I am not an expert; but I guess some interference seems to be happening from the beams coming out from different elements and so, in the middle there is some destructive zone or acoustic black "shadow" zone. Also, check if the central elements are working fine!

The image looks as though it is a snapshot at one instant in time of standing waves in a cavity; or perhaps the wave are propagating horizontally and or vertically.

In any case, what you may need to do is create an animation of the waves at small time steps over several wave periods, and watch the animation. Then you will see better what is going on.

As it is, there seems to be a horizontal null where the pressure is zero from left to right across the centre of the cavity. But is it a null, or did you just happen to take the snapshot at the time that the pressure was passing though a zero-crossing? The animation will tell.  And it will give a much better indication of what is taking place.

Ronald

Thank you Ronald and Vikash.

An additional information, I am propogating an ultrasonic standing wave field from the transducer as you can see from the image I have attached below.

The result is not a time-dependant study result, its a frequency domain result, or in other words its a stationary study result. 

But I want to know, what would you think that caused the horizontal null of zero pressure at the centre of the acoutsic cavity?

The particles that are suspended in the continuous medium (water) are solid polystyrnee particles. And usually for a standing wave field formation, a travelling wave first is produced from the transducer, then gets reflected at the end of the PMMA wall and a superposition principle takes place between the travelling and reflected waves. Hence, they form standing wave field.

Might it be because the PMMA wall is a poor reflector? 

Or is it caused by the mass fraction of the suspended particles which is taken as 0.001? 

Or could it be because there is no interface layer between transducer and the PMMA wall that caused a large acoustic interferance?

That diagram helps.

But in your earlier color plot, what is being plotted? It is the acoustic pressure for standing waves of a given frequency inside a cavity. But is it:

the instantaneous acoustic pressure in time domain, at a single time snapshot, or

the magnitude of the complex pressure in the frequency domain.

I think it must be (1), because magnitude (2) is always positive but your colour plot goes negative, and magnitude requires the relative phase in order to be complete, which is not considered.

The fact that it is a single frequency does not rule out the creation of a video from a sequence of time frames, just as you can make a video out of a sequence of time frames for a standing wave on a guitar string. I still think that creating such a video will inform you on the situation. Time-steps must be small relative to the period of the waves.

Given just one time snap shot, we really cannot tell what is going on, because nulls may not be nulls at all. The snap shot just happened to capture the acoustic pressure at the moment that it when to zero there.

If you cannot create a video, then try to plot the root-mean-squared (RMS) acoustic pressure field over a single period. Don't worry about the phase. This is easy to do if you are doing the modeling in frequency domain. And then you will see if nulls are really nulls or not. The RMS pressure will be zero (numerically very small) in the nulls.

Recall from your early physics lessons that the nulls are very important in understanding standing waves. The true nulls should make things much more clear.

Ronald