Subwoofer High xMax Distortion?

I'm planning a pair of subwoofer 'stands' with 10" or 12" drivers to go under my stand mounts that have 8" woofers. Range will be 20hz - 80hz.

In terms of SPL the super high Xmax drivers such as Dayton Ultimax UM10-22 seem impressive with 19mm xmax.

My question is, do these type of drivers also provide low distortion or is it SPL at all costs?

There is also the Dayton RSS266 variants with a bit less Xmax 12mm and a similar price.

Or I could step back again in SPL with a pair or 3 standard 8" woofers having about 6mm xmax, not subs.

I guess I'm asking if higher xmax sacrifices linearity at more 'normal' operating levels.
 
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This is just more of a question about driver design compromises. I'm imagining that a huge air gap in the motor on super high xmax drivers and a larger surroud / spider would sacrifice linearity, but then perhaps not.

Erin Hardison interviewed Lars Risbo a couple of years ago, and in video Lars deconstructs the original Purifi 6.5” high x-max midwoofer (PTT6.5X). He goes through to explain the motor system, which he and Carsten went to great lengths to optimise. But when they measured it; to their surprise, it only made little difference.

It was not until they went back and looked at the surround. And optimised that, that the distortion really dropped.

Have a look at around 45 mins-
https://www.youtube.com/live/o90AYyceaMI?feature=share

The surround affects the variation in the emissive surface area of the driver. Purifi calls this Sd(x) - the surface area as a function of excursion.

So my simplified one-line explanation about why, historically at least, low x-max drivers can sound better than high x-max drivers, despite having equal Vd, is largely related to the changing emissive surface area as the cone moves backwards and forwards.
 
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So my simplified one-line explanation about why, historically at least, low x-max drivers can sound better than high x-max drivers, despite having equal Vd, is largely related to the changing emissive surface area as the cone moves backwards and forwards.
And this is especially true with small drivers, because the ratio of surround Sd to cone Sd is relatively high. in other words, a good deal of the total Sd is surround. With larger drivers, such as 12, 15 (18?) the surround contributes much less to the total Sd of the driver. Nonetheless, the Purifi guys identified an important source of driver distortion that affects all drivers, but it becomes really important in small, big-displacement drivers in the 5 - 7 inch size.
 
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Yes and I think this is what has been evident, experientially at least, for a couple of decades, for hi fi enthusiasts.

Like why 6.5” mid bass drivers are not satisfactory.

And turned our noses up when the maketplace started to accept 3” to 5” drivers as “subwoofers”.

I mean sure they can manage some output at <50Hz. But even when we put them in parallel (or 4/8/16 in an elaborate series parallel arrangement); it just didn’t compete with a traditional large (12”+) woofer.

And our horror when the vast majority of the public started embracing speakers with 3” twoofers in cabinets only half a pint in volume. In mono. Via Bluetooth!

It turns out that 60Hz to 16Khz is really much of what is required for the majority of music listeners. As long as it reaches the right SPL for intended use.
 
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In terms of SPL the super high Xmax drivers such as Dayton Ultimax UM10-22 seem impressive with 19mm xmax.

My question is, do these type of drivers also provide low distortion or is it SPL at all costs?
Do understand that Xmax is different between mfgs as it is calculated/estimated differently.
and xmax is often where the BL/Field strength drops to 70%, and does not mean linear excursion necessarily.
Faital Pro being a typical mfg. using "optimistic" xmax values. There are also several mfgs that llists xmax, way past the point where the driver will actually perform.
For example: JBLs 2269H DD drivers, the coils will start "braking" and sound absolutely atrocious, within it's listed maximum displacement limits.
Or a AE TD15M, listed 6mm xmax, where the surround is stretched tight long before xmax is reached and you get nonlinearities in the output as the result if driven that far, the old 3,3mm xmax numbers are a better "guide".
Just some examples.

So with so much else in this field, xmax is sadly kind of relative also, and is measured/estimated differently by different mfgs.

Here a picture/ measurement of one of the more impressive high xmax subwoofers around (older 15" version) , with 39mm xmax each way.
You can see where the linearity drops off.
This driver having 20mm excursion with almost zero loss in field strength over the distance and a unusually linear suspension.
Sadly the Inductance/Le over excursion/position is not included.

Klippel.png
 
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Was this the Stereo Integrity HST subwoofer, with a ridiculous amount of excursion?

If so, here’s the rest of the data sheet :

https://stereointegrity.com/wp-content/uploads/2021/05/HST-15-MKIII.pdf



One thing Lars said in there (around 40 mins IIRC) was that (to paraphrase) it’s very hard to look at these things in isolation and applause (or blame) it as being the root cause of distortion.

