If Xmax shouldn't be exceeded, why have a Xlim?

Here are a couple of results I obtained this morning using a two tone sine wave signal source and then looking at the current frequency spectrum feeding a 'speaker which was driven a little over it's Xmax:
1) Twin input Frequency spectrum for reference purposes, 75Hz at +15dB and 330Hz at -5dB, cursor on the 330Hz fundamental signal:

2) (above) showing the spectrum of the Amplifier Output Current into the test 'speaker with 25volts RMS output which is well below any Amplifier clipping but exceeds the rated power of the test 'speaker and thus it's Xmax..Note the 'massive' amount of spurious current components which are both harmonic distortion products of the two input sine waves as well as numerous Intermodulation products; since the current drawn from the Amplifier by the 'speaker is not linear when Xmax is exceeded due to the change in impedance when the coil leaves the magnet gap....... sounded 'nasty'!

3) A 'reference' spectrum with 10volts RMS output from the Amplifier is below, cursor over the first Harmonic of the 75Hz sine wave (Edit: due to Amplifier crossover distortion) but noticeably reduced other distortion products. (please be aware of the scaling of this result)


Anyway, interesting results as to what happens when Xmax is exceeded in terms of high levels of distortion appearing in the current being delivered to or reflected back by the 'speaker's voice coil. It should be remembered that the Current is the actual 'driving force' for the coil and that this is directly proportional to the distortion of the sound output from the 'speaker.

Now to try and answer the point regarding having Xlim as well as Xmax; this could be obvious from the above as the tested 'speaker shows substantial increase in distortion above Xmax which might be taken as 'acceptable' up to a limit of Xlim.
IE: the acceptable (??) distortion limit is Xlim whereas the onset of distortion should be considered best as Xmax.
A useful link in fairly simple terms at:Xmax and Xlim

So there you are - with spectrum analysis pics to demonstrate what happens
Mik

Edited by audiomik - 13 January 2011 at 12:46am

thanks for all the good replies:)  and cheers MIK for doing the analysis.  I'll properly look at it when im out of work later on.  

someone in another forum wrote that its written in aes 2 standard 1984, that xmax is if the speaker still has so much cone control, that the distortion is not more than 10%...
dont know if its reliable, but wanted to let you know.


Edited by airbell - 11 January 2011 at 3:06pm

Also, if you trust hornresp for displacement (which I think should be fairly accurate for BR).

A Beyma 18SW1600 has an xmax of 5.5mm (regular measurement (Lvc - Hag)/2).

In a 185 litre 31Hz box hornresp has the beyma hitting its xmax @ 170W 45Hz

Even at Beymas 10mm quoted figure (Lvc - Hag)/2 + Hag/3.5), it will reach its xmax @ 600W 45Hz.

Re: "Now I know drivers are seldom loaded in infinite baffle or cb, but it is still quite easily possible to drive a reflex out of its xmax while staying well within the power rating of a driver."

thought I'd shown this rather conclusively in the current spectra displays above.

Suggest you look at getting some current sensing circuitry which differences the Amplifier input signal to drive a limiter using it's external side-chain input.
Problem solved....
Mik

Once you exceed the real linear Xmax where the coil starts to leave the gap there's less force so the cone moves less additional distance for each amp of current -- of course this is why the distortion increases.

So if you take an example of a 24mm coil in a 12mm gap, the real linear Xmax is 6mm, the quoted Xmax from manufacturers could be 9mm (added Hg/4) or 10mm (added Hg/3), or something else if they use measured distortion.

Hornresp might say you need 60V (450W) to reach 6mm, 90V (1000W) to reach 9mm, 100V (1250W) to reach 10mm, 120V (1800W) to reach 12mm, but only the first one of these is likely to be true. The further you go past the linear Xmax travel, the bigger the difference between the actual travel and that predicted by Hornresp.

It's still not a good idea to drive beyond the rated Xmax if you want decent sound...

Ian

S DeXter wrote: daywalk3r wrote: So, lets dissect those things one after another.. Found those numbers a bit odd so simmed it myself (185L/31Hz reflex) and I'm getting 5.5mm@260W and 10mm@860W for that particular driver, which is somewhat higher than what you got, but nothing excessively signifficant.. Nothing excessively surprising either though, as explained bellow.. What I found more odd is - why did you sim that driver in a box with a volume almost equaling its VAS, tuned it below its free air resonance (Fs), and then wonder what the heck is up with the power/excursion figures?  Could work if you added some mass to the cone, but otherwise those reflex enclosure parameters just seem not very ideal for that particular driver. You either need to reduce the box volume or tune it higher to make it work "as intended".. But back to topic: I believe the specified (some also call it "bloated") X-max figures, which most of the respected brands publish for their drivers nowadays are not necessarily just about marketing. Some of the brands (Beyma including) are not just getting those numbers out of thin air or by inventing random X-max formula (+Hag/x), but from measurements following certain AES standards, which set an upper limit on the allowed distortion of the reproduced signal (5 or 10%), and also a limit on the BL figure (=>70%). What I wanted to point out with this is, that the X-max figures usually seen on driver spec-sheets nowadays are allready taking into account that "extra headroom before bad things happen", so it's usually a good idea to stick to them when designing enclosures.. Howgh OK, i forgot that everything in speakerplans is disected with a tooth comb.... Yes that box is a bit low tuned, you are correct. However the power figures are correct for the sim.... Anyway the first point to mention from your post is that at Beymas quoted xmax, the driver is already distorting, you said it yourself - dont forget this is the point I am making and that is why I chose not to use the stated figure.  I also read somewhere, but do not quote me, (i will try to find it) that 70% BL requires double  power for same SPL.... The example I simmed was a worst case scenario but not totally unrealistic. So I have simmed the B & C Sub 18 with a 18NW100. Now there can be alot of dispute as to where we should draw the power figures for this. Again B & C appear to quote the xmax to the 'bloated' figures. (Lvc-Hag)/2 = 6.5mm in this case - this is what I would term as linear travel..... Hornresp comes up with these figures   At 700W has 10.6mm @ 48Hz At 1200W has 13.9mm @ 48Hz   Even at these powers, you can see you are definitely out of the linear travel. This driver will take an amplifier substansially bigger thermally.... at 2000W (on peaks) you will be nearer 19mm.