Plugging only one rear bass reflex port?

[Archsam]

My shiny new Revel M105 arrived this afternoon, thanks to the persuasion of Amir's recent review as well as the good folks on ASR whose opinions I valued highly. My wallet will recover eventually.....

The M105s replaced the spots previously occupied by my Harbeth P3esr, and straight away I noticed the bass was booming a bit due to the rear bass reflex ports being only 33cm from the rear wall, and the right speaker is also quite near the corner of the room while the left speaker is next to the open side. Previously my Harbeth with its sealed box design was quite happy there.

I tried using the supplied foam plugs to seal the ports, but the bass was reduced too much so I took them out. Then an inspiration came, and I plugged in only the port to the right speaker, thinking the room corner is worse in trapping / distorting the bass, and guess what, it sounds pretty good now.

My question is - will I go to audiophile hell by plugging only one port? Has anyone else done the same?

 

[tuga]

My shiny new Revel M105 arrived this afternoon, thanks to the persuasion of Amir's recent review as well as the good folks on ASR whose opinions I valued highly. My wallet will recover eventually.....

The M105s replaced the spots previously occupied by my Harbeth P3esr, and straight away I noticed the bass was booming a bit due to the rear bass reflex ports being only 33cm from the rear wall, and the right speaker is also quite near the corner of the room while the left speaker is next to the open side. Previously my Harbeth with its sealed box design was quite happy there.

I tried using the supplied foam plugs to seal the ports, but the bass was reduced too much so I took them out. Then an inspiration came, and I plugged in only the port to the right speaker, thinking the room corner is worse in trapping / distorting the bass, and guess what, it sounds pretty good now.

My question is - will I go to audiophile hell by plugging only one port? Has anyone else done the same?

I did that (plugged 3 ports in one of the speakers and 1 in the other) with a pair of PSB T2s asymmetrically positioned in the room:

 

[jcebedo11]

You could make the tuning frequency lower by rolling up paper and sticking it in both ports. It will look ugly. but because of the longer ports, it will effectively lower the resonance point. I have done this before with other speakers. It reduces the bass boom and also gives an extra low end extension.

 

[Archsam]

You could make the tuning frequency lower by rolling up paper and sticking it in both ports. It will look ugly. but because of the longer ports, it will effectively lower the resonance point. I have done this before with other speakers. It reduces the bass boom and also gives an extra low end extension.

Interesting trick. A long time ago I heard of (and tried) another trick by bunching drinking straws and cutting to length and pack into the port. I supposed it turned one big hole into lots of tiny tunnels, it did helped my monitor audio silver S2 tamed its bass, but only slightly.

Sounds like I need to play around a bit more.

 

[Archsam]

What do you know, the paper tube method made quite a noticeable improvement!

And when I slide it in and out the effect changes as well, I'm sure it has to do with the boomy frequency and wave length vs tube length, but even by me turning by ears its quite effective.

But WAF is in the negative territory - my better half is demanding their removal as I type this....

 

[jcebedo11]

DSP or some sore of equalization is another alternative. Also another alternative is plugging both ports + the use of a subwoofer. you adjust the amount of bass from the sub.

 

[raindance]

If plugging one port helps the boom, by all means, do it. I doubt it's a full solution though because the speaker that's in the corner will also be louder.

 

[tuga]

You could make the tuning frequency lower by rolling up paper and sticking it in both ports. It will look ugly. but because of the longer ports, it will effectively lower the resonance point. I have done this before with other speakers. It reduces the bass boom and also gives an extra low end extension.

But the mid-woof will still roll-off at the same frequency so you will probably get a wide dip in the lower-bass.
I would also recommend the use of reasonably thick paperboard with elastic bands wrapped around it to dampen vibrations.

 

[Archsam]

So far the paper tube trick is paying off - I cannot stop listening since the speakers were set up yesterday! The muddy bass boom is toned way down, without affecting the mid bass slam. This is everything I have hoped for - a supercharged Harbeth P3esr, with all the virtues in the midrange plus increased dynamics and clarity, and the soundstage is approaching electrostatic level of holography.

