January 16, 2012

S2 point source horn vertical directivity measurements

Previously I measured horizontal directivity but ran out of time to measure vertically.

Horizontal measurements >

Vertical measurements are much more challenging in this case. On my measurement rig, it means rotating the horn 90 degrees so that it is now tall instead of wide. This allows me to use the angle markings, but I found that without assistance the horn was unstable and my quickly rigged up struts were not up to the task. I would need a person to hold it in place carefully, or spend a lot more time making sure it stays up there. You will get some idea what I'm talking about if you see the setup for horizontal.

In the end I settled on a different setup where the horn was placed on the ground with thick foam on the first reflection point. So the horn was placed on the ground firing horizontally, and then the horn itself was rotated to the angles required, with the mic in place. I printed out a guide for the angles. I also placed foam around reflection points. This is certainly not ideal and the results show obvious problems, but still give some idea of what is going on.

Measurement steps are 10 degrees. So the first line (black) is on axis, the next is 10 degrees, then 20, all the way up to 90 degrees. 45 degrees is shown in black also. So the first 3 lines are within the defined 40 degree coverage angle. All other angles are what will be seen by the floor and ceiling. 

The peaks and dips are related to reflections - the horn itself is much smoother than shown. Yet you can see observe a trend in dispersion. Notice how the lines are bunched together at the bottom? This means it is an omnidirectional source down there. However, above about 260 Hz the dispersion does start to narrow slightly, shown by the lines becoming further apart. Overlooking the peaks and dips, the overall trend is a gradual broadening of dispersion up to about 1k. 

What does this mean?

Shown here is the vertical dispersion of this 90 x 40 degree horn. What this shows is that you want to arrange the horn it so that you are sitting with your ears within that 40 degree vertical window where the response is consistent. The floor and ceiling see the other angles. 

Horizontally, S2 controls directivity down low, but vertically the smaller mouth dimension means that this can't be maintained. So the ceiling will see more lower midrange than the side walls. I'd be inclined to leave side wall reflection points along (no treatment) but consider ceiling treatment that might absorb more lower midrange, like say an RGP BAD panel.


  1. Would moving the microphone up and down work ??

  2. That would mean with each measurement the distance to the nearest boundary would be different each time.


All comments are moderated.