Today, I find a question that if the speakers are set in parallel towards the wall. It has no complete destructive interference due to difference in distance from the point to each speakers, which leads difference in intensity, amplitude...(Answer is D)
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  [1]: https://i.sstatic.net/FpL3h.png

However, for the young's double slit there is also difference between the point and the slits. So, there should be difference in intensity,amplitude, isn't it? However, we consider it as they are having complete destructive interference so they have no intensity, dark region, at some point. 

Is it because the electromagnetic wave has high frequency, so we assume that there is no decrease in intensity, amplitude is negligible??