Can Anyone Explain What Happens When Plugging Speaker Ports

[springwood64]

My Espeks have two ports in each cabinet. A small one at the top and rear, behind the midrange driver, and a big one at the front and bottom that is linked to the woofer.

While the speakers sound lovely at most volumes, I've been getting high frequency harshness at high volumes.

On a hunch, I guessed that the harshness comes from interaction with the rear wall plasterboard, and that a bung in the top port could reduce the troublesome frequencies.

Knowing nothing about how speaker ports work, I cut a 25mm deep bung to target frequencies around the 13-14kHz. I also cut a 22mm bung as a control.

The bungs have a striking impact. The 22mm bung does not touch the harshness, yet the 25mm bung eliminates it. The difference is not subtle!

I measured the frequency response using my phone and a sweep tone, with and without the 25mm bung, and I can see a little dip around 13-14khz caused by the bung, but the higher frequencies are slightly boosted by the bung, giving a flatter frequency response overall than no bung. Below 13khz there's no difference in frequency response.

Is this to be expected? I can't find anything online that mentions this sort of behaviour when plugging ports. All the comments or videos I've found discuss bass response only.

I can't find any information that indicates that different bung lengths can filter selected frequencies.

[springwood64]

After a few listening sessions, it is clear that the rear-facing port bungs bring a delightful and substantial musical improvement.

I've ended up with a bung depth of 47mm for the best result.

I will probably refine the speaker position tuning again at some point in the future, as the port bungs have eliminated the harshness that made the previous tuning session so difficult.

However, the speakers sound so good now that I'm in no hurry to make any changes.

[springwood64]

While researching to try and understand what is happening when I bung the ports, I came across this site which provides a readable introduction to speaker construction.

https://www.sound-au.com/articles/enclosures.htm

Having read this, I think that when I bung the rear facing ports of the Espeks, I am not filtering any frequencies. Instead, I think I am changing the volume of the enclosure and the air suspension tuning of the cabinet. I think the effect of this is to flatten the frequency response for the mid range and the tweeter, and to provide an earlier, gentler, frequency roll-off from mid to bass. In other words the mid starts to roll off at higher frequencies than when the port is unblocked, and its responsiveness declines more slowly as the frequencies get lower.

[Spannko]

You appear to be having a conversation with yourself here Pete 😂, so I’ll join you and see if we can work something out together.

Going back to your first post, you made a 25mm bung with the intention of affecting frequencies in the 13-14kHz region. Measurement showed a reduction in level at these frequencies. That’s pretty impressive, well done! My own predictions rarely prove to be accurate. I didn’t reply to your first post because tbh I’ve absolutely no idea what’s going on there. However, thinking about how ports work, and I’ve very limited knowledge of this, I would expect that blocking a midrange port would have an effect on frequencies below the woofer/mid crossover frequency, and certainly no where near the frequencies you found to be effected. Remember though, I did say that my predictions are notoriously inaccurate! A port resonance will usually support frequencies below the drivers natural resonance frequency, extending the range of the drive unit (sort of - it’s not quite that simple). The Linn engineers would’ve taken this into account when designing the crossover, possibly by designing the bass to crossover at a slightly lower frequency than they would’ve had the midrange unit been built into a sealed enclosure …. possibly! Therefore, I would’ve expected to see a dip in output just below the bass/mid crossover frequency when blocking the mid port. Having said that, I don’t think you actually blocked the port completely, depending on the porosity of the foam used. It’s possible that you created some kind of hybrid pressure loading on the midrange unit, with my guess being more towards an aperiodic loading. That is, the enclosure is partially open to the outside atmosphere, but without the tuned, periodic resonance of a ported loudspeaker. Doing so may have added a bit of “warmth” just below the bass/mid crossover point, masking any distortions at higher frequencies. The ratio of (250-500 Hz):(500-1000 Hz) can be used to quantify how warm or clinical a loudspeaker may sound. It’s worth pointing out that this is a conventional HiFi way of describing sound which has no relationship to how musical a loudspeaker is.

My thoughts are that you are not really changing the volume of the cabinet, but you’re certainly changing the tuning, and your intuitive feeling that the frequency response of the mid has been altered could be bang on. There’s also the possibility that bunging the port has linearised the output of the midrange unit at the bass/mid crossover frequency by removing the nonlinear effects of the port resonance.

[springwood64]

Ha ha! Thanks for interrupting the monologue.

