Double The Power Is Not Equal To Double The Loudness

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

20 November 2005

Last week’s article demonstrated that the so-called Peak Music Power Output (PMPO) quoted on Hi-Fi systems is a completely meaningless quantity intended to make the machine look louder than it actually is. But one question was left unanswered: If a system is fitted with “more powerful speakers” would it sound louder?

The phrase “more powerful speakers” is inaccurate. The power of a music system is “made” by the amplifier. The loudspeaker’s job is to transmit the sound to the ears. Thus the power rating quoted on speakers is the maximum that they can take without burning out.

If the amplifier can produce, say, 10watts, it should ideally be fitted with loudspeakers that can handle at least that amount, i.e., 10W. Otherwise, when played at full volume, the amp will burn the speaker’s coil.

Now let’s follow the flow of power: The amplifier produces 10W; these go to the speakers and, assuming that there is no loss along the way, the speakers give out just the 10W of sound. Right? Yes.

Now suppose you changed the speakers and fitted new ones rated at say 50W and played the same amp at full volume. It would still make 10W thus you would get only the same 10W of sound out of your 50W speakers. Therefore, the system wouldn’t sound any louder.

The fact that loudspeakers are passive devices in a music system is often ignored. Thus many people expect that fitting “more powerful” speakers will make their HI-FIs louder. Indeed, suggestions have been made that the Ministry Of Transport should put a limit on the power of speakers that can be fitted in a matatu. Such action would not achieve the desired objective.

Still, what is the connection between the power output of an amplifier and the loudness of the music system? Would a 100W machine sound twice as loud as a 50W one?

The loudness of sound is measured in decibels (dB). Zero dB is dead silent. It cannot be heard by any human being. 135dB is so loud that it causes pain in the ears. An increase of 10dB is perceived in the ear as a doubling of the loudness. Thus 30dB sounds twice as loud as 20dB; 70dB is twice as loud as 60dB and so on.

In terms of power, doubling the number of watts increases the sound level by 3dB. This is in fact the lowest discernible change in loudness. Thus when someone says, “increase the volume a little bit”, you will probably raise it by about 3dB, in other words, you will double the power output from the amplifier.

Thus the difference between a 100W system and a 50W one is barely audible! To get double the loudness, the sound level should be increased by 10dB. This is equivalent to increasing the power by a factor of ten. Thus at maximum volume, a 100W system sounds twice as loud as a 10W machine.

Nevertheless, sometimes it is possible to increase the loudness of a system by changing the speakers. How? That is a story for another day.