Measuring resonances

[pixpop]

I'd like to find a method to separately measure horizontal and vertical resonance of a tonearm.

If I could just go out and buy the right equipment to do this, what would it be?

That probably won't be possible, so is there something I can make that will let me measure it?

My understanding is that the horizontal and vertical resonant frequencies need to be different, but I don't understand why.

[sreten]

Hi,

A test record with suitable tracks for determining such things.

There is no reason why the frequencies have to be different,
for most arms they are the same.

Generally speaking though why do you need to measure it ?
Usually just a question of sensible cartridge compliance choice.

/sreten.

[pixpop]

test record with suitable tracks for determining such things

I'm not sure I understand. How can a test record help with such low frequencies? I've looked at the instructions for my test records, and they don't mention anything about tracks for testing arm resonances.

why do you need to measure it ?

I'm designing & building arms.

[owen]

Its actually relatively simple using a test record - you just measure the cart output voltage - it will peak at resonance, and if you measure the output after the phono stage, it will remove the equalisation from the equation too.

Oddly though in the main, the resonances are usually measured using a mechanical 'exciter' (usually a solenoid coil), but by measuring the stage output, you could actually be getting a better insight into the arm/cart behaviour.

Just a thought


Owen

[Ghianni]

Track 15 on Test record QR2010.

[pixpop]

Owen,

Oddly though in the main, the resonances are usually measured using a mechanical 'exciter'

I think that's more what I'm talking about. I'm thinking of the natural resonant frequency of the arm as a system, that interacts with warps on records. I'm not talking about resonances in the audio band that would boost or cut certain frequency ranges. I understand how to measure that.

Do you have any more details on this exciter thingy? Would it be hard to make one?

[EC8010]

There are test records with subsonic sweeps recorded specifically for finding the arm/cartridge resonance. You just watch the vertical bahaviour of the arm, and when it tries to take off, you note the time from the beginning of the track and calculate the excitation frequency.

[Mark Kelly]

What do you actually want to measure? The resonant frequency doesn't tell you much about the arm unless you know the exact compliance of the cartridge.

It sounds to me like you actually want to measure the vertical and horizontal moments of inertia of the arm, which allow one to calculate the "equivalent mass" ( Icm / length^2, where length is effective arm length). The equivalent mass then gives the resonant frequency for a given cartridge compliance from the usual formula for simple harmonic motion.

The moment of inertia of the arm is reasonably easy to measure directly using a torsion pendulum. I made one out of a small stainless steel ruler and used a stainless rod as a calibration weight.

Torsion pendulum

[pixpop]

Mark,

this looks like what I'm after.

If I understand correctly, to take a measurement, you displace the load in such a way that the ruler twists along its long axis. It then oscillates, and you count the oscillations.

So, if I wanted to separate horizontal and vertical components, I would make the measurement twice, repositioning the clamp.

Is that correct?

Neil

[Mark Kelly]

Sounds good to me.

Caveat:I only used this to do a horizontal Icm, vertical seemed a little trickier (more things to avoid).