There are inductive sensors which can be retrofitted to turbochargers to measure their speed, like this: turbocharger speed sensor

Examples can be seen here.

Installation requires some careful and precise drilling and threading in addition to removing and re-installing the turbo itself, which is something I don't want to do for my personal experimenting.

Now I am wondering if the speed of the turbine/compressor can be determined acoustically. I mean, you can easily hear the distinctive sound of the turbo when it revs up/down.

The question to me is, what kind of sensor could be used to pick up the turbo's sound?

I'm not sure what frequency you hear from the turbo, but it must be either its rotational frequency (up to about 200000rpm/60s~3.3kHz), or the same multiplied by the number of blades (~13 -> 40kHz).

Installation of the pickup inside the engine bay will require, among other things, mechanical ruggedness and temperature resistance to at least 100°C.

Can a cheap piezo ceramic plate do the trick?

Are there special microphones for this kind of environment?

Knock sensors come to mind, but I'd prefer something contactless, especially not needing to be bolted to the turbo, see above :)

  • \$\begingroup\$ Would it be easier to measure the oil pressure and estimate the speed from there? \$\endgroup\$ Aug 10 '17 at 15:39
  • \$\begingroup\$ Just a couple of random thoughts. A microphone might work. With signal processing. But if you can pick-up Doppler shift from the vanes, that might be even better, because it would be unambiguous. Using sound alone would have the drawback of being "fooled" by harmonics. Maybe both, even. \$\endgroup\$
    – mkeith
    Aug 10 '17 at 15:40
  • \$\begingroup\$ Inside most turbos, temperatures will be many hundreds of degrees centrigrade and that is why different types of probes (not inductive) are used. \$\endgroup\$
    – Andy aka
    Aug 10 '17 at 15:42
  • \$\begingroup\$ @Andyaka Measurement is of course done on the compressor side, not on the turbine side :) Examples of sensors: jaquet.com/site/assets/files/1011/… \$\endgroup\$
    – JimmyB
    Aug 10 '17 at 16:07
  • \$\begingroup\$ @JimmyB OK I understand! \$\endgroup\$
    – Andy aka
    Aug 10 '17 at 16:08

For 40kHz you're in ultrasonic territory. You'll be hard-pressed to find an ultrasonic mic that's also rated for high-temperature.

This one is rated for a maximum of 100C, and shows a response curve to 80kHz: SPH0641LU4H-1


This one only covers up to 40kHz and 85C:


You might be able to create a temperature-shielded or cooled housing that's acoustically conductive, but YMMV.


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