Technical Specifications & Ambisonic Microphones
Spatial Mic is a 2nd Order Ambisonic microphone that records the entire sound field with 8 discrete capsules operating at the same time, mounted together in a near-coincident array. The Spatial Mic Converter plugin or internal DSP built into Spatial Mic Dante converts the unprocessed audio from the capsules to various formats like Ambisonics, surround sound and mono/stereo. The signals may also be digitally steered around the soundfield using Voyage Audio software.
The filters built-into the Spatial Mic Converter plugin have been carefully crafted from anechoic chamber measurements taken from Spatial Mic. These measurements are processed using ambisonic theory, which aims to take into account capsule array radius, spacing and microphone mechanical characteristics to produce coincident ambisonic output signals.
This arrangement has been designed with an understanding of the tradeoffs associated with each decision. In this article we will be exploring Spatial Mic technical specifications, ambisonic microphone behavior and other factors that go into the creation of this versatile tool.
Note that we will not dive deep into the math involved in the derivation of some of these specifications. We recommend starting with our Ambisonics Demystified journal article if you are unfamiliar with the difference between traditional channel based audio (mono/stereo & surround) and a spherical harmonic representation as used in ambisonics. For more advanced reading beyond the scope of this article, skip ahead to a selection of technical papers & books available to help you on your journey.
Table Of Contents
Connectivity
Setup is very important for multi-channel microphones. While many microphones provide analog mic level signals on separate wires, Spatial Mic Dante and USB both provide digital output.
All microphone amplification and analog to digital conversion is done within Spatial Mic itself for much better consistency than comparable microphones. While not only presenting a cost advantage, purity of signal can be maintained regardless of the end use case.
Both Spatial Mic USB and Dante provide single wire connectivity as follows:
Spatial Mic Dante
Plug a CAT5e/6 (shielded recommended) into the locking Neutrik EtherCon connector on the bottom of the microphone. The other end should plug into either a PoE injector or PoE enabled switch. The injector or switch may then be plugged into a Dante audio network.
The device complies with IEEE 802.3af PoE Class 0 power sourcing equipment that uses Alternative A or Alternative B options. The power consumption is 4.8W max.
Spatial Mic USB
Spatial Mic USB connects via USB 2.0 and may be bus powered via its USB-C connector when connected to a host, or alternatively get its power from the +5v mini-USB jack. When ADAT lightpipe is used, power must be provided from either USB jack.
Microphone Capsules
A great recording begins with a great acoustic sensor. That’s why the capsules for both Spatial Mic USB & Dante have been carefully chosen to provide the best performance in their class.
Spatial Mic USB & Spatial Mic Dante use completely different capsules. While both microphones feature 8 prepolarized cardioid (uni-directional) condenser capsules (AKA Electret) with a permanently charged backplate, specifications like size, Signal to Noise Ratio (SNR) & max SPL differ.
Spatial Mic USB features a capsule with SNR of 72 dB-A while the Dante version uses 78 dB-A SNR capsules. This equates to self-noise per capsule of 22 dB-A and 16 dB-A respectively.
Spatial Mic Dante features capsules with up to 136 dB-SPL handling capability for loud sound recording. Spatial Mic USB capsules have an SPL handling of 120 dB-SPL for THD < 3% (however in pad-mode, the ADC clips up to 131 dB-SPL to record loud sounds).
Here is the measured frequency response of the capsule used in Spatial Mic Dante:
How do these capsules compare to other ambisonic microphones? Take a look at a common capsule used in other ambisonic microphones and note the SNR of only 65 dB-A. While ambisonic microphones require combining and filtering the signals from all of the capsules at the same time, it stands to reason that the better the capsule we begin with, the better the final sound quality.
As we will learn in the next section, the per capsule self-noise influences the final self-noise for the entire 8 capsule array.
Array Microphone Self Noise
A common misconception is that the more capsules in a device, the noisier that device will be. In reality, an array like Spatial Mic can produce lower self noise than a single capsule on its own.
An array of N equal microphone capsules increases the array gain by a factor of N.1 Converted to decibels this would be 10Log(N).
Thus the SNR of the entire array can be written as:
(1) 
Where N is the number of sensor (capsules) elements. Using equation 1 and solving for an 8 capsule array with per capsule SNR of 78 dB results in a total array SNR of 87 dB. In fact, every time the number of capsules is doubled, SNR increases by 3 dB.
The above holds true for signals that add coherently and noise that adds incoherently.
With that said (and this is why it’s hard to actually quantify self noise of an ambisonic microphone) it becomes difficult to compare ambisonic microphones of different order because of bandwidth and the processing involved to create Higher Order Ambisonics (HOA). Think of it this way: First Order Ambisonics (FOA) obtained from an 8 capsule array has 3 dB better noise than if it were obtained from a 4 capsule array.
