:

http://www.subwoofer-builder.com/flare-testing.htm

i estimated port compression vs. port velocity on the cap sub using data-bass spl measurements vs. model port velocity:





port velocity will depend not only on the surface area of the driver and the cross sectional area of the port, but also on how much the driver is moving. an 18" with 8mm excursion doesn't need nearly the port of an 18" moving 32mm.
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post #4 of 44 Old 01-30-2016, 02:16 PM
 
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Originally Posted by Erich H View Post
I know there are programs out there that will figure port velocity and whether you can have chuffing issues on your subwoofer with certain port sizes, but has anyone actually tested the numbers?
The recommendation is to keep the port velocity no more than 5% the speed of sound, as more than that may cause audible chuffing. Operative word: May. It also may not. Since one doesn't want to find out the hard way that it does chuff most designers will opt to be on the safe side and stay close to that number.
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post #5 of 44 Old 01-30-2016, 02:54 PM - Thread Starter
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Originally Posted by LTD02 View Post
port velocity will depend not only on the surface area of the driver and the cross sectional area of the port, but also on how much the driver is moving. an 18" with 8mm excursion doesn't need nearly the port of an 18" moving 32mm.
Yes, I understand X-max would come into play, not just the size of the cone. I was mostly looking at 18's like the Dayton RS, Ultimax, and the cheaper SI-18 because those are a little closer to the Lab-15 real world X-max ratings, not the crazier 32+mm X-max models.

Databass does say there is some chuffing on the single 15" with 4" port, but that's when pushed hard, and of course no other material being played to mask any of it at that volume.

With some more searching there's also a similar commercial design using those dual 15's. No talk about chuffing issues on Databass. That's a surface area around 1650cm^2 which is around 30% more than an 18", but being tuned with a 6" port.



In your link he says this:

Quote:
An allowance for the masking effect of content can also be used.
The actual amount is based on the performance of sub's I've built, and on existing industry recommendations.
I estimate that reasonable values for masking allowance are an additional 15% for music and 30% for Home Theatre usage.
The equations developed below are based on designing for music, so a total allowance of 25+15 = 40% is used
So for home theater usage, the total allowance would be 55% more than what most programs would consider possible chuffing issues?

The only thing that seems odd about that write up is that there seems to be a decent amount of estimating going on after the point where they hear even the slightest chuffing with their ear next to the port. Basically adding 25% if your 8' away, then adding in a percentage for content being played.

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post #6 of 44 Old 01-30-2016, 03:17 PM
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I got 2 um 18's in 11cu ft enclosure with just 2 6" ports with zero chuffing. Im using inuke 6000 so it's getting plenty of power.
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post #7 of 44 Old 01-30-2016, 03:18 PM
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If your port is chuffing, it's likely already into compression territory, and that's just wasted efficiency.
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post #8 of 44 Old 01-30-2016, 03:53 PM
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Originally Posted by Brad07z06 View Post
I got 2 um 18's in 11cu ft enclosure with just 2 6" ports with zero chuffing. Im using inuke 6000 so it's getting plenty of power.

are you running a protective high pass?

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post #9 of 44 Old 01-30-2016, 04:16 PM - Thread Starter
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So going by the programs that figure port velocity, would a 6" Precision Port work okay with a RS-18 in a 6.5cuft enclosure? Are the programs figuring in the 'allowance for masking content'?

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post #10 of 44 Old 01-30-2016, 04:16 PM
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Quote:
Originally Posted by Erich H View Post
There's a commercial subwoofer that uses something 'similar' to an Eminence Lab-15 with a 4" Precision Port. It was tested by DataBass and the end result showed that chuffing wasn't really an issue until it was driven really hard, and if I recall, even then didn't seem like a big issue.
The design very likely had a high pass filter in place, and as tuxedocivic points out this will help the velocity and chuffing issues quite a bit. It's also possible (although probably unlikely) that there was some sort of dsp limiting built in. Even with the likely hpf and possible limiting the results are pretty bad as shown in LTD's post. The port alone is not necessarily entirely to blame as LTD points out, it could have been partly due to thermal issues and amp limits too, but the port is clearly too small and was likely mostly to blame.

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Originally Posted by Erich H View Post
In your link he says this:



So for home theater usage, the total allowance would be 55% more than what most programs would consider possible chuffing issues?

The only thing that seems odd about that write up is that there seems to be a decent amount of estimating going on after the point where they hear even the slightest chuffing with their ear next to the port. Basically adding 25% if your 8' away, then adding in a percentage for content being played.
The Flare It program on that website is very good, the data was empirically tested with a number of different examples. The conclusion is that 10 m/s is generally considered safe and anything above that will result in losses and at some point audible chuffing. The reason he gives masking allowances is because people want small enclosures with small ports and his masking recommendations are estimates of what he thinks the average person is willing to tolerate wrt audible chuffing. As he notes there are things you can do to mask chuffing and make it less objectionable, but as Notnyt very correctly points out, if there is audible chuffing there are losses. In fact by the point where you can hear audible chuffing the losses are already pretty bad.

