Fyi... A Beginner's Guide To Power
Sydney Morning Herald
Monday October 15, 2001
It's unclear when audio manufacturers first starting fudging their power figures, but it likely followed shortly after their discovery of marketing. Have you ever wondered why a 200-watt amplifier for your home is a major investment while every second car stereo, a fraction of the cost and size, claims to be 160 watts, 180 watts, 200 watts or more?
The answer is fudging. There are many ways to measure power and manufacturers are not above choosing the one that suits them best. Car audio makers are the worst, but lately fudging has been creeping into quality home audio as well.
The measurement that gives the highest power readings by far is peak music power output (PMPO), sometimes also called peak power or music power. This measures the maximum power that can be delivered for a split second when all the conditions are right. This is how an itty-bitty car stereo can claim to have 200 watts or more.
The same car stereo measured in watts RMS (root mean square - don't ask) might deliver 8 or 12 watts per channel; it's a far more accurate way of measuring power but much less helpful when your job is selling these things. RMS measures the maximum power an amplifier can deliver continuously - for days on end, if necessary. Most high-end home-audio manufacturers use the RMS measurement and it provides an accurate basis for comparison. This explains why a 200-watt home amplifier costs so much more than a 200-watt car stereo - the home amplifier's PMPO power would likely be well into four figures.
Just lately, however, a number of home-audio manufacturers, including some very reputable ones, have been providing RMS power figures measured with six-ohm speakers rather than the far more common eight-ohm ones. Six-ohm speakers have less electrical resistance so more power gets through - usually about 25 per cent more. As a result, the power figure is roughly 25 per cent higher. Such power readings are usually specified in the small print, either as being at six ohms or as EIAJ standard. To compare such a component with another measured into eight ohms, you need, as a rough rule of thumb, to divide the EIAJ figure by four and multiply that by three.
In other words, you need to do your homework and take what you're told with a grain of salt.
© 2001 Sydney Morning Herald