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Latest thoughts about the most important parametar - Xmax

Discussion in 'Amps and Cabs [BG]' started by Guja, Mar 5, 2008.


  1. Guja

    Guja

    Feb 26, 2007
    After seeing frequency analyzed bass guitar signal i got some thoughts.. It was more or less like this :

    1st harmonic was the loudest.
    fundamental was around 3db quieter than 1st har.
    2nd harmonic was also a little quieter then 1st har.

    That means that fundamental and first harmonic drain same amount of power from amp (fundamental is 3db down, but drains more power because it`s twice lower).

    So, the following looks true:
    1/3 of the amp`s power is used by fundamental
    1/3 is used by 1st harmonic
    1/3 is used by 2nd har, and all the upper harmonics.

    Here`s the example (for a 4stringer) : Delta 12LF in a 2.3cu box @45hz. Overexcursion occures at 70Hz with 110w input power.

    However, only 1/3 of the amp power amplifies the 70Hz signal. Another 1/3 goes to 140Hz, and another 1/3 to 210Hz. Those freqs require MUCH less excursion then 70Hz signal (210Hz almost none..).

    same applies to other freqs (notes)

    40-80-120 (80 uses the most X-max, but 40 and 120 use less)
    ...
    ...
    bottomline is that bass signal is not the sine wave, so Delta 12LF should work with 200-300W amp without farting even at highest usable levels.???

    Sorry, my English sucks....but I guess You get the point...
     
  2. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    One point to note is that there is no first harmonic per se, as that is the same thing as the fundamental. Another point to note is that a typical cab is tuned to 40-50 Hz. Excursion is at a minimum at and near the tuning frequency, and at a maximum generally one octave higher. That places the greatest stress on the driver in range of 80-100 hz, which coincides with the second harmonics of E1 to A1.
     
  3. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Your English is great.

    I reached a similar conclusion with my DIY speaker using a 2512-II driver. The traditional max-power graph is a worst case analysis because it assumes that the input signal is sinusoidal. As you note, a real bass waveform is a mixture of harmonics.

    We don't have a standard method of computing cone excursion for a real bass waveform. I have tried a couple different methods. The spreadsheet at my web page simply assumes that the first four harmonics have equal amplitudes, and computes the sum without regard to phase.

    I also did a "correct" calculation using Fourier analysis of a measured waveform, and the results were not much different.

    The problem with real waveforms is that they depend on the bass, player, and EQ settings. Still, we can probably expect typical speakers to handle more power than suggested by the max-power graph.

    Of course power compression is still an issue.
     
  4. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Just one more note that I should add. Obviously, any method that says "you can feed more power into your speaker" is associated with an element of risk. That's one reason why I have not heavily promoted my method. Also, a new generation of drivers similar to the 3015LF makes my analysis unnecessary.
     
  5. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    "real" waveforms are just the sum of fundamental and harmonics. So ANY freqhency that "hits on" a weak spot in the power handling MAY cause a problem.

    Sure, "on average" you have a certain input spectrum, with some harmonics higher and some lower. Sounds great, just make the speaker handle that.

    One problem....... it's called 'EQ".

    Turn up the low end, or worse yet, a slider on a graphic EQ. Now you have a filtered signal, probably a LOT more sinusoidal than it is naturally, because the EQ has filtered out harmonics and emphasized the fundamental (or maybe it boosted the second harmonic over the fundamental and 3rd. whatever........).

    All the "typical spectrum" information is now out the window, and you have a completely new power vs frequency relationship.

    So it's best to make the speaker handle as much as possible without over-excursion, and make sure tehre are no "low spots" that will create the equivalent of a "wolf tone".
     
  6. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Definitely true. I am assuming that those frequencies are of predictable amplitude, so that the overall cone excursion is also predictable.

    Perhaps this is a difference between commercial design and DIY. I have the luxury of designing a speaker to only handle what I am going to be putting into it. OTOH, I doubt very many commercial bass speakers can handle their rated power with a sinusoidal signal in the lowest octave of the bass.

    But your points are well taken. The method that I use definitely depends on the player's choice of EQ settings.
     
  7. seamonkey

    seamonkey

    Aug 6, 2004
    I've been playing around with some of this lately also. The mix of fundamental and harmonics depends on strings, pickups, how you play, processing, etc. In any case you're right it's not a sine wave. It's an asymmetrical complex waveform.

    A couple of extra points. Hearing changes at low/high volumes. If you have a lot of bass at low volumes, you need a lot less at high volumes to essentially sound the same to your ears. Less bass power ratio is needed when overall volume is higher. Some "smart" powered cabs are actually automatically turning down their bass at higher output levels.

    Another thing is the ear can be easily fooled by playing with the harmonics. It's the missing fundamental effect. For example pipe organs take advantage of this. For bass guitar strings, effect processing, etc., can actually sound like more bass to the ear where there is actually measured less. That is you'll see more measured harmonics and less fundamental and it will still sound bassy.
     
  8. Munjibunga

    Munjibunga Retired Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    X[SUB]max[/SUB]

    Oh, nothing. Just seeing how that looks.
     
  9. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    The way I see it, with most bass guitar speakers you're best off putting them in a small box that reduces their low frequency sensitivity and thus the risk of overexcursion. With a select few that have decent Xmax you can actually design a cab to give good bass extension without worrying about farting out, but there aren't many speakers that can do that.

    Alex
     
  10. ZolkoW

    ZolkoW

    May 8, 2006
    Hungary, EU
    maybe someone finds this interesting (I did) : I mailed Eminence for the T/S parameters of my EBS 210 (older one, not-neo).
    they replied with an Xmax of 1,6 mm !!! that's not much at all for a bass speaker.
    now I'm using this vented cab with a 300W rms head, sometimes at max volume.
    it can handle the 4 strings nicely. (I measured the amp's abilities, so I can say it's really 300W rms)
    some compression comes in at higher volumes, but it doesn't fart out!

    I think, I make the drivers exceed their Xmax, but not Xlim. I was very surprised about that 1,6mm...
     
  11. MikeyD

    MikeyD

    Sep 9, 2000
    Keep in mind that your analysis has to do with the "steady-state" waveform after the string is plucked. The initial transient of plucking, picking, slapping, or popping (note attack) - if you do a Fourier analysis of it - will show a much broader range of frequencies, though only momentarily. Those frequencies can include subharmonic content below the range of hearing, and it's why you can see cones "jumping" upon note attacks. While you might get only one cycle of overexcursion, it might cause driver damage in certain cases.
    - Mike
     
  12. Stromrider

    Stromrider

    Feb 16, 2008
    CT, USA
    Is it true that bass guitars generate subsonic (<30hz) frequencies? If they do, and i see no reason why they couldn't depending on the player of course, even a little bit of signal below the tuning frequency of the box can eat power and xmax. It would be a good idea to use an active filter at 25hz or so.
     
  13. Guja

    Guja

    Feb 26, 2007
    That`s why I`ll use an active 24db/oct high pass at 31Hz in my effect loop. DIY

    http://sound.westhost.com/project09.htm

    and the rest of the attack harmonics are mostly above 150Hz, and require little excursion. Still no farting :eyebrow:
     

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