Does port direction matter relative to the sub? - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 66 Old 09-08-2019, 08:21 AM - Thread Starter
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Does port direction matter relative to the sub?

I'm modelling something close to the VBSS build for end-tables (down-firing) and using round ports. To keep the air velocity below 30m/s at 1000W I've had to drop the tune to 18hz which leaves me two 4" vents at 27.5" long. I'd like to keep the ports straight to minimize friction which would mean having them fire out the back of the build. Are there any issues created by changing the location of the ports? I usually see commercial subs with either front or rear firing ports, not sure if this merely functional/aesthetic or if there is a solid reason.

Cheers!
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post #2 of 66 Old 09-08-2019, 08:44 AM
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Originally Posted by Aural_Sex View Post
I'm modelling something close to the VBSS build for end-tables (down-firing) and using round ports. To keep the air velocity below 30m/s at 1000W I've had to drop the tune to 18hz which leaves me two 4" vents at 27.5" long. I'd like to keep the ports straight to minimize friction which would mean having them fire out the back of the build. Are there any issues created by changing the location of the ports? I usually see commercial subs with either front or rear firing ports, not sure if this merely functional/aesthetic or if there is a solid reason.

Cheers!

At the wavelengths reproduced by the port, simply put, it doesn't matter where the port is located, as long as sufficient clearance is given to the port outlet. I would not place the port outlet closer than 8 inches to a wall in that case.

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post #3 of 66 Old 09-08-2019, 09:09 AM - Thread Starter
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Originally Posted by michael hurd View Post
At the wavelengths reproduced by the port, simply put, it doesn't matter where the port is located, as long as sufficient clearance is given to the port outlet. I would not place the port outlet closer than 8 inches to a wall in that case.
Thanks! That was my thought as well for the port location, just wasn't sure if a reflection delay could cause phase challenges between the ports and driver.
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post #4 of 66 Old 09-08-2019, 09:18 AM
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Originally Posted by Aural_Sex View Post
Thanks! That was my thought as well for the port location, just wasn't sure if a reflection delay could cause phase challenges between the ports and driver.

18 hz has a wavelength of 62.77 feet. Simply put, a reflection causing a null would have to be a boundary a bit over 15.5 feet away, and at these low frequencies, most of the LF will pass through or around the boundary, though a portion will be reflected. Also, at these low frequencies, you have superimposed reflections off of any other surface close to the port, all closely related in time. ( within 1/4 wavelength )



