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Old November 9th 19, 12:17 AM posted to rec.audio.high-end
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Default Motional feedback in speakers

On Friday, November 8, 2019 at 3:52:03 PM UTC-5, Peter Wieck wrote:
> Ummmmmm.....
>
> A speaker is a linear motor with a magnet, and a commutator
> (voice coil).


Sorry, no. The commutator in a motor IS NOT the equivalent of
the voice coil. The field windings of the motor are the equivalent
of the voice coil. The commutator, which is present only in motors
where the magnet is fixed and the windings are on the rotor, serves
two functions:

1. It's the way the current gets from the fixed input wires to the
spinning windings,

2. And it's the way that ensures that the polarity of the current
switches in synchrony with the relative position of the rotor
and the magnetic field.

And, mostly, it's what makes for DC generators.

> Just as in a PM Motor, when current is applied,
> the motor spins. DC motors spin according to the
> polarity of the power applied.


And, yes, without the commutator, an applied DC motor would cause
the rotor to spin 180 degrees, at which point, the relative polarity
of the fixed magnetic field reverses (because the windings are now 180
degrees "backwards" mechanically) and the motor wants to spin in the
opposite direction.

> Speakers move in or out depending on the polarity of the current
> applied. And, PM motors do, also, have a fixed resistance across
> the commutator just like a voice coil.


I'm sorry, you're absolutely wrong he the DC resistance
IS IN SERIES, not in parallel.

And forget the commutator, it's leading down the wrong path

> Now, when current stops being applied, the motor generates
> current - acts as a generator as it spins down. If it is
> unloaded, that current goes nowhere and does not add additional
> resistance to the motor spinning than normal bearing friction.
> However, if the motor is loaded, there will be additional friction.
>
> Similarly the (conventional) speaker. Try it some time with a
> sensitive VOM. The bigger the driver, the more easily this is
> observed. Just a few taps on the speaker cone will show you.


Peter, perhaps you've forgotten who I am: I've been doing this
stuff with speakers professionaly for a sizeable portion of
half a century at this point. And your analogy is STILL
inappropriate and flawed.

Be that as it may, you're omitting several VERY crucial points.
The most important one is that speakers are, first and foremost,
resonant systems. They are not motors that spin forever.

Secondly, damping is specifically the mechanism by which energy
is irretrievably removed form a resonant system: that is it's
fundamental definition.

And in ANY resonant system, the damping of that resonant system
is controlled by the total series resistance (be it electrical,
mechanical or acoustical).

Specific to drivers and speakers, the electrical portion of the damping
is the inverse function of the total series electrical resistance
in the electrical loop of the driver. And the single LARGEST series
electrical resistance in the VAST majority of drivers is the DC
RESISTANCE OF THE VOICE COIL.

Ignore this point, and, Peter, you WILL always come to the wrong
conclusion.

To go back to your motor analogy, it's NOT the difference between
the motor coil being open circuit or dead short, it's the difference
between open circuit and a fairly hefty series DC resistance.

> All and at the same time, DF is only one (1) single factor in
> how amplifiers interact with speakers.


And for the VAST majority of amplifiers and driver combinations,
it is among the LEAST significant of the bunch.

If you are so fixated on damping factor and you abjectly refuse
to consider it in it's correct context, then at least calculate
the right damping factor. The right damping factor, i.e., the
one that actually describes how the system is damped, is NOT
the ratio between the amplifier's output resistance and the
nominal impedance of the speaker, it is the ratio between
the voice coil's DC resistance, all divided by the SUM of the
amplifier's output resistance plus the voice coil's DC resistance.

Calculate it ANY other way, and you get a completely wrong answer
for damping.

> And, today in 2019, the issues that drove speaker design in the
> era after field-coil speakers were dominant up until the development
> of acoustic suspension are not particularly relevant as much
> evolution is taken for granted (and usually is granted). However,
> as one who spends as much time with electronics from the 1930s
> as from the the 1970s and up, I see all sorts of variations on
> how to control large speaker overshoot, sagging, and similar
> problems.


Please, what does overshoot and sagging have to do with one
another (especially as you have used "sagging" without a clear
definition of what you mean)?

And "overhang" is simply a function of the total Q of the system at
resonance. And the total Q of the speaker at resonance is a function
of the electrical and mechanical Q, to wit:

Qt = (Qm * Qe) / (Qm + Qe)

And the electrical Qe is a function of:

Qe = 2 pi Fs * (Mms * Re) / (B^2 l^2)

where Fs is the resonant frequency, Mms is the total effective moving
mass, Re is the DC resistance of the voice coil, B is the flux
density on the active portion of the voice coil gap and l is the
length of the voice coil wire in the active portion of the gap.

Now, adding the resistance provided by the amplifier changes that
electrical Qe:

Qe' = Qe * (Re + Rg) / (Re)

where Rg is the output resistance of the amplifier.

Clearly, this last equation shows that unless the amplifier output
resistance is significant in relation to the voice coil resistance,
it is the voice coil resistance that completely dominates the total
damping of the system.

THese are not my equations: go back and look at Thiele from the
the early 1960s, go back and look at Small from the early and
mid 1970s. If you wish to dispute these relations and the whole
issue of damping and damping factor, you'll need to argue it with
them with the same mathematical rigor that they formulated them
to begin with.

> A 15" Zenith speaker driven by a single-ended 6F6 is
> an entirely different animal than a 12" Long-throw woofer from an AR3a.


No, they most assuredly ARE NOT, not from the viewpoint of how
the physics of each work and how the mathematics describes those
physics very accurately, thank you.

Dick Pierce
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