Better port design?

[napilopez]

My Polks have an unusual port.

Exclusive Polk Power Port Technology
Power Port® is our patented design that smoothly transitions the air flow from the speaker’s port into your listening area. It also extends the overall bass port, providing greater surface area to eliminate turbulence and distortion, for bigger, more musical, deep bass impact. Plus, it offers 3dB more bass response output than a traditional port.

Unfortunately and ironically, for all Polk talked up its Power Port technology, the L200 had some of the worst port anomalies I've ever seen.

The end result didn't seem to affect the sound too much in my room, but I wasn't exactly looking for it when I listened to the speakers. But it is very evident in the measurements.

The spin overall is solid, but you can see the port anomalies at ~650 and ~1200 Hz:

Which are then more evident in the off-axis curves:

and then completely obvious in the nearfield measurement:

The absolute worst I've measured in this regard was probably the Amphion Argon1, which from my recollection just had a straight hard tube for a port directly behind the woofer:

The port's midrange peak is nearly as high as its bass peak:

It's also worth noting that I believe these port anomalies are actually about 4-6dB higher than they look in nearfield measurements because of baffle loss. Here's the above nearfield measurement when i apply baffle step correction:

But then I'm not sure how directivity comes into play for these port issues. Wouldn't the midrange peak be a bit more directional than the bass one, having less influence on the on-axis response with a rear-firing port like this one?

In any case it's evident the port anomalies are having an effect on the on-axis measurement too in the above two cases.

 

[tuga]

Unfortunately and ironically, for all Polk talkeds its Power Port technology, the L200 had some of the worst port anomalies I've ever seen.

Since so many of them show port issues, maybe all small standmounts should be sealed and sold in a bundle with a sub.

 

[TheBatsEar]

Don't use a port but a passive membrane. Yes, i know, a port costs 30 cent, a passive membrane maybe 70 cent, but to me it seems to be worth it.

 

[napilopez]

Since so many of them show port issues, maybe all small standmounts should be sealed and sold in a bundle with a sub.

While I definitely would not mind more bundled bookshelf+sub combos, it's evident this issue is completelymanageable in bookshelf speakers too. The KEF LS50 Meta is as small or smaller than the two aforementioned speaker:

The Neumann KH80 is one of the smallest speakers I've measured, and it laughs at petty port issues:

The Focal Chora 806 is rather cheap for a 6.5-inch speaker in Europe, at 600 to 700 euros I believe, and it has a rather basic-looking cabinet. But it still manages to keep the issue reasonably under control:

Don't use a port but a passive membrane. Yes, i know, a port costs 30 cent, a passive membrane maybe 70 cent, but to me it seems to be worth it.

A passive radiator is not necessarily better by default, as we saw in Amir and Erin's measurement of the Buchardt S400. Curiously, my review unit did not display this issue.

But on the Definitive Technology D9, which also uses a passive radiator, we see performance that isn't much better than a well designed port:

Now does it cost more to make a good port than it does to properly implement a passive radiator?

 

[KSTR]

THE classic paper by JBL on port design : https://jahonen.kapsi.fi/Audio/Papers/AES_PortPaper.pdf
Dealing with turbulence/noise/compression issues. Midrange leakage (and port "tube" resonances induced by it or the nearby midwoofer) is a different thing not exactly related to port design... it's enclosure design. Technics seem to have mastered it... at the cost of slight efficiency loss.

 

[dfuller]

Don't use a port but a passive membrane. Yes, i know, a port costs 30 cent, a passive membrane maybe 70 cent, but to me it seems to be worth it.

Passive radiators have their own issues if I'm honest. They ring worse than ports in the low end so the "overhang" is worse. Plus they're considerably more expensive.

Since so many of them show port issues, maybe all small standmounts should be sealed and sold in a bundle with a sub.

Yes and no. It is possible to design a smallish ported speaker without midrange weirdness, it just requires decent port design (see: Genelec, Dynaudio, Focal, etc)

 

[napilopez]

Yes and no. It is possible to design a smallish ported speaker without midrange weirdness, it just requires decent port design (see: Genelec, Dynaudio, Focal, etc)

Just as another example, the super cheap and rather small JBL 104 has 99 problems, but its port ain't one:

No visible effect on the spin:

 

[beaRA]

Unfortunately and ironically, for all Polk talked up its Power Port technology, the L200 had some of the worst port anomalies I've ever seen.

