Tuesday, February 17, 2015

Acoustic Resonant Cavities for Electronic Musical Instruments

If you don't plug in your electric guitar and play a chord, does it sound like an acoustic guitar?  Of course not.  Why does a plucked guitar sound different than a plucked violin of the same pitch?  Why does an upright piano sound different than a grand piano?  In the world of acoustics, we often talk about the physical model of an instrument, or the things that make up an acoustic instrument (size, shape, materials, etc.).  In the electronic world, there are many digital physical modeling synthesizers, where the mode of vibration, materials, shape, size, etc, can be programmed to approximate an acoustic instrument.  That is not what the discussion here is about.  Rather, I will talk about my experiments in the study of acoustic resonant chambers, or cavities, on electronic or recorded sound.

Usually, in electronic music, this task is achieved by filtering and effects.  Here, though, I am going to write about actual, physical, real acoustic bodies and objects to run the sound through.

One of the main challenges to this approach is the transducer.  A transducer is the device that changes the one form of energy into another form of energy - in this case, electronic into acoustic energy.  A speaker or microphone can be considered a transducer, as well as record needles, guitar pickups, piezo elements, etc., etc.

The transfer of energy seen through a resonant cavity can be as simple as a speaker resting on a wooden box, facing toward it, with a microphone placed inside.  Likewise, it is a common studio practice to send a signal into an ampliflier and then mic it back into the system - this is called reamping and when using this technique, the room size and microphone placement have just as much influence on the tone as the type of mic and amp used.
Although I use the above techniques from time to time, I mostly make use of a talk box when I run sound into chambers and other spaces, then use various microphones to change the sound back into electronic form.  A talk box is something that is used traditionally by guitarists and inserted into the side of their mouth to make their guitar "talk" - using their vocal tract as an acoustic cavity to shape the sound.
Recently, I have been building various small resonance chambers and cavities for this purpose. 
Here are some photos of some cavities that I made out of my mouth using dental alginate and a 2 part silicone rubber.  They are very strong and flexible - one being an "ah" shape and the other being an "eee".
I suppose that I could just use my mouth for this purpose, but holding it in a repeatable shape for the duration of a song can be difficult.  This way, I can also stretch and bend these molds in ways that would be impossible to do with my own mouth.
Another cavity that I have been using is a simple collapsible chamber that I made with a discarded CDr case, pictured on the right.  I used a silicone rubber margin to make it air tight, but it is still able to move back and forth, changing the dimensions of the cavity.  This sounds like a flanging effect, but smoother, deeper, and more natural, if that makes sense.  I don't have any automation to move it back and forth so it is not really like a flanging sweeping effect - it is more like a fixed flanger that is tunable.  This sounds amazing on white noise. 

So why all the bother?  One thing that I love about actual acoustic instruments is how certain ranges of the instrument change drastically in the tone.  In some cases, such as an oboe or clarinet, the change can be note specific - in other cases, such as a guitar, the change can be due to the area that is plucked or strummed, which string or strings you play, etc.  Using these types of chambers on electronic instruments brings them closer (and sometimes convincingly close) to a natural sounding instrument.  In many ways, I view it as the sonic missing link between the world of electronic and acoustic instruments.
Other things that I have tried have a lot to do with sympathetic vibrations, which will have to be a topic for a future post.

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