On the use of spherical microphone arrays in a classical musical recording
scenario
Johann-Markus Batke
Hochschule Emden/Leer, Germany, Johann-Markus.Batke@hs-emden-leer.deAbstract
The usage of spherical microphone arrays in virtual reality production environments have been seen to be steadily
increasing. This is understood to be a natural progression since the microphone array‘s output is easily converted to
the Higher Order Ambisonics (HOA) sound field representation.
However, for the recording of classical music, there is still much debate as to whether the application of sphericalmicrophone arrays makes any sense at all. Depending on the targeted playback format, various established coincident
single microphone setups, as well as spaced setups are preferred. Spaced microphone setups are particularly popularsince they ensure a certain degree of decorrelation for the resulting loudspeaker signals. On the other side, microphonearray recordings have a much higher degree of freedom with regard to spatial post processing.
This contribution investigates how advantages of both recording strategies can be combined. The recording of a string
quartet is used as a well known recording scenario for classical music. A spaced AB recording serves as the basis. Amicrophone array in a central position within the quartet another one distant from the quartet are used to enhance theAB signals. Different target formats including stereo, surround as well as 3D audio using HOA are discussed.

Keywords: 3D Sound, Higher Order Ambisonics, Production tools

1INTRODUCTION

Musical content production aims to reproduce a musical experience when the recorded content is played back on
loudspeakers or headphones. The spatial information in the content is an important component to reach this goal, ofcourse. Spatial information also plays a role for video production. The production of visual content for virtual reality(VR) or augmented reality (AR) often includes 3-dimensional (3D) sound field information. Here it is important thatthe spatial information is physically correct and matches the visual impression.
Also for music recording there is a growing demand in 3D audio representations, however the adaption to visualcontent is often not required. Traditionally channel based approaches like stereo or surround sound are in use. Channel
based representations refer to a specific loudspeaker layout like stereo (2.0) or surround sound (e.g. 5.1). Recording
music using microphones for such loudspeaker layouts leads to spatial microphone layouts. For classical music
recordings often the tonmeister aims to assign one microphone to one channel without further processing. This is thepopular concept of using a main microphone (or Hauptmikrofon) [11].
The audio aquisition for VR and AR is often done using microphone arrays. Unlike the music recording case,the output of a microphone array is always processed and adapted to the target playback system. Higher Order
Ambisonics (HOA) is often chosen as an intermediate representation [14]. As this representation enables spatial
modifications of the soundfield it is an interesting option the the music production case as well.
In the contribution we look at both approaches in a music recording szenario. The "Schunk string quartet" playing
compositions of Michael Schunk was recorded using a AB pair of microphones as well as two Eigenmike microphonearrays. The first is approach is to use the AB recording with additional microphone arrays. The AB serves as main
microphone, the desired result is a stereo recording. The purpose of the microphone arrays is the enhancement theexisting recording used as spot microphones and room microphone respectively. In turn the second approach is touse the microphone array in the middle of the quartet as main microphone and to enhance it using the AB pair. The
desired result is a 3D sound file, using HOA as a format.

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2TECHNICAL BACKGROUND

The delivery format for audio content often determines which recording approach is chosen. In general, audio content
can be delivered in a channel based, a scene based or an object based representation. The audio standard MPEGH reflects the existence of all this approaches [4]. In this contribution we look at channel based and scene based
representations.
2.1 Channel based production
The channel based audio production is based on a main microphone. Well known main microphone settings for stereo
loudspeaker layouts are XY, AB, ORTF, MS or Blumlein [10]. An coincident microphone setting like XY produces
an intensity based stereo image. The spaced AB setting produces a stereo image containing mostly time-of-arrival
information. A ORTF microphone setting produces both, intensity as well as time-of-arrival information. For 3D
loudspeaker layouts microphone settings like ORTF-3D are suggested that refer to a 3D layout with 8 loudspeakers[12].
Spot microphones are usually added in classical recording scenarios to obtain a certain degree of freedom for modifications of the audio content. The spot microphones’ signals need delay compensation that is dependent on the main
microphone’s position. The positioning of the spot in the resulting audio scene (stereo image, if 2D) is done using apanning tool that can address the targeted loudspeaker layout [7, 6].
Coincident microphone settings like MS and microphone arrays need further processing before the signals are playedback on the loudspeaker. In this context, the entire processing chain is termed as beamforming [8].
Stereo playback delivers good localisation information when the recording was performed decently [13]. An AB
recording delivers decorrelated signals resulting in good spatial imaging when played back on loudspeakers, howeverthese signals need additional processing when played back on headphones as the precedence effect is relevant toloudspeaker production [9]. The spatial imaging of coincident recordings works nicely if monitored on head phones.
As a drawback, such recordings may suffer from comb filtering when played back on loudspeakers.
2.2 Scene based production
Scene based production means the creation of a sound field descriptions using Higher Order Ambisonics (HOA) [14].
The recording of natural sound fields for HOA is accomplished using microphone arrays. For 1st order recordings,
such arrays are commercially available since the 1970th like the Soundfield microphone by Soundfield or more recently the Ambeo microphone by Sennheiser among others. For higher orders, microphone arrays like OctoMic byCore Sound or the Eigenmike by mh acoustics are available.
The output of these arrays is converted to the HOA representation providing full 3D audio information. In the contextof virtual environments like 360°videos or augmented reality scenarios the adaption of the audio scene to the visual
scene is feasible using HOA modifications like rotation or spatial warping.
Spot microphones can be used for HOA based productions as well. Like in the channel based case the delay between
main microphone and spot position needs compensation. The directional information is encoded by calculation aplane wave HOA representation. In combination with the HOA decoding step this is like panning for the channel
based case, this time with a panning function defined by the HOA decoding [1].
The playback of a scene based recording to a number of loudspeakers or a pair of headphones is enabled by decoding
the sound field. The spatial imaging depends on the loudspeaker layout or the binaural rendering [5].
To summarise, all spaced main microphone conecpts provide a limited degree of freedom for modifications, whereas
array recordings provide various options for post processing like sound field rotation, spatial warping and beamsteering.

