Singing Glass Contraption

Found this as I was searching for photos of Glass Harps on the internet.

Plasma Speaker sketchers

Sketches:

ignore the sars mask, that was a sketch-o

Alternate layout for 360 viewing.

Alternate lever interface to control arc gap.

Plasma Speaker

Last week, in addition to the two exhibit ideas that I refined, I also brought in another project that I've been developing. I'm very interested in sound, electricity (specifically high voltage), and the cultural and psychological effects of playing with danger. 

A plasma speaker uses a high voltage source (in the 10s of kV range) to produce an arc of ionized molecules in the air, creating a stream of plasma. This plasma is pulsed at a frequency too high to hear, but is modulated with an audio signal that can be heard coming directly from the arc.

Stanford's own Paul Demarinis used a similar phenomenon with the flame as a medium instead of an electrical arc.

I'd like to push this piece forward to a point that I can get some meaningful interactive elements. To do this, I need to ensure that no one receives an inadvertent shock from the high voltage sections, then I can concentrate on one of several variable elements. The distance between the electrodes controls not just sound quality, but also volume. The material, shape, and orientation of the electrodes makes a big difference as well. I could also allow the visitor to control the sound source. For starters, the distance between the electrodes will hopefully yield some interesting discoveries.

Can You Make Glasses Sing? Sketches

Musical Glasses

This is the original exhibit: 8 glasses ranging exactly one octave in pitch and two mallets. The idea is that the user will engage with it on a very visceral level--as an open exploration, my hope with this exhibit was that users would immediately know what to do and just play, experimenting with different tunes and potentially trying out different ways to make the sounds (mallets vs. wet finger on the rim). I care less about what users would say when they walk away from the exhibit and more about the smiles on their faces while playing with it.

In class today, I really enjoyed watching people in our group play with these glasses because it did two things. Firstly, it gave me confidence that this is a valuable idea, because although some people had reservations about this idea over my infinity mirror, they were still drawn to the exhibit and found themselves playing with it. Secondly, it gave me an interesting "nugget" of observation that I hadn't noticed before: people get more excited about being able to make the glasses "sing" by rubbing the ribs rather than the actual sound being made. I think this will take me in a really interesting direction this week--I plan on just watching people play with my glasses as I leave them out in my house. I will put the mallets there to see if they are used or ignored, but I expect they won't get much attention.

Sound Reflections

I knew about the electronic version of the effect demonstrated in this exhibit from building and repairing guitar effects pedals. For those of you familiar with guitar effects, this particular effect is called a flanger. The original name comes from when an audio engineer would stick a finger on the flange of a magnetic tape reel to slow it down slightly and then combine that signal back with the original. Perceptually, this gives an airplane-like whooshing sound. In more technical terms, the time delay will effectively alter the phase of the frequencies coming from the sound source based on the relationship between the delay time and the frequency in question. When combined with the original sound source there will be peaks where the phase is in alignment and notches where the delayed signal and original are out of phase. This creates what is known as a comb filter (http://en.wikipedia.org/wiki/Comb_filter), and altering the delay time will cause the comb to sweep through the frequency spectrum.

This can be done electronically by adding a very small amount of delay and combining it with the original 'dry' signal. The same effect can be accomplished through purely acoustic means by forcing a sound to travel in two different directions and then combine at the listener's ears.

Paul Doherty commented that he noticed this effect at the outdoor pull-up bars near the old exploratorium when an airplane went by and you were able to raise or lower yourself and hear the sound directly from the plane combine with the reflected sound off of the ground. His story reminded me that I've noticed the same thing when jogging on a smooth asphalt surface while a plane passes overhead. Even the slight bouncing up and down that happens when I jog is enough to trigger the effect. At the time I incorrectly attributed the change in sound to the doppler effect because the perceptual change can sound like an overall frequency shift up and down, but in actuality the notches in the comb filter are adding and canceling frequencies throughout the audio spectrum and this just sounds like a frequency shift.

When I first prototyped this exhibit, I placed a speaker playing white noise on a box on top of a table and moved my head up and down relative to the speaker and the table to hear the combination of the sound directly from the speaker and the sound bouncing off of the table. This demonstrated strong perceptual evidence of the effect but I couldn't shake the thought that maybe it had something more to do with the position of my ear relative to the speaker and not the filtering due to the the combination of two paths of the sound. To convince myself of the comb filter, I then kept my head and the speaker stationary and brought in a wooden plank to bounce the sound back towards my face. This worked extremely well, especially in that if I moved slightly out of the path of the bouncing sound (1-2ft to either side) the effect decreased to almost nothing, making the effect that much more convincing. I thought my work was done.

I did also listen to a sine wave at several different frequencies to see if it would cancel itself out as the plank was moved. The bouncing sound is not quite as strong, so I did not experience a completely silent node, but there was a noticeable increase and decrease in volume.

The problem that we encountered was that when I first set up the exhibit in class, the room was much smaller than where I had originally prototyped, and there were other sound-reflective surfaces in the immediate vicinity causing more reflections than just the one from the plank. This made it so that I could hear some version of the effect even when I stood several feet away from the person interacting with the exhibit. This takes away a lot of the surprise and wonder when it is your turn to try it out. In future versions, I'll ideally place the exhibit in a room that is acoustically dead so only the sound from the speaker and a single reflection off the plank can be heard.