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Offline syhprum

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« on: 06/11/2007 08:40:17 »
I recently purchased a loud speaker that has written on it 15 Ohms 100 Watts.
It consists basically of a coil of wire that would be hard put to dissipate 2 Watts without burning out, as the conversion of electrical power to Audio power is never likely to exceed 5% why this ridiculous specification.


 

Offline daveshorts

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« Reply #1 on: 06/11/2007 10:30:12 »
Any energy transferred to the cone of the speaker will not be dissipated in the coil not just the energy converted into sound. So it is not the efficiency of conversion to sound you are interested in but efficiency of conversion of energy to movement which is probably over 90% so only 10% of the input energy is dissipated and heats up the coil, as long as you don't put DC into the coil when it will burn out very easily.

Also speakers are normally quoted with a 'peak' power not an average RMS power so i can probably cope with 100W for less than a second. I think the RMS power is normally about 1/3 of the peak power.
 

Offline techmind

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« Reply #2 on: 06/11/2007 11:10:02 »
Also speakers are normally quoted with a 'peak' power not an average RMS power so i can probably cope with 100W for less than a second. I think the RMS power is normally about 1/3 of the peak power.
Totally second Dave on the fact that the power is going into motion of the cone, not primarily into heating the coil.
"Serious" loudspeakers should have a continuous power rating (or maybe RMS/peak), but cheap consumer speakers frequently do have truly ridiculous Peak Music Power Output (PMPO) specifications which really only means that the speaker cone has got the mechanical space to move a long distance (giving a thumping bass) - but the coil can only handle the power for very short duty-cycle.
 

Offline Bored chemist

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« Reply #3 on: 06/11/2007 19:24:12 »
"why this ridiculous specification?"
because 100W sounds more impressive than 2W so you can charge a bigger price for it.
I don't believe that any sensible domestic speaker system puts out 100w of acoustic power. Most are typically 2% efficient or so. To deliver 100 W they would need to dissipate 5KW
Also at 1 metre the sound intensity would be something like 8w/m^2 or about 130dB. Rather louder than most people could stand.
 

lyner

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« Reply #4 on: 08/11/2007 17:53:45 »
Don't forget that a large proportion of the lost power in a loudspeaker is not dissipated in the speech coil but in the crossover unit, the equalisation circuit and in the heavy  acoustic damping in the cabinet. A large horn loudspeaker is much more efficient.
Also, the 15ohms refers to the impedance - which may be considerably higher than the dc resistance of the wire in the coil.

The peak rating IS relevant and relates to how much the coil can actually move in a linear fashion and produce peaks of sound pressure.
Sound signal power is much harder to assess than a TV signal , which just peaks white at 1v.
There's that 'loudness' thing, with sound.
 

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« Reply #4 on: 08/11/2007 17:53:45 »

 

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