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Discussion Starter · #1 ·
So.... speakers almost always seem to come with sponge acoustic plugs for the bass ports. Well, bookshelves & centers anyway... whats the deal? Can someone shed some light on audio theory behind using them? What they're aimed for and why? Preferences?
 

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Ports give maximum bass extension. If you're using subs maximum bass extension is probably not necessary, so some manufacturers give you the option of plugging them.
 

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Discussion Starter · #3 ·
Ports give maximum bass extension. If you're using subs maximum bass extension is probably not necessary, so some manufacturers give you the option of plugging them.
I see... so "corking" the base port gives more of a sealed speaker effect? If I understood it correctly.

I do use a SW when I watch movies... LFE below 50Hz. Music is strictly 2.0, no sub. My B&W rears and center are currently "corked"... should I consider removing the plug? Not sure how much bass extension would affect the sound driving those speakers... since their allocated sound channels aren't really bass heavy anyway.

Although the thought of Darth Vader coming through my center without the plug now sounds intriguing! :eek:
 

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The port also affects the group delay of the low end. Plugging the ports will generally help with keeping the phase relation between the bottom end of the mains better matched with the top end of the sub-woofers output.
 

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The port also affects the group delay of the low end.
It does, but it's moot. Group delay is only audible in the most severe cases, and to be severe enough to be an issue the speaker design is intrinsically flawed and other issues would dwarf group delay. Much ado is made about group delay in some circles, but not by those with a good understanding of what it is.
Plugging the ports will generally help with keeping the phase relation between the bottom end of the mains better matched with the top end of the sub-woofers output.
That would only be the case if the mains were being run well below the box tuning frequency, where they shouldn't be run anyway. Even if you don't know what that tuning frequency is Audyssey etc will, because along with phase shift below Fb there's also a precipitous drop in sensitivity and output.
I see... so "corking" the base port gives more of a sealed speaker effect? If I understood it correctly.
Pretty much, but unless you're running them full range, or lower than you probably should when using subs, you shouldn't notice any difference.
 

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It does, but it's moot. Group delay is only audible in the most severe cases, and to be severe enough to be an issue the speaker design is intrinsically flawed and other issues would dwarf group delay. Much ado is made about group delay in some circles, but not by those with a good understanding of what it is.
I would agree if we were talking about a single speaker with one port. But in this case you are matching a mid-hi system with a low system and comparatively the differences, albeit small, will cause smearing if the difference is large enough.

That would only be the case if the mains were being run well below the box tuning frequency, where they shouldn't be run anyway. Even if you don't know what that tuning frequency is Audyssey etc will, because along with phase shift below Fb there's also a precipitous drop in sensitivity and output.
Audyssey can only set the phase relationship at one point. If the main speaker is tuned low enough I agree there will be no problem but if the port tuning is close (within 1/2 octave) to the crossover point then the relationship difference can be audible.

To the OP: We are debating minutia. I wouldn't worry about it.

If you are wondering what we are talking about, ref: https://community.klipsch.com/forums/storage/3/1027021/7805blauert.pdf
 

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So.... speakers almost always seem to come with sponge acoustic plugs for the bass ports. Well, bookshelves & centers anyway... whats the deal? Can someone shed some light on audio theory behind using them? What they're aimed for and why? Preferences?
The other respondents have hit on most of the reason, but there's one more - power handling.

A sealed speaker can take more power because the air in the box acts like a safety spring to prevent overexcursion. In the ported box, very low frequencies can push the driver to its excursion limits. With 2-speaker stereo, you'd likely notice the bottoming sound, and turn it down. When used with a subwoofer, the speaker no longer needs to generate very low frequencies, so stuffing the port regains some advantages of a sealed design, while not compromising on the ported ones.

Have fun,
Frank
 

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If you place rear ported bookshelf speakers close to the wall they will sound boomy. Plugging the ports will lessen that. But may also change the midrange a bit. Best bet is to move them away from the wall a foot or two if possible.
 

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A sealed speaker can take more power because the air in the box acts like a safety spring to prevent overexcursion.
That's really not the case. First off, when you limit excursion you also limit low frequency output. To counteract that one will probably use EQ to get the lows back again, and every 3dB of EQ boost doubles power. Second, a driver in a ported box has minimal excursion near the tuning frequency, where the port is doing all the work, while the cone barely moves. Between those two factors ported boxes have considerably less excursion than sealed in the lows. If you compare them at higher frequencies, an octave and higher above the tuning frequency, the excursion of sealed and ported are the same.
The only instance where sealed cabs have less excursion than ported cabs is well below the ported cabs tuning frequency, where a ported cab should not be run to begin with.
If you place rear ported bookshelf speakers close to the wall they will sound boomy.
That can occur, but more often than not it doesn't. The low frequencies that come from ports radiate omni-directionally, so where the port is on the cab seldom has any influence on response.
 

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Agreed. You'd need to change box size to see any change in power handling...
Power handling in and of itself doesn't matter that much anyway. You can make a box smaller, and in so doing increase the low end displacement limited power handling, but at the same time lose low frequency sensitivity, so the net result is a loss of maximum low frequency SPL. That's why when modeling a speaker the second most important chart after small signal SPL is maximum SPL, which takes into account frequency response, sensitivity and both thermal and displacement limited power handling.
 

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So.... speakers almost always seem to come with sponge acoustic plugs for the bass ports. Well, bookshelves & centers anyway... whats the deal? Can someone shed some light on audio theory behind using them? What they're aimed for and why? Preferences?
It depends how the speakers interact with the room you are in.


plugs.jpg


This is how the room I am using now measures with or without plugs. Blue is with the plugs in and orange with the plugs out. It measures flatter and sounds better with the plugs out, so plugs out it is.


However in my last house the room and placement I had gave a smoother flatter response and sounded better with the plugs in. (this time there was more of a peak and cancellation null around the crossover to the subs with the plugs out) So in it was.
 

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Discussion Starter · #13 ·
It depends how the speakers interact with the room you are in.


plugs.jpg


This is how the room I am using now measures with or without plugs. Blue is with the plugs in and orange with the plugs out. It measures flatter and sounds better with the plugs out, so plugs out it is.


However in my last house the room and placement I had gave a smoother flatter response and sounded better with the plugs in. (this time there was more of a peak and cancellation null around the crossover to the subs with the plugs out) So in it was.
That is a very informative graph and analysis you have there. May I ask what setup you used to perform this measurement?

On that note... can anyone else recommend any room characterization PC or Mobile applications?
 
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