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      07-31-2013, 03:33 PM   #117
NISFAN
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Drives: BMW M2
Join Date: Aug 2012
Location: Bedford UK

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Quote:
Originally Posted by bradleyland View Post
The "illusion" you refer to is called doppler shift. It can be corrected if you know the correct inputs, but we have to guess because we don't know the velocity of the car, nor the exact angle-of-travel relative to the recording source. If you're making a relative comparison, and the inputs are close enough, you needn't correct for it.

I used the 1:30 region for analysis.

Samples:
  • Pre-shift: 01:28.2, 00:00.3 duration
  • Post-shift: 01:29.2, 00:00.3 duration

Highest amplitude peaks (not corrected for doppler, Hanning window):
  • Frequency pre-shift: 289 Hz
  • Frequency post-shift: 232 Hz
Assuming negligible change in speed (cancelling doppler) between shifts, there is a 20% drop in engine revs, does that conclusion sound right?

I'm not going to guess on what the doppler effect is, as speed unknown. Assuming a gear change at say 7,000rpm, next gear is 5,600rpm. That is quite a long gear spacing if from 3-4th on a 7 speed box.

Little movie I took in the M6, very similar. Forward to 27 seconds.

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