Magnetic Resonator Piano
The magnetic resonator piano (MRP) is an electronically-augmented acoustic piano capable of eliciting new sounds acoustically from the piano strings, without speakers. Electromagnets induce vibrations in the strings independently of the hammers, creating infinite sustain, crescendos, harmonics, pitch beends and new timbres, all controlled from the piano keyboard.
The MRP installs in any grand piano and has been used in compositions, performances and recordings internationally. Videos can be found below and on this page.
Latest MRP news:
Extending the Piano
The piano is a percussion instrument whose design has remained essentially unchanged over a century. Sound is produced by hammers striking strings, and once a note is struck, the performer has no means to alter its sound before it is released. For example, the following gesture is impossible on the acoustic piano:
The MRP was initially designed as an extension of the piano, allowing the player to continuously shape each note in dynamics, pitch and timbre, while retaining the richness and nuance of the acoustic grand piano.
New Acoustic Sounds
The MRP also unlocks a sound world far removed from the traditional piano. When the keys are played lightly and no hammers strike the strings, the electromagnets elicit sustained, subtly evolving tones which have many of the acoustic qualities of the piano, including the sympathetic resonance between strings, but without the familiar percussion. It is also possible to explore the space "between the notes" by bending notes away from where the strings are tuned and exploring the harmonics of each string. The results of this mode of playing have been explored in compositions and gallery installations.
Principle of Operation
Piano strings are made of steel, which is a ferromagnetic material, meaning it can be attracted by a magnetic field. The MRP uses 88 electromagnetic actuators to cover the complete range of the piano (one actuator per note, though a note may include 2 or 3 identically-tuned strings). When current is passed through an actuator, the string is pulled upward; when the current is turned off, the string returns to its original position. By modulating the current at the frequency of the string or one of its harmonics, the string is made to vibrate sympathetically without ever having been struck by the hammer.
Audio signals for the electromagnets ultimately come from a computer, controlled by the performer's actions at the keyboard. One amplifier per actuator is used to achieve the necessary power level. The current instrument is capable of up to 20W per string for short time, limited by the heating of the magnets. The tones produced can be quite loud, especially in the middle and treble registers, comparable in sound pressure level to forte playing on the traditional piano. A wide dynamic range is possible which also includes tones barely above silence.
The instrument covers 88 strings, but requiring 88 audio channels would be prohibitive. The amplifier circuitry therefore includes a signal routing system where audio channels are dynamically allocated to each string as needed. A standard audio interface can be used, with the number of audio channels (up to 16) determining the maximum polyphony of the instrument. In practice, the instrument is often run with 6-to-10-note polyphony.
Accounting for Tuning
The tuning of each piano varies slightly, and the best performance is obtained when the actuator frequencies exactly match the tuning of the strings, with the bass notes being particularly sensitive to tuning accuracy. To adjust to the tuning of the specific piano, a pickup can be placed on the soundboard and a phased-locked loop (PLL) system used to lock the actuators into the existing vibrations of the strings. More detail on this process can be found in this paper.
References for Electromagnetic Actuation
Continuous control of every note is a primary goal of the MRP, whether it is used to subtly extend the sounds of the traditional piano or to create novel and unusual effects. MIDI keyboards represent notes solely by onset and release, and even where aftertouch is used, the degree of control is insufficient for the continuously-evolving MRP sounds. Instead, a custom optical scanner bar is placed on the piano keyboard which records the continuous angle of each key.
Originally, a modified Moog Piano Bar was used for this purpose but a new dedicated solution has been developed, detailed in this NIME 2013 paper. The scanner uses optical reflectance sensing on each key:
Data Mapping and Performance
Continuous key data can be used to capture the subtle details of how a key has been pressed, including dimensions beyond velocity. More information on these measurements can be found on the Research page. Continuous key angle also allows new techniques including partial key presses, light taps on the surface, vibrato and aFtertouch on the acoustic piano (since the keybed is lined in felt, which compresses under pressure).
A computer maps key angle to actuator behaviour in a manner designed to be intuitive for the performer. The pianist does not interact with the computer during the normal performance process, though it is possible for certain aspects of the mappings to be dynamically changed by the pianist or another person. The standard mapping uses key angle to affect the volume of a note, such that a slow key press will produce a crescendo from silence. Pressure on a key will change the timbre of a note, and rhythmic oscillation in key pressure will produce a vibrato effect. Lightly, repeatedly tapping the surface of the key produces a glissando up the harmonic series of the note, creating a shimmering effect. When one key is held and a neighbouring key is lightly pressed, the pitch can be bent up or down. Each of these effects is demonstrated in this video:
References for Continuous Key Position
Performances with MRP have been featured across the US, UK and Canada, including Philadephia, New York, Boston, Chicago, Palo Alto, Toronto, London and Aberystwyth. The equipment packs into two plane-friendly travel cases, setting up in a new instrument in about 2-3 hours (1 hour for a familiar instrument). The most recent public events include:
The MRP is also featured in the following commercial recordings:
The MRP has been used in new pieces and recordings by a growing number of composers and performers. Since it can be set up in any grand piano and fits in travel-sized cases, new pieces can be performed anywhere. Scores and recordings are available from the composers in most cases; contact Andrew McPherson if you are interested.
Two major projects have expanded the repertoire for MRP. A 2011 collaboration with Philadelphia composers and performers is described in this article in Computer Music Journal. In 2013, the MRP has been featured by the London Chamber Orchestra as part of the project LCO New: Inspired by Digital.
The magnetic resonator piano is featured in the new album "Field of Reeds" by These New Puritans, released April 2013. The recording took place December 2012 on a Boesendorfer 225 piano at Snap Studios in north London.
References for Musician Collaboration
The MRP project includes an ongoing collaboration with the Music & Entertainment Technology Laboratory (MET-lab) and Excite Center at Drexel University. In addition to the MRP equipment in London, a complete MRP is set up at Drexel, where it is used for research and for collaborations with performers and composers.
We are excited to launch a new project with Cunningham Piano in Germantown, PA, USA. Cunningham is Philadelphia's oldest piano company, and also produces its own model grand piano, the New Cunningham Piano. The collaboration will feature MRP technology in Cunningham factory instruments. Please contact Andrew McPherson or Youngmoo Kim for further information.