In short, it's really a non-issue.
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post #5 of 66 Old 09-08-2019, 11:56 AM
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What size driver are you using with two 4” ports? Hopefully not 18”.
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post #6 of 66 Old 09-08-2019, 12:11 PM - Thread Starter
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They are indeed 18's - Dayton UM18-22. BassBox shows the design peaking at 24.8 m/s, WinISD is showing 27.37 m/s - I'm guessing the truth lies somewhere in between.
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post #7 of 66 Old 09-08-2019, 03:30 PM
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Two 4” ports are woefully inadequate for that driver. You should be planning on no less than an 8” diameter.
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post #8 of 66 Old 09-08-2019, 05:46 PM - Thread Starter
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Originally Posted by SteveCallas View Post
Two 4” ports are woefully inadequate for that driver. You should be planning on no less than an 8” diameter.
Can you expand on that? A single 8" port would need to be 178" inches long and cuts the box volume down to internal 2.25 ft^3 from 6.6 ft^3 - unless I'm missing something obvious (entirely possible).
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post #9 of 66 Old 09-08-2019, 05:52 PM
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The further I stray from ideal (and reasonable) port diameter/length, the more I'd consider a rear/bottom firing port. You can absorb some of the extraneous noise that way.
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post #10 of 66 Old 09-08-2019, 08:01 PM
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6 cubes tuned to 18hz doesn’t seem right at first blush, though I haven’t modeled it. What are your design goals? If you want to be solid to the teens, you’ll want to go bigger than 6 cubes. If you can only do 6 cubes, you’ll want to tune higher or go sealed.
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post #11 of 66 Old 09-08-2019, 08:24 PM - Thread Starter
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There is a strong WAF factor in play here - I was able to sell her on end tables. The boxes external will be 21" x 23" x 32" - I dropped the tune from 20hz to 18hz as lowering the tune will drop the cross-sectional area needed for ports for the given volume. I'm trying to strike the balance that keeps me under 30m/s and gives me the maximum box volume (thus the round ports - save on MDF space).
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post #12 of 66 Old 09-08-2019, 08:50 PM
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If you can swing it, use a couple decent passive radiators, don't lose any internal space to port length, stop worrying about vent velocities all for the cost of a couple of drone cones and some extra holes cut in your box. While it's true that passive radiators have more losses than ported designs, this gets obscured when the ports become cumbersome, and remember, no vent noise/resonances is worth something. I like 'em.
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post #13 of 66 Old 09-08-2019, 09:43 PM
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Quote:
Originally Posted by Aural_Sex View Post
Can you expand on that? A single 8" port would need to be 178" inches long and cuts the box volume down to internal 2.25 ft^3 from 6.6 ft^3 - unless I'm missing something obvious (entirely possible).
Aha, and thus the dilemma of the vented subwoofer design! You're not missing anything, I go through this same struggle with every sub I design, battling a reasonable cabinet size with port volume to minimize port air speed. The more you attempt to achieve the ideal port speed, whatever you think that is, 20 m/s, 30 m/s, the more you fight an ever-increasing cabinet size to make up the volume consumed by using such a massive port. In the end you have to ask yourself, does a little port noise really matter that much to you? When you have to explain to your wife that this monstrosity of a subwoofer just doubled in size because that's the only way to achieve less than 5% port air speed, she's not going to be super sympathetic to your plea. I agree with the comments made though, dual 4" ports is probably too small for that sub. I'm seeing 45 m/s modeled in Unibox. You might want to rear-mount them anyway just to try and mask how much noise they're going to make. Minimum should be (3) 4" ports, (2) 6" ports or (1) 8" port to stay below 26 m/s (according to Unibox, though not sure why all the port air speed discrepancies among the programs).

If you can't afford the volume, then you may want to consider going sealed. UM18-22 models great in a 6 cube sealed box with only a few dB less bottom end at 20 Hz. But still pretty close to your vented design. Note that I do not own this driver, so my experience with it is merely academic. Others can maybe speak to the benefit of this driver in a sealed application. I've built a handful of both ported and sealed subs and they both have their merits. But you stumbled upon one of the major drawbacks to a ported design and that is that if you truly care about port noise, the port area has to be large, which means the ports have to be long for a low tune, which means the boxes have to be big. Most designers/manufacturers ultimately will compromise somewhere along the way. I've got a dual passive radiator design in the works to combat this very issue (like Chris mentioned above) - small box, low tune, high WAF, no port noise, but it does come at a cost, to your wallet! You gotta choose your battles.
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post #14 of 66 Old 09-08-2019, 09:59 PM
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Lowering the tune doesn’t decrease the cross sectional port area needed, it increases it, unless you are aiming for a steeply rolled off low end.

If you are limited to that size, just go sealed.
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post #15 of 66 Old 09-09-2019, 06:36 AM - Thread Starter
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Originally Posted by aktiondan View Post
Aha, and thus the dilemma of the vented subwoofer design! You're not missing anything, I go through this same struggle with every sub I design, battling a reasonable cabinet size with port volume to minimize port air speed. The more you attempt to achieve the ideal port speed, whatever you think that is, 20 m/s, 30 m/s, the more you fight an ever-increasing cabinet size to make up the volume consumed by using such a massive port. In the end you have to ask yourself, does a little port noise really matter that much to you? When you have to explain to your wife that this monstrosity of a subwoofer just doubled in size because that's the only way to achieve less than 5% port air speed, she's not going to be super sympathetic to your plea. I agree with the comments made though, dual 4" ports is probably too small for that sub. I'm seeing 45 m/s modeled in Unibox. You might want to rear-mount them anyway just to try and mask how much noise they're going to make. Minimum should be (3) 4" ports, (2) 6" ports or (1) 8" port to stay below 26 m/s (according to Unibox, though not sure why all the port air speed discrepancies among the programs).

