Headroom is the difference between the average level of the music, and the
peak level required for transients.
Consider the well-loved Tchaikovsky's 1812 Overture
- soft violins followed by military cannons!
The power difference required can be quite awesome as our ears are not linear with respect to discriminating power change effects on volume levels. (They are actually logarithmic.)
Let's do some maths:
an orchestra playing the 'body' of the 1812 Overture at, say, 90dB (max. industrial SPL for 8 hr / screaming child) .....
with (common) speaker efficiency of 89dB/w .....
listening at 4 meters (12 feet) .....
So the amplifier is called on to produce only 5.04 Watts
(Check calculations here: http://redgumaudio.com/help/efficiency.xls
)
As the 'industry standard' headroom for music is +15dB .....
to cover the
'bells and gongs' heard at the end of the Overture, the amplifier will now be required to produce
159.2 Watts
What about the cannons? Want to try for an additional 15dB on top of the standard headroom? After all, the last 15dB was fairly easy! :-))
The amplifier is now asked to produce 5035.7 Watts ..... or, for
most of us, just clip at its own (much lower) power limit!