Also note that Voyage Audio does not employ any dynamic processing to enhance the signal to noise ratio. For example, a manufacturer may choose to improve SNR by automatically switching to first order ambisonics below a certain audio threshold. In our opinion, if you are recording very quiet audio, it is instead best to use first order ambisonics output or the “LN” filters built-into Spatial Mic Converter plugin from the get go, without relying on any unnecessary dynamic processing.
Conversion Filters & Virtual Mic Patterns
As we discussed in the introduction to the article, when you record the unprocessed sound from the 8 capsule array of Spatial Mic, you place these 8 channels on individual tracks or within a single multichannel track. This audio is then converted using the Spatial Mic Converter plugin to ambisonics and 
then possibly further decoded as needed for the end format of choice.
While Spatial Mic Dante can encode the unprocessed capsule signals to first order ambisonics internally in DSP, the highest sound quality will come from using the Spatial Mic Converter plugin to transform the unprocessed audio from the microphone capsules. Using the Spatial Mic Converter plugin opens up output options for first and second order ambisonics, up to 7.1.4 ATMOS surround sound and a number of different virtual mic patterns. This will also give you the most flexibility in post to aim the microphone to the direction of interest.
At the heart of Spatial Mic Converter plugin are the conversion filters that transform the capsule signals to ambisonics. While different ambisonic microphones use different methods of filtering, after years of research, testing and refinement Voyage Audio has settled on a 64 element matrix that uses measurements from an anechoic chamber.
The Spatial Mic Converter plugin includes multiple filters. It is best to use your ears and decide for yourself which filter sounds the best for the specific piece of audio you are working on. For Spatial Mic USB, Type 1 and Type 2 filters use our original method of measurement and signal processing, while Type X and Y use a newer method. Type Y targets a more diffuse response while Type X relies on less processing. the “LN” filters target lower noise recordings. Spatial Mic Dante currently ships with Type A and Type B filters, the latter of which also targets a more diffuse, flatter response.
The output stage of the Spatial Mic Converter plugin can create various types of virtual microphones. Creating virtual mics from ambisonic signals sometimes seeks to maximize the focus of the pattern in the direction of the most energy. The coefficient that describes this is often referred to as rE. The virtual mic output stage can be thought of as a combination of the ambisonic signals, with various weightings applied. Let’s take a look at the different types available.
Figure 8: Traditional Omni → Cardioid → Figure 8 microphone patterns
Basic: Virtual mic patterns capable of second order cardioid (pattern = 2)
Max rE: Maximizes energy concentration vector by focusing energy signals in the direction of interest.
In-Phase: Full side-lobe suppression with no out-of-phase components
The following chart shows the characteristics of each virtual mic pattern type.
Decoded Polar Patterns
An ambisonic microphone aims to decompose the soundfield into its spherical harmonic components. After converting the unprocessed signals from the capsules, each audio channel of an ambisonic signal carries these components. Here are the measured First Order Ambisonic polar patterns from Spatial Mic Dante implying the Type A filter:
Note that these plots are taken from measurements and do not employ any octave band smoothing. Just like traditional microphones, the patterns shown do deviate from ‘perfect’ at some frequencies. Voyage Audio has chosen to rely more on sound quality from listening tests and user feedback versus applying large amounts of frequency dependent boost, which can create unwanted noise and other artifacts.
When it comes to polar pattern decoding for mono and stereo applications, here is the first order cardioid decode from Spatial Mic Dante:
These show how critical the signal path is from microphone to conversion – if even one analog preamp is slightly off as gain is changed, the decoded polar patterns and spatial resolution will be different than published. This is why the Voyage Audio Spatial Mic employs digital outputs with built-in programmable gain amplifiers (PGAs), along with sensitivity calibration stored internally within each microphone. With Voyage Audio Spatial Mic, the user does not need to worry about multi-channel analog gain staging and can instead concentrate on placement, decode style and other aspects of the audio capture system.
Going Further
Ambisonic microphone arrays present unique challenges to manufacturers and customers when it comes to understanding the technical specifications involved. We hope this article will illuminate more aspects of Spatial Mic. If you have any questions please feel free to reach out to us at info@voyage.audio. Finally, here are a few resources if you would like to continue learning about ambisonic microphones:
- Ahrens, Jens, Ambisonic Encoding of Signals From Equatorial Microphone Arrays. 2022.
- Rafaely, Boaz, Fundamentals of Spherical Array Processing. Springer Berlin, Heidelberg, 2015.
- S. Bertet, J. Daniel, and S. Moreau, 3D Sound Field Recording with Higher Order Ambisonics – Objective Measurements and Validation of Spherical Microphone. Paper 6857, (2006 May.).
- Zotter, Franz and Frank, Matthias, Ambisonics: A Practical 3D Audio Theory for Recording, Studio Production, Sound Reinforcement, and Virtual Reality. Springer 2019.
References:
1. H. Johnson and D. E. Dudgeon, Array signal processing: concepts and techniques. P T R Prentice Hall, 1993.
Learn more about Spatial Mic:
https://voyage.audio/spatialmic
Where to buy Spatial Mic:
https://voyage.audio/where-to-buy