The end result didn't seem to affect the sound too much in my room, but I wasn't exactly looking for it when I listened to the speakers. But it is very evident in the measurements.

The spin overall is solid, but you can see the port anomalies at ~650 and ~1200 Hz:

View attachment 124846

Which are then more evident in the off-axis curves:

View attachment 124847

and then completely obvious in the nearfield measurement:

View attachment 124848

The absolute worst I've measured in this regard was probably the Amphion Argon1, which from my recollection just had a straight hard tube for a port directly behind the woofer:

View attachment 124853The port's midrange peak is nearly as high as its bass peak:

View attachment 124849

It's also worth noting that I believe these port anomalies are actually about 4-6dB higher than they look in nearfield measurements because of baffle loss. Here's the above nearfield measurement when i apply baffle step correction:

View attachment 124852

But then I'm not sure how directivity comes into play for these port issues. Wouldn't the midrange peak be a bit more directional than the bass one, having less influence on the on-axis response with a rear-firing port like this one?

In any case it's evident the port anomalies are having an effect on the on-axis measurement too in the above two cases.

Polk redesigned the port (now called X-Port) for the new Reserve series. Any plans to test out the R200? I'm curious to see if they outdo their own Legend Series in the port resonance area.

 

[napilopez]

Polk redesigned the port (now called X-Port) for the new Reserve series. Any plans to test out the R200? I'm curious to see if they outdo their own Legend Series in the port resonance area.

Yep, I've requested a unit and hope they've addressed this.

 

[mhardy6647]

This thread reminds me of the -- somewhat fleeting -- popularity of the passive radiator as a "portless" way to make a Helmholtz Resonator.

Polkmonitor7Asn4900 by Mark Hardy, on Flickr

I think I've already posted my port noise complaint (bad) joke in some other thread on these august forums, so I'll spare all y'all this time...

 

[Kvalsvoll]

Just as another example, the super cheap and rather small JBL 104 has 99 problems, but its port ain't one:

View attachment 124862

No visible effect on the spin:

View attachment 124863

This is sort of what you can hope for in a proper design - mid leakage around -15 to -20dB down, does not show up in on-axis.

An additional disadvantage with ported is that internal damping inside the cabinet can not be too stuffed, because then the port loading will no longer work due to too high acoustic losses. Then it becomes difficult to achieve good damping of cabinet internal reflections and resonances - kind of similar to having a room with too little absorption. This sound radiates out through the port, and also through the driver cone.

Integration with the bass-system is more difficult to do properly with a ported, because the phase-shift and GD is much higher. The port can then be damped by inserting a foam plug or similar. Here, the benefits of the port is usually not present anyway, because the port loads only at frequencies below crossover.

So, yes, it makes sense to design small speakers as sealed.

 

[Putter]

This is sort of what you can hope for in a proper design - mid leakage around -15 to -20dB down, does not show up in on-axis.

An additional disadvantage with ported is that internal damping inside the cabinet can not be too stuffed, because then the port loading will no longer work due to too high acoustic losses. Then it becomes difficult to achieve good damping of cabinet internal reflections and resonances - kind of similar to having a room with too little absorption. This sound radiates out through the port, and also through the driver cone.

Integration with the bass-system is more difficult to do properly with a ported, because the phase-shift and GD is much higher. The port can then be damped by inserting a foam plug or similar. Here, the benefits of the port is usually not present anyway, because the port loads only at frequencies below crossover.

So, yes, it makes sense to design small speakers as sealed.

If it's going to be used with a sub. (There, fixed it for you) I do actually agree with your statement. However the economics of selling small speakers typically make it a difficult sell given that most buyers are unwilling to buy a sub, at least initially.

My thought would be to look at center speakers, many of which are sealed

 

[More Dynamics Please]

... My thought would be to look at center speakers, many of which are sealed

Many sealed centers like the Infinity RC263 tested on ASR are designed to the old THX sealed satellite spec to begin rolling off at 12 dB/octave to -3dB @ 80 Hz to match up with the 80 Hz crossover designed into most AVRs -- 24 dB/octave low pass and 12 dB/octave high pass. But of course listening tests without subwoofer support of sealed speakers that are -3 dB @ 80 Hz invariably produce criticism of "weak bass."