The drop in lower bass output is a good thing - I am trying to be respectful to my 3 adjoining neighbours

Happy to leave it alone and just enjoy the music now. Thanks to all!

 

[q3cpma]

Just a friendly reminder that rear ports don't radiate "toward the rear".

 

[Frank Dernie]

Interesting trick. A long time ago I heard of (and tried) another trick by bunching drinking straws and cutting to length and pack into the port. I supposed it turned one big hole into lots of tiny tunnels, it did helped my monitor audio silver S2 tamed its bass, but only slightly.

Sounds like I need to play around a bit more.

That was the standard port type on Celef and Proac monitors 40 years ago!
My Proac EBS are like that.

 

[Archsam]

Just a friendly reminder that rear ports don't radiate "toward the rear".

Yep good one. I started to read up on speaker ported design since I played with the paper tube trick yesterday. So the port tube is using the air inside to resonate with the sound pressure generated from within the speaker cabinet, and 'whistles' along with the frequency produced by the woofer?

I see that with the paper tube trick, I am effectively extending the length of the port tube and thus altering the resonate frequency. This is as far as my knowledge takes me - I am still trying to understand how the longer tube affects the frequency, and to what extent?

Exciting stuff.

 

[Archsam]

That was the standard port type on Celef and Proac monitors 40 years ago!
My Proac EBS are like that.

How are those ports constructed? I don't supposed they used drinking straws.....

 

[Frank Dernie]

How are those ports constructed? I don't supposed they used drinking straws.....

They did.

 

[Archsam]

They did.

Now I have seen everything.....

 

[Frank Dernie]

Yep good one. I started to read up on speaker ported design since I played with the paper tube trick yesterday. So the port tube is using the air inside to resonate with the sound pressure generated from within the speaker cabinet, and 'whistles' along with the frequency produced by the woofer?

I see that with the paper tube trick, I am effectively extending the length of the port tube and thus altering the resonate frequency. This is as far as my knowledge takes me - I am still trying to understand how the longer tube affects the frequency, and to what extent?

Exciting stuff.

A reflex speaker adds an extra mode to the system resonance(s). The mass of air in the tube resonates on the compliance of the air in the box at a frequency often tuned to be the same as the main driver resonance. In principle, at this frequency all the sound should be coming from the port and the driver itself is at an node, so stationary.
This is a bit hard to get your head round if you aren’t familiar with dynamic systems.
A crankshaft ”damper” uses the same physics but for a different reason. Here at the tuned frequency the crank resonance is suppressed by being at a node and the mass of the “damper” is going like mad on its rubber spring.

 

[Frank Dernie]

Now I have seen everything.....

Hard to get camera at the right angle so rubbish pictures but

 

[jcebedo11]

I also tried flexible plastic binders. I used a black color, since I had a black speaker before. This is less ugly and also more sturdy than paper. I cut the piece I need and then tape it after rolling it up. It is more expensive than paper though. https://www.amazon.com/Blue-Summit-...9Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=

 

[jcebedo11]

about ports and tuning frequency. If you double the length of the port and the port is the same diameter, the resonance frequency is cut in half. Im guessing the resonant frequency of your speakers are around 50hz. doubling the length is brings it now to 25hz. You can also change it to somewhere in between. Im guessing the one of the lengths or widths of your room to be 10 or 11 feet. If it is, your room as a resonant point of around 50hz. This is probably what is causing the boom, because both the port tuning and the room mode coincide together at around 50hz. I had a room with this very problem before.

 

[Duke]

If you double the length of the port and the port is the same diameter, the resonance frequency is cut in half.

That has not been my experience, either modelled or measured. The relationship between port length and tuning frequency (or number of ports and tuning frequency) is not straightforward. Here's an online calculator, I can't vouch for its accuracy (it's not what I use) but you can plug in some numbers and observe the trends:

http://www.mobileinformationlabs.com/HowTo-1Woofer-Box-CAL Port lenth 1.htm

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As for plugging ports, in most cases it works just fine (theoretically you could run into chuffing at high SPL's). I try to incorporate multiple pluggable ports into most of my designs. Here's a photo of the back of a bass cab, and you can see that two of the four ports are plugged, which can help in a boomy venue: