mh acoustics’ em64 Eigenmike spherical microphone array contains 64 professional-grade omnidirectional microphones mounted on a rigid spherical baffle. The em64 allows for tremendous degrees of freedom in projects requiring recording soundfields in any location. From acoustic measurements to 360° recording and complex soundfield analysis, the patented design enables multiple use cases explored in this article.With the Eigenmike em64, mh acoustics updated its original em32 model, bringing a state-of-the-art higher-order Ambisonics capture tool to the market (Photo 1). The em64 array integrates microphone programmable gain preamplifiers, 24-bit A/D conversion, and microphone calibration, all within a single unit. For control, processing, and recording of the microphone signals, mh acoustics provides its native EigenStudio application and a set of VST plug-ins.
The solution is the product of an eclectic engineering team with decades of experience in digital signal processing, acoustics, and product design. Led by Gary W. Elko (President), Jens M. Meyer (Chief Technical Officer), Tomas Gaensler (Chief Scientist), and Eric J. Diethorn (Chief Engineering Officer), all of whom have previously worked at Bell Labs, Lucent Technologies, and later spin-offs Agere Systems and Avaya Labs, the mh acoustics team also developed a number of proprietary technologies and products in the areas of microphone arrays, noise suppression, single- and multi-microphone electronic wind-noise removal, and multichannel acoustic echo cancellation.
Ambisonics
Ambisonics is one of those concepts in audio engineering that many struggle to understand and is easier explained in practice. Very briefly, Ambisonics is the logical upshift of mid/side stereo with more channels and signals, which are called spherical harmonics, attached to an imaginary sphere. The output signals from the microphone array cannot be used directly and must be rendered to a channel-based output format first (i.e., stereo, 5.1, etc.). The processing can only be done digitally, due to the required management of high channel count audio and precise control of signal levels.
Higher Order Ambisonics (HOA) enhances both spatial resolution and the effective listening area and enables the reconstruction of a sound scene regardless of the actual speaker layout used for playback. While first-order Ambisonics typically uses four channels (W, X, Y, Z), higher orders can utilize more channels, allowing for more precise localization of sound sources. Each higher order includes all lower-order channels, making it easier to down-mix if necessary.
As of this writing, there are only a handful of HOA microphones on the market. Most available products stick to first-order Ambisonics.
Ambisonics itself is an old technology. It traces back to the work of Michael Gerzon and Peter Fellgett in the 1970s. And yet, the full potential of Ambisonics is still underutilized in audio production and broadcast. Thankfully, in recent years multiple academic and commercial organizations have released many plug-ins that allow for creation and spatial manipulation of the Ambisonics soundfield.
The challenge of working with Ambisonics remains the right combination of hardware and software required for engineers working in different fields. With the original em32 (32 mic capsules) introduced almost 10 years ago, mh acoustics set out to meet many of those requirements and has now upgraded the solution with the em64 (64 mic capsules), effectively doubling the amount of mic capsules for soundfield capture.
What Is It?
The Eigenmike em64 is a multi-microphone array that turns the mathematical model of Ambisonics spherical harmonics into reality and utilizes an 84mm diameter sphere as a receiver. The 64 low-noise electret capacitors, sourced from Sennheiser, are strategically placed equidistant on the surface. The output of each of the microphone capsules is amplified, converted to digital, and frequency corrected, all inside the housing. The interface to the outside world is an audio-over-IP connection with the Dante protocol. The network cable provides power via Ethernet (PoE+), control data (gain settings), and firmware updates to the microphone‘s signal processor. Since no application-specific integrated circuit (ASIC) is available for accomplishing so many microphone-specific and network-related tasks simultaneously, the em64 uses an FPGA, running a custom firmware from mh acoustics. The design includes a secondary Ethernet connector for redundancy, which has not yet been implemented at the time of writing.
In contrast to other special-purpose microphones such as intensity probes or boundary effect microphones, the em64 is not a one-trick pony, reduced to its Ambisonics capabilities. It supports a long list of potential use cases. Probably the most obvious being augmented reality and virtual reality (AR/VR) — the only market segment where Ambisonics is commercially explored, allowing a high precision sixth-order Ambisonics soundfield to be captured and controlled interactively by the user.
In an engineering context, the calibrated mic signals allow for SPL measurements, reverberation time measurements, capturing three-dimensional impulse responses, and complex sound field visualizations (e.g., with the software IRIS). For sound design applications, recordings in rooms can be re-reverbed, re-oriented and re-recorded, while the “focus” of the recording can be broadened and narrowed. Binaural and channel-based surround format extractions are free in orientation for the specified output channel format. Be it Quadrophonics, 5.1, or 7.1.4 — the em64 can render to any format. Music producer and YouTuber Benn Jordan released a brilliant video about the creative use of Ambisonics recordings and impulse response creation. It is aptly titled: “Ambisonics reverb is insane.” There is not much to add here, just that insane is meant in a positive way.
When used in binaural mode, the signals from the em64 allow freely rotating the recording — just in software, without the need for complicated mechanics and moving parts. And even mundane use cases are not only possible but rewarding such as with voice-overs or trivial omni recordings. The creation of virtual microphones with varying polar patterns in the recording is realized through the included software suite. Ultimately, we get a whole collection of microphones with configurable polar patterns, enabling attractive possibilities for video and film production, where engineers can refocus or dampen sound events in post-production.
