Free Field Measurements

Measuring large objects during operation
Setup for mining bridge
Result
Setup for mining bridge

During the last 20 years, the environmental noise protection is gaining special prominence to improve the quality of life. It is known how difficult and resource-taking it is to measure, analyze, and find the different sound sources located in big free field areas. This problem is enlarged by the difficulty to reach the area with the equipment. In order to solve this obstacle, the Array Star 48 will be presented.

 

The study case presented revolves around a stripping shove of a strip mining excavation machine during operation, in a coal strip mining. The aim of the measurement session was to reduce the noise pollution received in a village nearby. For this purpose, finding and eliminating the loudest noise sources was essential.

Using the software NoiseImage for identification and localization, the results show that the main noise emissions were located at the powerhouse and on the jip of the excavator. The subsequent analyses allow a deeper look into the emissions at the working level of the shovels.

System Characteristics

Array Star 48

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

Data Recorder

  • 192 kHz Sampling frequency
  • 48 to 168 channels per 10 inch rack (24 channels per 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

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 whole 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 the 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 900Hz. The sources of these emissions are the shovels themselves, squeaking shortly after turning at the redirection respectively.

Conclusion

The Acoustic Camera as a self-sufficient system provides the opportunity to carry out free field measurements of large objects from several hundred meters distance without taking the complexity of the object into account. Both broad-wide and band emissions can be easily located and analyzed in time and frequency.

With the measurement results, the client can take action in order to remove the sound sources that the Acoustic Camera has located, focusing the effort to decrease the noise at the powerhouse, the guide block at the top of the jib, and the connection of the shovel and the guides.

In this case study, the Acoustic Camera has demonstrated to be the best solution to the problem, giving a fast diagnose of the problem, and an accurate and efficient analysis of the noise.