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3PRACTICAL CONSIDERATIONSIn the string quartet recording investigated in this paper we have an AB pair of microphones and two Eigenmike
microphone arrays available. The AB pair and one Eigenmike are positioned near to the string quartet and could be
used as main microphone. To gain inside to specific advantages and drawbacks of each method, both options areinvestigated.
3.1 Material and methods
The recording took place on 3018-02-03 in the "Schweizer Kirche" in Emden. This church is medium sized andprovides acoustically a good environment for classical music recordings. Figure 1 gives a view from outside andFigure 2 from inside to gain a general impression.

Figure 1. The "Schweizer Kirche" in Emden is the recording location for the recordings investigated in this paper.
The church has a decent reverb appropriate to classical music. Photo by Matthias Süßen
As shown in Figure 2 the string quartet is seated in front of the apse. The AB pair of microphones is in front of thequartet, one microphone array (EM1) is surround by the musicians. The second microphone (EM2) is located apartin the middle aisle.

Figure 2. The string quartet is seated in front of the apse. The AB pair of microphones is in front of the quartet, thefirst microphone array is surround by the musicians.
The positioning of the microphones while recording is shown in Figure 3. Note that the pictures show a slightly
different positioning.

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Figure 3. Positioning of the microphones used for recording: in front of the string quartet there is an AB pair ofomnidirectional capsules. Inside of the quartet and apart the microphone arrays EM1 and EM2 are located.

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The recording devices are summarised in Table 1. All soundcards where synchronised using the clock signal of
the soundcard serving as a master. The recording software is Reaper by Cockos. The master slave settings for
simultanious start/stop of all three Reaper instances needed for recording all microphones was dropped due to network
issues, instead all programms were operated individually. The HOA processing is enabled by the "IEM plug-in suite"
as provided by the "institute for electronic music" from Graz [2].
Table 1. Hardware and software used for recording and production.
#221331

hardwareomnidirectional microphone MK 2
microphone array Eigenmike
soundcard Fireface 800
misc. notebooks
software
Digital audio workstation Reaper
HOA VST plugins

providerSchoeps
mh acoustics
RMEApple
Cockosiem Graz

3.2 AB as a main microphone
When the AB pair serves as main microphone, the quartet as well as a certain amount of reverberation is captured.
The "perceived" listening position could be located in front of quartet and AB pair.
The signal of EM1 is then used as source for spot microphone. Four beams are directed to the individual players. Thebeams are calculated at order N = 3. The beams result in four signals that are treated like spot microphones. For thestereo production they are delayed and added to the main signal using the stereo panning tool as provided by Reaper.
The second array EM2 is used in the same manner as EM1. The obtained signals contain room reverberation that ismixed into the main audio image. For surround the EM2 is used as source for the surround loudspeakers.
If a HOA based scene of the AB recording is created, i.e. for headphone playback, the four individual signals can beemployed as object sources that are positioned in the scene resulting by the AB recording. The positioning needs tobe done according to the actual placement of the players. The AB pairs’ signals can be used as main sound sourcesat the respective positions of the AB array supported by 4 individual sources. As the HOA scene is newly created, asufficient high order can be chosen to ensure a sufficient spatial resolution.
3.3 Microphone array as main microphoneUsing the microphone array EM1 as main microphone sets the listening position to it’s location in the recording
scenario, that is the center of the string quartet. The AB pair can be added to this audio scene as two additional plane
wave sources [14]. This is accomplished using the stereo encoder plugin from the IEM plugin suite. It is practical tochose the same Ambisonics order as used for the microphone array encoding (N = 3). As the AB pair is made withomnidirectional capsules, also distance filtering can applied to the pressure signals. They would then appear as point
sources in the resulting scene.
Another option is to translate the listening position of the microphone array to the frontal position as chosen in theformer section. Translations of sound fields are computationally costly in terms of HOA, though [3].

4SUMMARY

A string quartet recording was recorded using a spaced AB setting of microphone. Additional microphone arrays
where used to capture direct sound and reverberation of the room. Two approaches were discussed: firstly the more
traditional approach of music recording, that is the use of the AB pair as a main microphone. Secondly one microphone array was taken as main microphone, the sound field was supplemented by the AB pair.

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