The Core Sound TetraMic is the first portable, coincident, stereo and surround-sound Ambisonic soundfield microphone under $1,000, and it compares very favorably with similar soundfield mics selling for $3,000.
This pencil-sized microphone offers amazing sound and flexibility at an affordable price. Some applications include stereo or surround recordings and broadcasts of musical ensembles, sound effects, news events, sporting events, environmental sounds, and general ambience.
How It Works
The TetraMic is a soundfield system that utilizes a patented four-capsule microphone and decoding software.
On the top of the tiny mic are four sub-cardioid electret-condenser capsules of 12 mm diameter mounted 90 degrees apart.
The capsule grilles define the faces of a tetrahedron. Below the capsules is a slim brass handle with a built-in 6-pin tiny XLR connector.
Other cables and connectors (described later) let you connect the mic to a recorder of your choice.
The signals coming directly from the four capsules are called A-format. While these signals are not usable as they are, they feed into Core Sound’s processing software which creates a four-channel signal called the soundfield B-Format.
Those four channels are known as:
X (front-back)
Y (left-right)
Z (up-down)
W (omnidirectional, a reference for the other three channels).
The X, Y and Z channels are effectively three figure-eight patterns at right angles to each other.
By summing and differencing those B-format channels in varying amounts, the processing software can create a wide variety of mono, stereo, mid-side, or surround polar patterns—even in post-producton after the 4-channel recording has been made.
Surround formats include 5.1, 6.1 and 7.1; loudspeakers in a square, and loudspeakers in an octagon. Almost any loudspeaker arrangement can be user-defined.
Although the capsule diaphragms are separated about 1.3 inches, the processing results in phase-coherent signals, as if the capsules occupied the same point in space.
The four B-format channels capture the 3-D sound field all around the microphone, as picked up at a single point. These signals can sum to mono with no phase cancellations.
The processing also matches the capsule sensitivities, and equalizes the capsule signals to give them a wide, flat frequency response.
So you start with a four-channel recording of the mic-capsule signals. Then using the processing software in post, you can effectively point or steer the “effective” or “virtual” microphone(s) as desired.
The software allows modeling of essentially any number of coincident first-order microphones, each pointing at any arbitrary angle and each having an independent pickup pattern.
For example, suppose you set the software to create a virtual figure-eight mic. As you move a slider in the software to turn the virtual mic off-axis, you can hear the recorded source become more distant as the null of the figure-eight pattern sweeps toward the source.
Or suppose you create a virtual Blumlein array of two figure-eights angled 90 degrees apart. As the sound source moves from center to 45 degrees to the right, you hear the sound image move the same way to the right monitor speaker.
When the source moves beyond 45 degrees, you hear the sound imaging becoming out-of-phase and diffuse, just as you would with a real Blumlein pair.
It’s as if you had a mono mic, stereo mic, or surround mic that could be rotated, tilted, or zoomed at will—after the recording is made.
The Physical Mic & Cables
A TetraMic system of mic, cables and adapters can be connected in various ways. Figure 1 shows a typical system.
The TetraMic is connected to an extension cable, which goes to a breakout cable, which is connected to four PPA phantom-power adapters with XLR plugs.
.
The signal at those XLR connectors is low-Z balanced.
The PPA adapters plug into a mixer, audio interface, or mic preamps that supply 48-volt phantom power, or they can plug into an Audio Technica AT8531 power module, and many others.
The front of the ruggedly constructed mic is indicated by the “Core” logo on the machined brass handle.
A Switchcraft TB6M connector in the mic handle mates with a TA6F in the extension cable, and this extension cable plugs into a “four 3-pin mini-XLR-F breakout cable”, which divides into four 3-pin mini-XLR plugs.
The signal chain of this connection option is:
TetraMic > extension cable > breakout cable > PPA phantom power adapters > mic preamp with phantom power.
Because the mic-capsule signals are medium impedance unbalanced, it’s best to limit cable runs to 20 feet before converting to low-Z balanced with the PPA adapters.
Core Sound offers another connection option:
TetraMic > extension cable > dual 5-pin adapter cable > Core Sound 4Mic power supply/mic pre/A-D converter > 2-track digital recorder with S/PDIF input.
