Critical Listening and Troubleshooting

This session consisted of critical listening and troubleshooting during the set up and operation of a small line array configuration. Phase and dispersion characteristics were identified, measured, and analysed, and potential eradication methods were discussed.

The d&b audiotechnik T10 speakers were configured in stereo line array, 3 boxes a side, with 4 paired B4 subwoofers. This session focused specifically on critical listening to detect physical or auditory issues and then prove the occurrence of these subjectively perceived behaviours with objective scientific graphical evidence on Room EQ Wizard using a Behringer ECM8000 microphone.

The first issue encountered was during a song test. I detected a significant lack of HF on the right hand side of the front of house PA. The speakers were checked and it was found that one of the boxes was still set to point source not line source, as the T10’s can conveniently be configured in point or line source. This would majorly affect the dispersion based on the rotation of the waveguide as that affects the propagation. The array was partly disassembled and the wave guide rotated to line source. The song was played again and the HF in the stereo environment was more even, but potentially uneven on the left side, so the left array was checked to make sure the same waveguide issue hadn’t occurred on the left hand side, it hadn’t. The next identified issue to remedy was the splay angles on the line array rig. The first set of angles were all set to 0, which resulted in the middle of the three boxes being overworked in terms of dispersion in the room when walking back from the speakers in a straight line. The rigging was adjusted to splay angles 9 on the middle/top and 14 on the bottom/middle box. This created a more even dispersion and a more curved array compared to the previous flat facing set up, as seen in the below images.

More listening took place and the measurement mic was set up. Using a frequency sweep from 20Hz-20kHz, I audibly identified some phase issues while the sweep played through the pa. The graph further indicated comb filtering, however, to be certain, the speakers were angled in slightly, purposefully increasing the chances of phase cancellation and moving the crossover point forward in the room. Another graph reading was taken confirming that phase cancelling, or comb filtering, was an issue. Phase cancellation can be reduced by making sure the dispersion and waveguide are set correctly, and angling the speakers away from each other where possible. It's also important to be mindful of subwoofer cancellation if the boxes aren’t spaced correctly.

Various configurations shall be measured and analysed for optimised sound behaviour in the sound hall in the following sessions.

The following graphs depict the readings of intentional increased comb filtering taken. The red graph is the initial natural reading where comb filtering was identified. The green graph depicts increased comb filtering once the speakers were angled in slightly, and the blue graph shows a further increase after the speakers were angled in further. The right column of graphs shows the first increase of comb filtering laid on top of the initial natural reading. Then the further increased comb filtering laid on top of the initial natural reading, and finally the extreme comb filtering increase layered upon the mild increase in comb filtering.