Introducing the world’s first light guitar, a revolutionary musical instrument that uses light to make music. Invented by Dr. Russell Stoneback, a recent physics Ph.D graduate from the University of Texas at Dallas, these new instruments generate light waves that are converted to sound to produce music.

Although the light we see appears to be totally unrelated to sounds we hear, the physics of light and sound are similar. So similar, in fact, the light guitar behaves remarkably like an acoustic guitar. Traditional string instruments use the resonance of wood and sound waves within the instrument to create music. The light guitar replaces the traditional oscillations of wood with oscillations of currents in metal. These electrical currents resonate much like the wood in an acoustic instrument, producing electromagnetic (light) waves that resonate within the light guitar body. The resonance of these light waves functions similarly to sound waves in an acoustic instrument. Since the function of acoustic instruments is to make music, the light waves produced by the light guitar are also music, albeit directly inaudible. To hear this light music, the light waves are measured and converted to sounds.

While the light guitar is similar to acoustic instruments, light is more complex than sound. Because of this complexity, the light guitar actually behaves like two acoustic guitars. To understand this, consider sound in an acoustic instrument. Simplified, the behavior of these waves is determined by the properties of the wood and the shape of the instrument. If the wood is extremely rigid, the resonances of the instrument will have a particular distribution. In the opposite extreme, if the guitar was made of a material with no rigidity (ex. latex) the resonances of the instrument would be different.

Waves in the light guitar can operate in two different and simultaneous modes. One of these modes behaves similarly to sound in a rigid box, while the other behaves like sound in a box with no rigidity. The combination of both of these behaviors in a single instrument can give light instruments a richer tone not possible with acoustic instruments.

To emphasize the fact that electromagnetic waves behave similarly to sound waves, the first prototype light musical instrument was shaped like an acoustic guitar, constructed using pure aluminum, stainless steel foil, copper, and acrylic glass. The acoustic guitar shape makes the light guitar sound similar to an acoustic guitar.

To achieve similar behavior between sound and light, the sound and light waves need have the same size (wavelength). Since light travels much faster than sound, the frequencies of the light guitar must be much higher than the sounds produced by acoustic instruments. Thus, oscillations of acoustic instruments ranging from 20 - 20,000 cycles per second are replaced with electromagnetic waves oscillating a million times faster (20 MHz - 20 GHz).

This large difference in frequency makes it impossible to play a light guitar directly. One method to  get the sound of the light guitar uses a computer to modify input music such that the music sounds as if it came from the light guitar. Software can exactly recreate the light guitar (at acoustic frequencies) by incorporating measurements of the instrument’s response over a range of frequencies (the transfer function).

The conversion from the high operating frequencies of the light guitar to the audible range is simple on the computer. Each measurement of the guitar includes two pieces of information, the frequency the measurement is performed at and the response of the instrument. The recorded frequencies of the light guitar are reduced to acoustic frequencies through simple division.

The light guitar presented here is a prototype and does not reflect the sound of a well tuned instrument. The equipment used to measure the light guitar was rented, eliminating opportunities to  refine the instrument. Measurements of the light guitar could only be performed up to 6 GHz, or an acoustic equivalent of about 6 kHz. So while the light guitar software exactly reproduces measurements, the measurements are incomplete. The light guitar plug-in (free) is compatible with macintosh computers running OS X. A more complete description of light guitar physics can be found in my dissertation. Let me know what you think, russell (a) lightguitar.org