I've noticed that when referring to the data sheets of drivers, the value for Xmax seems to contradict dimensions that it can be worked out from, for example, taken from the data sheer for the PD1850
Xmax = 11.05 mm
but
Magnet Gap Depth = 12 mm / 0.47"
Voice Coil Winding Depth = 28 mm / 1.1"
I was under the impression that Xmax corresponded to: (coil length - magnet gap)/2
So that would mean an Xmax of +/- 8
Where is the other value coming from?
Thanks
Xmax = 11.05 mm
but
Magnet Gap Depth = 12 mm / 0.47"
Voice Coil Winding Depth = 28 mm / 1.1"
I was under the impression that Xmax corresponded to: (coil length - magnet gap)/2
So that would mean an Xmax of +/- 8
Where is the other value coming from?
Thanks
Would it be a value that's measure in by running tests on the driver itself then? For the driver above for example, I'm just wondering how they get to such a specific value of 11.05?
Usually manufacturers add 1/4 or 1/3 of the magnetic gap depth to the xmax value so (coil length - magnet gap)/2 + (magnet gap/4). We end up at 11mm applying that to the PD1850, i don't know where the additional .05 comes from.
There is still significant magnetic density (although sharply falling) outside the "geometrical" gap (what you calculated). and voice coil is still getting "some" push out there; in principle that may give you extra/free X max.
In general narrow gaps are "tighter"; and wide ones (as often used in a woofer/subwoofer) are lossier and extend beyond.
And car subwoofer gaps are monstrous.
Nothing is free, you pay for that with much reduced efficiency/sensitivity.
In general narrow gaps are "tighter"; and wide ones (as often used in a woofer/subwoofer) are lossier and extend beyond.
And car subwoofer gaps are monstrous.
Nothing is free, you pay for that with much reduced efficiency/sensitivity.
How does Beyma calculate the Xmáx?
The Xmax is calculated as (Lvc – Hag)/2 + (Hag/ 3.5 ), where Lvc is the voice coil length and Hag is the air gap height. This value indicates, as a guideline, the linear excursion capability of a speaker.
While B&C have this
limits are customarily indicated by Xmax, the maximum linear excursion. This value is typically measured according to the AES2-1984 standard, corresponding to a maximum of 10% total harmonic distortion (THD) with a sinusoidal signal (though most manufacturers, including B&C, now typically provide data for Linear Mathematical Xmax, not measured Xmax). Recent research shows that this method can yield ambiguous results, and even different numerical values for the same loudspeaker. The main limit of this measurement is that it looks at the output signal instead of the physical features of the driver itself. On the contrary, the most up-to-date instruments for distortion analysis can measure the variations in loudspeaker parameters when they are fed with high-level signals. In this way, an excursion limit can be fixed, beyond which the parameter’s variation becomes excessive.
The “X var” value reported in our data (generally after the traditional “Xmax” value) is measured this way. Beyond this excursion limit, the magnetic field seen by the voice coil, or the total suspension compliance, or both, drops to less than 50% of their small signal value, producing high distortion levels, strong variations from small signal behavior and power compression. The new technique yields different results from the standard measurement based on THD. B&C Speakers believes that this added information gives a more accurate and reliable description on loudspeakers behavior in actual operating conditions.
B&C 15SW115 Xmax 13.5
(34-12)/2+ 12/4.8 = 13.5mm
Xvar distortion spec = 13mm
18SW115 Xmax 14
(34-14)/2 + 14/3.5 = 14mm
Xvar Distortion Spec = 16mm
So no real consistency there? Hopefully the Xvar figure is more helpful - a bit like an updated AES distortion Xmax limit
When I look at my old School (from last century)
Fane 15XB Xmax 7.5
(22.86-9.9)/2 +9.9/9.9 =7.48mm
So if this was specified in this century would the Xmax magically increase to keep up with the competition?
For PD 1850
(28-12)/2 + 12/4 = 11mm
If your not confused yet then maybe you have not paid enought attention?