The Core Sound 4Mic ($899) is a handheld, battery operated preamp and A/D converter that provides four discrete outputs or a matrixed 2-channel signal that can be decoded later.
When used with the 4Mic, the TetraMic can record the Ambisonics information to a standard two-channel flash-RAM recorder like the M-Audio MicroTrack.
So for about $2,000, you can carry a versatile, handheld surround recorder in your hand.
The 4Mic provides two output data formats: four channels on two S/PDIF outputs (optical or coaxial), or four channels multiplexed onto a single S/PDIF output.
According to Core Sound, ” We’ve found that a few commercial mic pre/ADCs will do the job. One is our 4Mic. Others are the MOTU Traveler, Prism Sound’s Orpheus, Metric Halo ULN-8, and Apogee’s Ensemble. The MOTU Traveler can be used as an excellent quality stand-alone mic pre-amp, in addition to its FireWire interface for PCs.
Its line level outputs can be connected to a Sound Device’s 744T four-channel recorder’s line inputs to make fine sounding recordings. Apogee’s Ensemble is one of the few mic pre/ADCs that, like our 4Mic, can be battery powered.”
“TetraMic works great with the RED Digital Cinema Camera. Its microphone pre-amp inputs are four mini-XLR jacks. We offer 6-foot cables that plug into the TetraMic PPA (XLR-F) on one end, and the RED camera mic inputs (mini-XLR-M) on the other. For the first time, cinema cameras are now able to record surround sound in-camera, and also track individual sound sources during post-production!”
Included with the TetraMic is an effective shock mount which slips onto the handle. It mounts to any standard 5/8-27 threaded mic stand. A fixed stand mount without shock mounting is also available.
Core Sound offers wind screens to reduce wind noise when recording outdoors with the TetraMic.
Manufacturer’s specifications:
Frequency response (after DSP correction): 30 Hz - 18.5 kHz +/- 2 dB.
Self-noise per capsule: 19 dBA. In my opinion, this may be a little too noisy when recording very quiet sound sources.
Maximum SPL per capsule: 135 dB SPL.
Sensitivity per capsule: 7.0 mV/Pa nominal (-43 dB re 1V/Pa).
Software
Once you have purchased the TetraMic, Core Sound provides a link to download three types of processing software (for free), and the calibration files for your particular TetraMic. I’ll describe the software below.
VVTetraVST and VVMic VST plugins
If your DAW can use VST plugins, these let you monitor the recording in real-time, fully decoded. You can record either A-format (four channels), B-format (also four channels) or files decoded to any specific microphone and playback configuration (from 1 to a very large number of channels).
You insert the plugins into a surround bus in your DAW that is fed by the four A-format tracks that you recorded.
The VVTetraVST plugin corrects the capsule frequency responses with downloaded calibration data (an advantage over many other mics).
It also converts from A- to B-format.
.
The VVMicVST plugin (Figure 2) decodes the B-format signals so you can set the mic parameters (for example, the number of microphones, the angles at which they point, each mic’s directivity) and match your playback system configuration.
Figure 3 is a screen capture of Reaper DAW software set up for use with the TetraMic.
The four A-format mic signals are on tracks 1, 2, 3, and 4.
Those four tracks feed into a multichannel track 5, which contains the VVMic VST and VVTetraVST plugins.
Track 5 is soloed so you hear only the decoded signal (the virtual microphones).
.
VV Mic for TetraMic
This is a stand-alone application which changes A-format signals to B-format, and decodes the B-format signals into mono, stereo, or surround mic signals.. Here’s one way to use it:
1. Make a 4-channel recording of the TetraMic signals.
2. In your DAW, export each channel’s recording as a mono file.
3. Launch VV Mic. Multi-select those four files and import them for processing.
Here’s another way:
1. Make a 4-channel recording of the TetraMic signals.
2. In your DAW, pan each capsule’s signal to a different surround channel.
3. Export a multichannel file.
4. Launch VV Mic, and import that multichannel file for processing.
Figure 4 shows the main screen of VVMic.
.
The input files can be A-format or B-format. You can output a processed file (in B-format or decoded as a virtual mic) or just listen in real-time.
The Rotate screen lets you rotate, tumble (roll vertically) or zoom the sound images.
In the Decode screen (Figure 5), you can vary the elevation, azimuth, width, directivity and gain of the polar patterns.
