A question about bias in general

[PRR]

> They bridle at the suggestion that their driver would need an offset.

The latest 65 pound $1,100 12" car-woofer from CerwinVega has an offset adjustment. In car-sound competitions (or some of them), there is no sin in distortion. An offset adds even-order distortion, which is efficiently radiated, and reads higher on the SPL meter.

Or in current contaxt: the motor is biased away from its most linear operating point.

 

[BYacey]

How does one determine the proper midpoint of the coil excursion to bias the coil at? With any speaker as the compliance nears the end of it's linear travel there is a certain amount of "give" or stretching before mechanical damage occurs. Obviously this is no longer within the linear operating range. So I am curious to know by what method this could be measured with any degree of accuracy? This is a very interesting topic.

 

[BYacey]

After thinking about this for about 30 seconds it occured to me that this could be accomplished by adjusting the magnet pole piece position within the speaker basket in relation to the coil. I guess every speaker would have to be individually tested because the spider and surround mechanical resistance would vary on every cone assembly, not to mention the variables introduced by the person assembling the cone into the basket.

 

[bcarso]

That's amazing PRR. Shows I've been away from car audio for a while (and not missing it a bit I might add!).

"So I am curious to know by what method this could be measured with any degree of accuracy? This is a very interesting topic."

There are two systems available I've heard of. One of them is Wolfgang Klippel's and another I've forgotten the name of. The loudspeaker is moved pnuematically in at least one of them I believe. The claimed accuracies are a little optimistic IMO but you get there. There was a review of the products in I think audioXpress (Paul S. do you recall the issue?), followed by a haughty letter from Earl Geddes (who spends a great deal of his time reminding everyone that it is DR Earl Geddes), berating everyone for not recognizing him as the supposed originator of the idea. So deftly and politely did the authors respond that it stayed my hand from sending something a bit testier ;-).

 

[PRR]

> How does one determine the proper midpoint of the coil excursion to bias the coil at? With any speaker as the compliance nears the end of its linear travel there is a certain amount of "give" or stretching

Two issues: centering the magnetic system and centering the stiffness system.

For the magnetic system: use a limp spider and a stiff spring-scale. Run some DC in the coil, and measure the force as you move the coil through the gap. It will be zero when the coil is far outside the gap, rise to a plateau while in the gap, and fall off again on the other side. For an ideal gap, the top of the plateau is flat, the sides are equally steep. For most practical speaker magnetic gaps, the backside (where the magnet is) has a lot of leakage, so the force/position curve is not symmetrical.

In about any sane use, the mechanical compliance should be overwhelmed by box air stiffness. If the box isn't stiff, either by sealed-air or by port resonance, it isn't loading well and isn't going to make much output. However competitive car-sound is not sane, and they do crank the spiders hard. Spider compliance is never linear or symmetrical. If air-stiffness dominates (if system resonance is much higher than free air resonance) that does not matter. Otherwise, at high excursion the suspension nonlinearity will "rectify" the signal and produce a "DC" displacement. Competition woofers may balance syspension flaws against magnetic gap flaws.

> adjusting the magnet pole piece position within the speaker basket in relation to the coil.

Generally, you take existing parts, assemble a prototype, and mark where the gap covers the coil. Then do the math to center (or near-center) the coil on the gap.

> I guess every speaker would have to be individually tested because the spider and surround mechanical resistance would vary

Resistance does not matter. If you mean stiffness, that's not important for centering, only for Fs which in small-box systems is rather a phantom number. What you want is the spider position at assembly, and also after shipping, after break-in, and at high excursion. There is too much variation to really precision-center a speaker. And it isn't really needed. You want a long range of constant electromagnetic coupling and fairly constant compliance, so small offsets shouldn't matter except as a slight reduction of maximum excursion. And while buyers may shun an 11mm travel in favor of 12mm travel, small differences don't matter enough to hear.

