There is alot of debate on this topic. Many believe there is no way to "blow" a sub by underpowering. Others strongly believe by experience that underpowering is the main cause for blown subs as well as other type speakers. Well both are right.....and wrong? how can this be?
It mainly depends on how you view the "cause and effect" and certain technical aspects. Other considerations include the type of amp, quality of the speaker (this doesn't apply just to subs), and how hard [over driving] the listener operates the system.
Now those like myself that think "underpowering" causes problems are not trying to imply that this is the only way to damage a sub. There are thermal and mechanical limits with any speaker and that of course is always a consideration.
Some "blow by underpowering" theorists believe that clipping is the cause of the damage. Some "impossible to blow by underpowering" theorists believe that there is no reason an amp should clip regardless of power and/or that clipping won't damage a speaker. Some simply blame it on the user stating that they are playing the system too loud...which can be the case but "too loud" is another subjective issue, ie: maybe the listener is hard of hearing.
As for the technical aspects regarding clipping you can find a ton of info about that here, other sites or any textbook that deals with A/C- D/C and audio. A clean sine wave viewed on an o-scope or on paper will be curved at the tops and bottoms. This picture of a sine wave, or audio signal contains the following info: Amplitude (power, this is where you see the clipped signal although other properties are involved), Frequency (cycles per second expressed in hertz or hz), period (time) and wavelength (size of the sound wave).
If you are looking at the sinewave from an amp that is clipping the top and bottom curves will appear flat, or cut off. Basically the amp can't produce any more "clean" signal at this point - Some engineers state that the output at that point is DC, or that a portion of it is. All transducers (speakers) don't really take well to DC, not a big fan. When an amp is clipping even though its clean output hit a "ceiling" (won't play the speakers any louder no matter how much more you crank the volume control) the amp is putting out more then its "peak" rated power. Many engineers claim up to 50% higher, some believe even higher and some say "nonsense". Again, confusion so who do we believe?
Heres my take on it (or opinion if you wish). If the amplifier had enough "clean" power to begin with no clipping would occur. Regardless, whichever theory you side with we are trying to prevent toasting a woofer. Been there, done that.
Many audio engineers tell us that a high powered, quality amplifier won't clip, or at least is very difficult to clip and that you can safely run more rms power to the speaker then it is rated for. I completely agree with this, but within reason.
For example say we have a 50watt RMS amp. If we replace it with a 100watt RMS amp we can expect an increase of 3db output from the speaker. (note that for every doubling of power, or # of speakers you can expect a 3db gain) We need a gain of 10db to be twice as loud. So just to make a 50watt rms speaker play twice as loud we would need to run over 400watts rms to it. Now that may be a bit [sarcasm] overkill in the headroom department but I would see no major crisis in doing this. The potential is there to blow speakers but it would sound so horrendous before they ruptured that even the most ignorant of the ignorant would want to turn it down, unless they are purposely trying to destroy equipment. Start getting greedy and apply 500w - 1000w to that same speaker and you can too easily exceeded it's thermo/mechanical ability. The speaker may not nessecarily blow from clipping in this fashion but can basically self destruct. Been there, done that as well. You will release the "voice coil smoke".
Think of it this way. The speed limit is 65 mph on the highways where I live. But I would not really buy a car that has a top speed of just 65 mph. I want a little "headroom". Now in comparing this to an amp would be why lowering the gain control (assuming it was set properly to begin with) will help, but is in a way it's just a band-aid. The same analogy applied to the car would be that I put an egg under the gas pedel so I can't speed or floor it. It will work, yes, but not always leave a very satisfied customer. (plus I would crack the egg and have a mess..I'd like more power... please)
In my day to day system installs, I find that most of my customers like it LOUD !!!! They trust me to deliver what they want. Afterall, they are paying good money (master installers don't come cheap). Now if a customer has 2 woofers that are rated @ 1000w rms each but want me to install an off-brand flea market amp that says 2000 watts on its casing they assume this will work for them. Well it will amplify and produce audio. The problem in this case is the specs are an outright lie, or at the very least peak @ high distortion levels. And by "distortion" I don't mean the good harmonic kind like with heavy metal guitar. I mean any unwanted audio, or "change in waveform" which is the literal definition of distortion when it occurs with electronics. Hmmm..change in waveform..can this mean clipping?
Now on to reality, perhaps this amp can deliver 100w rms of clean power. Everything I have ever been told about audio would lead me to think this will cause clipping. The amp will send too much "dirty" power to the woofer(s) and cause destruction as the user attempts to enjoy the "2000 watts" of power, which is what they are expecting since the amp is labled so and the woofers are rated to safely handle it.
So, from that example you can see "cause and effect". The fact that the amp is not powerful enough can send it into clipping, which in turn will send too much "dirty" power (possibly DC) to the woofers. Sooner or later this will cause a problem. If the amplifier had sufficient rms power to begin with the woofers will..... and DO, last for years. Once again, been there, done that, over and over again...oh what a life.
CAUSE: The amplifier didn't have enough clean power.
EFFECT: Blown woofer(s)
Another factor was the user, agreed. The user likes it really loud. This problem would not occur at moderate listening levels-
CAUSE: User cranked the wiz out of it because they like it loud
EFFECT: Blown woofer(s)
And we haven't even talked about transient response, damping, voltage issues and a host of other goodies. And we won't talk about that here to avoid information overload....we don't want to "clip" our brains :)
My feelings about this is to simply use an amp with at least the rms power rating of the speakers (if you can find anything with an accurate or honest spec...good luck), or go higher, within reason. Of course if the end user is not a bass head you can certainly get away with less power. The key is you can't be general in this industry, there are too many factors to take into consideration, especially when dealing with car audio where 2 ohm loads are commonplace, and loads as low as 1ohm and less are sometimes utilized. (this is rarely the case with home audio so I don't pay alot of creed to home audio or musical instrument specialists when dealing with mobile electronics as too many variables exist and the same physics don't always apply, in what other industry do they have off the chart mega wattage and insane spl levels with 12 volts???).
With all this said I hope some clarity can come out of this topic without there being any fights or name calling.
Yes, I might be pathetic and have no life (well, this industry is my life) but the things I sometimes hear regarding this topic bug the crap out of me.
Note that some info here is just my opinion but it is based on books, schooling, seminars, experience and actual events I see all too often. Other info is from professional sources and engineers in the industry.
Replacing speakers until I upgrade the customer to the proper amp,,,, and only then the speakers blow no more. That in itself is enough proof for me and regardless of what anyone believes is causing the problem....I found a solution that WORKS! MORE RMS POWER!! My credentials should be apparent.
Thanks for listening everyone. Any feedback, positive or negative is welcome, just no name calling please.