Rational Acoustics



David
April 4th, 2014, 01:31 AM
I measured 6 identical 2 way passive speakers located in a large open room about 140' by 100' with 40' ceiling. All of the boxes are mounted along the long wall at the same height and downward angle. They are about 20' apart and the end boxes are 20' from the side walls.

Box 1 has a cable length of 50' and box 6 is about 150'. The cable is 4 conductor 14g, 1 pair to the low driver and 1 pair to the high frequency driver. Processing is done in the amplifiers (Lab Gruppen).

The measurements were done using the same output channels of 1 amplifier with the microphone about 25' from the speaker being measured.

Speakers 1-4 measurements were very similar. However speakers 5 and 6 had a noticeable change in the phase;especially in the higher frequencies. I moved the microphone and remeasured several times with the same result.

I went and listened to the speakers, not really expecting this to be a major issue...I was unpleasantly surprised. Walking the room intelligibility was good and even from speakers 1 to 4. In the area between 4 and 5 there was a noticeable decrease in quality. As I moved down toward the end of the room things improved.

I assume that the phase change is due to the length of the speaker wire. It is the only thing different about the 2 effected speakers.

So my question is - Can I do anything in the processing to help resolve this? I played around with some filters without much luck but I was running out of time so I had to leave my playing for another day. Running new wiring is not an easy option.

Basic processing information
Crossover 12" driver 70hz and 1.86khz.
High driver is 1.98khz
All filters are LR24.
1 Allpass at 1.75khz

Thanks for your time, advice and comments!

David


786

PhillipIvanPietruschka
April 4th, 2014, 03:09 AM
David,

It is unlikely the differences you are seeing are due to cable length. If they were I would expect the differences would appear first and most profoundly in the VHF, or perhaps the VLF, but not in the mid range.

Given that these are bi-amplified speakers, and the largest differences in the phase response is in the mid range, I would be checking the wiring to see if the polarity of the HF drivers is identical through out, and if that is the case, check the processor for polarity flips on various outputs.

I think this is a likely culprit, at least for the white trace. The orange one is a little harder to answer, though I have seen traces like that simply from being off axis to some speakers.

Im not sure that this will be the extent of your problems, but it is where I would start looking.

As a next step you could check and measure each driver in each speaker individually and look for any distinguishing differences.

Best,

Phillip

Chris Tsanjoures
April 4th, 2014, 09:46 AM
I measured 6 identical 2 way passive speakers located in a large open room about 140' by 100' with 40' ceiling. All of the boxes are mounted along the long wall at the same height and downward angle. They are about 20' apart and the end boxes are 20' from the side walls.

Box 1 has a cable length of 50' and box 6 is about 150'. The cable is 4 conductor 14g, 1 pair to the low driver and 1 pair to the high frequency driver. Processing is done in the amplifiers (Lab Gruppen).

The measurements were done using the same output channels of 1 amplifier with the microphone about 25' from the speaker being measured.



It seems to me that you have more variables in the measurement than wire length. If you are trying to get clear shot of what each speaker is behaving like - use a near field measurement. Instead of 25' away - try 6'. I would then mark the box position and measure each box from the same position, with the same cable, from the same amp channel/signal path.

Have Phun!

- Chris

Riley Casey
April 13th, 2014, 10:11 PM
Box six is wired out of polarity with the other boxes. Box five has either a different woofer than the others, a capacitor in series with the woofer, something wrong with the woofer or some other difference. Compare woofer five to the others, swap if it is the same and see if the problem follows the woofer or stays with the cabinet / cable.