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"Under-powering" a Cab

Discussion in 'Amps and Cabs [BG]' started by Woodboy, Apr 6, 2004.

  1. Woodboy


    Jun 9, 2003
    St. Louis, MO
    My dealer used this term and it doesn't make sense to me. I think he wanted me to buy a cab that has been on the floor for a while. Say you've got two cabs, one rated at 250 watts and the other rated at 400 watts. The sensitivity is the same at 103db. If you hook up a 300W amp to either cab and don't run the amp to clipping, there should be no difference, (other than the different character of the cabs), right? To me a good analogy is this: If you drive a Ford Pinto and a Ferrari, which have different "power handling" abilities, at 50 miles per hour, both will do just fine. It is when you ask the Pinto to go 100 m.p.h. that it will suffer. As I understand power ratings, they are derived in the lab by passing voltages thru the speaker and seeing how much can pass without damage. It is not measured as a program measurement, meaning real music being amplified. (Some might call amplified music an oxymoron :) ) If you want to fill an auditorium with bass, you need a high power amp and a high power speaker. If you want to fill a small club with bass, you need a medium power amp and can use a high power speaker, if you like the sound of it best. Right?
  2. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    There is a phenomenon like that, but you usually find it with blown tweeters on stereos during a party, for example.

    I think it only very rarely happens with bass speakers, if you blow a speaker there it's usually something other than underpowering.
  3. No, you've got it right. There's no such thing as underpowering a speaker in any meaningful sense. You can only be underpowered for your playing situation. In the scenario you mentioned, with the two cabs of equal efficiency but different power handling, if 300 W is enough to get the job done comfortably, without tapping out, then you *cannot be underpowered*, regardless of what the cab rating is. In this setting, there is no intrinsic benefit to be derived from a lower cab power handling rating in and of itself. Such a rating usually works fine, but it doesn't make anything better.

    Conversely, if 300 W isn't enough to get the job done with either of those two cabs of equal efficiency, and you're hitting the amp's limits, you must, by definition, be underpowered for your playing situation, *regardless of whether the cab is rated for more than the amp or less.* Here too, you get no intrinsic benefit from having a cab whose power handling is lower than the amp's output. You won't always experience damage with a lower cab rating, but you certainly won't have any special protection against it either; if anything, you'll be more at risk, not less, because less hardy (i.e., lower-rated) components will probably start to get damaged earlier when you push an amp harder, all else being equal.

    To me, the key is to have enough, or more than enough, amp for the job you want to do. Once you've got that, the cab's power handling rating doesn't really matter, as long as it can handle *at least* the amount of your amp's power that you use (ideally nowhere near 100% on any regular basis).
  4. Yup. :D
  5. When people talk about blowing a speaker because they were "underpowering" the cabinet, what they are really talking about is that they have got into a situation where they really didn't have enough amplifier to get the job done, and cranked it up to the point where the amplifier started to go into severe clipping. It is the clipping that destroys the speakers in this situation, not the fact that the amplifier didn't have enough power for the cabinet.

    In reality, it is hard to damage a speaker from too little or too much amplification power (except in really extreme situations) under normal playing circumstances.
  6. Yes and no. ;) What you are saying needs to be qualified a bit more...
    In fact, it isn't the clipping as such that damages speakers but the fact that amps will deliver more than their rated power when clipped. Say an amp is rated for 400W@1% total harmonic distortion (THD). 1% THD is often quoted as the 'clip point.' At 100% THD, the power output would be something near 800W. This can be shown by the fact that square waves (clipped waveforms approach this) have twice as much energy as sine waves of the same amplitude. This can be seen by calculating the areas under the curves of the two wave types.
    So it's easy to see how a 400W amp driven into severe clipping could damage the drivers of a cab rated at say 500W, but there's a limit to this. A 50W amp isn't likely to damage the drivers in a 500W rated cab because its theoretical maximum power is only 100W, which the drivers can easily dissipate. Things like cabinet design and EQ curves can influence this too. You can melt the voice coil of a driver in a vented box with perfectly clean power at a fraction of the driver's rating if I concentrate the energy at the tuning frequency of the cab (where the air column damps cone movement almost completely).

    So really, damage from clipping isn't a case of underpowering, it's actually a case of overpowering!
  7. MikeBass

    MikeBass Supporting Member

    Nov 4, 2003
    Royal, Oak, MI.

