The Can and Feedback Microphone
October 12, 2011
This system builds upon past work I have done with feedback and performance systems. Specifically, I sought to create a way of tracking audio motion to let brief moments of sound into a system, and then to find ways of tracking audio motion to meaningfully control the sounds in the system.
My solution for determining when to let sound into the system involved tracking high gain peaks at a contact microphone or piezo microphone. The contact mic is placed inside of a paint can attached to the bottom surface. Over the top open end, I stretched a piece of rubber and tightened it around the opening of the can. This provides an object with two surfaces of distinctly different sounds that a performer can “play” and achieve decent acoustic sounds with. The sound is then routed through SuperCollider , which tracks the audio signal from the paint can drum. The audio from this drum is never introduced into the digital audio signal in the system, but is instead used purely for control signals. Upon receiving an audio signal that registers as a 1.0 or higher SuperCollider opens an envelope on a second microphone for a brief moment.
For a second microphone I have been using a Shure SM58 dynamic microphone placed at a height equal to that of the performs mouth and pointed directly at the center of the output from the stereo speakers. The performer is free to choose between: creating his/her own sounds by singing into the microphone at the time of the peak strike on the paint can drum, striking the paint can drum by the microphone in order to allow the acoustic sound of the drum through or simply allowing a brief moment of controlled feedback to occur.
Upon entering the system, this moment of sound is routed directly to the output, and simultaneously to two separate delay lines. The first line (DelayLine1) is made of two delays, each outputted to an individual speaker and consisting of 5 and 6 second delays, respectively. The mic signal is also routed to another delay line (DelayLine2) with delays of 2 and 3 second, respectively. DelayLine2 also has its individual delays output to separate channels.
The paint can drum serves a second role once sound is in the system. In addition to initiating sounds in the system, the paint can drum serves as a amplitude output control for DelayLine2, by rubbing scratching or otherwise playing the drum in a way as to create an amplitude signal between 0 and 0.9 in SuperCollider. Upon receiving amplitude data from the paint can drum, SuperCollider scales this data to control the output of DelayLine2 to the main speaker outputs. This allows the performers to “play” the system by playing the drum.
The additional microphone also is always having audio tracking data being extracted from its signal. Similar to the paint can drum, the additional mic’s amplitude is being tracked and mapped to the amplitude output of DelayLine1. DelayLine1’s output is routed to the main speaker output and also into the input of DelayLine2. This amplitude tracking on the mic creates an additional feedback loop by allowing the amplitude from the speakers to make it more difficult for the mic amplitude to become 0. Due of this, sounds never stop immediately, when heard from the speakers, but instead fall off.
The mic signal is also being watched in SuperCollider for amplitude peaks similar to that of the paint can drum. Upon receiving a peak signal, SuperCollider triggers an envelop over the DelayLine1’s delay length. Over the course of 10 seconds the delay length is shortened by a scale of 1 : 0.05 (one line goes from 3 to 0.15 second delay and the other from 2 to 0.1 second delay). This creates increased tension through shorter delays and a raising pitch effect.
Finally the mic is also tracking frequency. This scales roughly to the overall pitch of the entire system output. This scale should also be adjusted within the code for a performers vocal range. By creating higher pitched sounds in the mic, the system roughly raises the entire pitch of the output. Likewise by creating lower pitches in the mic the system lowers the pitch of the system output.
I’ve tried to create an audio motion capture process that is interesting to play for me through interconnected processes. This is a system that takes some practice to use, but allows for rich audio outputs and control over relative pitch and amplitude of the two delay lines. It is also worth noting that I purposely never introduce the sound from the paint can drum and instead intend that the sound of the drum being played and used for control, the sounds made by the performer in the mic and the sounds from the speakers mix in the room to create the full ensemble sound. This is also a system that I envisioned as being capable playing with other instruments because of its ability to have a lack of defined rhythm and the built in amplitude frequency spectrum controls.
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