:: View Point ::
often given three power ratings, usually
described as continuous, program, and
peak. In reality these are just different
ways of saying the same thing, but with
varying crest factors. Continuous power is
defined by the AEs standard IEC 268-5
as the amount of power a loudspeaker can
handle over a long time period (hours)
when driven with pink noise ( 6 dB crest
factor) that is limited to the driver’s operating bandwidth. This is a pretty good
“worst case” scenario, as any music that
you are trying to reproduce that has only
6 dB of peak to average ratio is going to
sound like noise anyway.
A program rating for the same loudspeaker is then usually listed, and it is 3
dB higher, or twice the wattage. At least
one major manufacturer doesn’t even list
the continuous (AEs) power rating for
their loudspeakers, because the number
that is twice as large looks twice as good
to the consumer. They list program and
then peak, which is 3 dB more than program and twice the wattage again. what
is important to realize is that all of these
numbers are simply estimates and the
higher numbers simply represent the
amount of amplifier power one should
apply when reproducing signals with
more dynamics, headroom, crest factor, or
whatever you’d like to call it. These numbers assume that the average power of the
signal will be the same at each power rating, but as each signal has a progressively
higher gap between its peaks and its RMs
value, more peak amplifier power output
can be applied to reproduce those peaks
without overheating the driver.
A more relevant extrapolation of loudspeaker power capability might list even
more power levels, but at the lower end
of the scale. A driver capable of 100 watts
when driven to its limits with the AEs
specification is potentially only able to handle 50 watts when driven with a sine wave,
which only has 3 dB crest factor. Apply a
dC current (0 dB crest factor) and it will
likely handle less than 25 watts. no manufacturer in their right mind is going to list
a loudspeaker with a power handling of “ 25
watts dC” when the same driver can be
said to handle “400 watts peak,” so we are
all left to do a little math on our own. Fortunately for everyone, the amount of dC or
sine wave power a driver can take is pretty
meaningless, since music is never made up
of waveforms like that. The peak power rating is much more accurate as an indicator
of what amplifier power rating to choose in
order to get maximum performance from a
driver. Furthermore, there are many more
options available to cool a loudspeaker that
is being driven with a highly dynamic signal, so gains can be made in terms of power
handling there as well.
let’s try and take a real world example. This short sample (Exhibit 4) from
a live board recording of the band All
Time low performing their song “Jasey
Exhibit 4 - Phillip Graham.
Ray” shows what a real world waveform
looks like. The thick green lines represent the RMs value of the waveform,
the voltage going towards driver heating, which is The Enemy. As you can
see, the actual peaks of the signal go
much higher than the RMs value… 14
dB higher, to be precise. To show how
significant this is, an amplifier capable of
providing 1,000 watts of power will be
at the limits of its rail voltage reproducing this signal, while only putting out 40
watts RMs. That’s a pretty enormous
difference, especially in today’s world of
100+ watt rated compression drivers and
1,000+ watt rated cone drivers.
These sorts of differences are exactly
why in many pro systems it is not at all
uncommon to have an amplifier capable of
delivering 500-1000 watts or more to the
high frequency section, while that section
can only handle a hundred watts AEs.
In the case of a compression driver being
driven at 100 watts RMs with the waveform in Exhibit 4, the amplifier would
have to be capable of providing 2,500
watts to reproduce the peaks! Providing
less amplifier power simply clips off the
tops of the waveform, creating distortion
that is much less pleasing the to ear.
In the real world we never really know
what kind of signal we’ll be feeding our
loudspeakers. what allows truly pro systems to operate with so much “excess”
amplifier power is very carefully set limiters. If the system processing is set so that
the limiters keep the long-term power of
the signal at or below the RMs capability
of the loudspeaker, one can imagine how
it would be possible to use an unlimited
amount of amplifier power without danger. Because the limiters are making sure
that the heat-generating average power of
the waveform doesn’t get to damaging levels, the “excess” amplifier power can safely
be used to reproduce peaks and dynamics,
ensuring that the snare can still pop without allowing the low frequencies from the
kick drum to overwhelm the loudspeaker.
