title: The SpHEAR project update: Refining the OctaSpHEAR, a 2nd order ambisonics microphone
Creators: name: Lopez-Lezcano, Fernando
Publisher: name: EAA Spatial Audio Signal Processing Symposium
Year: 2019
Subjects: ambisonics microphone
polar patters
frequency response

description: This paper presents an update of the *SpHEAR (Spherical Harmonics Ear) project, created with the goal of using low cost 3D printers to fabricate Ambisonics microphones. The initial four-capsule prototypes have evolved into a family of full-featured high quality microphones that include the traditional tetrahedral design and a more advanced eight capsule microphone that can capture second-order soundfields. The project includes all mechanical 3d models and electrical designs, as well as all the procedures and software needed to calibrate the microphones for best performance. A fully-automated robotic arm measurement rig is also described. Everything in the project is shared through GPL/CC licenses, uses only Free Software programs, and is available on a public GIT repository (https://cm-gitlab.stanford.edu/ambisonics/SpHEAR/). The new measurement system, based on a low cost robotic arm with custom hardware modifications and custom software, enabled us to automatically measure the microphone response over a substantial subset of the full spherical response. In the case of our microphones, we can capture 108 of the 180 points of a suitable spherical t-design (those measurements includes full horizontal plane coverage and limited elevation coverage - between -30 and +43 degrees elevation). The limitation in vertical coverage is due to the limited length of the arm itself, which was a necessary compromise as longer arms were only available on substantially more expensive robotic systems. The arm selected also runs on fully free software so it is easy to modify and customize.The automated measurement software, written in SuperCollider, interfaces with the modified firmware in the arm and handles the playback of sine sweeps and recording of the capsule response, and can aquire a full set of more than a 100 measurements in around 40 minutes.With that new data we were able to fully calibrate the array, so that we are now recovering 8 of the 9 spherical harmonics that make up a 2nd order Ambisonics signal set ("R" aliases to "W" with this particular capsule arrangement, and cannot be independently extracted).We will also describe the tradeoffs that are involved in choosing what to optimize for during the calibration of the system. Graphs will be included that show the raw and calibrated performance of the microphones (frequency response, polar patterns at different frequencies and elevation angles, etc).Our more optimal measurements also revealed some problems with the original mechanical design of the 8 capsule microphone, which was a simple extension of our 4 capsule tetrahedral design. The 8 capsules created a resonating cavity that unnecessarily degraded the polar patterns of the recovered B format components at mid and high frequencies. A new mechanical design avoids this problem, and the new prototype exhibits better polar patterns. We will compare the old and new designs.A comparison is also done between the first order performance of both the four and eight capsule designs. The 8 capsule design shows more even performance in the horizontal plane, without the dips or peaks at 45 degrees that a tetrahedral design exhibits.We will also measure and characterize some of the commercial microphones available to compare their performance with our microphones, specially with regards to the precision of the calibration and the accuracy of the recovered B format polar patterns.