    You got it!
    You also have a handle on something else that your local sales guys (and most for that matter) forget.
    Just for example, a single 15 can for the most part, only be so loud. Reguardless of it power rating. There are other factors that come into play like the cabs/speaker sensitivity, maybe X-max and so-on (the real big tech guys,thats your cue!).
    But the bottom line is, with all things the same as you described, it will only go so loud reguardless of it's rating.
    More power handling = the Pinto to Ferrari example. They both can do it, but one may die quicker!
    Same goes for your amp.
    Power does not equal volume (well it does, just not much). More square inches in speakers you add equals more volume A LOT quicker.
    A 10,000 watt won't be any louder than a 100 watt amp thru a single 15 cab.

    Sorry if I am over simplifying these concepts or getting off base from your question. But it sounded like somewhere in there the sales guy maybe was talking volume from what you asked :meh:
  8. Ericman197


    Feb 23, 2004
    It'd be a lot louder... until the speaker explodes :meh:
  9. sloppysubs


    Nov 24, 2002
    Swansboro, NC
    ive had a similar question on my mind for a while.

    what if you ran a 150 watt @4 ohm amp into a total of 950 watts (2 8 ohm cabs 600 and 350 watts a piece). would that do any damage to either the amp or the cab? thanks
  10. Thunderfunk


    Mar 27, 2004
    McHenry, IL
    Yes and no to everything above. A larger cone pushes more air with less excursion (X-Max). If you exceed the max excursion, the voice coil leaves the magnetic flux and I believe the inductance goes down, meaning it draws a lot more current. (Need a real speaker expert here).

    You can also have mechanical problems with the voice coil getting larger with heat and touching the magnet gap, or becoming mechanically unstable and doing the same.

    The comment about clipping causing problems has to do with the fact that a square wave carries more power (think of it as the area of the wave form). AND--the cone wants to stand still across the top of the square wave (DC), again drawing a lot more current. Clipping blows speakers, and especially tweeters, and is a big advantage to bi-amping. If the bass (woofer) amp clips, that square wave doesn't get into the tweeter.

    Ultimately, heat is the problem. You can drive a 1,000 watt speaker with a 100-watt amp, it just won't be as loud as it could be. You could also possibly blow the speaker depending on what frequency and wave form you're trying to pass, but it's not likely.

    You can also drive a 100 watt speaker with a 1,000 amp. You just have to be careful not to push it too hard.

    As for blowing up an amp with a certain speaker, that would have more to do with minimum impedance, and the capacitance of the load. It's not likely.

    The real idea would be to not carry the weight or expense of a 1,000 watt speaker if you only have a 100-watt amp.

    Underpower a speaker? Just a waste of money, but not technically a problem.
  11. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    "Ultimately, heat is the problem."

    That's a good general statement that applies to "all" speakers, not just bass speakers (or tweeters).

    There are two ways an underpowered amp can damage a speaker. The first way applies mainly to speakers with built-in tweeters/horns/crossovers. Usually, the high frequency driver is rated at a lot less power than the woofer. When the amp starts to clip, the bass frequencies are "compressed" because they've reached the maximum power that the supply will provide. But, the high frequencies don't get attenuated this way, they keep increasing in output power as the volume control is turned up "even more", with the eventual result that the tweeter may fry because it's getting too much power.

    The other way, and more common with woofer type speakers, is that any high frequencies that don't move the coil go directly into heat. So, let's say you're shooting a 2kHz signal into a woofer that tops out at 1 kHz, even though the actual power being delivered may be well within the speaker's rating, the coil will heat up more quickly, with the result that the next good low B that comes through might fry the winding (through whatever means, electrical or mechanical).

    But JMX is right, both of these phenomena are relatively rare in bass (instrument) speakers. Although, those of us who have SWR cabs with blown tweeters have probably experienced a close encounter of the first kind. The most common cause of blowing bass speakers is overexcursion, meaning very powerful bass transients that cause the coil to jump out its frame (and the cone sometimes follows shortly thereafter).
  12. Agreed, except for the cone wanting to stand still bit. A Square wave is still an inherently AC signal, thus a square wave of X-frequency will move the cone in and out the same number of times per second as a sine wave of that frequency. Also, a speaker's cone momentum won't allow it to stop moving at the crest of a square wave instantaneously. In fact the cone follows a sort of ragged sinusoidal path when driven by a pure square wave signal. The cooling ability of the piston is barely derated at all under these conditions. The amp does lose the ability to control cone movement which isn't really a good thing :D, but it probably won't blow up speakers.

  13. My experience has been that this is not a significant issue with real signals containing many frequencies in cone speakers. When you consider that 97 or 98% of the power put into a speaker is converted into heat anyway, a small bit more energy converted into heat isn't that big of a deal.

    90% or more of the damage I see to cone speakers is mechanical damage from overexcursion, torn surrounds being the most common thing.

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