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3. Rumble and motor presence
Rumble and presence refer to motor vibrations with frequencies higher than that of a hum.
Rumble: motor vibrations in the 250 Hz octave (175-352 Hz) ;
Presence: motor vibrations in the 500 Hz octave (353-707 Hz).
Figure 22 shows the vibration levels of an in-line 4-cylinder combustion engine in a frequency-engine speed diagram (Campbell diagram). It clearly shows that engine vibrations are not limited to the hum zone (harmonic 2). In the 250 Hz and 500 Hz octaves, vibration levels can also be very high.
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