Meeting Topic: Microphones: The Physics, Metaphysics, and Philosophy.
Moderator Name: David Weinberg
Speaker Name: Ron Streicher, Past President of AES
Other business or activities at the meeting: Round table presenting of audio interests of the attendees
Meeting Location: NPR Headquarters, Washington DC
DC Section Meeting Summary- 22 Sep 08.
On 22 September 2008 27 members and guests converged on the boardroom at NPR Headquarters to listen to and watch Ron Streicher in his presentation called Microphones: The Physics, Metaphysics, and Philosophy. Ron is past president of the Audio Engineering Society, and is an independent audio consultant and recording engineer, specializing in live performances. For 18 years he was associated with the Aspen Music Festival — 10 of those years as Audio Production Manager. It would be difficult to find anyone more heavily engaged in classical music recording than Ron. His book, The New Stereo Soundbook is now in its third edition.
Before the presentation, Dave Weinberg, section Chair, invited everyone present to introduce him/herself and share their audio interests. There was some useful "networking" that resulted from this exercise, and it will be continued at future AES meetings.
Ron began his presentation by describing a recording as a "sonic illusion." The corresponding audio production is the science and art of creating that illusion, which hopefully convinces you that you are there at the concert hall or performing space, or the performers are here in your space. He likened the recording to a painting, where silence is the canvas, sound sources are the colors, and the microphones are the paint brushes. The illusion may be a re-creative one or a creative one: with the latter, the final illusion comes together only after it emerges from the loudspeakers. A fundamental key is the proper selection and use of the microphones. Ron briefly outlined the various types (dynamic, condenser, ribbon), and their basic polar patterns (omni, cardioid, dipolar). He then applied the types and patterns to various recording situations, using some of the unique microphone qualities to enhance or limit characteristics of the spaces they are used in. For example, the null of a dipole microphone can be "pointed" at an interfering sound source rather than simply pointing the main lobe at the performer. Ron illustrated this with the common situation of recording of a choir situated behind the orchestra: ordinarily the orchestra would dominate the recording, but by placing the dipole over the orchestra the null can be pointed at the orchestra to maximize the pickup of the choir while minimizing the orchestra. Several other examples of null-pointing were shown. It is important that the microphone response off-axis is acceptably flat in order to not spoil the sonic illusion. Ron reminded us that polar patterns are three-dimensional.
Other concepts that were discussed included the critical distance of microphone placement, where direct energies and the total of all other energy (reverberation, ambient noise, etc.) are equal; the inverse square law; comb filtering; multiple microphone spacing distance rules; and some practical considerations concerning rigging and visual issues. His demonstrations included multiple microphone comb filtering, effectiveness of shock mounts, and the audibility of pattern shapes by close-talking while rotating the microphone.
Ron next covered some psychoacoustic issues. How we hear stereo depends upon differences in the intensities and/or time of arrival of sound as it arrives at the two ears. He described and demonstrated how panning a mono source may be achieved not only by varying the amplitude between the two channels, but instead by delaying one of the signals (the image shifted to the left with introduction of as little as 0.2 ms of delay in the right channel). Signal delays are also useful in large venues to create the illusion that the sound is coming from the stage and not from the nearest speaker.
As usual, there were a number of questions following Ron's presentation, and he willingly stayed beyond the planned time for several one-on-one conversations.
Note: Photo sent separately to Bill McQuaide. Caption: Ron Streicher demonstrating comb filtering effects using spaced microphones.