Alexey Korf post at Lencoheaven about tonearms
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mr_petit
High on Vinyl
Actually not Isaac Newton, but Robert Hooke.
The formula that he traced back to....1954, and he found a bit too old, is literally Hooke's Law, a physics law in classical mechanics that stood the test of time for about 350 years....
If Korf could proove Hooke's law wrong, he'd win the nobel prize next year.
Also what he says about stress strain on rubber is not entirely correct. Indeed the spring constant in rubber can change with excursion, so it's not entirely linear, but there are 2 aspects that he doesn't addresses:
1. with a resonant excursion, the movement always goes through the 0 axis, there the spring constant is it's nominal value. So imho it doesn't change the natural frequency (and that is what we're talking about), it merely changes the movement to a not perfect sinus.
2. there is no proof or investigation that in a resonance, the excursion is so high that there is any non linearity to speak of.
And these are very easy tests everyone can do for themselves. Different test records have different amplitudes for the resonance tracks. You can see that by eye, or record it and wave analysis with audacity or so. I have measured it with very low amplitude resonance tracks (for instance on the firato test record) or very high amplitude resonace tracks (for instance on the tacet test record) and both show the same resonance frequency for a certain cart in a certain arm, and it's also the same frequency that is shown when recording a piece of music.
So this completely debunks that part of Korf's story.
And as far as the dampening "calculation" in his calculator: it is based on an assumption
With no damping parameters to fill in, it can't actually calculate. It has to assume a number for either Q or damping.
The formula he claims to use, only states that Q is greater than 0,707. yeah that's pretty much a given for a passively damped system. But Q can just as well be 0,707 as it can be 1 or 1,5.
So 2 cartrides with the same compliance will always give the same graph.
But that's not what happens in real life; 2 cartridges with the same compliance can have massively different damping properties.
I've measured my DL-160 to my AT440mla; both have about the same compliance, and thus the same resonance frequency in a certain tonearm, but if I measure the output of that resonance, compensated for their output (I first measure a reference tone and relate the output of the resonance to that reference tone) there's over 6dB difference. So the denon DL-160 suspension rubber has over twice the damping as the AT-440mla has.
And yet Korf's calculator say's they're the same.....Because it can't calculate it.
Of course with the classical formula that follows Hooke's law you can't calculate excursion, only the natural frequency, but at least it doesn't pretend it can.
It's nice that Korf tries to make something that can, but the way he tries to is all wrong.
Also his conclusions about vertical vs lateral resonances are all wrong. He uses the HFNRR test record to come to his conclusions, but that vertical track is fubar. 20 and 19Hz are completely overamplified, and Len Gregory quickly turns down the volume; way to far, but at least 18Hz and lower are somewhat playable, but he has turned down I think the wrong gain as the waveforms are still very distorted. Anyone who has this record can check that for themselves.
But Korf thinks these are resonances when they're just the actual signal. So he hasn't really studied his source material in this matter, otherwise he would have seen that.
He should have used a better test record for these purposes (It's beyound me why he uses the HFNRR record in the first place, there is so much wrong with that record....).
So if you really want to know if your cartridge has good damping properties, just compare it with another cartridge. Record the same piece of music with audacity and look at the level of the resonance signal compared to the level of the music signal (say at 1khz or so).
That way you can see which one of the cartidges has better damping properties of it's suspension at around 10hz.
And sometimes you can also see the difference when playig a resonance track of a test record.
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adnick
Addicted Member
Agree with @JP, don’t find either position polarizing, also agree that Korf glosses over details and/or doesn’t get detailed enough, but that’s a minor complaint that can be applied to both camps.
Find each view edifying and have personally experienced similar results (confirmation) using my own tonearms & cartridges….
VR
Andy
mr_petit
High on Vinyl
Yes I know, I'm quoting him. Not Newton but Hooke, but that last name was not included in the discussion I believe
Nor that it was about 350 years old I believe.
