Does Plugging Bass Port Affect Pace, Rhythm and Timing?

Hi, I recently purchased a used pair of Monitor Audio Silver S1 bookshelf speakers for, well, my bookshelf. :\  As expected, the presentation is a little muddy, and the highs not as extended as they might be otherwise on these speakers known for the quality of their tweeters.  I am about to try plugging the ports to see if any improvement could be had. I was wondering if there is any downside to plugging the ports on a bass reflex design as far as bass accuracy and speed. 
Hi Rwinner,

Not really. Actually my S1's came with plugs. Reducing the bass is a good idea when you have speakers not really designed for close boundary locations. It will help even out the bass response. 



my best friend has s1's in his study . He found them muddy until he moved them away from the wall, need about 10 inches behind them & some distance in front, improved dramatically. 
Mine are about 6" from the wall right now.  I'll try moving them out 4 more inches first before plugging the ports to see what that does. The only down side is esthetics - having the front of the speakers out over the edge of the shelf. But I could learn to live with it if the sound improved dramatically! 
OK, I will do that, and see how it works out. I don't have the original port plugs, but I do have some large chunks of foam material I could use (or just go with a pair of socks). 
Are you using any type of footers? Lifting the speakers off the shelf will really tighten up the bass.
This is a little off topic. But I’ve wondered if plugging the ports on speakers that do no come with plugs can damage them by causing excess woofer excursion than the speaker was designed?
Hi Jetter, 

It actually works the other way. Below the port resonance, driver excursion increases dramatically faster than with a sealed cabinet. 

Below resonance, the tube acts as a simple vent, eliminating the air-spring (i.e. acoustic suspension) portion of the cabinet. By plugging the cabinet, the air-spring remains working at all frequencies, controlling excusrion but also, of course, reducing bass up to a point. 

At around -10 to -20 dB sealed and ported speakers have about the same output.



Thanks Erik, so its the opposite of what I was thinking.  Maybe its time to go to the local craft store and purchase some of their foam cones. 


One thing to consider is that the speaker designer chose the internal volume of the enclosure based on the fact that it is ported. He would have probably used a smaller enclosure if he was using the same woofer in a sealed application. With the port plugged, there will be unintended consequences---you will have changed the performance and behavior of the woofer.
OK, guys - I plugged the ports, and it really reduced that mid bass bloat by a lot. The music is much more balanced now, and those sweet highs are much more to the fore. I think I'll live with this configuration for a while before I try anything else like moving the speakers from the walls with and without plugs. Thanks for all your help!
@prof - That’s actually an often repeated myth.

The overall efficiency is determined by the driver and crossover. The cabinet and port affects the bottom end of the driver’s response. Plugging the port does raise the -3 dB point up to about an octave or so.

I should point out that when I bought MA Silvers they came with factory foam plugs for the bass port, so this technique is quite normal.

@bdp24 - You are kind of right, in that an optimized sealed cabinet is usually quite smaller than an equally optimized ported cabinet, BUT that assumes they are going to be used the same way. If you are going to take a stand-mounted speaker, and then put it on a bookshelf, plugging the port is a very very good way to compensate for the additional bass reinforcement.



Thanks erik,

It's been a very long time since I have been the details of speaker design.  I burned out on the technical side of this hobby years ago and don't follow it near as much (that's one reason it's taking me forever to get around to setting up my subwoofers.  To do it right is such a hassle I never get around to it.  I just sit down and listen to my system as it is...)

I'd always noticed, though, that plugging the ports of a speaker seemed not only to tighten up the lower response, but also seemed to reduce the sense of dynamics, making it a bit more lifeless.  So that seemed to at least subjectively ratify that myth.  Perhaps that was just due to the loss of lower bass...(though I have both the larger and the slightly smaller version of the Thiel 3.7 and 2.7 speakers, the only difference being a 2" smaller woofer on the smaller speaker and not as deep bass, but it certainly sounds as dynamic or more than the bigger speaker with the lower bass....)
If you want to experiment, go to a fabric shop and buy some Bonded Dacron(polyester batting).   You can then control the vent function, by varying the amount of Dacron(tight or loose wrap/longer shorter roll) in the tube.  1" to 1 1/2"  thick works great and most places sell it by the yard(cheap).
I found out something interesting. The muddiness seemed more than anything else traceable to the fact that the tweeters were a good 12-15 inches above the height of my ears while listening. I tried tilting the speakers down by inserting a shim on the rear bottoms. This helped some - but when I turned the speakers over on their sides - voila! Now that the tweeters are almost even with my ears, the high frequencies and mid-range really came alive.

I didn't realize tweeters of this sort were so directional, but apparently 15 inches off axis is enough to severely attenuate the highs. I still keep the ports plugged due to the closeness to the wall. Pretty well balanced now, although I may have done something to the imaging by turning the speakers over on their sides - more experienced members than myself may be able to chime in on this.
That will do it, and depends how far away.

You could also try turning the speaker upside down. Many are measured on tweeter axis, so this will restore the vertical relationship.

Of course, it only matters if you can hear it.


If you want to tighten up the bass..plug it.
Speakers should not go into "wobble mode"...the bass must be tight and controlled..not doof doof but bang bang!
Good one rwinner..putting the boxes down will help..why your tweeters are directional is a bit disturbing...mids and tops have to point and disperse the audio to your man chair.
I was given a pair of Advent indoor outdoor 2 ways from USA
stereo a few days ago...comments please..
Eric Squires...good one if they are standing high and the tweeters are recessed not dispersing the tops to your ears!
Any idea where i can procure the original main drivers for the advent indoor outdoor mini?
The one seems a bit burnt, want to replace both and compare to the jbl's.

Monitor Audio dual ports some of their speakers specifically for boundary tuning. My Silver 300 7G were a bit thick on certain vocals. Plugging both was too much, plugging the top port, too little, and plugging the bottom port, perfect, with no loss of extension or dynamics. Ported speakers roll off at 24 dB/Octave, sealed ones at 12dB/Octave. Porting at best gives another half octave of bass before rolling off. As for being below the tweeter axis, it's not the tweeter, rather you are in a phase cancellation lobe that's a function of driver spacing, crossover frequency and slope. Regardless, the fixes which you discovered are correct: the goal is getting more on axis with the tweeter. Actual tweeter dispersion is a function of driver diameter and wave guide (if any).  Remember those wave tank experiments in HS physics? The narrower the aperture the wider the dispersion, but the lower the output on the other side. Exactly the same with speakers. 

LOL@ people who would rather ask a forum something rather than just try a simple thing themselves.