Soundfield Microphones and Choir Broadcasts

Last November I was involved with a number of choir concerts that had to be broadcast. As anyone who has done broadcast knows, there is a definite battle with trying to get the ideal sound while also keeping sight lines clear from the array of microphone stands. Additionally, with choirs there is the issue of getting a cohesive ensemble sound that isn’t too distant, but also doesn’t have individual voices standing out. It is atypical to mic a choir with the same density that one might set-up microphones within an orchestra.

The Covid-19 pandemic has added a further wrinkle to these issues. Social distancing has mandated that musicians be larger distances apart from one another. For orchestras and wind bands, this has tended to push more musicians towards the walls of the stage, or caused a shift towards more chamber music. For choirs — which compact more people into smaller spaces than instrumental ensembles — this has caused them to spread out, and with the number of people often involved this spread can require more space than a stage allows.

This was the case for the choir concerts that I was involved with. The first set of concerts were recorded to tape in a number of sessions, one per choir (5 choirs total). The smaller ensembles were able to perform — with social distancing — on-stage. For the larger ensembles, the decision was made by the directors to disperse the choir into the audience area.

An… “interesting”… effect of the scheduling and the logistics process (tech was brought in after many of the decisions were finalized) was that we had limited time for any soundcheck and transitions between ensembles. For the ensembles on the stage, the permanently installed recording array in the venue proved to be usable; however, we were still left with the issue of how to approach the ensembles that were in the audience seats (and balcony, as we found out very late into the process).

In an effort to minimize the number of microphones used, as well as gain coverage and flexibility, I put forward the idea of using an array of soundfield microphones to cover the audience area. Between my personal collection and the university’s equipment store, there were 4 ambisonic microphones available to us (3 NT-SF1 and an older Soundfield, with the hardware decoding unit). We came up with a plan to put out the microphones in the space in a way that would give us adequate coverage of the area: a 600-seat auditorium, with the balcony.

The soundfield microphones allowed for the dynamic synthesizing of virtual, directional microphones aimed and balanced in any direction. Using this flexibility we were able to create virtual microphone arrays that were suited to the disposition of each ensemble — with the four ambisonics microphones bringing 16+ virtual microphones to the table.

This flexibility allowed for us to manage the changing choirs, and adjust as the directors moved voices around in the hall to suit the blend they wanted based on where they were standing.

This technique was revisited for a combined orchestra/choir concert a couple of weeks later. The venue for this concert was a 3,500 seat performance venue, and featured the full orchestra and the combined choirs. The orchestra was placed on stage, and the choir was dispersed into the auditorium.

In this venue, the height difference between the stage and the floor became a concern. Additionally. the space available for placing microphone stands was more limited compared to the smaller venue. Finally, the need for cameras to be able to see both the seating area and the stage enhanced sightline concerns.

Three microphones were placed in front of the combined choirs. The height was set to compromise between the sound of the stage and the choral sound (further complicated by the effect of the masks the choir wore). Virtual microphones were again used to balance different parts of the stage and choral sounds. We also placed a soundfield microphone on-stage, in front of the conductor to provide some detail for the orchestra, and to reinforce instruments towards the back of the stage.

Since this concert was live, I wrote some automated processing (using Max/MSP) to handle the synthesis of the virtual microphones. We brought the A-format signal from the microphones into the console, then used a USB card in the console to transfer the sound to a laptop which performed the A-to-B-format conversion and the synthesis of the virtual microphones, and then transferred the decoded signals back to the console for mixdown and output to the production center.

The effect was quite successful, despite a very truncated load-in and sound-check, and allowed for a quick break down of the equipment. While many people have found the “immersive audio” aspects of these microphones interesting, I think that their practical applications within traditional studio recordings are also worth exploring and that they have been an under-used resource for audio engineers.