If you can't afford the volume, then you may want to consider going sealed. UM18-22 models great in a 6 cube sealed box with only a few dB less bottom end at 20 Hz. But still pretty close to your vented design. Note that I do not own this driver, so my experience with it is merely academic. Others can maybe speak to the benefit of this driver in a sealed application. I've built a handful of both ported and sealed subs and they both have their merits. But you stumbled upon one of the major drawbacks to a ported design and that is that if you truly care about port noise, the port area has to be large, which means the ports have to be long for a low tune, which means the boxes have to be big. Most designers/manufacturers ultimately will compromise somewhere along the way. I've got a dual passive radiator design in the works to combat this very issue (like Chris mentioned above) - small box, low tune, high WAF, no port noise, but it does come at a cost, to your wallet! You gotta choose your battles.
The struggle is real. When it comes down to it I'm not sure how often I'll be seriously pushing these things so there is something to be said about port-noise in real-world application. When you're modeling what input power do you use - RMS or max?
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post #16 of 66 Old 09-09-2019, 06:43 AM - Thread Starter
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Originally Posted by SteveCallas View Post
Lowering the tune doesn’t decrease the cross sectional port area needed, it increases it, unless you are aiming for a steeply rolled off low end.

If you are limited to that size, just go sealed.
You're correct - I had my wires crossed. By dropping the tune I think I was getting closer to my high-end filter so it helped against the velocity.

The three by 4" may be the way to go as it doesn't look like I lose too many of those pesky dB's and at max power I'm still in a reasonable range for m/s.
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Lowering the tune doesn’t decrease the cross sectional port area needed, it increases it, unless you are aiming for a steeply rolled off low end.

What?! lol

Tuning lower DECREASES airspeed, which in turn would allow for less port area.
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post #18 of 66 Old 09-09-2019, 11:11 AM
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What?! lol

Tuning lower DECREASES airspeed, which in turn would allow for less port area.
Negative -at a given sound pressure level.

Remember, you need 4x the displacement for every octave you drop (to keep same SPL).

Real world balance enters in here when we consider the commonality of source material; drop the tune nice and low or after driver response is already rolling off and you'll get less material most of the time, which means the vent is doing less work most of the time, which means vent velocity is reasonable more often than not (most of the time). Some room gain can also help bury this stuff, but end of the day, all else equal, lower tune = higher vent velocity.
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post #19 of 66 Old 09-09-2019, 11:18 AM
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Negative -at a given sound pressure level.



Negative - I never said "at a given sound pressure level"

Box size kept the same (as the user is trying to do)... you lower the tune, you lower the air speed. Which, then if you wanted, you could use a slightly smaller port.

Although, the scenarios are rare in which I would ever go small on a port. 99% of users here suffer from too low of port area. (myself included!)
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Originally Posted by lknhomeaudio View Post
Negative - I never said "at a given sound pressure level"

Box size kept the same (as the user is trying to do)... you lower the tune, you lower the air speed. Which, then if you wanted, you could use a slightly smaller port.

Although, the scenarios are rare in which I would ever go small on a port. 99% of users here suffer from too low of port area. (myself included!)


lol

You're going to be unhappy if you drop your tune to use a smaller port thinking it'll stop chuffing. Particularly when you run room correction.
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post #21 of 66 Old 09-09-2019, 11:51 AM
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lol

You're going to be unhappy if you drop your tune to use a smaller port thinking it'll stop chuffing.
Not sure why you're laughing lol

Go to your favorite box modelling program, lower the tune, watch airspeed drop, be amazed.

The end!
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post #22 of 66 Old 09-09-2019, 03:59 PM
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My statements stands correct. If you want a steeply rolled off low end and compression, go for it, but then what’s the point of going to the extra work of porting.