.
All these actions are instantaneous and work smoothly.
.
Measurements
I measured the frequency response of the TetraMic on and off axis. Using Reaper DAW software with VVMic VST and VVTetraVST,
I was able to decode the TetraMic’s signals in real-time to create a single omnidirectional, cardioid, and bidirectional virtual microphone to be measured.
Using TEF software, I first measured the frequency response of a loudspeaker at 1 foot with a laboratory reference mic, with the TEF set to 1 msec receive delay.
Then I measured the frequency response of the same loudspeaker at 1 foot with the TetraMic, with the TEF set to 41 msec receive delay (because the latency of the TetraMic software was 40 msec).
I manually differenced the two response curves to create the plots shown in Figures 6, 7, and 8.
As the plots show, the frequency response of the TetraMic’s virtual microphones is flat within 1.5 dB over most of the audible range.
Data below 200 Hz is omitted because it included sound reflections off the test-room surfaces.
The TetraMic has the flattest and most extended response of any velocity (cardioid or fig-8 type) mic, thanks to the digital EQ in the software.
.
.
In Use
According to Core Sound, “One stand-alone digital recorder that folks have used with great success is the Sound Device 744. While it only has two microphone pre-amps (two short of TetraMic’s four outputs), the MOTU Traveler’s four mic pre Line outputs can be plugged into the 744’s Line inputs.”
.
On its web site, Core Sound offers tips on recording with several different systems that include the TetraMic, MOTU Traveler, a PC, Sound Devices 744, and a 2-channel recorder.
I used the TetraMic to record speech walk-arounds, acoustic guitar in a live room, and an old-time string band with the mic in the center of the group.
The TetraMic fed a PreSonus Firepod audio interface and a computer running Reaper DAW software. I had tried the software with Cakewalk Sonar Producer, but they were not compatible.
Overall, the sound quality of the TetraMic was excellent. Image focus and placement sounded natural. Localization was precise, yet with plenty of ambient detail.
I heard clean, clear sound, plenty of low end and depth, and no audible distortion. Movement of images across the stereo stage was very smooth.
When used outdoors with a windscreen, the mic picked up little or no wind noise.
The string band recording sounded natural but a little lacking in upper-midrange presence compared to a typical close-miked commercial recording. It was hard to hear the guitar strings being strummed.
The response curves do show a small dip in that range. However, it was easy to get a brighter sound with equalization.
While listening to that string-band recording, I set up one virtual cardioid mic in VVTetra VST. As I rotated the cardioid in a circle using the Azimuth setting, I heard each instrument in isolation. Very cool!
According to Core Sound’s president, Len Moskowitz, “Folks are using this function for post-production of panel interviews to highlight specific speakers, and also in movie sound post to follow a moving sound source around. It won’t replace a boom operator but does provide another powerful tool for the sound crew. For music, you can highlight a soloist, like a spot mic.
“You can also do this function more than once (for multiple sound sources), and then mix the resulting decodes together.
“So do a Blumlein decode for stereo ambience and mix in spot decodes for soloists. You can also move a microphone’s pattern null around to exclude noise sources, even ones that move around.”
I received excellent technical support from Len Moskowitz at Core Sound and from the software’s developers, David McGriffy and Richard Lee.
The TetraMic is a huge advance in recording technology, and a great value. Highly recommended.
Core Sound provides a 30 day trial period. All TetraMics returned within 30 days of shipment qualify for a full refund (minus shipping and handling charges) provided that they are returned in as-new condition. TetraMic is sold with a 1 year limited parts and labor warranty.
You can find out more about the TetraMic at www.core-sound.com, and company owner Len Moskowitz can be reached at .(JavaScript must be enabled to view this email address)
AES and Syn Aud Con member Bruce Bartlett is a recording engineer, microphone engineer and audio journalist. His latest books are Practical Recording Techniques (5th Ed.) and Recording Music On Location.
.
More recording articles by Bruce Bartlett on PSW
Simulating A Live Drum Solo In The Studio
Remastering Jazz Classics: The Dave Brubeck Quartet, Art Pepper, and Sonny Rollins
Deconstructing Hip-Hop To Hear How The Mix Comes Together
Recording Microphone Techniques To Produce Warm, Spacious Stereo
Return to article