Development of Ambisonic Microphone Design Tools - Part 1 (Poster)

Abstract

Poster for Associated AES Paper

Full text

Development of Ambisonic Microphone Design Tools —Part 1
Authors : Charles Middlicott, Dr. Bruce Wiggins
University of Derby, UK
E Brief # 489
EB07-7
[1] Mathworks Inc, “〖MATLAB〗^® App Building R2018a” http://bit.ly/MathworksGUI
[2] H. Tokuno, O. Kirkeby, P. Nelson, “Inverse filter of sound reproduction systems
using regularization” IEICE Transactions on Fundamentals of Electronics,
Communications and Computer Sciences, Vol. 80, No. 5. 809-820. (1997).
[3] Heller, A.J; Benjamin, E. M; Calibration of Soundfield Microphones using the
Diffuse-Field Response. In Audio Engineering Society Convention 133, 8711, 2012.
AMATLAB•routine has been developed that simulates the response of an array given
a specific set of design attributes, these are set by the user via a graphical user
interface (GUI). These attributes (shown below) are used to generate a set of impulse
responses (IRs) that can be used to evaluate an arrays theoretical performance.
Introduction
A theoretically ideal ambisonic microphone array would have…
oCoincident Microphone Capsules
oCapsule w/ Flat Frequency Response (Regardless of the angle of incidence)
It isn’t physically possible to have coincident capsules as multiple capsules cannot
occupy the same physical space. Additionally, capsules even when matched, won’t
exhibit the desired frequency and polar response across the audible frequency range.
Q : How can we Attempt to account for these physical inadequacies?
A : Informed Design & Pre Filtering / Post Filtering
With the advent of spatial audio for virtual reality and 360°video, software tools have
enabled users to synthesize higher order ambisonic material in Digital Audio
Workstations (DAWs) in order to create immersive, full-sphere, sound fields.
The availability of microphones capable of capturing such signals has not grown at the
same pace, or affordability. This work aims to provide the tools needed to successfully
develop such arrays; gaining a greater understanding of there performance capabilities
and limitations.
The AADT is an application that packages the array simulation routine into a standalone
GUI using the MATLAB•GUI Development Environment [1].
The current incarnation of the AADT utilizes the Equal-Angle sampling scheme in the
horizontal plane only (thus simulating only circular arrays). The reason for opting for
horizontal only capability in the first instance was so factors such as spacing, calibration
and filtering could be evaluated with greater ease. Findings could then be used to inform
the development of a 3D spherical array.
The AADT is currently capable of generating plots of the following five responses.
An Ideal Ambisonic Microphone
MATLAB®Array Simulation
The Ambisonic Array Design Tool (AADT) AAET (Continued)
Figure 1 –AADT –Polar Response Figure 2 – B-Format Polar Response
Figure 3 - Model of
3D Printed Prototype
Fig. 4 –AAET – B-Format Polar Response
References
Contact Details
Charles James Middlicott
University of Derby, UK
Web -charlesmiddlicott.co.uk
Email -charles@charlesmiddlicott.co.uk
DEMO THE APP OR SCAN
THE QR CODE TO
DOWNLOAD THE
STANDALONE GUI
oAmbisonic Order N
oArray Radius (mm)
oArray Type (2D / 3D)
oSampling Scheme
oCapsule Directivity Factor
oDesired IR Length (samples)
oSample Rate (kHz)
oVirtual Source Distance (m)
oSource Increment (Degrees ˚)
Future Work
Implementing Additional Features
oFull 3D Simulation capability
oScattering on a Rigid Sphere
oSampling schemes
ØNear Uniform
ØGaussian
ØSpherical T-Designs
oFrequency dependent directivity factor
oAdditional Pre-Filtering Methods
ØAveraged Gain Matching
ØDiffuse Field Equalization [3]
oPost Filtering of SH Signals
oCapsule Specific Radius Calculator
Simulated Capsule Signals
ØTime Domain
ØFrequency Domain
ØPolar Response (See Fig 1)
B-Format Signals
ØFrequency Domain
ØPolar Response (Fig 2)
The Ambisonic Array Evaluation Tool (AAET)
The Ambisonic Array Evaluation Tool differs to its counterpart
in that its purpose is to evaluate / validate a physical arrays
performance against a comparable simulation.
In conjunction with the development AAET application a five
channel prototype array was developed (Fig. 3). A set of IRs
were measured in an hemi-anechoic chamber, These IRs were
loaded into the AAET via a .mat file for direct comparison.
Along with the array performance being evaluated it is possible to visualise the
response of the individual capsules, pre and post filtering.
Two initial approaches to pre-filtering have been implemented in this application.
These are the optimisation of the capsule signals by means of on-axis capsule
calibration, with the desired on-axis response being either flat across the audible
frequency range or matching of capsules to an individual capsule response.
These filters are
calculated within the
application by utilising
Nelson -Kirkeby
Inversion with
Regularisation [2].
The effect of this Pre-
Filtering can be viewed
by selecting the desired
filtering approach from
a drop down menu