One of the challenges in the field of free field measurements is the transport of the equipment to the location of the measurement. Furthermore, the system needs to be able to conduct a long distance measurement without neglecting the complexity of the object.

The Acoustic Camera as a self-sufficient system can localize and analyse the time and frequency of broad band and narrow band emissions.

In this case, the measurement of noise emissions of a bucket excavator in a coal mine has been performed to reduce noise pollution in a village nearby.

System Characteristics

Microphone array Star48 AC Pro

  • 48 Microphones
  • 3.4 m diameter
  • Aluminum array structure 
  • Dynamic of the microphones: 35 dB -130 dB
  • Recommended mapping frequencies: 100 Hz - 13 kHz
  • Typical measurement distance: 7 m - 500 m

Data recorder mcdRec

  • 192 kHz Sampling frequency
  • 48 to 168 channels per 10 inch rack (24 channels per measurement card)
  • Ethernet interface > high
  • Transfer rate > 80 MByte/s, network-compatible
  • Digital card with 12 extra channels for recordings of RPM, rotation angle, reversal point, etc.
  • Integrated PC with Windows XP Embedded

Software

NoiseImage 4: Spectral Analysis , Acoustic Movie

Power supply

The mobile power supply Slim MPS enables independent usage of the Acoustic Camera for up to 4 hours.

Measurement

Application Area

Acoustic analysis of large objects in free field during operation.

Measurement Object

Stripping shovel of a coal strip mining.

Measurement Set-up

In order to cover the whole strip mining excavator during the first measurement, the Acoustic Camera was set up at 150 m distance from the noise source. To get a close up of the derrick jib, a second measurement was carried out where the Acoustic Camera was set up at 100 m distance of it.

For a detailed analysis of the upper-side of the jib, a third measurement was done at the bank. The entire measurement session and the main analysis were performed within 3 hours directly on the mining ground.

A high-capacity, portable battery was used as power supply offering a continuous operation of more than 4 hours.
All data was treated with an A–weighting filter in order to minimize the background noise created by wind and other machines nearby the strip mining.

Results

In the first measurement, the powerhouse was identified as the loudest source. The second loudest source was detected during the second measurement at the derrick jib. The measurement shows the main emissions on the redirection at the top of the jib and some reflections on the ground which also adds to the overall noise level in the strip mining.

The third measurement, at the working level of the bank, pointed out that the broad band rattles were emitted by hits caused by the guides of the shovels while running onto the guide block right below the redirection. This phenomenon can be seen in the spectrogram. The detailed analysis of the broad band rattles are calculated from the marked areas of the spectrogram.

The time- and frequency-selective filtering also allows the analysis of the squeaks emitted around 900 Hz. The sources of these emissions are the shovels themselves, squeaking shortly after turning at the redirection, respectively.