building.md 11 KB
The *SpHEAR Project
Requirements
Hardware
Basic
A 3D printer (we use an Ultimaker Extended Plus)
A PCB (printed circuit board) prototyping system (we use the OtherMill computer controlled mill)
Tools
X-Actor knife or similar with set of blades (for trimming and cleaning up 3D prints)
Needle file set (for trimming and cleaning up 3D prints)
Soldering iron with temperature control and fine tip (for assembling the phantom power interfaces)
Long nosed pliers
Wire stripper
Wire cutter
Digital Multimeter with hFE measuring function (transistor forward current gain - for matching phantom power interface transistors)
Headband Magnifying glass (this project has a lot of very small things you need to see, solder and manipulate)
Calibration
Excitation loudspeaker: preferably a wide bandwidth single driver small speaker. This is used to measure the response of the completed Ambisonics microphone (and reference microphone) when it is calibrated.
Reference microphone: a flat omnidirectional reference microphone, preferably with its own calibrated response. We use this as a reference for calibrating the TinySpHEAR Ambisonics microphone.
A Big Room: you need a relatively quite big space for doing the calibration measurements (or an anechoic chamber, of course). If the room is not anechoic the shortest distance between a reflecting surface, the microphone and the speaker will determine the usable impulse response length (in the space I am using I get 5 to 6 mSecs which is apparently sufficient).
Software
All the software used to design and build the microphones is Free Software under various licenses and runs under the GNU/Linux operating system. At the time of this writing I'm running this software on Fedora 24 in my laptop (most of the software described below is available either from the Fedora repositories or from the Planet CCRMA repository).
3D Printing and Printed Circuit Design
- Openscad: 3D design language and renderer. All 3D models in this project are created in Openscad.
- 3D printer slicer: this is a tool that takes the STL (STereoLithography) output from a rendered Openscad object and outputs the G-code that the 3D printer understands. You will need software that supports the 3D printer you have access to. We use Cura as that is tailored to the printer we have (Ultimaker Plus Extended).
https://ultimaker.com/en/products/cura-software
- Kicad: schematic capture and printed circuit layout software. This is used to create the phantom power printed circuit boards that connect each capsule to one input of a four channel preamplifier or recorder. The Gerber format export from the printed circuit layout software is used to mill or etch the printed circuit boards.
Calibration
- Aliki: an integrated system for Impulse Response measurements, using the logaritmic sweep method. This is used to capture the impulse responses of the reference microphone and the finished TinySpHEAR Ambisonics microphone array which are then used to calibrate the microphone.
http://kokkinizita.linuxaudio.org/linuxaudio/downloads/
- Octave: a scientific programming language with a powerful mathematics-oriented syntax with built-in plotting and visualization tools. Most of the calibration software is written in Octave. In addition to the base software you will also need to install the "Signal" package.
https://www.gnu.org/software/octave/
- DRC (Digital Room Correction): a program used to generate correction filters for acoustic compensation of HiFi and audio systems in general, including listening room compensation. This is used to calibrate the excitation speaker using the calibrated reference microphone
http://drc-fir.sourceforge.net/
- SOX: the Swiss Army knife of sound processing programs. Used during the calibration process to convert soundfile formats.
- Ecasound: a software package designed for multitrack audio processing. Together with Python and Pyecasound it is (optionally) used to measure the final FIR calibration matrix as deployed in TetraProc.
Running
- TetraProc: TetraProc converts the A-format signals from a tetrahedral Ambisonic microphone into B-format signals ready for recording. The calibration process generates a 4x4 matrix of FIR filters that can be directly loaded into TetraProc. TetraProc is a very capable front end for the TinySpHEAR Ambisonics microphone.
http://kokkinizita.linuxaudio.org/linuxaudio/downloads/
Building
As outlined in the introduction this is not an easy to build project. It is assumed in these instructions that you have previous experience with 3D printing, precision printed circuit circuit and connector soldering skills and have a steady hand and a lot of patience. The reward at the end of the road is a calibrated Ambisonics microphone with very good performance.
You will need access to a 3D printer. Plan on doing many experiments as you learn the ins and outs of the particular printer you are using. You will probably need to tweak model parameters so that all the parts fit together nicely.
There are at this point two different types of microphone bodies depending on the type of electrical interface you choose. The simplest one connects the capsule array directly to a standard clip on microphone stand and includes no electronics. The interface to the phantom power supply is just a few components that usually fit into the shell of the XLR connector. This is very simple but the output is not balanced so cable runs have to be short, and is very sensitive to noise in the phantom power supply (for example it leads to unacceptable low frequency performance when using a Zoom F8 recorder).