So the excursion determines the Vd but it is just one factor in the cause of distortion. Because the excursion is affected by the BL linearity, which is affected by the motor design, magnetic strength/direction and the voice coil winding method and number of layers which affects the moving mass with affects the sensitivity.

The cone profile affects the frequency response which is affected by the surround termination which may be affected by the... And so forth.

Doing this all by trial and error takes years. With the advent of software and simulations it now takes weeks instead.

The traditional ( voice coil height - gap length ) / 2 doesn’t fully represent the available excursion, and may overestimate, and sometimes underestimate it.

@Arez
Did you ever see Klippel reports for the TC Sounds drivers?
 
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Back to the original questions.
My question is, do these type of drivers also provide low distortion or is it SPL at all costs
It's very much driver dependent. You can't really say that all high excursion drivers are good or bad at distortion, sound quality, etc. There are examples of both.

Sometimes durability is more important than linearity/low distortion, so that is given priority (car high SPL drivers being an example).

a pair or 3 standard 8" woofers having about 6mm xmax, not subs.
8 inch woofers tend not to go very low unless they're specifically designed for subwoofer duty. 8" ported/passive radiator woofers can be satisfying for music use at moderate levels, but they are not normally going to be mistaken for a subwoofer unless you use a pile of them. DSP, high power, high excursion, etc. are typically used for smaller subwoofers.

Stepping up to a 10 inch woofer is typically a pretty big performance improvement over an 8. For highest output/lowest distortion you generally want to use the biggest woofer you can. How much output you need, how large the enclosure can be, etc. are also important factors, obviously.

I'm comparing the same size drivers here just one with higher xmax than the other and asking if that xmax capability compromises linearity at more normal output levels.
Generally, in higher quality drivers, I would say no. Even if a high excursion audiophile driver can't maintain low distortion at full excursion, it's likely to have more output that's acceptable vs. a driver designed for lower excursion (i.e., for the same distortion level, the high Xmax driver will often produce more output). One way to think about this is that linearity typically degrades with increasing percentage of Xmax used.

Also keep in mind that your ear isn't as sensitive to distortion at lower frequencies, so subwoofers are often given positive reviews at distortion levels people wouldn't accept in midrange/treble.

Are there distortion measurements for these sub drivers anywhere?
I haven't seen comprehensive test data by a third party. Since that's lacking, you can look at subjective user reviews to try to get a feel for their performance. Or look at high excursion drivers that have more data available.
 
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Right, but I'm comparing the same size drivers here just one with higher xmax than the other and asking if that xmax capability compromises linearity at more normal output levels.
My inference - apologies that I was less than clear - was 'the bigger the better' (all other things being equal). Comparing drivers of a similar quality, a 12" is going to use less excursion, less power and have improved linearity versus a 10" to produce the same spl, which is a Good Thing. Further, as Sd increases, acoustic impedance matching of the cone to the surrounding air improves, increasing efficiency.

Acoustic impedance of the air surrounding a speaker can be considered low low; the acoustic impedance of a speaker cone being high in comparison, and as we know, mismatching impedances does nothing for efficiency. This mismatch decreases with increasing cone area.
Importantly, radiation impedance is inversely proportional to the driver's radiating area.

This is why we put horns on compression drivers to achieve more output - the horn being an acoustic transformer. If we could make a tweeter with the same area as the horn mouth we wouldn't need one! This physics is equally true at low frequencies, where large cone area makes the impedance mismatch less and improves efficiency, the speaker having to move less air (lower Vd) for a given spl.

I would suggest trying the largest driver that your budget and space allows, considering one of the many truly excellent professional PA offerings at, frankly, bargain prices compared with the marked-up hifi market.
Once you have heard 'big driver' bass there is really no going back, but choose your enclosure wisely!

Move a lot of air - GENTLY!
 
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What level are you expecting at the 20Hz you mention, and in what room size? 20Hz is quite a tall order for the driver and enclosure sizes - are we talking hifi or home theatre? One of my very earliest subs was 190l using a 10" Audax driver which measured flat to 20Hz but would easily exceed Xmax with a 60W amp at fairly modest levels in my 3m square teenager's bedroom! You have to shift a lot of air to reproduce 20Hz at higher levels than normal domestic listening (say 80dBA).
Do you plan for one or two drivers, and is the budget £300 per driver or the total?

Also, I would take the 'rms' ratings for the Dayton driver and similar with a large pinch of salt since no testing standard is specified. Note also the tiny cone area and woeful efficiency typical of huge Xmax car audio-inspired drivers! Watching the surround contortions of drivers like this at high excursion in slow motion is most amusing, but also quite worrying in terms of distortion since it forms a significant portion of the radiating area...

In conclusion, we are putting the cart before the horse somewhat in being ignorant of your requirements, so please fill us in a little before we can make a 'best guess' of a specific driver.