[sci.electronics] Cables and stuff

torkil@psivax.UUCP (Torkil Hammer) (09/25/87)

In article <1953@tekig4.TEK.COM> brianr@tekig4.UUCP (Brian Rhodefer) writes:
# [about current rather than voltage source drive of speakers]
#
#Moreover, forces generated by currents passing through magnetic
#fields tend to be simple products of the currents and field strengths.
#A speaker's voice coil, then, should then have a linear current-to-force
#transfer function.  As I'd expect voice-coil force to translate directly
#into induced air pressure, it seems to me that a current-mode output
#signal would also be desirable due to a more nearly linear transduction
#of audio voltage signals into air pressure signals.
#
#Am I mistaken in thinking that this is exactly the kind of transducer
#that an amplifier/speaker system ought to be?
#
#There must be some reason, though, for amplifier outputs to be made
#the way they are;  could someone kindly explain why?

This is correct as long as we keep to 1st order effects.
2nd order are caused by inhomogenities in the magnetic field.
Note that the constant of proportionality between current and sound
pressure [which is the 'transfer function' of the speaker] depends on
frequency, listening angle, distance and environment such as furniture.
Since the electrical impedance (ratio between voltage and current) is
equally independent of the current (1st order again) and dependent on
frequency, all what you get is different overall transfer functions
and approximately the same linearity.

Voltage feeding (i.e low output impedance of the power amp) has one big
advantage over current feed: It dampens the bass resonance of the speaker.
The bass resonance is mechanical and formed between the membrane mass and
elasticity.  Since the speaker is a 2-way transformer - electrical on one
side, mechanical/acoustical on the other - this resonance transforms
into an equivalent parallel L-C resonance.
The resonance is below the useful frequency range of a well designed
speaker.
The resonance is dampened by a low impedance across the electrical
terminals of the speaker.
Which in turn is desirable because it cuts down on 2nd order effects
as well as limits the voice coil movement at a useless frequency.

#
#An Inquiring mind which may not really want
#to know, but sure loves to ask dumb questions,
#
#Brian Rhodefer

Not that dumb at all.
If you want to go into details, note that the term 'linearity' applies
to the ratio between output and input; mathematically, acoustically, and
electrically.
Frequency independence is best called something else, such as
'flat transfer function', because it is an altogether different concept.

Garden variety (non-audiophile) ears usually find non-linearity much
more annoying than frequency dependence.
Frequency dependence can be tuned by means of an equalizer [though
purists know that the phase response usually gets more messy in the
process], while non-linearity can not be cancelled that easy.
Furthermore, non-linearity is usually inherent to the speaker, while
frequency dependence is environment as well as speaker.

It should be obvious that I speak as an engineer, not an audiophile.

torkil hammer