AFMG Network Forum

Glenn Leembruggen · Member

Hello Folks
We have an encountered a strange discontinuity when mapping the direct field SPL of a line-arrays which have been set up with xglcs, but power summation selected for mapping. One of the arrays is delayed.

The disontinuity has the form of contour lines being cut through by a "dog bite".

In the area where the discontinuity occurs, the arrival-time graph in Probe shows the direct fields arriving only 60 ms apart, but the level of speaker 1 (undelayed) is 0 dB at the discontinuity. Either side of the discontinuity, the level of speaker 1 is approx 60 dB. Speaker 2 (delayed is approx 70dB) in the discontinuity.

Any thoughts?
thanks!
Glenn

AFMG Pedro Lima · Forum Moderator

Hi Glenn,

Could you please send a picture os small video so that I can better understand and evaluate your problem? Maybe you could send the packed project.

Please send to pedro.lima@afmg.eu

Best Regards,
Pedro

Glenn Leembruggen · Member

Hi Pedro
I have just sent you an email with the project and pictures.
thanks
Glenn

Peter Patrick · Member

Hi Glen,
I just ran a standard mapping routine on a pair of line arrays using a Bosch *.xglc over an audience area 300m long and found no discontinuity at all.

I can only enquire ... What does the map look like if only S1 is mapped ??
and ...
What happens if you change the *.xglc setting ??

In other words ... could it be in the *.xglc ??

AFMG Pedro Lima · Forum Moderator

Dear Glenn,

The "problem" you have reported is actually a combination of three different effects that are confusing you: Shadow, "Low" Resolution and the sound of some speakers running out of the "80 ms time frame" of DirecSPL calculations.

Let me try to explain.

1. As shown on the Picture 1 below, the speakers on your project are very close to a wall causing the demonstrated shadows (whenever Map with Shadow is on, which was the case).

On the left side of the left shadow you can see that both speakers contribute to the displayed sound field, since those data points are on their "line of sight". On the center (between the two shadows) only A1 is acting, because B1 cannot "see" those data points. That's correct because the wall is a barrier for the sound in that case (which is in accordance with the geometrical acoustics assumptions used in EASE). On the Probe display below the mapping you can see this effect. Only the impact for the speaker A1 is being shown.

Similary occurs on the right shadow. But this time no speakers can see those data points. The wall is "blocking their view". Thus there is no direct sound for those data points. The shadow is one of the causes for the "weird" discontinuity you've seem.

Picture 1:

2. On the upper left of the Picture 2 you can see a mapping with a resolution of 0.5. The resolution sets the patches sizes. That is, the size of the little squares you can see on the mappings. The higher the resolution the smaller these little squares. Notice that the right shadow is quite jagged. If you increase the resolution, as can be seeing the upper right mapping, you can see that the effect is less prominent and the line is more consistent. The lower mappings display the same result but with isolines. That's one of the reasons for the "dog bite" effect you've seen. So, just increase a bit the resolution as needed and the effect is supressed (be careful with the calculation time).

Picture 2:

3. Last, but not least, there is one more effect that caused the "unusual" discontinuity and "dog bite" effect shown on the Picture 3. Here the direction of what seems to be the "shadow" line is apparently changed.

That is actually not an extension of the "shadow line". When perfoming Direct SPL calculations there is a time frame to be considered after the very first arrival of sound at a specific point. Sound (or impacts) that arrive whitin a time frame of 80 ms are considered as Direct Sound. Whereas others are discarted.

On the picture 4 you can see that on the left of the discontinuity (red line) the sound of the speakers B1 and C1 arrive after aproximately 11 and 69 ms after A1. That is within 80 ms. Hence, their sound fields are contributing for the displayed SPL. On the other hand, on the right side of the discontinuity, C1 arrives after aprox. 90 ms. Out of the 80 ms time frame. Thus, C1 is not contributing to the sound field nearby that location. That said, one can verify that the red line actually marks the "limit of functioning" for the speaker C1.

That's why you can see that line, that looks like a shadow.

Maybe reviewing the defined delay times is enough to "fix" that.

I hope the information above can still help you.

Best Regards,