In the previous post, we have measured how the bearing acts as a conduit for vibrations originating in playback. Turned out, those are quite significant but, understandably, are of lesser intensity than tonearm vibration.
Today, we'll look at vibrations that the bearing itself might generate. Because the spindle rotates in the busing at 33 1/3 RPM, any periodic noise should have a frequency of 0.5 Hz or its multiples. Thus we'll first look at the low frequency readings.
Low frequency vibrometry presents a bit of a problem. First, that's where most of 1/f noise is. Second, the signals are very weak. And third, the fact that the natural frequency of the bearing parts lies a lot higher doesn't help the detection.
Still, we've dialled the sensitivity all the way up, did a few measurements, and I think we have some valid data on the chart.
As predicted, there's a hump at 0.5Hz. But we're really at the performance limit of the accelerometer, and can't say much about it except that it's there. What about higher frequencies, maybe there's something in the tens of Hertz range?
It's a bit of a monster, right?
"Stopped" is a measurement taken with the platter stopped. "Running" is taken with platter running under motor power, "Music" is same as "Running" only an LP is being played. And "by hand" is a measurement taken with motor switched off, belt taken off, and platter rotated by hand to reach about 33 RPM.
And there's no difference between those measurements whatsoever.
How about even higher frequencies? Shall we go all the way to 20kHz?
Nothing but noise here. Whatever vibrations we are registering can't even excite the sensor's own resonance at 10 kHz.
Is the bearing really that quiet? A quick check with a stethoscope confirmed the opposite. There is some noise, generated by the bearing surfaces and the motor, but our accelerometers are not picking it up. We are simply using the wrong tool.
We are going to continue this series when we get meaningful results from a tool that we think fits the purpose better — a miniature measurement microphone. It's already on its way to us, and I can't wait to test it.
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