John Wayland, of EV and ShoeBox Subwoofer fame, offers this advice regarding rear speakers. This is from the Yahoo! Honda-Hybrid discussion digest #685 (message 2).

Watts are watts, it makes no difference how they're generated, whether from a high current, low voltage 12V system, or a low current, high voltage 120 vac home system. There is a huge difference, however, in the way manufacturers in different market segments can manipulate the audio output in watts ratings, and in this area, 'there can be' a massive difference between '12V watts' and '120V watts' as Ronetele (sure wish people would use a real name) has put it. This again, is something I've covered before, but for Ronetele and other newbies, I'll recap.

The only way to really compare audio watts, is using the 'xxx watts rms' standard, at a 20-20khz response with less than .1% distortion. Higher end home stereo's use this rating system, and so do higher end car audio outboard amplifier companies. As a matter of fact, in the past 10 years or so, car audio amp makers have surpassed the once strict standards of home audio makers, in that hi end type car audio amps are very conservatively rated, where many of the more mass produced home audio makes have slipped in their credibility and are today, resorting to over-rated claims.

Sadly, in the car stereo world, the makers of head units (all of them) have been allowed to play games with their head units' audio output ratings, and unlike the ratings of the car stereo outboard amps, these ratings are 'pie in the sky' fake watts. Ronetele's Blaupunkt probably puts out less than 12-13 watts per ch. at the more conservative 'rms' rating (look at the owner's manual and read the rms ratings at 1% or less distortion). I'm not picking on his choice of brand...Sony, Pioneer, JVC, Alpine, Kenwood...they all do this.

Conversely, excluding the cheeseburger off brand car amps, higher end car amps are rated completely different, and a 40W X 2 (or 4) car amp will be rated at 40 watts rms @ .05% distortion, from 10-25khz. This is in no way, comparable to the over-blown head unit's 40W X 4 ratings, where the audio signal can get as dirty as 20% distortion and has an awful frequency response at this level of garbled power. If you again, look at the spec sheets for this same head unit, there should be the more conservative rating posted, and it will read something like 12 watt rms @ 1% distortion, from 20-20khz. But, this still isn't even close to the high caliber ratings of a hi end car amp.....look at the ratings of this same head unit at a more critical .05% distortion (if it can even get to this level of cleanliness), and the true comparable audio output power is something closer to 5 watts per ch. at .05% distortion, from 20-20khz.

If there is any doubt on the above, the proof is easy to find. Most of the head units that have over-blown output ratings of 40W and even 50W X 4, have just a 10 amp power fuse. Since amps X volts = watts, it doesn't take a brain surgeon to figure this stuff out, and even when the key is on and the car has more than 12V on tap, say 14V, this is only 140 watts of 'power consumption' at the point where the fuse will blow. In order to keep that fuse from blowing, it's rated at slightly higher then the normally expected full power draw of the device it's supposed to
protect, thus, a car stereo head unit that has a 10 amp power fuse, most likely, pulls a max current of closer to 9 amps...now, in reality, we've got 9 X 14V = 126 watts of 'power consumption'. You can see already, it's impossible to deliver 200 watts of 4 ch. audio power, if the thing can only access 126 watts! When you factor in the power needed to spin the drive motors, run the myriad of electronics inside, and run the illumination of the head unit, there's about 8 amps left for audio amplifiers to suck...now we're looking at 8 X 14V = 112 watts 'power consumption'! But wait, now we have to factor in the class AB audio amps that all head units use...these are at best, 50% efficient, meaning that only half the power consumed, makes it to the speakers (the other half goes out in wasted heat, hence the thick aluminum backside of the head unit). So here's the final breakdown, using 50% of the 112 watts of power consumption, 56 watts divided by 4 = 14 watts per channel. Keep in mind, that this 14 watts per ch., is at best, at the 1% distortion level, a level one can easily hear (well, perhaps MP3 users won't notice it), and it is still not even remotely comparable to an outboard amp putting out 14 watts @ .05% distortion....not even close!

Completely the opposite, look at the power input fuse (fuses) of a high quality car amp rated at 40W per ch....you'll see 40 amps of fused protection, and power input terminals beefy enough to accept 50 amp rated 8 gauge power wire! Here's two more examples. The Eclipse amp I offer in my audio kit, is rated at 50W X4 at 4 ohms per ch., can put out 70W X 4 into a lower 2 ohm load load, and has twin 25 amp power fuses. My Phoenix Gold amps in my soundoff car have 50 amp fuses per amp.

The 40W X 4 head unit weighs about one lb., has a thin 18 gauge power input wire, and has a single 10 amp power fuse...the 40W X 4 car stereo amp weighs about 7.5 lbs., takes a thick 8 gauge power wire, and has twin 25 amp power fuses....see the difference?