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Old 06-20-2006, 11:32 PM   #1
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Default Here'* a tough one......

My Eaton Supercharger turns at 18,800 rpm'* at redline. The whine is remarkable. We've often been under the impression that the whine is the sound of the gears (straight-cut gears like an Indy-Car gearbox, remarkable similarity to that sound.

But this frequency lies outside the perception of humans.
http://en.wikipedia.org/wiki/Audio_frequency

To make it more difficult, each supercharger rotor has 3 lobes, so the frequency of that (if they actually make sound) is tripled.

What are we hearing?
Why does my dog hate riding in the 93 when I romp the throttle?
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Old 06-21-2006, 12:19 AM   #2
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My guess would be heterodyning. We aren't hearing the actual frequency of the supercharger, but rather a resultant frequency created by the mixing of the frequencies created by the rotors and maybe the lobes? Your dog can hear the higher frequencies also present.

That'* my theory.

http://en.wikipedia.org/wiki/Heterodyne
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Old 06-21-2006, 08:09 AM   #3
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So you are saying that the sound is the gears on the front of the charger. Why mention the 3 lobes ...they are not the gears....they are on the rotors.

I thought the reason the M62 sounded different from the M90 was the slight differences and the sound was caused by the air being compressed and the way the air is dragged into the SC over the inlet...possibly like a whistle when you look at the shaping of the SC inlet and how the air is pulled in.....most wouldn't have the visual of this unless they pulled a charger apart.
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Old 06-21-2006, 09:27 AM   #4
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The sound is the gears primarily. But the lobes will also produce audio frequencies.

The difference between the M62 and M90 is the speed they turn at. The M62 is always turning faster.

An M62 with a 2.2" pulley is spinning at more than 18,800rpms at 6000 engine rpms.
An M90 with a 3.4" pulley is only spinning at 12,100 rpms at 6000.

The drive ratio difference is why the M62 also gets into boost quicker.
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