What Makes Some Music Systems Louder Than Others?


The Sunday Nation


27 November 2005


Why is it that one music system can sound louder than a second one of the same power rating (real power, not the P.M.P.O. nonsense)?

The loudness of the speakers depends largely on their sensitivity. That is, how many decibels do they produce when supplied with one watt of power? Determination of that quantity is not a straightforward matter. It is made difficult by the fact that the further away you are from a source, the quieter it sounds.

Indeed, if you are standing, say two metres from a speaker and then you moved to four metres, the sound level would drop by 6 decibels (dB). If you moved further to eight metres, the loudness would go down another 6dB. Every time you double the distance from the source, the intensity of sound goes down by about 6dB.

Because of this variation of intensity with distance from the source, the sensitivity of loudspeakers is measured at exactly one metre in front of the box. Very sensitive speakers will produce between 80dB and 100dB when supplied with one watt of power – and their manufacturers will proudly quote the figure!

Ordinary, run-of-the-mill speakers (the kind you’ll get in most cheap boom boxes) will struggle to deliver anything above 70dB from one watt. For this reason, one has to really play the system at higher volume levels to get any meaningful sound out. Thus, in order to distract your attention, the manufacturer will put a large sticker on the front saying something like 1,500W (PMPO)!

Now, suppose you have a good speaker that give 80 dB from one watt: How many dB would you get if the power was raised to two watts? 160dB? Wrong. If the power is doubled (from one to two watts), the sound intensity increases by only 3dB (see last week’s article). Thus two watts will produce just 83dB from these speakers. Four watts would add another 3dB making a total of 86dB…and so on.

To raise the sound level from 80dB to, say, 90 dB (double the loudness) would require 10 watts from the amplifier. And to push it up to an annoying 100dB will need 100W. If our good speaker cannot withstand this much power, its coil will burn and nothing will be heard! But keep in mind that these sound levels are recorded at only one meter from the speakers. When you are further away, the story is different…

At a typical five metres from the music system, the sound intensity is about 14dB lower than that at one metre. Thus, driving 10W through these very sensitive speakers will produce 90dB at one metre, but this drops to 76dB when the listener is five metres from the machine.

Now, if we have two of these good speakers and pumped 10W into each, how much sound do we get while listening from five metres?  76 x 2 = 156dB, right? Wrong again! The power has simply doubled, therefore the sound level has gone up by 3dB to 79dB. This is why there is very little difference in loudness when one speaker dies off.

If a second speaker makes so little difference in volume level, why then do music systems come with two speakers? That is a story for another day.

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