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No ports = blown speakers?

Discussion in 'Amps and Cabs [BG]' started by The Gaz, Sep 24, 2008.


  1. The Gaz

    The Gaz

    Sep 24, 2008
    just curious if a non-ported cab (i.e. 4x10, 2x15) is more likely to blow a speaker at high volumes with a lot of low frequencies being pushed through it. i presupposed that it might. any clue?
     
  2. Trevorus

    Trevorus

    Oct 18, 2002
    Urbana, IL
    Hmm, not sure, but there are some cab gurus here that might know!

    billfitzmaurice should be able to chime in!
     
  3. I'm no techie, but ports certainly ease up on the PRESSURE, so, I go for ported cabs. Nothing's blown yet, but that's not exactly proof, just correlation.
     
  4. The Gaz

    The Gaz

    Sep 24, 2008
    makes sense. like farting without an a**hole?
     
  5. Yes, you're the techie, although proctologically inclined: Imagine pressure accumulating up front (speaker) with no posterior relief, yes, colon w/spinchter in a sense.

    Gaz, you are a genius!
     
  6. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Sealed cabs enjoy a linear, predictable speaker cone movement. There’s 2 ways to increase movement of the cone. One is to simply turn up the volume. The more power you feed it, the further the speaker cone will move. No surprises there. The other way is to turn up the Bass EQ.

    A ported cab works very differently. It has distinct areas where speaker cone movement increases. The smaller hump occurs slightly higher than the cabs tuning frequency and is usually well within x-max. Therefore this hump is of little significance.

    As we decrease frequency, speaker movement firstly reduces as we approach the cabinet’s tuning frequency. This is because the port starts to take over. Indeed at the tuning frequency (Fb), an audio signal without harmonics (ie a sine wave) results in speaker movement being zero. The port takes the entire load.

    Further reduction of frequency sees the speaker cone movement commence once again. It is in this region where the frequencies being fed to the cab are lower than the cabs tuning frequency where we begin to run into problems.

    Before long speaker cone movement will reach X-max, the limit of the speaker’s suspension travel. The lower we go, the worse it becomes, and we don’t have to go too much lower before the speaker reaches X-mech and is physically damaged (blown).

    Therefore with vented cabs, we have to be more cautious and more aware of the program material that will be fed to them. When designing vented enclosures, thought has to be given to what sort of signals the vented enclosure will be expected to reproduce, particularly if it is going to include lots of low frequency material. Then you need to try to tune your cabinet to a frequency which is at or near, preferably below the lowest frequency the speaker will need to reproduce.

    So you want to use a cab tuned to 70Hz for Bass guitar? I’d think again. The lowest note on a standard 4 string Bass guitar has a 41Hz fundamental. Those low notes are surely going to get that speaker moving too far.

    At low SPL, none of this matters.

    So which one is most likely to blow first when fed low signals at high volumes, sealed or ported? It depends........
     
  7. The Gaz

    The Gaz

    Sep 24, 2008
    Just the meticulous type of answer I was searching for. Thanks.
     
  8. Sadly, I'm going to have to take a week off and borrow a few textbooks to understand the answer, but that's me with anything tech-related.

    I am grateful for those who know how "these things" work. Just today such a dude, and a guitarist yet :scowl:, figured out what was making my bass sound fuzzy. And he can fix it, so I don't have to be a doofus and take it to the shop.
     
  9. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Actually that's one of the few in depth explanations I could understand without a textbook. I now actually understand a concept I thought was beyond my scope. I just bettered myself!!!
     
  10. I will read it a few times. However, it could very well be that although that posting is one of the few in depth explanations you could understand without a textbook, I will not be that fortunate. I am very, very challenged in this dept. If I had my own bass-tech.... "If I was a rockstar...all day long I'd...if I was a wealthy man...I wouldn't have to work hard...." And so on. Y'all be good; that's what really matters :bawl:
     
  11. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Maybe a diagrams will help.

    This first pic is a graph of how a speaker moves in a sealed cab. The red line is the point at which the speaker runs out of suspension. So anything above the red line = distortion, or worse. Notice how much worse the problem becomes as the frequency gets lower? In this case, anything below 100Hz is going to cause problems, making this a particularly badly designed sealed cab, but it illustrates the speaker excursion characteristics of sealed cabs pretty nicely.
     

    Attached Files:

  12. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    The next graph shows the speaker movement in a typical ported cab. I deliberately tuned the ports to 50Hz, just to show what happens at the port tuning frequency. That's right, the speaker cone movement drops significantly as the port does the work. But look what happens below 50Hz, that's where it gets ugly.

    Does that help?
     

    Attached Files:

  13. I appreciate the help! And I believe that even I will find enlightenment in it. Cool. :hyper:
     
  14. So a five-stringer like me can really BLOW it with a ported cab; is that it?
     
  15. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    If you add a lot of bass boost then yes. But the vast majority of 5-string basses don't put out quite as much lowest note fundamentals as you'd think so usually a ported cab will perform better than a sealed one. Note that some ported cabs are tuned lower, maybe around 40Hz and the Acme Low-B models are actually tuned to 31Hz (but it takes an unusual woofer to work well with such a low tuning).

    Alex
     
  16. I always thought it was easier to blow a ported cab. Just because the speakers are more likely to be over excerted.
     
  17. Thank you for elaborating on the LOW B issue as pertains to a ported cab. That's what I have, of course, so I'm doggedly pursuing some info and you and others are giving me great help that even non-gearhead me can understand.

    This is very helpful Alex.
     
  18. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    Also Xmax is the point where the speaker distortion exceeds 10% but not the point where the speaker breaks. That point is Xlim which should be two or more times Xmax in a typical pro audio speaker (in hi-fi speakers Xlim and Xmax tend to be much closer). Once you go beyond Xmax the excursion does not increase proportionally because the motor power decreases so it will take many more watts than a theoretical calculation would suggest before you reach Xlim.

    Alex
     
  19. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    That's right. The fundamental frequency of the low B is 31.5Hz. But it you played a Low B (on an electric bass) and looked it under the microscope of a spectrum analyser, the 31Hz fundamental is usually only about half as strong as the first harmonic of 63Hz. To prove it, play a low B with the EQ set flat, then play it again with 31Hz cut on the EQ. Sure a lot of sound disappears. Now play it again with 63Hz cut out instead. Notice it sounds even emptier than when you removed 31Hz! That's because there's more 63Hz present than 31Hz fundamental.

    Ported cab design takes advantage of this. If it didn't, a low B note at high wattage would destroy a speaker instantly, which we all know isn't the case in reality. But yes, a 5 stringer is more likely to blow a speaker from over excursion than a 4 stringer, especially if you like you sound with a lot of bass, and/or if you don't like to use a lot of speakers to share the load.

    Mind you, how often are you on the low B? If you're down there a lot, you might want to re-think your playing a little :)
     

  20. Although your elaboration on the topic here far exceeds my knowledge on the topic (FAR), I gather that you are sharing good news with me. How very astute of me.

    Is there a textbook--seriously--is there a good reference for my ilk that might help me become more attuned to the baffling new phenomena, words, etc. that I am reading on this thread (and many, many others)?
     

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