In Flare It there are two limit lines, the core limit and the chuffing limit.

The core limit refers to the ability of the interior of the port to pass air competently. The chuffing limit refers to the ability of the port exit and entrance to introduce the air into the enclosure (at the entrance) and into the room (at the exit) without audibly chuffing, which is violent air vortexes at the port entrance and or exit resulting in inefficiency and noise.

The core limit can be defined by the Reynolds number of the port, which takes into consideration the length, shape, aspect ratio, surface smoothness and air velocity (although I have no idea how Collo came up with his core limit information). If the Reynolds number is more than zero there are losses, although the losses don't become apparent until the Reynolds number is quite high. At some point when the Reynolds number gets high enough the efficiency of the port drops dramatically and the port becomes very restrictive, beginning to dramatically block the amount of air that can pass. This is why you should not exceed the core limit in Flare It.

The chuffing limit, as previously explained, is the ability of the port exit and entrance to allow a laminar smooth flow of air without creating vortexes bad enough to cause audible chuffing. A healthy sized flare on both exit and entrance can ease this transition tremendously. The flare can be either a roundover or a chamfer up to about 45 degrees or so (so slot ports can be flared too). The problem is that losses start well before chuffing becomes audible so it's best to keep velocity at or under 10 m/s.

I assume the reason you ask is because you want to make ported kits. This is pretty easy to do, just design them to keep velocity under 10 m/s at xmax with the required high pass filter in place and no one will ever have a problem unless they don't use the high pass filter (in which case they will probably damage the driver anyway). You could probably get away with as high as 18 m/s and maybe even more without audible chuffing (you can test it pretty easily, just build it and listen to material with tones at the tuning frequency at full power) but it's best to keep it under 10.

I can't remember if WinISD will simulate flared ports with a high pass filter in place but the upcoming version of Hornresp will be able to (new feature), I think TL.app can do it, Akabak can certainly do it, MJK's worksheets can do it and Flare It will give acceptable port sizes but not a simulation.

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post #11 of 44 Old 01-30-2016, 04:17 PM
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Quote:
Originally Posted by Erich H View Post
Are the programs figuring in the 'allowance for masking content'?
Absolutely not, simulators will just report velocity and it's up to you to decide what's safe and what isn't.
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post #12 of 44 Old 01-30-2016, 04:20 PM
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Originally Posted by Brad07z06 View Post
I got 2 um 18's in 11cu ft enclosure with just 2 6" ports with zero chuffing. Im using inuke 6000 so it's getting plenty of power.
I know from experience even 4 4 inch aeros would have been more than fine. My brother used to be a iasca record holder and he built a lot of boxes. One port is ideal though for maximum spl. Also a good way to make an aero from wood is an octoport. (sp)

http://www.google.com/webhp?sourcei...F-8#q=octoport

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post #13 of 44 Old 01-30-2016, 05:13 PM
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Quote:
Originally Posted by Erich H View Post
So going by the programs that figure port velocity, would a 6" Precision Port work okay with a RS-18 in a 6.5cuft enclosure? Are the programs figuring in the 'allowance for masking content'?

the programs, such as winisd, don't specify what is good or not good. they simply specify the air speed given a driver, enclosure, port configuration, and power level. generally, keeping the port velocity in the upper 20's meters/second (m/s) of air speed at max power is ok. lower 20's m/s at max output would be better (that is where most of the larger martysubs are). upper teens m/s is optimal from an air speed/chuffing/compression perspective, but there are other tradeoffs that need to be considered.


the 460ho driver in a 6.5 cubic footer tuned to 20hz with a 6" port with 2nd order high pass and a couple db added back right on the tuning frequency at 1100 watts of power looks something like this:





air speed looks like this (winisd calls it a "rear port" but that doesn't actually have anything to do with the direction the port aims out of the sub):



at 30 m/s air speed, there would be about 3.5db of compression (maybe a little more from other effects), but something like that.


i would call that a 'borderline' design. it may make an untoward noise here and there, and it will certainly have some compression at high spl, but overall it probably functions ok.
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post #14 of 44 Old 01-30-2016, 05:16 PM
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I generally use a 4th order bw filter a couple hz below the tune depending (see how excursion looks).

