Velocity Vector Comparison Project

Summary:

Sound intensity probes are expensive due to their high precision and calibration. In this project, we set out to provide an alternative solution by using a cheaper tetrahedral mic array, applying our own calibrations, and calculating the velocity vectors. We used a Zoom H3-VR ($250)and compared it to results measured with a Microflown intensity probe ($12,000).

Microflown:

First, we collected and calibrated data from the Microflown. Using a MATLAB script, we played a sine sweep from 100 Hz to 12.8 kHz from a small speaker using a two-inch driver. The Microflown comes with a calibration document to adjust for inaccuracies and after applying calibration values to the recorded values, the probe gives a “ground truth” accurate measurement. The Microflown outputs four channels, measuring acoustic pressure and the acoustic velocity vector (p, Vx, Vy, Vz).

Zoom H3-VR:

In Zoom H3-VR tetrahedral mic array, the four capsules sample the pressure field. Because the mics are in different locations, they pick up sounds at different times and phases. The first step was recording the same data using the H3-VR. To compensate for differences in the capsules, amplitude calibration was applied. We rotated the H3-VR so that each capsule faced the source and then measured the relative difference between the recordings. We then determined the scaling multipliers and applied corrections across the recordings. The calibrated values were then used to calculate velocity vectors.

Results

Below is an example of the results we achieved. Overall, mathematical calibration allows for somewhat effective intensity mapping at lower frequencies, but it lacks precision. Upper-frequency performance is bound by physical spacing of the capsules because the spacing of the capsules causes phase differences which grow as the frequency increases.

We worked to address the reflections in our initial data by switching to a WhisperRoom booth and gating the recordings, which was able to improve accuracy of results. The ideal response would be a straight line. In this case, the loops in our results were mostly caused by lack of sound insulation in the measurement enviornment.