AES Section Meeting Reports

District of Columbia - March 11, 2011

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Transcending The Acoustic Space
by Fred Geil (AES-DC section Secretary)
At the 11 March 2011 meeting of the AES-DC section in American University's Katzen Arts Center, "Transcending the Acoustic Space" was the subject of a presentation by David v.R. Bowles, who explained that in the pursuit of realistic-sounding recordings, simple "direct-to-stereo" is often inadequate to provide a believable acoustic space — the sound in the space between the notes. He promised to reveal some of his techniques for achieving the goal of spatial believability.
Jim Mastracco, AES-DC executive committee member, introduced our guest: Bowles received audio training from Tony Faulkner, John Eargle and the Edgar Stanton Audio Recording Institute at the Aspen Music Festival. He received Bachelor and Master degrees in violincello from the Juilliard School and a certificate in early music performance from the Royal Conservatory in The Hague, Netherlands. For many years he performed and recorded as cellist with period-instrument orchestras and chamber-music ensembles. He is an officer of the AES-San Francisco section and a National Academy of Recording Arts and Sciences (NARAS) member. He has recorded 500+ performances resulting in more than 75 commercial releases. Bowles is founder and head of Swineshead Productions (Berkeley, CA).
Bowles began with a question: Do we want to capture the natural acoustics of the space, or be creative? The answer: Both! His sound clips demonstrated how the acoustics were captured and/or augmented for several solo instruments (violin, cello, bassoon) and ensembles (piano trio, trumpet with chamber orchestra).
He also played a recording of a nine-member vocal ensemble.
His basic microphone array varied among the several configurations, but typically consisted of a XY cardioid pair with flanking omnis, plus spot microphones as needed.
Room ambience was captured by rear-facing directional microphones located near the main array.
He recorded each mike on its own track, without processing; the acoustic "space" was enhanced/created in post-processing. Most of the demos were of location recordings.
Apart from the microphone array, important considerations included the acoustics of the spaces, captured or added electronically, and the acoustic properties of the instruments — primarily directivity.
An example of instrument directivity revealed that a violin with more of an "edgy" sound can be tamed by facing the microphone at the scroll rather than looking to the side from above at the top plate (table) of the instrument.
To further illustrate the effect of instrument acoustics, Bowles recorded four sessions at the Green Music Center (Sonoma State University, CA). The sources were solo violin, solo cello, solo bassoon, and piano trio. Each of these examples was accompanied by his descriptions of the respective microphone placements. He also illustrated the use of a cello platform, which decouples the instrument from the stage. In recording the solo bassoon, considerations included key noise and the fact that the sound at the top of the instrument differs from the sound lower down; this resulted in the need to blend the two microphone positions during post production. Another demo featured a piano trio in two different spaces, plus the different sound of period instruments versus modern ones.
Hall characteristics affect the ratio of direct to reverberant sound at the microphones. If the hall is dry, microphones can be placed farther from the instruments; however the greater the distance, the more monaural the sound becomes — more of a problem with large ensembles. As partial compensation, additional outboard microphones can widen the sound stage (Bowles prefers a fairly wide sound stage).
Ambience can be enhanced by processing the microphone outputs to emphasize the rear lobes, as well as by introducing rear-facing mikes into the mix. These derived outputs can be further enhanced with electronic reverb.
Recording a trumpet-soloist with chamber orchestra presented Bowles with additional problems. No solo microphones were needed for the trumpet, but the english-horn soloist required his own microphone. This required careful level-matching of the two instruments' recordings. Also, different movements were recorded on different days and times, plus the platform could not accommodate all of the instruments, which required separate miking of the off-platform players; because the off-platform players and mikes were to each side of the platform, this helped widen the sound stage. His success in overcoming these problems was evident in the playback.
The problem of a "dry" space that needed reverberation was presented; the group was a nine-voice choral ensemble. There was not enough hall sound picked up by the microphones, including the rear-facing surrounds, so Bowles created an artificial reverberation using the outputs from the main microphones processed through an external reverb processor. The result sounded quite authentic.
Bowles last demos came from the scoring stage at Skywalker Ranch: Nadia Salerno-Sonnenberg (violin) with the New Century Chamber Orchestra, and Elizabeth Blumenstock (violin) with Nicnolas McGegan conducting the Philharmonia Baroque Orchestra. Several diffusers were used, and platform decoupling was needed for the basses because the cable space under the floor gave them too much resonant support. With different mike placements, the first recording gave a close-miked 'studio' sound while the second had a more open 'church' sound.
The meeting continued with questions and active discussion about many aspects of the recording process and related topics, including headphone monitoring, microphones, sample rates, control room acoustics, and microphone impedance matching.
We thanked Bowles for filling in that space between the notes for us.

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