I'm no longer convinced that the bung targeted the frequencies I originally thought were problematic. While the sweep tone trace does show a dip with the 25mm bung, it's a pretty noisy trace across the frequency range, and there are similar magnitude differences at different frequencies, greater and lesser. I think I was guilty of applying an unjustified interpretation on the results I recorded.

To add to the complexity, the foam bung has changed shape, shrunk a bit to the diameter of the port and is not as tight. The original dramatic difference between the 25mm and 22mm bungs has disappeared. Now I need both bungs in to achieve the effect that the 25mm alone had originally.

So maybe that means the shrunken bungs need more friction in the port to have the same effect, and bung depth is only important in that it affects the friction necessary to lock the bung in place.

The difference for the listener is really impressive, which is why I'm so curious as to what is actually going on.

[Spannko]

You’re obviously onto something, even if we don’t have an explanation for why you’re hearing what you hear!

[markiteight]

target frequencies around the 13-14kHz.

Did you mean 1.3-1.4 kHz? 13-14 kHz is into the topmost octave of human hearing and beyond the operating range of the midrange driver, and well outside the frequency range affected by the port.

You're pretty much correct in your assessment of what's happening when you plug the port, but there is a lot more going on. Isn't that always the case?

Over the years I've experimented with plugging the ports on various speakers with mixed results. Some improved, others didn't. But I was just shoving socks into the ports with no consideration for the effectiveness of the seal or the change of internal volume.

Speaking of volume, your bungs are changing the volume of the enclosure. When you plug the port the airmass inside the port becomes part of the overall airmass inside the enclosure. By adjusting the length of the bung you're changing the volume of air inside the port, thus the overall volume of the enclosure. The difference is minuscule, but it is a non-zero change. It's fascinating that you experience a significant change in musical performance with such a small adjustment.

[springwood64]

Did you mean 1.3-1.4 kHz? 13-14 kHz is into the topmost octave of human hearing and beyond the operating range of the midrange driver, and well outside the frequency range affected by the port.

The reason I targeted 13-14 kHz was because I thought that the harshness must be caused by frequencies at the upper limit of hearing. I agree that my original thinking was almost certainly wrong.

But I was just shoving socks into the ports with no consideration for the effectiveness of the seal or the change of internal volume.

I did try some socks initially, and they reduced the harshness, but they also reduced the musicality. It was a useful test because it demonstrated that bunging the port can tune the sound.

[chefren]

The air inside a speaker cabinet acts as a kind of spring or suspension that the speaker driver operates against. In a sealed cabinet (also called acoustic suspension) the air inside the speaker doesn't leak into our out of the speaker when playing. So when the speaker driver moves outward, the air pressure in the cabinet decreases which introduces a force sucking the driver cone back in, and when the driver moves inwards, the air pressure increases, which pushes the drived cone outward. This helps the amplifier keep the driver cone under control which is why you often hear tighter bass from sealed speakers.

If the cabinet leaks, for example through the reflex port, this reduces the control the amplifier has over the cone, but since a port is tuned to a specific frequency, it will only affect this frequency. In addition the port itself will produce sound of that frequency, like when you blow on the opening of a bottle. This will provide more bass, typically the speaker designer has leveraged this to allow a speaker to go lower by selecting the appropriate port tuning to a frequency at the lower end of what the driver would normally play to.

The tuning frequency depends on the ratio between the port diameter and length, which is why a larger port also has to be longer to stay at the same frequency

A partially blocked port will allow some leaking of air, but it will also dampen any audible noise from the port meaning you get a kind of in-between situation. It matters what you damp the port with, tightly packing it with airtight material will basically disable the port completely, while something leaky like a sock will allow it to work partially.

However it's not just about the port, because speaker drivers are designed to work in different types of cabinets, a driver well-optimized to work in a small ported cabinet will not work optimally in a sealed cabinet of the same size.

So why am I writing this? Just to point out that every driver is different, every port is tuned differently and the cabinet sizes vary too. If you get better results partially or completely blocking your speaker ports, great! But that does not mean the same thing will happen with other speakers.

Tweeters are usually sealed constructions and not affected by the pressure changes caused by a bass driver inside a shared speaker cabinet, as those would overwhelm the small and light tweeter dome. So if you hear differences in the upper range, my guess is that you are damping some resonances, for example the port could resonate at higher frequencies and you now managed to damp this and can't hear them anymore.

Note that you can also get speaker bungs with holes in the middle for partial blockage.

