Ian, G3SEK wrote:
"Examples include a loudspeaker---."

Good transducer example. Its problem is abysmal efficiency, even if
better than the usual incandescent lamp.

The loudspeaker`s efficiency can be improved by a better match to its
medium. The usual loudspeaker is small in terms of wavelength. A result
is that it is capable of exerting much force on a small area of a very
compliant medium, air. Air could better accept power exerted over a much
larger area, especially at low frequencies, with less force required to
make the air move..

We have a high-Z source and a low-Z sink in the loudspeaker and air.
Conversion from electric power to mechanical power can be more efficient
through better impedance matching. Two solutions are often used for a
better match, a larger loudspeaker or a horn between the loudspeaker and
its air load. The larger speaker is directly a better match. The horn is
an acoustic transformer. They both improve energy conversion efficiency.

Best regards, Richard Harrison, KB5WZI