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 Forum index » Discussion » Composition
Capturing Feedback in a Bottle
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Acoustic Interloper



Joined: Jul 07, 2007
Posts: 2013
Location: Berks County, PA
Audio files: 86

PostPosted: Sun May 25, 2008 1:15 pm    Post subject: Capturing Feedback in a Bottle
Subject description: Persistent virtual feedback loops that capture and store acoustical feedback
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I'm not sure if this belongs in Composition, but it is definitely sound design, so here goes.

See diagram below. I have been working with various delay FX within Live and Max/MSP lately, mostly with repetitive delays of multiples of my banjo finger picking speed (finger picking, the original digital delay). The delay loops inside the computer have the usual wet / dry and feedback parameters. Live's built-in delay FX max out at 95% feedback so that they won't, in themselves, go into oscillation.

I had been working my compositions and sonics out with headphones , partly to hear "what will come out the loudspeakers" (using isolating headphones to minimize the dry, unprocessed banjo or guitar that comes from having the instrument right next to me), and partly to spare my family from my less successful experiments.

Anyway, a few weeks ago I moved the setup to a room where I can drive a set of amplifiers/speakers, so I can get the sound out into the room and check for feedback problems. Because of the feedback within the software signal paths, even though it is less than 100%, this setup is extremely sensitive to acoustical feedback in the room. If the room, speaker placement and mic placement tend to feed back and go into oscillations, then the software feedback gives them an almost impedance-free path in which to go into oscillation. The particular mic I am using is very sensitive and perhaps not as directional as I would like, but it reproduces banjo tone rings wonderfully, so this is partly an exercise in figuring out how to use that mic in live performance.

Anyway, sure enough in a real room with loudspeakers, there is enough regenerative feedback to start oscillations. My son Jeremy pointed out the low frequency aspect of this, and suggested using the mic's switchable built-in high-pass filtering, which I finally got around to doing yesterday. That helped some, but I also added an EQ within Live's virtual feedback path, set up as a high pass filter that allows me to notch out the room's + instrument's worst resonant frequency range. It worked, so that if I then increased mic sensitivity or output drive, the next resonant frequency for feedback was higher up. So far, nothing new for most people on this forum, I imagine.

But part of the virtual feedback setup had been the ability to foot switch from one nested virtual delay path to another while playing. Previously, with headphones, I had already been "trapping" live samples with 95% delay feedback via the footswitch. The two differences now are:

1. Room acoustical feedback gets trapped within this live sample, and

2. I can use EQ and other FX to processed this "trapped" room feedback. I can play the room feedback, not just immediately as guitarists have been doing for years, but also by trapping and processing it!

Which leads to the point of this post: Has anybody used or seen the technique of going around and capturing acoustical feedback of rooms at resonant frequencies of those rooms, and using this as the basis for a virtual instrument? Presumably captured as persistent samples, although what might be even better would be to capture the acoustics of the rooms as physical models for later acoustical simulation of those rooms, i.e., regenerating the acoustic feedbacks rather than simply sampling them.

I think there may be a research paper and some novel instrument design here.


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Acoustic Interloper



Joined: Jul 07, 2007
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Location: Berks County, PA
Audio files: 86

PostPosted: Tue May 27, 2008 5:44 am    Post subject: Reply with quote  Mark this post and the followings unread

I played with this a bit more over the weekend, and at least for me, the utility of it is probably more for improvisation than for persistent sound capture. Basically, I just let the room feedback build up to some (possibly annoying but) safe level, trap it in a virtual software feedback configuration while killing the mic -- so now I have delayed, recycling acoustical feedback, and then proceed to process it into a phrase within a piece. As it dies out, I can do it again.

I have something new to practice Smile

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PostPosted: Tue May 27, 2008 5:14 pm    Post subject: Re: Capturing Feedback in a Bottle
Subject description: Persistent virtual feedback loops that capture and store acoustical feedback
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Acoustic Interloper wrote:
Has anybody used or seen the technique of going around and capturing acoustical feedback of rooms at resonant frequencies of those rooms, and using this as the basis for a virtual instrument?


There is a classic non real time version, sort of: http://en.wikipedia.org/wiki/I_am_sitting_in_a_room

Quote:
[...]featuring Lucier recording himself narrating a text, and then playing the recording back into the room, re-recording it. The new recording is then played back and re-recorded, and this process is repeated. Since all rooms have characteristic resonance or formant frequencies (e.g. different between a large hall and a small room), the effect is that certain frequencies are emphasized as they resonate in the room, until eventually the words become unintelligible, replaced by the pure resonant harmonies and tones of the room itself.


Hearing forward for the results of your experiments!

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also .. could someone please turn down the thermostat a bit.
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Acoustic Interloper



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PostPosted: Tue May 27, 2008 7:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

Listening to the original recording now, thanks for the reference!
Quote:
"I regard this activity not so much as a demonstration of a physical fact, but more as a way to smooth out any irregularities my speech might have," referring to his own stuttering.

The stuttering is especially interseting to me because, when I first started practicing entraining my finger picking to audio delays fed back into my ears (yet another feedback loop, from fingers through banjo to headphones to brain to fingers), I mentioned it to my engineering nephew, and he said that his mother / my sister-in-law, who is a speech therapist, had told him about using delayed audio as a therapy for stutterers. Searching "sound delay audio feedback stuttering" leads to a number of interesting articles and software products.

This project started out as trying to find a way to minimize feedback when I play at EM2008, but, if I can refine and practice the technique somewhat, I may switch mics and monitors off and on and try to use it. If not by EM2008, then EM2009, for sure Exclamation

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