xmax discussion

[dadydolas 10+ year member]

F is the frequency you are looking for the SPL at. So..
SPL = 20 * Log (3L * 30hz^2) + 48.4 = 117dB at 30hz if your driver displaces 3L of air.

SPL = 20 * Log (3L * 40hz^2) + 48.4 = 122dB at 40hz with the same displacement.

John

lamens terms

 

[NDMstang65 10+ year member]

More movement = more potential linear/clean output.

Theoretically, of course.

 

[vehementSPL 10+ year member]

Why are you still bumping this, you've gotten your answer already.

agreed

 

[Electrodynamic 10+ year member]

how 'bout that sticky mods? //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif

Probably won't happen though.

 

[newusername 10+ year member]

We found that you could take a driver with the same physical overhang, say 14mm overhang, but vary the top plate thickness and get different results. For example a 10mm gap with a 38mm coil has 14mm overhang but acts totally differently from an 18mm gap plate with a 46mm coil. Both have 14mm overhang, but the BL curves are totally different and you have much more usable excursion on the driver with the thicker top plate. Likewise, taking a driver with the same 38mm coil and using the two different 18mm and 10mm mm gaps gives nearly the same results. Going by traditional standards of just coil length minus gap and divide by two, you'd think that one would have 14mm and the other 10mm, but in reality they are very similar with the edge actually going to the shorter overhang driver in terms of usable excursion. This is about the time people started throwing out the generally accepted ideas on Xmax and started investing more time and money in FEA analysis and real world testing.

Yes, and it should be clear that this is because the flux that jumps the gap (from the top plate to the pole piece) doesn't just exist in the gap...there is a surrounding area above and below the gap called the "fringe field" where a considerable amount of flux can be found (this varies with motor geometry). The use of a larger top plate can result in higher flux density in the fringe field, which means BL linearity at the outer ends of the coil's movement is improved.

The improved BL linearity only really applies, however, if the gap is already fairly saturated. If it isn't, the taller top plate will result in an increase in flux in both the fringe field and in the gap...the net result is a minimal improvement in BL linearity outside the gap but a large improvement in BL and efficiency.

And a taller top plate comes with a few downsides...most notably cost (steel is expensive and becomes moreso everyday) and the potential for flux modulation. But like everything, it is a balancing act where you choose what your target is and build to that....the final result is a strong function of the assembled parts and a weak function of the sum of the individual parts.

So that all said, what does Xmax really mean? It's a good way to tell how much range of the driver is really useful while keeping distortion inaudible. In speakers, the low end limit for being able to notice distortion is considered to be about 10%. In reality most people can't hear distortion and notice the audibility up until way over 25% even. That seems high, but while at Michigan Tech, the regional AES vice president, Tom Nousaine, came and did a discussion on distortion. Nut just subwoofer distortion, but distortion full range. He played a track of Tom's Diner by Suzanne Vega. This was a blind listening test where the track was played over and over again with progressive amounts of distortion added. The trick was to say when you could hear audible distortion, but we didn't know the amounts until later. In a room of about 20 people, all audio geek AES members, we all sat and listened and would say "ok, the 4th time i could hear it". It turned out that only a couple in the room claimed to hear distortion at the point where 10% was added. Most claimed at 25-50% distortion.

This means that if distortion isn't audible until 10% we can use the driver up until 10% distortion and it will sound clean. Therefore taking the point at which Bl drops to 70% of it's rest value is usually considered the point where the motor will cause the driver to reach 10% distortion and the reason that point is chosen for Xmax ratings.

The challenge here is differentiating audibility...for example, if I excite a driver with a sine wave of a fundamental frequency of, say, 40 Hz, I can clearly tell when one driver has more distortion than another. It is plainly audible to my ear at medium to high amplitudes. But although Fourier described the nature of complex tones as a series of sine waves, hearing distortion in music can be exceedingly challenging...especially in the rock genre where distortion is an intended by-product of the original performance...it's that sonic signature a guitarist looks for.

But I do believe there are great strides made on this issue...Drs. Earl Geddes, Lidia Lee, and Floyd Toole (to name a few) have done great work in this field. Even Dr. Klippel (whose system is actually more production focused than distortion focused) has put in time attempting to decode the ears perception of complex tones.

However, the motor isn't the only thing that causes distortion. Parts physically making noise during their motion, resonances in parts, power compression, etc can all cause distortion to rise over 10% well before the motor is the limit. Klippel had all kinds of other standards to simply measure 10% distortion points. In many cases this would correlate quite well to the 70% Bl mark, but some drivers would hit 10% distortion above and below that point. IMO this is the best way to truly rate a driver by looking at all the factors that contribute to distortion and seeing which brings it to an audible point first, then rating Xmax based on that point. Not everyone has money for a Klippel though //content.invisioncic.com/y282845/emoticons/frown.gif.a3531fa0534503350665a1e957861287.gif

I think the Klippel machine is a great way of identifying the source(s) of distortion and provides a great opportunity for extensive R&D, but as our understanding improves, FEA is growing in popularity particularly considering the price of Klippel's Distortion Analyzer, which poses some problems in it's nature as a statistical fit analyzer and it's sub-par inductance measurement. But I suppose you know that already. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

so basically
worry less about xmax in vented apps

and port it to specs

and buy good woofer that can control the cone

Hmmm....what controls the cone? Oh right! The motor (BL) and the suspension (Cms). Again, this is why linearity is important and why Xmax is still relevant in a ported enclosure. The only difference is that you need less Xmax for a given level of output in a ported enclosure.

And it is important to understand the typical tradeoff between efficiency and Xmax. You can give up some Xmax for efficiency and gain output so long as you don't need the Xmax you gave up.

Someone correct me please if I'm way off. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[jmanpc 5,000+ posts]

Thanks for the read, guys. This has been very educational. //content.invisioncic.com/y282845/emoticons/graduate.gif.d982460be9f153bb54e5d4cb744f6ae8.gif

 

[Electrodynamic 10+ year member]

Someone correct me please if I'm way off. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

Nah, you're pretty much spot-on. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[audioholic 5,000+ posts]

Good post Neil, as always bud.

xmax, while definitely much less important than in sealed apps, gets underappreciated in vented systems imo.