[Rutger]

The air inside a speaker cabinet acts as a kind of spring or suspension that the speaker driver operates against. In a sealed cabinet (also called acoustic suspension) the air inside the speaker doesn't leak into our out of the speaker when playing. So when the speaker driver moves outward, the air pressure in the cabinet decreases which introduces a force sucking the driver cone back in, and when the driver moves inwards, the air pressure increases, which pushes the drived cone outward. This helps the amplifier keep the driver cone under control which is why you often hear tighter bass from sealed speakers.

If the cabinet leaks, for example through the reflex port, this reduces the control the amplifier has over the cone, but since a port is tuned to a specific frequency, it will only affect this frequency. In addition the port itself will produce sound of that frequency, like when you blow on the opening of a bottle. This will provide more bass, typically the speaker designer has leveraged this to allow a speaker to go lower by selecting the appropriate port tuning to a frequency at the lower end of what the driver would normally play to.

The tuning frequency depends on the ratio between the port diameter and length, which is why a larger port also has to be longer to stay at the same frequency

A partially blocked port will allow some leaking of air, but it will also dampen any audible noise from the port meaning you get a kind of in-between situation. It matters what you damp the port with, tightly packing it with airtight material will basically disable the port completely, while something leaky like a sock will allow it to work partially.

However it's not just about the port, because speaker drivers are designed to work in different types of cabinets, a driver well-optimized to work in a small ported cabinet will not work optimally in a sealed cabinet of the same size.

So why am I writing this? Just to point out that every driver is different, every port is tuned differently and the cabinet sizes vary too. If you get better results partially or completely blocking your speaker ports, great! But that does not mean the same thing will happen with other speakers.

Tweeters are usually sealed constructions and not affected by the pressure changes caused by a bass driver inside a shared speaker cabinet, as those would overwhelm the small and light tweeter dome. So if you hear differences in the upper range, my guess is that you are damping some resonances, for example the port could resonate at higher frequencies and you now managed to damp this and can't hear them anymore.

Note that you can also get speaker bungs with holes in the middle for partial blockage.

Good information.

Can only add:
A ported speaker, If its done well, may benefit from the backpressure from the tube making the driver standing almost still at the tuning frequency and one octave above it. Below the tuning frequency the cone has no help from either the box or the tube. In such cases it might be beneficial to use a highpass filter below the tuning point . This is how its done in the exakt implementations.

A ported box speaker can have higher spl and lower distortion at, and above the tuning frequency of the port. Compared to a closed box.

[Spannko]

…… and a closed box creates a more realistic portrayal of bass notes, plays more in tune, and the bass goes lower, without the artificial upper bass boost of the ported fart boxes!

[Rutger]

…… and a closed box creates a more realistic portrayal of bass notes, plays more in tune, and the bass goes lower, without the artificial upper bass boost of the ported fart boxes!

You surely get that artificial upper bass boost If the boxes are way to small, and the tuning frequency to high . Its difficult to make a ported loudspeaker that is tuneful, but its not impossible. The key to get good results is using lower tuning alignment than the school book teaches us, ( making it more behaving as a closed box speaker ) and to use a high pass filter right below the tuning frequency.

A ported speaker should, in my opinion, never be tuned above the lowest tone of a 4 string electric base, maybe thats why Linn never had any real success with ported speakers? ( majik 140 is one exception, tuned at 36 Hz. )

[chefren]

This basically means it has to be an active speaker. Due to the low frequencies involved, a passive speaker level filter would have to use a very large, expensive coil and any high pass filter would have to deal with two large impedance peaks in the region it's operating in (one from the port tuning frequency and one from the driver's free air resonance (Fs) frequency). This really isn't feasible in a passive speaker.

Tuning a port to at around 30Hz or lower to avoid the lowest notes of bass guitars or say pianos successfully does require a larger speaker/driver already that what is modern today.

[Spannko]

…… and a closed box creates a more realistic portrayal of bass notes, plays more in tune, and the bass goes lower, without the artificial upper bass boost of the ported fart boxes!

You surely get that artificial upper bass boost If the boxes are way to small, and the tuning frequency to high .

Of course! I took it for granted that we were talking about optimally designed enclosures. Just so we’re clear, my definition of an optimally designed enclosure is one which allows the drive unit to sound its most harmonious. I certainly don’t mean an enclosure designed to conform to a particular Qtc value, for example.

[springwood64]

Thanks for the contributions that explain the interactions of ports and port bungs.

I have now had a few weeks to get used to my 'new' speakers, and I thought it might be interesting to summarise the journey and describe the current 'destination'. Most likely my experience brings nothing new for those who understand speaker design, but for me it's been very surprising (and at times a bit stressful).