Three 4” ports still doesn’t equal one 8” port in cross sectional area, you run the risk of crowding the port openings, and if you want flared ends, it will be more expensive than the single 8”.

Only trying to help. Raise the tune or take a look at sealed.
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post #23 of 66 Old 09-09-2019, 04:42 PM - Thread Starter
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My statements stands correct. If you want a steeply rolled off low end and compression, go for it, but then what’s the point of going to the extra work of porting.

Three 4” ports still doesn’t equal one 8” port in cross sectional area, you run the risk of crowding the port openings, and if you want flared ends, it will be more expensive than the single 8”.

Only trying to help. Raise the tune or take a look at sealed.
I hear you - just trying to find the balance as an 8" port needs 58.28" for 20hz; even bumping it to 25hz requires 28.08". For the three 4" at 20hz they end up at 37.67". For the impact to box volume the 8" eats 2.2ft^3 and the 4's about 1.4 ft^3. It looks like either one will perform below the 20m/s at 1000W so I may as well go with the three 4's. I appreciate the input.
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post #24 of 66 Old 09-09-2019, 05:33 PM
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Quote:
Originally Posted by lknhomeaudio View Post
Tuning lower DECREASES airspeed, which in turn would allow for less port area.
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Originally Posted by lknhomeaudio View Post
Box size kept the same (as the user is trying to do)... you lower the tune, you lower the air speed. Which, then if you wanted, you could use a slightly smaller port.
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Originally Posted by lknhomeaudio View Post
Go to your favorite box modelling program, lower the tune, watch airspeed drop, be amazed.
If you keep box size and port area the same, then lowering tune does appear to decrease airspeed, but if you're doing that, you're not yet striking the proper balance between port resonance and port velocity. If you can simply increase the port area, you don't need to change tune to reduce port velocity. Instead, the issue that normally occurs during design is that you've already maximized port area with the constraint being the lowest port resonance you're comfortable with. If you're not happy with port velocity at that point, the solution is to raise tune and upsize the port to whatever is now possible at that higher tune while holding the same port length constraint. Overall, the port velocity decreases.

Decide the maximum port length you're comfortable with based on the resulting port resonance (for me, it's about 48"). Maximize port area for that port length and desired tune, and now you've reduced port velocity as much as you can for that particular box and tune. If you aren't happy with that velocity, you can increase the size of the box or increase the tune. Both of those changes support more port area for a given port length. If you lower tune, port area has to go down, and velocity goes up.

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OP, I suggest you start by looking at the Max SPL graph and deciding which tune you like best with 6.5cf and the UM18. From there, decide the lowest port resonance you're comfortable with, maximize port area at that port length, and then see what velocity looks like (after entering power, and adding a proper HPF to keep excursion in check). If you're happy with velocity at that point, you're done. If not, your only choice is to make the box bigger or tune higher.

If you just tell me what your desired tune is, I'll lead you the rest of the way.

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Originally Posted by aktiondan View Post
Aha, and thus the dilemma of the vented subwoofer design! You're not missing anything, I go through this same struggle with every sub I design, battling a reasonable cabinet size with port volume to minimize port air speed. The more you attempt to achieve the ideal port speed, whatever you think that is, 20 m/s, 30 m/s, the more you fight an ever-increasing cabinet size to make up the volume consumed by using such a massive port. In the end you have to ask yourself, does a little port noise really matter that much to you? When you have to explain to your wife that this monstrosity of a subwoofer just doubled in size because that's the only way to achieve less than 5% port air speed, she's not going to be super sympathetic to your plea.
In all of my sub designing, I've never had port volume make a major impact like you're describing. Were you designing something really small and possibly trying for an unreasonably low tune for such a small box? Even in that case, it's not the port volume that's the problem, it's the port area that you can't make any bigger without increasing length beyond your limit.

