Linear excursion & magnet force

[Ronkel]

Hi everybody,

I've found that there are woofers which have a very great 'linear excursion', many Adire driver for example. The largest linear excursion I've found is 33 mm (one way).

As far as I know, the magnet force at the end of the xmax (of probably far before) must be reduced by such an extent that the excursion must be far from linear.

Philips long ago produced speakers with a built-in Motion Feedback amplifier which responds to this effect and corrects it.

My question is: how great is this decrease in linear excursion, looking at excursions of 20 mm + ?

(Sie durfen auch auf Deutsch antworten)

[kboe]

Hello Ronkel.
Here is a bit of theory:

If you have an airgap of certain depth e.g. 10 mm and a voicecoil with a length of e.g. 20 mm this should give you a linear excursion of 5 mm one way.
within theese 5mm absolutely NO decrease of magnetic force should appear. only from the point where MORE windings of the coil leave the airgap on one side than ENTER on the other side you begin loosing magnetic force.

so as far as I know, the more or less linear excursion is even BIGGER than theory because the magnetic field does not end right at the end of the airgap but extends slightly beyond it.

BTW Visaton has developed a methode of measuring the linear excursion by means of a laser-distance measuring device.

another approach is measuring the excursion for 10%THD
regards
bernhard

[tiki]

Hello,

sorry, if you would remind, that the Bl-curve tends to be not flat in almost all speakers and the curve isn't really horizontally, the linearity decreases with increasing amplitude, before the vc will leave the gap. It is a matter of allowed distortion level. You will have to measure distortion over frequency _and_ sound level.

A symmetrical Bl-curve will produce odd harmonics only, an asymmetry of it will produce even harmonics and (by internal feedback effects) odd harmonics too. A flat Bl curve results in a good (Bl related) distortion performance inside the "linear" region, but outside of it, the distortion will have wider spectra of harmonics due to the steep edges of Bl.

Regards, Timo

edit:
Please, measure the intermodulation distortion (bass frequency sweep and voice frequency sweep at different sound levels) also if possible, these distortions sound much worse than harmonics, described before, because they consist of higher frequencies and are not related (means nonharmonic) to the programm frequencies.

[maha]

Kann man nicht deutsch weiterreden. Die Germanismen nerven.

maha

[Ronkel]

Naturlich kann mann auf Deutsch weitergehen... ich kann's gut lesen .

I don't own such a woofer, nor does it fit my budget. I just would like to know more about this subject.

Does anyone have an idea of how great the thd may be with the enormous xmax of an adire tumult woofer (33 mm)?

[tiki]

Hallo,

wenn man keine unabhängigen Messungen dieses Chassis findet, hilft nur selbst messen. Bestellen, messen, bei Nichtgefallen zurückschicken.
Ich meine bei Klippels aufgeschnappt zu haben, daß ulkigerweise just im Carhifi-Bereich insbesondere auf lineare Bl(x)-Kennlinien zunehmend Wert gelegt wird. Das hat zur Folge, daß die Reihenfolge der dominanten Verzerrungen bei solchen Subwoofern durchaus anders sein kann, als bei z.B. Mitteltönern oder TMTs. Bei großen Auslenkungen und damit bei den meist niedrigen Kennempfindlichkeiten notwendigerweise großen Eingangsleistungen/-strömen können die Le(x) und/oder die Le(i)-Nichtlinearitäten dominant werden. Zumindest wurde dieser Effekt eindrucksvoll an einem Beispieltreiber (in der ähnlichen Größenordnung wie der Adire) vorgeführt. Leider ist so ein großer Treiber auf deren Beispielseite nicht aufgeführt.

Möglicherweise schreibst Du direkt an Klippel, ob "anonymisierte" Daten eines solchen Treibers verfügbar sind. Soweit ich weiß, setzen sie in einer ihrer Meßmaschinen einen sehr dicken Aura-Treiber zum "Durchschütteln" von Sicken ein, der wurde ganz bestimmt vorher vermessen.

Gruß, Timo