The main point of my post is actually more of a reaction of what @ripblade says here:
Korf's calculator doesn't take the inherent damping properties of the elastomer in consideration. They make an assumption, that every cartridge's suspension has exactly the same damping properties.
And they couple that damping to the system resonance frequency. (you also get the same graphs when the resonance frequency is the same, so for example a 12mm/N compliant cart in a 25gr system and a 20mm/N compliant cart in a 15gr system)
And that is just not how it works.
The only thing the korf calculator shows is that there is a relation between the acceleration of a resonance and and the frequency, if the amplitude of an acceleration is the same. Yeah, that's a given. The higher the frequency, the faster the level changes.
But I don't think that's the true meaning of what he want's to show or what is important. In the end it's the amplitude of the resonance that is messing up the music.
I think that is part of the reason why they put one or two pilot tones with the resonance track. Obviously to hear the resonance, but also to display it's effect.
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adnick
Addicted Member
mr_petit
High on Vinyl
oops, you're absolutely right.
, missed that completely 
.But the next 4 paragraphs of my post do add to the discussion I believe
actually now I remember that @tnsilver and I talked about this very thing a while back in this topic:
https://audiokarma.org/forums/index...-of-resonant-frequency.1003384/#post-15785849
JP
7480 74111110101115
I must’ve missed where they delivered that message.
ripblade
Addicted Member
Assumption or no, at least he is considering it, unlike the other calculation.
Resonance and damping are interlinked. In a closed system, if you change damping the resonance changes. Every speaker designer knows this. I don't see the moving generator of a cartridge to be substantially different in that regard.
And anyway, if you plug numbers into Korf's calulator you'll find that mass and compliance are still relative in much the same way as the standard calculators. Low mass still needs high compliance and vice versa. Two ways to compute the same result.
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JP
7480 74111110101115
You're forming a conclusion that they didn't state. While their goal may be reasoned it would appear to be rather lofty. Achieving a fundamental resonance of 18Hz** would require a low compliance cartridge on a low mass arm - to an extent that pushes reason. System damping wouldn't be exclusive to the arm, and wouldn't necessarily be along the lines of what people typically think of. They then point out that rigidity of the arm and fixtures shouldn't be sacrificed, which happens to be the number one tradeoff in low mass arms.
Boy, while I'm trying to figure out how to make the right trade-offs to get as close to this impossible goal as I can, it'd sure be nice if there were some test kit that could help me out all these pieces together. In the late 70's audio was still engineering driven and fidelity to the recording was the goal. Regardless, choosing a different goal doesn't alleviate the need to measure, and it doesn't change what to measure.
**I note on page 3 that they cite two references that the optimal tonearm resonance is at or slightly above 10Hz. Do they really mean arm/cart resonance in this prescription for '18Hz system resonance' in the conclusion?
JP
7480 74111110101115
How much does the fundamental resonance elongate in an underdamped system?
Reasonably. They are tensioned in opposition and the bend radius is appreciable.
That's sorta true, a high compliance cart may not fit the bill but I can hit 13Hz on my DIY arm with a VMN40ML if I stick with low-weight hardware, I should have no problem doing 15Hz+ with a VM95XX (the paper suggests a range of 15-18Hz, not "18Hz").
Maybe you're right, perhaps B&K's motive was to sell gear to engineers that would never be able to achieve the targets suggested in the paper.
I just sorta doubt it.
JP
7480 74111110101115
Yes, I used the extreme number to make the point easier rather than elucidating on the entire range. Regardless, it's inescapable that higher fundamental resonance requires a give and take between low EM with low compliance. Are we debating that now?
The document is categorized as an application note, which is commonly defined as a detailed description of how products or systems are used in specific applications, typically in a tutorial style and for a specific audience. That's not to say in general that info it contains is incorrect, only that it's not authoritative, nor is its intended purpose to be such. It's not a technical paper that delves in to any of the topics it covers, it just accepts them as fact, cites a few things, and explains how BK products solves the problems they've presented. IOW, it's a marketing brochure for engineers, which is exactly how it reads.
