By Frank Lockwood
*The recordings described here were done at the Church of St. Mary Magdalene in Toronto from January to May, 2009. This article is based on a presentation given by Frank Lockwood to the AES Toronto chapter in January, 2014. *
Traditionally, classical music has been recorded on location to benefit from suitably reverberant acoustics found in concert halls and churches. It is often recorded direct to two-track/stereo using little or no processing beyond high quality microphone pre-amplification. The thinking is it’s best for the technology to “get out of the way” so that the purest sonic representation can be captured with the closest thing to a “straight wire” path between microphone and final distribution.
As part of a larger CD recording project, this recording started out following these precepts of “minimalist” recording, but then, due to the music’s unique requirements, departed from it in significant ways, particularly in the post-production.
This CD featured the music of composer Healey Willan, as performed by the Choir of the Church of St. Mary Magdalene, where Willan was director of music and composer in residence for nearly 50 years. This unique acoustic environment and Willan’s music are inextricably bound together, and so capturing the total sound of that choir in that environment was a crucial aim for this recording. The CD was to include some pieces that had never been recorded before, including “The Reproaches.”
Impulse Response Reverberation Capture
While teaching audio and music recording at Ball State University in Indiana, I developed and refined a method of collecting the acoustic reverberation signature of performance spaces – the Impulse Response. Once captured, it can be used with an IR Reverberation plug-in in a DAW, applying that particular character to any recording. Given the sound quality in St. Mary Magdalene and its importance to this project, I was eager to do the same thing here.
I went into the church in December of 2008 to make the necessary recordings. The process begins with a test signal, a sine tone swept from 20 to 20,000 Hz, played through speakers in the hall. A recording is made with microphones placed at various locations in the hall to pick up the test signal along with the reverberation it excites. These recordings are then subjected to a Deconvolution process, which transforms them into something that sounds like a single impulse with a trailing reverberation tail. When loaded into an IR Reverberation plug-in, the character of the hall’s acoustics can be applied to any recorded sound I might wish.
My location recording equipment included a Millennia Media HV3D microphone preamp, RME Fireface 800 audio interface, and a MacBook Pro running Merging Technology’s Pyramix software. A pair of DPA 4003 microphones was used and test-tone playback was through a pair of Dynaudio BM5a speakers.
At first, the microphones were mounted on a single stand, with their capsules spaced 40 cm. apart. Recordings were made six, 12, and 20 metres from the speakers. Then the microphones were placed on two separate stands four meters apart, again capturing the sound at distances of six, 12, and 20 metres. A third method used the Optimized Stereo System, invented by Jürg Jecklin, which places a 30.5 cm. disk between two omnidirectional microphone capsules placed 16.5 cm. apart. These dimensions are roughly the same as the average human head, such that the disk baffles sounds arriving from the sides, affecting frequencies of 700 Hz. and above, but without the limitation to headphone playback of binaural or “dummy head” recordings.
In the diagram, you can see that the sound file of the Test Signal is sent out in digital form to the audio interface where it is converted to an analog signal and played through the powered speakers sitting in the room. This sound travels into the room, directly towards the microphones as well as to the walls, ceiling, and floor where it is reflected, creating the reverberant sound. The signal is captured, converted into electrical signals amplified by the preamp, then digitized by the interface and sent to the computer for storage as a sound file.
Here is a view of the session in Pyramix. Track 1 is the test signal, output to the speakers. Tracks 2 and 3 show the recording obtained by the microphones. The waveforms are displayed in a logarithmic (decibel) scale, approximating how our ears react to sound.
This image shows the result of the Deconvolution process. Rather than a swept tone, the sound is of a single impulse or click followed by the decay of the reverberation tail.
Part 2 will run in the June 2014 issue of Professional Sound. An expanded version of this entire article, with audio examples and more pictures, can be found online at www.LockwoodARS.com/production. Frank Lockwood is the owner/operator of Lockwood ARS. Based in Toronto, he specializes in classical and acoustic music recording, editing, mixing, restoration, and mastering.