Example 3: “Measurements on a real system.”
When measuring a real PA for live or installed use, only one of the arrays should be measured. Additionally, the microphone should be placed approximately halfway between the source and the maximum distance to be covered, assuming reasonable coherence is obtained at that location.
When we choose this center point for phase alignment we make sure that significant phase change will not occur at any other listening position within the audience area (unless we are really close to the array), be it closer or further away from the speakers.
Make sure the floor reflection does not create poor coherence at the crossover frequencies. This is a common occurrence when using a microphone stand.
In this example we are aligning phase between a DAS Aero 50 line array and some DAS LX218A subwoofers.
The level difference between the two will make the shared frequency range narrower or wider. In our example the two systems overlap in the 45 Hz to 125 Hz range.
The DAS LX218A subwoofer is a self-powered system incorporating signal processing (crossover and equalization), but we will use an external processor in order to add delay for alignment to the mid-highs.
A common mistake is to filter a self-powered system at the same crossover frequencies used in the equivalent passive system, this way the slope corresponding to the external processor adds to the slope provided by the integrated crossover.
So we end up, for instance, with a sort of 48 dB/oct filter instead of one that is 24 dB/oct. In our example, no filtering is used at the external processor. Only the filtering that is built into the subwoofer is used.
The DAS Aero 50 mid-high is a 3-way line array system. Each of its three bands has a factory specified delay time applied on an external processor.
The procedure is the same as for the previous examples:
1) Enter 20 ms as the delay time for each of the outputs in the processor (a different delay time can be used).
2) Let’s first work just with the mid-high. We’ll use the “Delay Finder” utility to add the required delay to the channel with the reference signal, i.e., to synchronize the reference signal to the measured signal.
3) Measure the frequency response for the complete system before doing phase adjustments. At worst, we will see significant cancellation in the frequency range being shared by both enclosures. The top graph in Figure 20 shows the magnitude frequency response that we are trying to improve on. It is shown below that the low coherence above 100 Hz is due to the different arrival times for the mid -high and the subwoofer, with similar levels. The same effect would be seen with a single source and a reflection.
4) Mute the subwoofer output, and unmute the mid-high output in the processor.
5) Measure the mid-highs and save the curve. In our example, the curve in Figure 21 is obtained, showing measurements for the mid-high box. In this case the system, normally complemented with subwoofers, uses a very low crossover frequency.
6) Mute the mid-highs, and unmute the subwoofer output.
7) Do not use “Delay Finder” again! (i.e. do not synchronize the reference signal to the measured signal again).
Remember that we are comparing phase on both outputs, i.e., we are measuring the difference in time arrival between the two signals as a function of frequency. Therefore the synchronization delay for the reference signal should not be changed on the measurement software again.
Keep in mind that we chose the mid-high box as our timing reference because it is the signal from which the best impulse response can be obtained.
8) Measure the subwoofer, and compare the phase curve with that of the mid-high box. The result can be seen in Figure 22, where phase can be seen to be significantly different between the subwoofers and mid-highs. The Aero 50 and LX218A sub overlap acoustically in the 45 Hz to 125 Hz region for the level ratio we selected.
9) Add or subtract delay from the subwoofer output until the two phase curves overlap around the crossover frequency, as seen on Figure 23. When lowering the delay time on the subwoofer output, the green trace shifts upwards (and reappears on the lower part of the scale) on the SATLive phase graph, decreasing its slope in the pass band. Do not forget to save the curves.
In this example, the phase trace for the subwoofer shows a steeper slope at the pass-band than the mid-high system. It is evident that some delay will need to be removed from the subwoofer until the phase curves overlap as much as possible in the band that they share.
When reducing the delay time for the green curve on Figure 22 it will shift upwards, eventually disappearing and appearing again on the lower part of the graph.
We need to bear in mind that the phase graph only shows values between +180 degrees and -180 degrees. If a larger range were to be used, the traces would not zigzag as they do here.
10) Measure the system frequency response and compare it with the initial measurement.
If phase has been correctly adjusted, subwoofers and mid-highs will sum in phase, and this will be reflected on the m/divagnitude frequency response.
Figure 24 compares the system combination before (blue trace) and after (red trace) adjustment. Cancellation and poor coherence that were previously seen around 125 Hz have disappeared.
As noted previously, low coherence was due to the fact that the same frequency band was arriving at different times, which is equivalent to a reflection of similar SPL.
Once the correct amount of delay is added, the shared band from the two systems arrives at the same time and coherence goes back to normal.
11) Take the lowest delay value and subtract it from the subwoofer and mid-high so that at least one of the outputs has a delay time of 0 ms.
The DAS Aero 50 mid-high is a 3-way line array system with external amplification. Resulting delay times are as follows:
Subwoofer: 14.458 ms
DAS Aero 50
Low: 20 ms
Mid: 25.9167 ms
High: 26.0104 ms
What we are really after is the time difference for the outputs to be phase aligned. Therefore, the lowest delay of all bands (14.458 ms in this case) needs to be subtracted from each of the bands.
Final delay times will thus be:
Subwoofer: 0 ms
DAS Aero 50
Low: 5.542 ms
Mid: 11.4587 ms
High: 11.5524 ms
Once the final delay times have been entered, it is good practice to run the measurement again to check that everything is correct. Before adjusting a real system for the first time, it makes sense to practice this procedure as often as possible and with whatever combination of gear we happen to lay our hands on, until we have mastered the technique.
Using scaled-down systems will allow us to become familiar with the procedures until we are confident to try larger systems.
Joan La Roda is a member of the DAS Audio Engineering Department.