 

[Kvalsvoll]

If it's going to be used with a sub. (There, fixed it for you) I do actually agree with your statement. However the economics of selling small speakers typically make it a difficult sell given that most buyers are unwilling to buy a sub, at least initially.

My thought would be to look at center speakers, many of which are sealed

In that case, a port makes sense. It is possible to get much higher spl capacity at low frequencies, better efficiency and reduced distortion. A compromise - but that is essentially what any audio design is, a choice of compromise.

Some of my speakers would not be possible without some sort of acoustic port loading.

 

[ROOSKIE]

We have seen recently that many cheap speakers have port resonances. It looks to me they do use basic port (a tube).
I have seen a few better designs, KEF or Yamaha comes to mind. Do you have other examples? I am surprised that
designers are not doing better, this is a very well studied fluid dynamics problem.

Yamaha example (from this website):

View attachment 124787

KEF example (from LS50 meta manual):
View attachment 124788

Most of what you see here is designed to reduce port chuffing, turbulence from high port seed/velocity with low bass.

Port pipe resonances are completely different and as I understand it, every port has them. There is not a way to avoid that. There are ways to reduce their affects by various means. My understanding is that if you have the time and R&D budget, you can locate an area where the exaggerated waves being leaked out of the port are in a "null" inside the enclosure and locate the port termination there.
With a certain tuning strategy (based on length of port)you can also drive the pipe resonances above 1k - the further up the better - where internal stuffing can reduce them. The stuffing has drawbacks and some designers prefer to leave the stuffing aside, and accept the resonances.
Cabinet shape can affect the severity of the pipe resonance and standing waves in the cab.

In any case, rear ported speakers are benefited(&advantaged over front ports) by using some room treatment behind the speaker. Room treatments can address some of this. Also pulling the unit further front the front wall has an affect as it delays the bounce from the pipe resonances.

The port tube can be designed to cancel the pipe resonate frequency, there are several methods purported to work. One involves making small holes in the port tube to help can say break up a 1khz wave but since the holes are small the bass waves do not "see" the holes in the tube.

It looks like Polk's design is a take on finding a way to breakdown the higher frequency waves before they exit the port while leaving the bass alone.
To my eye it looks very promising but I am no expert.

Excited to find out. Go Polk.

PS. These pipe resonances are 1 good reason for a 3 way.

 

[tuga]

The Harbeth Monitor 30.1 is an "evolution" of an early 80's BBC design.

No fancy ports in sight (just a tube with no flange flaring - see below) and yet the roll-off is just about perfect.
Why is that so?
The optimised port location?
The front-facing port?
The larger cabinet size?
All of the above and/or something entirely different?

https://www.stereophile.com/content...-anniversary-edition-loudspeaker-measurements

source

 

[tuga]

Maximizing Bass Reflex System Performance Through Optimization of Port Geometry
Bryce Doll, University of Central Florida
https://stars.library.ucf.edu/cgi/viewcontent.cgi?article=1956&context=honorstheses


Yamaha (NS-3000/NS-5000) bass reflex port design patent
https://patents.google.com/patent/EP2779692A2/en

 

[fredoamigo]

Surprised also that the problem of shapes is not yet solved although the flares are taking a small lead without anything being yet written in stone by science.

https://www.aes.org/tmpFiles/elib/20210418/20683.pdf

 

[Eetu]

A short video on Genelec port design:

 

[KSTR]

Port pipe resonances are completely different and as I understand it, every port has them. There is not a way to avoid that.

There is a way, and that is spread the apparent port length, this also spreads the tube resonances. Neumann does this in the KH120, by having cut-outs in the port covered with dampening material. Penalty again is reduced port efficiency and of course the dampening is probably not perfectly linear (not independent of pressure and air velocity it is subject to). See https://www.audiosciencereview.com/forum/index.php?threads/neumann-kh120-disassembly-pictures.11975/
EDIT: Genelec's port design (in the 80xx models) is somewhat similar.