That said, as LTD02 modeled, that port velocity is not ideal.
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post #15 of 44 Old 01-30-2016, 05:24 PM
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this is roughly what the mini-marty looks like with 460ho and 1100 watts, 2nd order high pass and a couple db compensating eq right on the tuning frequency. that is just a slot port with no roundover. a little higher velocity can be used with a port with roundovers (which was a good chunk of collo's data). this is at full power. at 50% power air speed is less than 10 m/s from 21hz up, so under almost all conditions there will be very little port compression and no chuffing sounds. it's a pretty big cab though. there are reasons for it to be the largish size that it is.


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post #16 of 44 Old 01-30-2016, 05:50 PM
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Quote:
Originally Posted by LTD02 View Post
at 30 m/s air speed, there would be about 3.5db of compression (maybe a little more from other effects), but something like that.


i would call that a 'borderline' design. it may make an untoward noise here and there, and it will certainly have some compression at high spl, but overall it probably functions ok.
This is probably pretty similar to how it would be designed if it was a commercial product, with an undersized port and compression issues..

But IMO (and it's just IMO) this is completely unacceptable. Once you start losing 3+ db due to port compression you might as well just stick to a sealed box. It won't go quite as loud but it can go lower and it can be smaller. And it will never make any chuffing noises.

Having said that, a steeper high pass filter and some large roundovers on both ends of the port would get that down into a somewhat acceptable range. If it's a kit you could include a huge flared port (or build a flared slot port into the flat pack) and specify it should ideally be used with a 4th order hpf and it might be ok. If I was going to build it though I'd use a much larger port, or more likely some monster roundovers (flares) like Danley's ported box, the CS30 model.





There's also the option of a reverse tapered tl when port sizes get out of hand, they function very much like a ported box at a 10:1 taper ratio, they are similar size to ported, and the terminus can be flared if necessary. Not really any more complicated to design or build than a ported box either, and roughly the same amount of wood since the internal divider(s) act as bracing.

EDIT - CS30 link - http://www.danleysoundlabs.com/produ...s/cs30/#slide3

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post #17 of 44 Old 01-30-2016, 06:31 PM
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Is a slot port less prone to chuffing compared to round ports?
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post #18 of 44 Old 01-30-2016, 06:38 PM
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Flared aeros help cut down on chuffing. My 6" aer ports flare to 8" at each end. You can usually get away with less port area with flared aero port vs slot.
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post #19 of 44 Old 01-30-2016, 06:52 PM
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Quote:
Originally Posted by Brad07z06 View Post
Flared aeros help cut down on chuffing. My 6" aer ports flare to 8" at each end. You can usually get away with less port area with flared aero port vs slot.
They have new aeros with dimples inside the port that are suppossed to help reduce velocity but how much I dont know. You can also purposely under tune it as well but I think this wears the sub out faster because it uses the motor to support it under tuning.

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post #20 of 44 Old 01-30-2016, 07:19 PM
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I build a lot of car audio subwoofer boxes for folks who want to get as loud as possible (for whatever reason). Port velocity and port sizing is a lot more complex than just the size of the sub and xmax. Port velocity in affected by the size of the enclosure, the power applied, and the tuning frequency, as well as the size of the sub.

The folks I build boxes for want to wring out every last bit of performance, so I try to keep port velocity as low as possible. There is no penalty to sizing ports on the large side, other than they take up more space. I try to keep port velocity under 22 m/sec if possible. Sometimes this results in ports that have port area half of what the cone area is. Modeling software (WinISD, Hornresp, etc) are very useful.

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post #21 of 44 Old 01-30-2016, 07:36 PM
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I build a lot of car audio subwoofer boxes for folks who want to get as loud as possible (for whatever reason). Port velocity and port sizing is a lot more complex than just the size of the sub and xmax. Port velocity in affected by the size of the enclosure, the power applied, and the tuning frequency, as well as the size of the sub.

