Plugging a port or ports on a speaker, can this damage them?

Discussion in 'Speakers' started by Tony V., Jan 10, 2019.

  1. Tony V.

    Tony V. Moderator
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    A thought that recently came to me, given some manufacturers (SVS comes to mind) supply plugs to block the port giving the speaker a tighter bass output. Can this be done to any speaker that are ported? What are the drawbacks or advantages of doing so?
    Is there a difference between a speaker design that has the port on the rear vs the front?
     
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  2. Kal Rubinson

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  3. Tony V.

    Tony V. Moderator
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    Yes, I read that post but it does not speak to plugging a port on a speaker that dose not come with or say that you can plug the port.
     
  4. Kal Rubinson

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    You are right. For years, people have been advised to "put a sock in it" if they perceived problems with vented speakers placed close to walls. However, I doubt if any competently designed and optimized ported speaker would be improved by blocking a port, whether the company recommends it or not.
     
  5. Kerry Armes

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    There are really only two things that can damage your speaker in a home environment. Either the speaker had a defect from the factory or the speaker has been over-powered. Over-powering can cause damage in two main ways. One is causing a thermal failure of components due to the heat. Typically, this causes the enamel that coats the voice coil wire to get hot enough to melt and then you get a short in your voice coil. The second is by over-excursion. Over-excursion can cause a driver to incur physical damage to the voice coil (knocking wire loose) or the suspension (spider or surround separations).


    In a ported system, woofers unload below port tuning. Unloading is when the box provides no restorative force on the driver and the driver shows extreme excursion. So if you have a ported pox tuned to 50 Hz and played a 30 Hz tone, you would see some extreme excursion from the driver. A sealed box does not do this. It as the woofer tries to move more, which is required to play lower tone, the air inside the box tries to resist that movement and therefore will not see the same excursion. Here are two graphs with a typical 6” woofer (the Dayton RS180) tuned to 45 Hz and at 30 watts input power to illustrate the difference between ported and sealed in the same size enclosure.

    Graph1.gif Graph2.gif

    So the point is that there is that sealing the port will not damage your speaker. In fact, you are probably reducing the chances of damage by sealing up a ported speaker.
     
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  6. Tony V.

    Tony V. Moderator
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    Thank you Kerry, thats the info I was looking for.
    Ive seen that exact extreme excursion you mention on my Mission 765 speakers in my livingroom. They are rated down to 36Hz and using the song Funhouse from Flim & the BBs for example during the part where the train whistle and then door slam is heard the cones move alot but no sound is heard or felt as I am sure the sound is well below its output ability.
    I know a sock was mentioned by Kal but what is your recommendation to use as a plug?
     
  7. Kerry Armes

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    I like socks too if you want to seal it up pretty tight without worrying about it being permanent. They also come in black to match your port ;) You can use foam, but foam won't block as much air so it will act more like an aperiodic enclosure.
     
  8. Kal Rubinson

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    Hmmm. Unless the item is an imperfect plug, it should not make any difference. The socks have unspecified acoustical properties but, if they fit tightly, they should do the job, make no damage and be removable. I do not believe that anyone makes audiophile socks....................yet.

    I like the PSB plugs because they are solid thick rubber with finger grips for easy removal.
     
  9. tesseract

    tesseract Senior Admin
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    The calamity and vibration going on in a ported enclosure is much greater than in a sealed, due to the tune causing resonance.

    I once returned a pair of review speakers with the socks still in the ports. The manufacturer gave me a good ribbing over that one, heh.
     
  10. JStewart

    JStewart Active Member
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    Thanks for a good laugh. :)
     
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  11. Kal Rubinson

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    I'm guessing they were ribbed socks.:rofl:
     
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  12. tesseract

    tesseract Senior Admin
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    @Kal Rubinson :heehee:

    Ha! Well, I told them they were clean, at least. Put me out two good pairs of socks, though.
     
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  13. BadJRT

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    Kerry,
    In the graph's you showed I don't see where SPL is depicted. I'm curious as to what effect less excursion had (due to port plugging) on SPL, in that particular test.

    Also, how does plugging a port compare with high passing a ported tower at say 40, 50, or 60 Hz? Isn't a port tuned by a manufacture for maximum spl at a certain frequency and then there's rolloff on each side? So if you high pass a speaker above the ports tuned frequency, what role is the port then playing, acoustically speaking?

    If I were going to run a 2.2 system and high pass the towers, that would allow me to position the towers slightly closer to a wall without plugging the ports? Or could a combination of both plugging and high passing end up being best? Are there ramifications to these tweeks I'm not considering?

    Disclaimer: Keep in mind I'm trying to learn and I'm possibly asking questions that don't fully make sense.

    Thanks, Brian
     
  14. Kerry Armes

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    Brian,


    SPL is difficult to answer because it depends on what frequency you are looking at. In the example I showed, the input power on both the sealed and ported were the same so at frequencies below the fb of the sealed enclosure, the ported will have more max SPL output. Ports or passive radiators make a system more efficient at lower frequencies. Above the roll-off frequency of the sealed box, both systems should be identical.


    However, the true answer is very complex once you start looking at a musical signal with varying frequency response. The dip in excursion for the woofer in the ported graph coincides with the port resonance. At that point, the driver is almost completely stationary, and the port is providing almost all the output. If you were to only measure at this specific frequency, the ported design would have drastically more output. But once you start looking at frequencies below the port tuning, the sealed design controls the cone better and prevents the excursion from becoming as extreme. Below the port tuning, you might only be able to send 10-20 watts to a small bookshelf woofer before it starts to distort. In addition, the excursion doesn’t just cause distortion at low frequencies, but over the entire bandwidth of operation.


    The question most people are probably wondering now is why wouldn’t most ported speakers distort like crazy given what I just said. Well, most music content really likes at around 50 Hz or above and there is almost know bass outside of synthesized material below 30 Hz. So if a typical bookshelf is tuned to 50 Hz, you have low excursion centered at 50 Hz but some to each side because of the port output. This is right where a majority of bass content actually lies, so most music won’t cause the speaker to reach over-excursion at the 10-20 watts input power.


    High passing, unless you are using something with controllable slopes, can be a little more difficult and usually won’t give you the same roll-off as a sealed system, especially as you get closer to the port tuning. However, you could use it in combination with a low tuned speaker to get a sealed roll-off with even less excursion than a typical sealed design. The port tuning is only affected by box size and port dimensions. High passing won’t affect the port itself but will decrease it’s overall SPL. But think about it this way, using a 12 dB slope high pass at 80 Hz just sends the speaker 12 dB less output at 40 Hz. You could have the same thing happen in music. You could have a 40 Hz tone played at 90 dB and then the next one at 80 dB with no adjustment to your volume knob. High passing with your receiver is essentially doing the same thing.


    Typically, you want to high pass more to limit the excursion on a speaker than to allow you to move it closer to the wall. Remember, people still high pass sealed speakers. This is so you are sending most of the bass content to the subwoofer and your mains can get louder before they start to distort. Excursion is what causes the majority of distortion. So plugging and high passing could help but it depends on what your problem is.


    I think I mentioned before, but moving a speaker closer to the wall can affect the area all the way up to about 2 kHz. High passing doesn’t fix this. Some speakers are designed with less baffle step so they can be put closer to the wall and not start to sound thick. Plugging the port can sometimes help, but it is only affecting a very limited frequency bandwidth. Depending on where the problem lies, only one of two things can really fix it: room correction or proper placement.
     
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