Lorentz Lap Brass
Adam Schmidt
Graduate Student, Rackham/SMTD

Musical Instrument/Device


The Lorentz Lap Brass is a new Electromagnetically-Actuated Musical Instrument/Interface that utilizes the principles of electromagnetics and optics to sense, modulate, and generate vibrations in the instrument’s brass strings. The instrument exemplifies an affordable and accessible method for integrated sonic/haptic interaction via a low-cost setup that employs Lorentz Force Actuation. This style of actuation is not nearly as common as electromagnetic coils, though it presents a rich opportunity for cost-effective tactile feedback, infinite sustain, and feedback control in musical applications. Custom bridges with built-in optical transducers allow for sensing of the user’s interaction with the instrument’s two strings.

Electromagnetic actuation of musical instrument strings is popular among music technology practitioners, often utilized to produce novel artistic effects in augmented guitars, pianos, or other stringed instruments. Effects such as infinite sustain or harmonic control are commonplace on steel-stringed instruments since non-invasive electromagnetic coils can be added to the existing system to induce string vibrations. Some popular examples are the commercially available eBow, Moog Guitar, and Fernandes sustainer system. However, unlike magnetized steel strings, brass strings (commonly used in instruments such as traditional harpsichords) are unaffected by the changing magnetic fields that electromagnetic coils create. To actuate brass strings, a less common style of actuation must be utilized: Lorentz Force Actuation. 

To generate vibrations in the strings (producing sound and/or haptic feedback), audio signals are sent to a 5-Watt audio amplifier module typically used to power small 4Ω or 8Ω speakers. Rather than connecting to speakers, the output terminals of the amplifier are connected to each end of the brass string. The amplified signal pushes current back and forth in the string, generating a weak magnetic field. However, an alternating current on a string is not enough on its own: to induce vibration, the strings’ weak magnetic fields interact with the much stronger magnetic fields from neodymium permanent magnets located directly below the strings. 

The vibration of the strings is picked up with a custom optical sensor made using an ITR9608 photoelectric switch. As the string vibrates, it changes the amount of occlusion between an infrared LED and the light-sensitive resistor. When placed in an optimal position, this voltage is directly analogous to the position of the string and can be used like the audio signal output from an electric guitar. 

Each string has 2 custom 3D-printed bridge mechanisms that allow for precise placement of the string within another optical sensor’s field of operation. The signals from these sensors are used to manipulate and actuate the haptic and sonic feedback in the instrument, creating new sounds that are manipulated digitally but heard acoustically. 

Though it was outlined and directly inspired by Dr. Andrew McPherson’s 2012 paper “Techniques and circuits for electromagnetic instrument actuation”, hinted at by inventor Paul Vo in US Patent 6216059B1, and notably used by Alvin Lucier in his performances and recordings of Music On A Long Thin Wire, this style of actuation had yet to be utilized in a new musical instrument until the Lorentz Lap Brass. Like a lap steel guitar, The Lorentz Lap Brass can be played with a guitar slide and/or your fingers. Musical innovations of the past, such as the Ondes Martenot or Theremin, demonstrate the musical and expressive potential of somewhat simple and direct control of pitch and amplitude, suggesting this new haptic instrument, with enough practice, has similar potential.