An under powered amp can be driven into
clipping more easily.
Clipping creates high frequency
harmonics in the output. When an amp is driving a full-range
speaker, the
crossover dutifully passes along the
harmonics caused by
clipping to the high frequency
driver. It causes more
power to be delivered to that
driver. If it exceeds the
power handling or thermal capabilities, it'll damage the
driver. An under powered amp is unlikely to damage low frequency drivers.
Perhaps the more important issue is that a clipped signal may maintain the same peak
level but represent an increased average
level. This also applies to people who compress or limit a signal to reduce the peak levels and then turn it up, doing so may maintain the same peak levels but increases the average
level. And it is the average
level that typically relates to thermal failures, one of the most common failure modes of electronics and speakers.
On a more general note, you cannot actually underpower a
speaker - what you can have is an underpowered
system for an application. If you get all the output,
headroom etc. you want and need with 1
Watt of
power then the
speaker can be properly powered for the application with just 1W of
power and not be "underpowered" regardless of the
speaker power rating being much greater. On the other
hand, if in trying to get sufficient output from the
system you are
clipping the
amplifier before reaching the rated
power of the
speaker or providing
power greater than that for which the
speaker is rated then the
system is underpowered. The
point is that it is the performance of the
system in a particular application or in trying to obtain specific results that determines whether a
system may be "underpowered" rather than just the device ratings.
On a related note, "
headroom" is essentially the difference between the maximum levels the
system can support and the levels occurring during operation. I bring this up as it is easy to say that having
headroom is good but that is true only if the
system operation is such that the difference
headroom is used as
headroom and not used to try to push the
system harder. A savvy operator could run a
system with an
amplifier rated well above or below the capability of the related speakers by controlling the actual operating levels, however a less experienced operator could easily turn that same situation into a disaster by trying to get greater
system output than the
system is intended or able to provide.