Work in Practice
The em64 hardware comes in a robust Peli case that was used by mh acoustics without further adjustments directly for shipping (Photo 3). The microphone can be set up quickly. The microphone assembly comes with an expectable 1/4” screw thread, suspension holder and a Rycote “dead cat” for wind and dust protection (Photo 4). But a complex product like this depends in equal parts on its documentation, support, hardware, and software. The digital documentation and website are helpful in the upbringing. The package includes a comprehensive, A4 printed manual that strikes an excellent balance between readability, detail, and instruction depth. mh acoustics also provided excellent support during the initial stages of our installation. For our review, Gary Elko himself not only assisted us with the setup, but also patiently answered all our questions.
To control and record with the em64, there are several options. The em64 can be integrated into an existing Dante network for routing and recording. In a smaller setup, we can use the Eigenstudio 3 standalone software (Figure 1) from mh acoustics, where recordings can be made, the preamps controlled, and virtual microphones are created and monitored. Another option is to use the Eigenmike in our DAW, where the Eigenunits plug-ins (Figure 2) can be used to control the preamps. The GUI offers only a limited number of control options, such as gain and pad, but no virtual microphones can be created.
Audinate’s Dante Controller and Dante Virtual Sound Card are distributed with the em64 and are crucial for operation and recording on a PC (Photo 6). I have some experience with A2B, AVB, and MADI networks, but Dante was new to me. That did not pose a challenge. The configuration of the network and routing was smooth, and all was running within minutes. Dante Virtual Soundcard provides ASIO/CoreAudio connectivity to all audio applications. Most of the options are self-explanatory.
Once all end points are defined and routed, the multicast stream from the em64 takes about 10Mb/s of bandwidth from the network. That is also the bandwidth required for recording the 64x 24-bit/48kHz audio datastreams. Dante is the perfect choice for interfacing, since it is on a good path to become the audio via IP de-facto network standard. The only thing that was a little disappointing is that the em64 only comes with a one-time activation coupon for Dante Virtual Soundcard that is not transferable between computers. Nevertheless, a transferable license is available for $33 USD from Audinate.
If you want to use the em64 with a stand-alone field recorder, you are out of luck. Currently, there are no suitable recording devices with Dante inputs that would allow using the em64 in the field. Gary Elko told us that they are working on a stand-alone recorder but have yet to decide if that product will ever come to market.
How Does It Sound?
The sound experience can be accurately described in one word: Wow! For the test recordings I have calibrated my headphones to playback the same SPL as the em64 receives. What quickly became clear to me was that the microphone capsules are useful for measurements and recording purposes. Most measurement microphones do not sound good, and studio microphones are normally voiced to add “color,” becoming unusable for serious measurements. The em64 delivers a perfect balance between precision in terms of frequency response and perceived sound quality.
When I decoded the Ambisonics audio signal to different formats, I was profoundly impressed by the clarity and precision. When I listened to a test recording that I did in my office where I opened and closed a door, I almost had a heart attack. Although I knew I was listening to a recording that I had just made, and no one was in the house, the sound of the door opening was so realistic in my headphones that I had a hard time discerning reality from illusion and spilled the coffee over my desk, when I instinctively turned to the door.
The em64 uses a coincidence miking technique and comes with the flavor associated with these types of recordings (Figure 3). It delivers both depth and width, but it is more sharp than wide. The feeling for depth from spaced pair A/B recordings techniques was not achievable, but this facet was not expected to be there.
The workflow sounds complicated, but the software tools from mh acoustics are real enablers for a productive workflow; well documented and easy to understand. I could listen to anything I wanted on the product without actual physical access to the precious prototype. With the em64 set up, I was simultaneously able to make calibrated SPL measurements, virtually rotatable binaural renderings, channel-based renderings, acoustic camera imaging, multi-point array measurements, and audio quality evaluations with ViSQOL! All from one microphone and one set of recordings!
The em64 makes it possible to re-live an acoustic scene in a convincing way and gives engineers a tool for analysis of the sound field. Since all that can be automated in a DAW, all it takes is the initial effort to set it up. With this workflow, cross-functional product development becomes easier for everyone involved, and decisions can be made more efficiently.
Conclusion
The em64 is a great piece of technology. I wish everyone in the acoustics community would try this microphone. It is such a practical, versatile and flexible tool, both for creative use and engineering purposes. The use cases are simply endless, and I am confident that we will see more software tools helping to extract all the information from the captured sound field.
The em64 is a no-compromises professional product, handcrafted and assembled in the US. That comes with a price tag: $15,000 US plus tax is no pocket money, but understandable for the effort undertaken and the relatively small target customer group. Maybe an entry-level or license model product with a smaller feature set could cater to a larger potential customer base in the future? I really hope mh acoustics is successful with this product, improves it further step-by-step and iterates into even higher Ambisonics orders. aX
Resources
Eigenmike em64, www.eigenmike.com
IRIS, www.iris.co.nz
B. Jordan “Ambisonics Reverb is insane,” www.youtube.com/watch?v=MvS3HixNb1E&t=828s
mh acoustics, https://mhacoustics.com
VRTONUNG, https://www.vrtonung.de/en/ambisonics
mh acoustics em64 Pros and Cons
Pros:
Versatility
Build quality
Complete system for higher order Ambisonics
Software quality
Cons:
Price
This article was originally published in audioXpress, February 2025