Here's an example of a technical paper (that also addresses a long-standing issue [it was offered in 1979]): https://pearl-hifi.com/06_Lit_Archi...s/Lipschitz_Stanley/Lipshitz_on_RIAA_JAES.pdf
And the 15-18Hz target assumed as tonearm resonance still doesn't make sense to me.
The document is categorized as an application note, which is commonly defined as a detailed description of how products or systems are used in specific applications, typically in a tutorial style and for a specific audience. That's not to say in general that info it contains is incorrect, only that it's not authoritative, nor is its intended purpose to be such. It's not a technical paper that delves in to any of the topics it covers, it just accepts them as fact, cites a few things, and explains how BK products solves the problems they've presented. IOW, it's a marketing brochure for engineers, which is exactly how it reads.
Here's an example of a technical paper (that also addresses a long-standing issue [it was offered in 1979]): https://pearl-hifi.com/06_Lit_Archi...s/Lipschitz_Stanley/Lipshitz_on_RIAA_JAES.pdf
And the 15-18Hz target assumed as tonearm resonance still doesn't make sense to me.
Nope just pointing-out that you needlessly picked the extreme (18Hz), whereas 15Hz+ would be quite easily achieved with a low mass arm and medium compliance cartridge.
And yet "that's not to say in general that info it contains is incorrect."
I've actually been trying an aftermarket stylus on a Shure M75 and I'm getting 18.1Hz on the combination. I need to work on my damping but I do like the sound. Tracking of hot passages is impressive for a cheap bonded elliptical.
Not at all. This isn't an ad, meant to satisfy the egos of audiophiles. For the engineering crowd I'm sure this was pretty exiting, showing new possibilities using the B&K gear and methods, whatever direction oneself wanted to go. I wouldn't be surprised if this would have been a part of the more modern tone arm designs showing up a couple of years later.
When that paper was written in 1977 the only high-mass/low compliance crowd would had been a dozen or so of graybeards banging on about their neolithic Garrards and 3012's on their way to the terminal ward. By then, the scientifically based future was still low mass / high compliance. Isn't history of technology fun?
mr_petit
High on Vinyl
His calculator doesn't consider damping imho, it only shows the relationship between frequency and energy for a certain mass at a certain amplitude.
But that's the point. You can't change the damping on a cartridge, unless you swap out the suspension. With speakerdesign, you vary the size of the encosure. But if you want to stay in the speakerdesign analogy: it's like designing an enclosure without knowing the Qts (or it's components that make up the Qts), so just use a Qts of 0,707 and calculate the box to that.....
No designer does that.
No, not in the same way as standard calculators.
For example if you take a cartridge with a compliance of 8mm/N in a 22gr system, the resonance frequency will be 12Hz.
Korf calculates a resonance frequency at 8 or 9Hz:
I assure you, this is not correct.
Also if the excursion picture were accurate, at it's resonance it would display 11 or 12mm excursion where as on lower frequencies it would display 10mm excursion. Anyone that looks at resonance tracks played can see that this is not the case and that the difference is way higher and that the peak is way 'spikier'. You can also see that in a spectrogram (and these are usually in dB, so linear it's way more)
This is how such a combination looks like with a resonance track (I didn't use the 8mm/N compliance and 22gr mass as a coincidence):
So if any, Korf's calculator is also off on that aspect, with easily a factor 5 or 10.
Therefore I also expect that the Q factor of the suspension of a cartridge (any cartridge) is way way higher than the 0,707 that Korf uses. It's probably 2,3 or 4 times as high. That also results in a much higher and narrower resonance peak.
So to me the Korf calculator is pretty meaningless. I don't think it calculates the eigenfrequency of a resonance correctly (so the frequency where the amplitude is highest) and I don't think it shows the relation between peak amplitude at the eigenfrequency in relation to lower and higher frequencies.
To make such a calculator work, cartridge manufacturers would have to publish Q factor or damping factor data, like how speaker chassis manufacturers do.
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