Finding Reflection Direction using Polar ETC

By Dale Shirk

How do you find the surface that is causing the reflection? In this article,  Dale Shirk shows a technique using Polar ETC.

Polar ETC is a program module for the Goldline TEF™ measurement system. By taking 6 measurements with a cardioid microphone and computing the differences between them, a quasi-3D display is generated that shows the arrival time, arrival direction, and strength of reflections in a room. This is very useful for determining which surface is causing a reflection, and for determining certain acoustic metrics, such as Lateral Fraction.

The Hardware Photo of the hardware

The cardioid measurement mic needs to be accurately oriented in four horizontal directions, North, East, South, and West, if you will, plus up and down. These 6 measurements must be taken with the capsule at the same point in physical space. Typically this is done with the mic mounted in a sort of gimbal arrangement with the capsule at the intersection of the axles. This allows the mic to be aimed in different directions.

I am proposing a simpler arrangement using common items found in most audio practitioner’s junk box. Using a side address studio condenser microphone with a cardioid pattern and some simple mic mounting hardware, we can cobble together a rig to measure polar ETC.

A boom arm of any variety, is mounted onto a mic stand at a 45-degree angle. Onto the end of it we put a 45-degree angle mic mounting tube. See Fig. 1. We put the studio mic with a rotating base on the end, observing the following geometry. Photo of the mic, boom, base and mounting tube

  • 1. The mic must be vertical.
  • 2. The boom arm must be at a 45-degree angle.
  • 3. The center line of the boom arm must pass through the mic capsule.

These relationships are drawn onto Figure 2.

In my example I used a Shure KSM-44. I added a male and a female coupler to extend the mounting tube enough to get the proper relationships. Under strong light you can see the actual capsule in most studio mics (Figure 3). I marked the base of the mic and it’s swivel adapter at 90 degree increments using graph paper as a guide (Figure 4).

Photos of the mic and Polar ETC software

The four horizontal aiming directions are achieved by rotating the mic in it’s base. The vertical aiming is done by rotating the boom arm 180 degrees as shown in Figure 5, to take the up and down measurement. Be sure you start with the boom arm slid all the way down so it doesn’t change length when you rotate it.

Although a studio condenser mic may not have perfectly flat response or perfect cardioid polar patterns, most will be quite well behaved over the range of 250- 8000 Hz where we are most interested in arrival directions.

The Result

Figure 6 shows the composite ETC of all six measurements. Any reflection (vertical spike) that exceeds the threshold (red line) is displayed as a dot on the 3D display. The user can use a cursor to select specific reflections, whose azimuth and elevation are then indicated on the display. ds