Body

The Vibraphone

Marv began life back in the 1930s as a Deagan model 145 vibraphone, an instrument typically used in Jazz by players like Milt Jackson, Lionel Hampton, and Gary Burton (among many others). The instrument consists of 37 tuned solid aluminum keys, ranging from F3 to F6 (174.6 - 1396.9 Hz), all suspended by cord in a "black and white key" configuration similar to that of a piano. Each key is suspended over a resonating pipe whose closed pipe fundamental resonance frequency is tuned to the fundamental of the key hanging above it. In other words, if you blow across one of these pipes, as you would a beer bottle, the note produced will be the same as the note heard by striking the key above the pipe. This similarity causes amplification through additive synthesis, thus making the note fundamental and some harmonics sound louder. Each pipe is fitted with a butterfly valve attached to a rod (one rod each for the "black" and "white" keys) which rotates via a variable speed electric motor. As these valves rotate, the amplification is allowed (open valve) or blocked (closed valve) in a sinusoidal pattern, causing a tremolo effect.

This photograph is a beautiful example of Deagan 145, although it is not actually of Marv. Marv started as the same make and model as the instrument pictured here, but a picture of Marv's youth did not exist that was this well taken. Thanks to Bruce Wells for the lovely example of a Deagan 145.

The vibraphone keys are struck with yarn-wrapped rubber-core mallets, chosen by each vibraphonist for a particular sound. Different mallets may be used by a particular performer in the same set, even on the same song, depending on the sound desired at any given time. One of Marv's key limitations is to one set of "mallets", the tips found on the actuators, as will be discussed. A vibraphone's keys are dampened by the vibraphonist via a foot pedal which controls a single dampening bar. This pedal operates similarly to the pedal on a piano: if the pedal is depressed, all notes will ring freely. If the pedal is released, notes will be stopped. This is a key limitation of the human player that is improved upon by Marv's design, which employs individual dampening of each key, only possible through computer control.

Three key components of Marv's design:

The original instrument was not compromised in adding the automation mechanics - when they are removed, the original vibraphone is just as it was, with the exception of four hidden 1"x1"x1/16" notches in the wooden key racks to ensure a snug fit with Marv's striking assemblies.
The automation mechanics are easily installed and removed. No special expertise is required - they simply slip on and off the instrument. The whole mechanism can be installed in under five minutes, most of that time being occupied by removing and replacing the keys.
The automation mechanics do not interfere with human play of the original vibraphone. While Marv is playing, a human vibraphonist can easily play along with, accompany, duet with, or be accompanied by Marv, with no special training or adjustments.

Striking Assembly

Marv is equipped with an individual striking solenoid for each key, mounted to the wooden key racks by two custom designed aluminum solenoid racks, one for the "white" keys and one for the "black" keys. (See Attachments for the formal design of these racks) The racks are made from 4"x4"x1/8" 6061-T6 aluminum L-bars, bolted together to form a custom C-shape, with room for flush-mounted hangars to attach the assembly to the wooden rack. Each rack has a 3/4" hole on the top flange for each solenoid: 22 for the "white" keys, and 15 for the "black" keys. The bottom flange is fitted with neoprene padding to catch the solenoid plungers, which are pulled away from the instrument's keys by gravity after being charged and striking.

The solenoids are Ledex 195207-228 push-type solenoids, the same solenoids used by Dr. Godfried-Willem Raes in his automated vibraphone Vibi (2001/2005). This was by no accident - Dr. Raes' instrument is well-documented online, and certainly provided guidance on this design decision. Indeed, upon finding the website for Vibi while in the early phases of design, this solenoid was chosen based on Dr. Raes' successful implementation and the ready availability of these parts from a nearby distributor. We wish to thank Dr. Raes for his previous work; while our implementation differs significantly from his, the guidance on solenoid choice was essential as we began the design of Marv. His inspiring family of musical automata are masterworks of musical robotics, and we hope to have more interaction with him and his work in the future.

Each striking solenoid is fitted with a Delrin tip, both to protect the aluminum keys and to improve the tone of the struck keys over that produced by the stock metal solenoid plungers. Delrin was chosen for its medium hardness and excellent machinability, and produced a reasonable tone in testing. The tips will be subject to improvement as work continues on Marv, beginning with the addition of a very thin felt to further soften the higher overtones produced in the keys.

Dampening Assembly

The keys are individually dampened by another set of solenoids for each key. These are mounted to the frame exactly as the existing single dampening assembly was: a long rod, with mounting arms attached to the "black" keys rack, and a master spring assembly in the center. The entire assembly sits between the "white" and "black" key racks, beneath the innermost distal end of each key. The assembly consists of 37 individual pull-type open-frame solenoids (Solen 121e10941), all mounted to a 1"x1.5"x1/8" 6061-T6 aluminum "Tee" extrusion. Each solenoid unit is fitted with a return spring and neoprene padding to provide the dampening force and transmit that force to the keys, respectively.

The new damper assembly sat nearly two inches lower than the existing assembly, which necessitated the manufacture of new mounting arms and a new master spring assembly. Each of these components were made from steel bar stock, which had to be heat treated and shaped. The mounting arms each had a 90 degree bend to makethe deeper assembly contact the keys correctly. Each arm also had a 90 degree twist to allow for the flat mounting of the arms to both the vertical member of the mounting T-bar and the horizontal extrusion of mounting pins to the mounting brackets on the "black" key assemblies. The master spring assembly had four precise bends, two of 84 degrees and two of 96 degrees.

For a thorough description of design choices, see the formal design document in the Attachments section below.