Re: RMS and Peak in car audio.

Dear Doug,

First let me clarify terms here.  RMS is NOT really the right measure to
use for audio power- it's the one lots of people use, but it not strictly
correct.  What is usually meant when they say "RMS" is the average power
delivered (in the case of the amp) or handled (by the speaker).  RMS really
means "Root Mean Square" which is a particular way of averaging out the
total power in a complex waveform, but the RMS method is often NOT used to
do these measurements.  Take a look at Wikipedia's article:
http://en.wikipedia.org/wiki/Audio_power

That said, your question has to do with the various ratings of the speaker
and amplifiers.

A speaker rating of "60 W RMS" usually means "able to handle 60 Watts
average power applied as a pure sine wave, for a long time, without
overheating and being damaged."

The amp rating of "50 watts RMS" usually means "able to deliver an average
of 50 Watts of power as a sine wave without too much distortion to a
resistive load, for a long time, without overheating and failing".

In each case exactly how the averaging is done, how long is a long time,
and how hot is overheated, and how much distortion is too much, all will
vary.  So you get different specs.  Some honest specifiers try to
standardize the tests.  Take a look at: 
http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&tpl=/ecfrbrowse/Title16/16cfr432_main_02.tpl

But not all suppliers are honest.

The speaker "peak" rating of "120 W" means that if you drive the speaker
hard enough that more than 120 W is delivered to it, even instantaneously,
it may fail.

With music, there are peaks of the signal which have to be faithfully
reproduced.  These peaks are when the highest instantaneous power is needed. 

The 200 W rated amplifier will (probably) be able to deliver more power
than the 50 W rated amps in the receiver.  Maybe enough to blow out the
speakers on program peaks.  

The 50 W rated amplifier channel might not do a good job delivering enough
peak power to keep distortion low on sound peaks.  But it might.  It
depends on how loud you want to play it!  For my taste, 50 W is REALLY LOUD
for a car stereo.  But many would disagree...

As with many questions in audio, this might come down to a subjective
judgement.

The basic question I would ask is whether the built-in amp (50W x 4) is
loud enough, without objectionable distortion, for the way you use it?

Only if it's NOT, then you might want to do something.

Hope this helps.

Best,

Barry.