Take the Red Pill (BassEQ) BassEQ Demo Clips
Video: Sony 85" X900F @ 80" eyes-to-screen (49.4° viewing angle)
Audio: Denon AVR-X4400H 7.2.4 Atmos
Mains: Fusion-15 LR, Custom Tapered Ported Volt-6 Center, Ported Volt-10 Surrounds, Custom 45°/45° Double-Angled Ported Volt-6 Atmos
Subs: The Two Towers (HT18 32cf 11.5Hz x 2), UM18 4cf x 2, Crowson MAs x 4
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post #27 of 66 Old 09-09-2019, 07:31 PM
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Originally Posted by lknhomeaudio View Post
Not sure why you're laughing lol

Go to your favorite box modelling program, lower the tune, watch airspeed drop, be amazed.

The end!
You're hamstringing the design when that's your "method". It's not an efficient alignment. Dropping the tune when the design is already rolling off *DOES* reduce the vent airspeed... but it's called having less output You could just signal shape to remove some output down low and it would do exactly the same thing.

Conversely, like I said, if you add some boost down low because "she needs a little help" (not that anyone here at AVS has or would do such a thing!!!) and you think your lower tune/reduce vent velocity key to victory method works, you'll be sorely surprised that there isn't a free lunch and you'll be in chuff-city.

Sim racing is fun, to a point, but no one listens to sims. Nearly all of us use room eq of some sort, and target a response of some sort vs. plopping 'er down and we get what we get. Real world you can do exactly what you suggested as long as you high pass or use DSP to remove output such that the vent isn't a risk anymore. IMO that's an extremely poor design choice for ported.

Just for fun, run the 6 cuft sealed model next to a ported 6 cuft design with a single 2" diameter port 30" long. Response is nearly the same! With 1000 watts vent velocity doesn't exceed 24m/sec until under 12hz. Doesn't make it a great idea.

Chris

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post #28 of 66 Old 09-09-2019, 07:35 PM
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Originally Posted by aron7awol View Post
In all of my sub designing, I've never had port volume make a major impact like you're describing. Were you designing something really small and possibly trying for an unreasonably low tune for such a small box? Even in that case, it's not the port volume that's the problem, it's the port area that you can't make any bigger without increasing length beyond your limit.
Yeah, very small and low is a PITA with ports unless you do some cumbersome folded slot. Go model the tangband w6-1139 for an example of "hard to port". Luckily it's a candidate for multiple drivers giving a touch more wiggle room, but you'll get the idea. Great candidate for a PR.
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post #29 of 66 Old 09-09-2019, 08:33 PM
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Quote:
Originally Posted by aron7awol View Post
In all of my sub designing, I've never had port volume make a major impact like you're describing. Were you designing something really small and possibly trying for an unreasonably low tune for such a small box? Even in that case, it's not the port volume that's the problem, it's the port area that you can't make any bigger without increasing length beyond your limit.
Right, so as the port area increases, so does the required length for a given tune and therefore the total port volume increases as well. And that's a problem when you have a target enclosure volume/size you're trying to achieve that the port volume keeps eating up. And yes, this is usually more of a design constraint when you've got a small box and a low tune. But it doesn't have to be unreasonably low. Some drivers model better in small enclosures with a low tune (or at least relatively low, depends on your definition of low).
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post #30 of 66 Old 09-09-2019, 08:43 PM
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Originally Posted by aktiondan View Post
Right, so as the port area increases, so does the required length for a given tune and therefore the total port volume increases as well. And that's a problem when you have a target enclosure volume/size you're trying to achieve that the port volume keeps eating up. And yes, this is usually more of a design constraint when you've got a small box and a low tune. But it doesn't have to be unreasonably low. Some drivers model better in small enclosures with a low tune (or at least relatively low, depends on your definition of low).
But what I'm saying is the issue isn't that the port volume is eating significantly into the total volume, it's that you can't make the port any longer without running into resonance issues. In OP's case, for example, I modeled a 5"x4" port @ 48" long. That's only ~0.6 cf, a perfectly reasonable amount of space for a port to take up. That came out to a tune of ~18Hz. If he wants a lower tune, I can't increase the port area (assuming 48" length is the most I'm comfortable with), I have to actually decrease it, which reduces the port volume. Port volume is a non-issue here.

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