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Mat Nieuwenhoven Mat Nieuwenhoven is offline
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Default Motional feedback in speakers

On Thu, 7 Nov 2019 05:56:12 -0800 (PST),
wrote:

snip
One of the big problems with drivers is that the suspension stiffness
and the magnet's BL product is, for the most part, highly non-linear:
the stiffness increases with excursion, the BL product decreases
with excursion.


Is this BL change true also for underhang/overhang voice coils? The
german magazine HobbyHifi (very technical about homebuilding
speakersystems) stated several times that (paraphrased) some speaker
have a fair amount of excursion before the cross-section of voice
coil and magnetic field changes.

snip
Let me give one example that shows why this is true: look at ANY
bass reflex system: let's assume, for the moment, that it is
"optimally" tuned, i.e., that its frequency response is that of a
"perfect" lossless B4 alignment: it's dead flat down to cutoff,
and it rolls off at 24 dB/octave below that. Now, look at what
the cone is doing as it goes down in frequency through the region
of cutoff:

Acceleration: Constant with frequency as you go lower until
you approach the enclosure tuning frequency, at which point
is approaches 0, ad blow which it increases back up to
the same constant as above tuning.
Velocity: Increases as inverse of frequency until you approach
the enclosure tuning frequency, then goes to 0, goes back up for
a bit then starts decreasing with decreasing frequency.
Position: Increases as the inverse square of frequency until
you approach the enclosure tuning, at which point it approaches
0, then below it starts increasing with decreasing frequency.


I don't get this. If velocity is 0 at the tuning frequency, the
speaker is making no sound at all. I'm pretty sure it keeps moving.

I though bass-reflex boxes below the tuning frequency behaved as open
enclosure, more open the lower you get in frequency. If you keep
applying the same power, you can damage the speaker due to too large
excursions, because it has typically a flexible suspension and relies
on the air in the box to damp its movement.

Well, I "was there" when the the phillips was being marketed, and at
the same time there was an actually reasonably well-implemented B6
system from EV, designed by, I believe D. B. Keele. Neither speaker was
"outstanding", but both were quite reasonable systems, performance-wise.

Both died in the market for a variety of reasons, but, especially with
the Philips, customers resisted it because they wanted to use it with
THEIR amplifier. Often times, they already had a system and wanted to
upgrade speakers, and looked at their existing amplifier as now being
a "waste". And, with the EV, that little EQ box, no matter how technically
sound the approach was, was just an obstacle for most people. To many,
it reminded them of the Bose 901, and for many, that was just too much.


Interesting story. I had two of the smallest Philips MFBs a few years
back, was not impressed with the sound.

Regarding feedback, I remember there was an hobby project long ago to
have a very small R between speaker and GND (GND also being the amp's
ground), and using the speaker's back EMF as feedback to correct
excursions. There are some later publications from W.Kippel about it.
The idea is that the speaker's voice coil itself is the sensor. Will
this feedback method work? And only for closed boxes, or for
bassreflex also?

Mat Nieuwenhoven