The Korf Blog

The inside story: our research, development and opinions

3 July 2017
Vibrometry on a budget, part I
As I am preparing a tonearm prototype for measurements, I think it is only fair to speak about what exactly we will be measuring and how. If you're a dedicated DIYer, you might actually be interested in doing some of these measurements yourself!

Using accelerometers to fine-tune a turntable and tonearm was a bit of an industry secret, at least until Hi-Fi World published a teaser on one of the ways of doing it.

Since then, there was a run on Bruel & Kjaer Type 4517 accelerometers and suitable dynamic signal analyzers, the venerable HP3561A being probably the best for the task. While not expensive by laboratory equipment standards (you should be able to buy both in decent calibrated condition for less than 3000 euro), this combination is out of reach for all but most affluent hobbyist.

B&K Type 4517
Also, while B&K 4517 is undeniably excellent, no single type of accelerometer can give you the whole picture of what is going on with your tonearm. Armtube vibrational modes, yes. Bearing resonance, maybe. Low-frequency (10Hz or so) resonance — definitely no.

Broadly, there are 2 types of miniature accelerometer that can reasonably be applied to audio measurements: charge and ICP. ICP stands for Integrated Circuit Piezoelectric, and is practically same as IEPE industry standard. ICP/IEPE works a lot like phantom powered microphones, while charge accelerometers have no electronics inside. Here's a quick comparison:

Hewlett-Packard
HP 3561A Dynamic Signal Analyzer
Ideally, we need both. A moderately sensitive charge accelerometer for low frequency and modal resonances, and a very sensitive ICP one for spot measurements.

Thanks to eBay, these accelerometers and a specialized amplifier (called "signal conditioner") can be had for very little money. They would probably be somewhat out of calibration, but 1) it's not prohibitively hard to do yourself and 2) what we are mostly interested in are differences rather than absolute measurements — and an accelerometer that is slightly "off" would still highlight these differences perfectly.

I am using an Endevco Model 133 signal conditioner bought second-hand for about $250, an Endevco Picomin 22 charge accelerometer (about $100 used) and a high sensitivity Endevco 7250 ICP accelerometer. Equivalent models from PCB Piezotronics or Dytran are just as good. If you get the accelerometers on eBay make sure the cables are included. The connectors are decidedly non-standard and very expensive.
Endevco Model 133 is a Swiss Army knife of signal conditioners, working equally well with charge and ICP accelerometers. It has widely adjustable gain, which is very useful if you intend to use a PC-based data acquisition system. It is also multi-voltage, so you can buy one in the US and use it worldwide. Its manual is available for free at Endevco website once you register there. If you're really financially constrained, you can feed its output straight into a PC sound card and get somewhat acceptable if uncalibrated results (as long as you don't measure low frequency content).

While some people use super glue to attach accelerometers, I recommend using special waxes to do it. They do no damage to device under test, and give more time to properly position the accelerometer before setting. A sample pack of these special waxes can be had from https://strongholdwax.com/product-category/stronghold/. It's free and would last you a very long time.
For the basic measurement of main arm tube modes, the accelerometer is attached to the headshell. To get an idea of how strong the tonearm vibrations are, it's fun to record the output of the accelerometer and then play it back over the speakers. It gives better sound fidelity than some cartridges I tested! Here's a recording of a frequency sweep made from Endevco Picomin 22 accelerometer.
Next week, vibration analyses of 2 real-world arms, with an explanation of what the observed results mean for the sound!
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