 

[BYacey]

Thanks PRR,
To me this really seemed like splitting hairs while rampantly chasing a rainbow. I wonder if there is anything significant to gain by putting a small dc offset on a power amp output, other than coil heating.

 

[Kev]

[quote author="BYacey"]Thanks PRR,
To me this really seemed like splitting hairs while rampantly chasing a rainbow. I wonder if there is anything significant to gain by putting a small dc offset on a power amp output, other than coil heating.[/quote]

to publish a better THD spec and plot

to make the most of available pos and neg cone excursion. The area below driver resonance where the cones excursion rises very rapidly. These techniques benefit the IN-SANE world of Car Sub Competitions
AND
the hi tech world of the DEEP sub ... not only performance to 20hz but also heading for 15 and 10.
Places where the normal cone and coil techniques just shouldn't go.

As for coil heating.
We found that the heating was marginal when thing where in good control. Please note that when a coil starts to leave the magnetic field the coil heating is very rapid.

Drivers used in sealed boxes that are deliberately TOO small based on the normal calculations and active electronic are employed to force a flat frequency response.
Add that to multiple drivers where the phase response needs to be matched to get good mutual coupling.

I still say ... that for normal music production these techniques bring little benefit for the efforts put in.
but for the extreme systems it may be the only way to keep control of the parts ... especially multiples.

 

[BYacey]

As the cone nears excursion limits the driver becomes non linear; I suppose if volume level is the only goal this isn't a concern....

Reminds me of a time when I had a coil that was rubbing on a JBL 2225. Since the cone was a lost cause I applied 120 VAC to the coil to see how long it would last; It made a hell of a racket for about 15 seconds while it hammered from one limit to the other. Loud, but not too musical.

 

[Kev]

[quote author="BYacey"] a JBL 2225. Since the cone was a lost cause I applied 120 VAC to the coil to see how long it would last; ...[/quote]
:roll:
mmm
120 was probably a little high and the 50/60hz was probably a little low. Getting near the tunned resonance of the box/speaker and so cone excursion will be high.

Move up an octave or two to around 200 ... even 400 and lower the peak to just under 100volts and the driver may well have lasted the one hour thermal test.

ear protection required ... the neighbours wont like it either.

 

[BYacey]

This was just a raw basket , no box. It was a rather disturbing noise... :shock: It's also hard to get the power company to supply other line frequencies for my convenience. They have a very limited sense of humor.

 

[Kev]

[quote author="BYacey"]This was just a raw basket , no box. It was a rather disturbing noise... [/quote]
yep
but would have been fun to watch

It's also hard to get the power company to supply other line frequencies for my convenience.

err
yeah
I was just indicating I have done 100volts at a the higher frequency and ...
at those volts and frequencies a 10 volt DC bias can be useful in RE-centring the cone in the basket.
This can change the THD.

 

[bcarso]

My friend John Norris, who coincidentally (see last post in rbb thread od student's) worked with Arnold at Cambridge on nonlinear dynamical systems before his wife got a job with the Rand Corp. and he became a programmer -( , has done some work on the jump-out effect and how to prevent it.

He works for Nvidia now, so it is not clear if the work will go much further.

I have my own ideas using a version of position sensing (not integrating accelerometer signals), but I disclosed them to H*rm*n before I knew better. They will of course do nothing with them, but I can't use them anymore. Bugger. (^(

 

[Kev]

[quote author="bcarso"]but I disclosed them to H*rm*n before I knew better. They will of course do nothing with them, but I can't use them anymore.[/quote]

:green:
lol

I tend to prefer the extended former with the optical sensor approach ... for both total displacement and off-set monitoring.
Could never get ONE unit to acheive the results I wanted so had to use two arrangments back there.
... same idea though.

It was all long ago and as I said ...
I never found that it was worth all the effort for general quality audio performance ... and good sound reproduction.
Great for pushing the envelope and the IN-sane world of car sub competitions.