I'll start at the end. My Espeks now do the most important job of music replay much better: emotion and engagement. How do I measure this? Everyone in the house responds to music now - whatever the volume - by singing or humming along or dancing. It is striking, and a step change from before.

Other aspects have changed too: when I stop dancing and singing, and listen, I hear more detail, even in records I've owned since a child. I can hear the character of individual instruments and singers, if I choose to listen for them. I listened to Paul Simon's Graceland last night and was really taken by the drumming on That Was Your Mother, and then again by the wonderful layering of percussion on Grace Jones' Nightclubbing.

While there is an optimum listening spot, it really doesn't matter where in the room I listen unless I want to immerse myself in the image, which is sometimes fun to do for some records, like Marillion's Misplaced Childhood.

Now I dare not touch any aspect of the speakers for fear of losing this magic! To me it seems mysteriously disproportionate that all this messing about with port bungs, speaker positioning, base washers, etc can arrive at such a beautiful result.

I haven't experienced this level of engagement and emotion with any other system I've listened to (all at dealers). It makes me wonder how you can possibly judge a speaker performance unless you are sure it is set up optimally, since set up can transform the performance.

Anyway, to recap the journey, I've had my Espeks since around 2008 (I think) and early on my brother made me some limestone bases which, after a bit of experimentation with washers, improved the mid to lower frequencies.

I've tuned speaker positions and driver bolt torques a few times and gained incremental improvements, but I now know that I was a long way off the best these speakers can do.

After the Covid period, I re-soldered the Linn Knekt plugs on my K400, created jumper connections out of soldered K400 and brass washers to replace the Espek connector plates, replaced steel washers on the bases with brass (another surprising improvement), and then most dramatically replaced the foam driver gaskets with silicone sealant.

This last step was the cause of some anguish. It brought much more precision and information, but at higher volumes the speakers sounded unpleasantly harsh.

Finally re-tuning the speaker positions and adding foam port bungs catapulted the speakers into their current wonderful configuration.

[beck]

I'll start at the end. My Espeks now do the most important job of music replay much better: emotion and engagement. How do I measure this? Everyone in the house responds to music now - whatever the volume - by singing or humming along or dancing. It is striking, and a step change from before.

Great job done. A good hi-fi makes the music come alive to anyone. Not just to those with “Golden ears”. : )

Enjoy Your system!

[springwood64]

Enjoy Your system!

Thanks Beck!

[lejonklou]

I agree, well done!

I'd love to hear the system. Any clips recorded since you did the final re-tuning?

[Rutger]

This basically means it has to be an active speaker. Due to the low frequencies involved, a passive speaker level filter would have to use a very large, expensive coil and any high pass filter would have to deal with two large impedance peaks in the region it's operating in (one from the port tuning frequency and one from the driver's free air resonance (Fs) frequency). This really isn't feasible in a passive speaker.

Tuning a port to at around 30Hz or lower to avoid the lowest notes of bass guitars or say pianos successfully does require a larger speaker/driver already that what is modern today.

Yes, the high pass filter below the tuning frequency has to be active. Dsp or analog.
I meant a 4 string electric bass ( lowest E is 41 Hz ) .
There are some instrument going lower, thats true.

A good tuning frequency should be about 36 Hz or lower, meaning that many ”modern” cabinets are to small to be suitable. You get a boom-box with ported speakers and small cabinet volume.

[Rutger]

…… and a closed box creates a more realistic portrayal of bass notes, plays more in tune, and the bass goes lower, without the artificial upper bass boost of the ported fart boxes!

You surely get that artificial upper bass boost If the boxes are way to small, and the tuning frequency to high .

Of course! I took it for granted that we were talking about optimally designed enclosures. Just so we’re clear, my definition of an optimally designed enclosure is one which allows the drive unit to sound its most harmonious. I certainly don’t mean an enclosure designed to conform to a particular Qtc value, for example.

You can have a better musical soundresult with an optimal closed box with 4 drivers than a ported box with only one driver of equal size and xmax. The spl at the tuning frequency will however be about the same. The closed box is un-efficient, but its advantage is much lower group delay avoiding one note bass.

So, a ported box is a compromise to get higher spl in a cheaper way.

To be ontopic: I have heard the espeks in a smaller room, and it was hard to make them sound really good in the base. Im not surprised they sound better plugged during such conditions.

[springwood64]

I'd love to hear the system. Any clips recorded since you did the final re-tuning?

Good point. I will dig out an earlier clip as a 'before' and record a new one to compare.

[springwood64]

Check out https://www.lejonklou.com/forum/viewtop ... 542#p74542