Objective

The objective of this investigation was to visualize noise sources on a coffee grinder during operation using 3D nearfield acoustic holography

Test Procedure

1. A flexible 56-microphone array was used to conduct pressure measurement at about 1cm from the grinder surface
2. One of the array microphones was used for phase reference
3. The coordinates of each microphone were measured using a 3D digitizer
4. Acoustic pressure data was acquired for about 30 seconds
5. Acoustic quantities were reconstructed on the surface of the coffee grinder

Results

The following screenshot shows the 3D distribution of Sound Pressure Level on the surface of the coffee grinder for the peak frequency 397 Hz.

In the left screenshot below, Intensity distribution for 397 Hz clearly indicates acoustic energy flow from two opposite sides of the grinder.

The right screenshot for normal surface velocity pattern indicates a rigid body type motion at the mid section of the grinder, since velocity is in the normal direction on the right and opposite to normal on the left.

Conclusion

The metal cup that supports the motor transfers the vibration due to any imbalance to the plastic casing which produces noise.