The folks I build boxes for want to wring out every last bit of performance, so I try to keep port velocity as low as possible. There is no penalty to sizing ports on the large side, other than they take up more space. I try to keep port velocity under 22 m/sec if possible. Sometimes this results in ports that have port area half of what the cone area is. Modeling software (WinISD, Hornresp, etc) are very useful.
Just checked out your FB page. You've done a lot of amazing enclosures!
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post #22 of 44 Old 01-30-2016, 08:10 PM
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I have an SI18 in a 6ft3 net ported cab with a single 5" x 5" square port 24" long. Running on a 3000dsp at almost full tilt. I can only get port noise with a sine wave at full power and even then it's masked by the stuff vibrating in the house. With a movie playing, I have never heard it. Mine models a little on the high side for air velocity, I think 28ms. The port noise is only going to happen at the very lowest frequencies at full power. That's only going to happen in a handful of movies for a few seconds. Getting that air speed down gets real tough in a smallish cab with 20+ mm of Xmax.
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post #23 of 44 Old 01-31-2016, 03:41 AM
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I remember one time getting a demo of the solid from b & w when it came out and it was a standard port with no flare and when the music was playing the air out of the port shot quite a distance away that i felt it and as far as i could tell you werent able to hear it. Ive never felt a port do that and I had been around audio for quite some time already at that point.
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Couple reasons. ...
As the port compresses it saves your driver.
...
No, it's the other way around, when the port is working it loads the driver and keeps it under control, but when the port chuffs, the loading is lost and the driver will make larger movements. At frequencies below the port tuning, where the driver has no loading from the port anyway, there will be no help from the port, be it chuffing or not.
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post #25 of 44 Old 01-10-2019, 09:50 PM
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Originally Posted by paa_ View Post
No, it's the other way around, when the port is working it loads the driver and keeps it under control, but when the port chuffs, the loading is lost and the driver will make larger movements. At frequencies below the port tuning, where the driver has no loading from the port anyway, there will be no help from the port, be it chuffing or not.
No, unloading is not the same as port compression. At port tuning where impedance is at a minimum, excursion from the driver will be the lower and port velocity will be higher. Compression occurs when the port is too small to move the necessary amount of air, which would further restrict driver movement.

Also, the post you replied to was from 2016... but I digress.

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post #26 of 44 Old 01-11-2019, 05:19 AM
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...

Compression occurs when the port is too small to move the necessary amount of air, which would further restrict driver movement.
...
Also, the post you replied to was from 2016... but I digress.
Compression from a too small port does not restrict the driver movement, it increases the driver movement.

Compare the cone displacement of a vented box with a closed box:
The vented box has less cone displacement around the vent tuning frequency than the closed box. The more the vent restricts the flow the more it approximates a closed box. And thus the cone displacement increases with overloaded ports.

I know the post was a bit old when I came across it, looking for LAB12 info, but I think this information is important to get right, since I have never seen the proper explanations about it on the english speaking forums.

Anyway the double-tapered port in the Danley sub is a great way to get a port that can pass lots of air in a short length. It's basically at the opening areas where the vortexes starts, in the middle of the tunnel the air can travel much faster without problem. Roozen has done much better work than Harman/JBL on tapered ports and port chuffing, and even though the Harman paper mentions Roozen, it is obvious that they have neither properly read the Roozen paper, nor understood what he wrote. Danley on the other hand seems to understand more than most of us.
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post #27 of 44 Old 01-11-2019, 03:05 PM
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Quote:
Originally Posted by paa_ View Post
Compression from a too small port does not restrict the driver movement, it increases the driver movement.

Compare the cone displacement of a vented box with a closed box:
The vented box has less cone displacement around the vent tuning frequency than the closed box. The more the vent restricts the flow the more it approximates a closed box. And thus the cone displacement increases with overloaded ports.

I know the post was a bit old when I came across it, looking for LAB12 info, but I think this information is important to get right, since I have never seen the proper explanations about it on the english speaking forums.
Hm, both sides of the argument are compelling, but I'm still not sure which is correct. It's not like the port resonance is going away or changing, but at the same time it's not going to behave linearly. Can you explain in more detail?
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post #28 of 44 Old 01-11-2019, 03:45 PM
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As stated, even if you cant hear audible chuffing because you are seated far enough away from the sub or the source mayerial is really loud, you are still getting compression. All that extra work to go ported for nothing.

If your enclosure isnt big enough, you cant get away with using enough port cross sectional area for a low tune, or else your length becomes too long and you get a resonance.
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post #29 of 44 Old 01-11-2019, 04:07 PM
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Originally Posted by SteveCallas View Post
As stated, even if you cant hear audible chuffing because you are seated far enough away from the sub or the source mayerial is really loud, you are still getting compression. All that extra work to go ported for nothing.

If your enclosure isnt big enough, you cant get away with using enough port cross sectional area for a low tune, or else your length becomes too long and you get a resonance.
extra work to go ported? What extra work?

For nothing? It's not like a little compression is making the port useless, it's still far more output than sealed.
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post #30 of 44 Old 01-12-2019, 06:38 AM
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Quote:
Originally Posted by notnyt View Post
Hm, both sides of the argument are compelling, but I'm still not sure which is correct. It's not like the port resonance is going away or changing, but at the same time it's not going to behave linearly. Can you explain in more detail?
Paa_ is correct. As your port compresses it isn't responding in a linear manner to the power inputs. It starts to lose loading on the driver and the driver excursion will start to grow. If a speaker is pushed into deep vent compression the driver can produce significant excursion if enough power is applied. I've seen it many times. Most vented subs are operating like that to some extent at maximum output.

Nearly every ported sub that is tuned low and uses a high excursion driver is woefully under vented according to textbook vent theory and maximum vent airspeed. Subs seem to work just fine with undersized vents is the good news.
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