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.
When
comparing SPL figures it is helpful to remember that at medium SPLs (sound levels)
and mid frequencies, a tenfold increase in watts offers only an approximate doubling
in loudness to the ear. But at the lower bass frequencies and at higher SPLs,
considerably smaller changes in wattage, say just x3 to x5, have the same doubling
effect.
No comments:
Post a Comment