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Speakers: Xmax vs. Power handling

Discussion in 'Amps and Cabs [BG]' started by Rockin John, Mar 11, 2003.


  1. I presume there must be a relationship between the two. Without any real knowledge, here, I presume the cone is designed to remain within it's linear excursion limits at the peaks of a sine wave that's driving it?

    So, a driver that will handle (say) 100 Watts RMS with an Xmax of (say) 2mm, will handle 283 Watts PP and the cone will travel +/- 2mm about it's at-rest position, = 4mm?

    Is that how it works, oh speaker gurus?

    Ta.

    John
     
  2. There are two cone movement specs: Xmax and Xmech.

    Xmax is the shorter of the two and specifies the amount of linear cone movement where the voice coil remains fully within the magnetic gap.

    Xmech is the maximum safe cone movement before physical damage occurs from bottoming out or where the voice coil jumps out of the gap entirely.

    Cone movement is directly proportional to frequency. Movement increases 4x for every 1 octave lower, at the same loudness level. This increase is predictable by calculation, as long as the movement remains within the Xmax limit. This is "Displacement Limited" cone excursion. The manufacturer's power rating is the Thermal Limited amount of power the cab will take before melting the voice coils. This is not frequency dependent.

    Open air or sealed cabinets do not modify cone movement as vented boxes do. A vented box tuned at 31 Hz will have almost zero cone movement, compared to maximum movement in a non-vented box. Vented boxes typically achieve maximum cone movement about 1/2 octave higher than the tuning frequency. The vented box may, or may not have less cone movement over the entire bass range compared to a sealed box. The individual driver and box in question must be plotted and compared to a sealed box with the same driver.

    Short Xmax drivers are usually intended for vented boxes, and have higher efficiency. The vent compensates for the short Xmax at low frequencies. A large vented cabinet will be required for low frequency extension. Long Xmax drivers are lower efficiency, run in smaller boxes, and have more low extension. Subwoofers typically have very large Xmax values. The amount of loudness you can make is directly related to the amount of air displaced. A large cone area and long Xmax makes the most noise.

    Brian Steele's spreadsheet models and Robert Bullock's DOSBOX program will both calculate cone excursion. Steele's spread sheet is easier to use and provides both a visual plot and numerical output of displacment and thermal limited movement. Both are available on my site.
     
  3. OK, Bruce, and thanks. A very informative and comprehensive reply.

    Whilst I obviously misunderstood the connection between Xmax and power, there obviously is a given amount of watts input that will produce a given amount of cone movement. And that will depend on i/p frequency and on the cabinet type and size. However, it seems to me that the ideal is to keep within Xmax for faithful sound reproduction, and within Xmech to prevent another trip the bank!!!

    Here’s my train of thought. I wondered about giving the amp system (say) a 6dB uplift at 30-ish Hz to help compensate for the roll off of the cab: a bit of bass boost, for short. This idea’s nothing new, though. I thought that because I drive my speaker at a much lower power than it’s rated maximum – say ¼ or ½ - I should be well within Xmax at all frequencies. That should allow me room to give some boost.

    Eg. A Kappa 15LF @ 600 Watts RMS, driven with 200 Watts RMS max.

    According to my “theory”, is this a sensible idea, or not?

    John
     
  4. Your ears will tell you, if you are willing to listen. Distortion increases rapidly once Xmax is exceeded.

    EQ'd alignments have been around forever. The ELF system is just an EQ'd sealed box. Where you get into trouble is the increased cone movement required in the lower notes. The solution is playing at a lower volume level, or adding more drivers.

    Depending on the venue, bass boost may be undesired. I play a supper club once a month that has all brick walls and glass windows, which makes it a booming, reverberant nightmare. I cut my EQ -6dB in the 43 ~ 55 Hz range to avoid a nasty boom that I otherwise get.
     
  5. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    I was reading recently that another factor that determines the power rating is the strength of the advesives and their behavior under movement and heat. Is this true?

    John just remember that agressive EQ can kill speakers even if they haven't exceded their power rating. Like Bruce says, you'll usually get an audible warning.
     
  6. I accept what you both say. It was just an idea: as I build all my stuff it's easy to do. There never was the intention to have a fixed boost.

    Funnily enough - as discussed on another thread - my favourite sound is the sound of the bass direct into a slave with no EQ.

    John