By far the best design (and of course the most complicated) uses a balanced phantom power interface derived from the well known Shoeps interface (by Zapnspark). The main body of the microphone assemble houses four of these interfaces. There is an upper "flare" that snaps into place and holds the array of four capsules. Four shielded cables connect the capsules to the printed circuit boards. There is a second lower "flare" that has two design options. The simplest one directly connects a 4 pair balanced snake cable to the printed circuit boards (microphone and cable are a single unit), the second option houses a 12 pin DIN connector that enables the microphone to be disconnected from the cable, but is more expensive.
For our prints we have used an Ultimaker Plus Extended with PLA filament.
If you want to build the phantom power printed circuit boards from scratch you will need access to PCB prototype manufacturing equipment. We have used the small OtherMill computer controlled mill with success.
Parts list
This is a probably incomplete of what you will need a complete microphone assembly. Some of the items below are optional and depend on which type of mount you want to build.
electret capsules: in our prototypes we have used either the Primo 10mm EM182 (approximately U$S 8.50 each) or 14mm EM200 capsules (approximately U$S 16 each). The 3D models are dimensioned for both capsules. The 10mm capsules allow for the smallest array radius (around 9.6mm) and highest transition frequency. The 14mm capsules have a lower transition frequency as the array radius is bigger (around 11mm), but their low frequency response is much better - and they are more expensive.
components for building the Zapnspark phantom power interfaces. See the full list in this spreadsheet:
pcb/zapnspark/zapnspark_parts_list.xlsx
Plan on buying at least three or four times the number of transistors needed (8 for a complete four capsule microphone), you want to be able to match them by measuring their hFE using the digital multimeter so that each pair has the closest hFE values possible. You will get the best balanced performance if they are matched - that is also why some of the resistors are 1% tolerance.
Please be aware that the geometry of the body of the microphone assembly is optimized for the exact size of the printed circuit board and those exact components (specially the diameter of the electrolitic capacitors). If the components you buy are different the completed printed circuit board may not fit inside the microphone body.
- Mogami W2490 Ultraflexible Miniature Microphone Cable - for connecting the capsules and 12 pin connector (if used) to the phantom power printed circuit boards, the 3D models are dimensioned to use this cable
http://www.mogamicable.com/category/bulk/ultra_flex_mini/
- Mogami W2931 Analog 4 Pair Audio Snake Cable Black - for connecting the microphone to the XLR audio connectors, 3D models assume use of this cable
http://www.mogamicable.com/category/bulk/snake/
- Amphenol Part Number: T 3635 002, male to cable connector, for cable diameter 6-8 mm, solder termination, gold plated contacts (only the inner core is used, housed in the bottom flare of the microphone assembly).
- Amphenol Part number: T 3636 002, female to cable connector for cable diameter 6-8 mm, solder termination, gold plated contacts (this is used to build the cable that splits the microphone output into separate XLR connectors).
Switchcraft or Amphenol XLR connectors (four), male to cable
screw and nut for shock mount
black short hair ties for assembling shock mount
3D Printing
There are two top level Openscad files, each one optimized for one of the two Primo capsules we have used:
- TinySpHEAR_EM182.scad (Primo EM182 10mm capsules)
- TinySpHEAR_EM200.scad (Primo EM200 10mm capsules)
We assume that you are building a complete microphone assembly with phantom power balanced interfaces and a 12 pin DIN connector. If you want to build a microphone with a different capsule copy one of the files and start by modifying the capsule dimensions as measured from your selected capsule.
Each file has one line statements that render each component of the microphone and are "commented out" by a leading "". If you load the file into Openscad and remove the leading "" from one of the lines you should be able to preview the part by pressing "F5" (or do a full rendering by pressing "F6").
For building a TinySpHEAR four capsule Ambisonics microphone you will need:
- microphone body ("render_tetra_body"), holds four phantom power printed circuit boards
- upper flare to capsule array ("render_tetra_capsule_flare")
- lower flare to DIN connector ("render_tetra_connector_flare")
- two top capsules ("render_tetra_capsule_top")
- two bottom capsule holders ("render_tetra_capsule_bot")
TinySpHEAR shock mount with adaptor for Quik-Release KQ-2B microphone adapter:
- adaptor to Quik-Release clip-on stand ("render_tetra_shock_mount_stand")
- shock mount external ring ("render_tetra_shock_mount_external_ring")
- shock mount internal top ring ("render_tetra_shock_mount_internal_ring_bot")
- shock mount internal bottom ring ("render_tetra_shock_mount_internal_ring_top")
- four shock mount internal ring connectors ("render_tetra_shock_mount_ring_connectors")