Why power is the most abused engineering quantity
The Sunday Nation
Nairobi,
11 May 2014
Power makes people go crazy. Not just political or even managerial power, but engineering power as well. Lately, I have seen adverts for music systems quoting ridiculously high power values – in the tens of thousands of watts. These figures are, to say the least, laughable!
There was one system from a leading manufacturer that quoted 23,000W output. Now, music amplifiers operate at an electrical efficiency of about 25%. This means that this system has to consume about four times the power it purports to give out. That is, 23,000W x 4 = 92,000W.
According to my power bill, the Kenya Power & Lighting Company limits my supply at 3,000W. Therefore, if I tried to operate the said music system in my house, the main fuse would blow out immediately! I would be trying to draw out 30 times the power that I am allowed.
Obviously, there are people who are safely using this particular music system in their houses without causing any blackouts. Therefore, I am justified to suspect that the power figures are not true.
Perhaps what the manufacturer is quoting is the maximum power handling capacity of the loudspeakers. But that would also be suspicious.
The bigger question is: why do manufacturers do this? The answer is that people mistakenly believe that the higher the power of a music system, the louder [and clearer] it is. This is simply not true.
Whereas there is a relationship between power and loudness, our ears do not hear watts; they hear decibels. So the critical factor that determines how loud a music system can get is its audio sensitivity. That is, how many decibels does it produce from one watt of power. A very good system should give above 80dB per watt when listened from one metre away.
But music systems are not the only gadgets where power has been used to hoodwink. Car manufacturers also prominently flaunt the horsepower (or kilowatts) generated by the engines. Here again, there is a relationship between power and speed but it isn’t straightforward.
Power alone cannot tell you how fast a car is: it is more likely to tell you how quickly the car will pick up speed – its acceleration. But even this is not straightforward since it depends on the weight of the car.
Put a 300HP (225kW) engine in a 1,500kg car and you can ideally get from zero to 100km/h in less than 3 seconds. Put a similar engine in a 15,000kg truck and now the time to reach 100km/h is over 20 seconds!
Furthermore, the power output of a car does not tell you anything whatsoever about the top speed! To get that, you would need to also know at what rate of revolution the engine generates this power, the gearing ration in top gear and the size of the wheels.
Clearly, just like the in the case of a music system, knowing the power output of an engine isn’t very helpful. For this reason, I think that power is the most commonly abused engineering quantity!
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