For the most part, the processing of audio
signals can be performed with only minuscule power, either input or dissipated.
Analog signals pass through the majority of the overall signal path at average
levels in the order of 100mV to 1 volt.
Load impedances may be as high as 100kΩ but
even if as low as 5kΩ, only 120μW (a hundred and twenty microwatts; or about a
tenth of a thousandth of one watt) would be dissipated. At this rate, it would
take about eight million hours or hundreds of years of playing, for the load to
absorb or use one unit (1kWh) of electricity!
Most loudspeakers used to reproduce audio
are highly inefficient. Typical efficiencies of common direct radiating
speakers are 1% to 0.05%. By comparison, the efficiency of an internal
combustion engine (considered highly inefficient by ecologists) is between
2500% and 50,000% greater.
A medium sized car uses about 70kW to move
4 people or hundreds of pounds of goods, and its own weight – altogether at
least half a tonne, at speeds of say 70mph.
In some sound systems, to move just the
weight of air molecules to reproduce a bass drum, as much as 7kW of electrical
‘fuel’ can be burned in bursts. And yet a loudspeaker only needs to convey 1
acoustic watt to the air to recreate music at the highest practical sound
levels in a domestic space, i.e. about 120dBSPL.
And a tenth of this level (0.1 acoustic
watts) will still suit most of the loudest passages in the less extreme forms
of music. If speaker efficiency is taken as 0.1%, and 1/10th of an acoustic
watt is enough, then an electrical input power of 1000 times this is needed,
i.e. 100 watts.
The highest SPLs in music can be
considerably greater than 0.1 acoustic watt. Loudspeaker drive units exist that
can handle short term electrical power bursts (the norm in much music) of 5000
watts (5kW) or more.
With 2% efficiency, today’s most capable
drivers can generate 100 acoustic watts each. With horn loading, efficiency can
be raised to 10% or more, allowing one drive unit to produce 500 acoustic watts
for large scale PA. This allows fewer sound sources to be used, improving
quality.
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