Understanding the Reactable: An Innovative Music Tool

Blocks are a timeless symbol of childhood creativity, curiosity, and intelligence. By stacking or arranging these simple building pieces, children can unleash their imagination and construct endless structures. Adding colors, letters, or images enhances their ability to create patterns, words, and designs.

When a table is introduced beneath these blocks, the act of building transforms into a collaborative experience. While a table might seem like a basic item, it is, in fact, a space for connection, conversation, and collective effort. Many schools use large round tables to encourage teamwork, and architects and artists utilize tables as spaces to design complex projects or works of art.

Combining the concepts of blocks and tables, four graduate students from Universitat Pompeu Fabra in Barcelona, Spain, added music to create the reactable. This interactive music synthesizer resembles modern electronic dance music but offers a unique twist: participants manipulate sounds with movable blocks on a round table. By rotating or shifting the blocks, individuals (or groups) can alter sounds, beats, and notes to craft their own electronic soundscapes.

Beyond the ease of manipulating sound, the reactable adds a visual dimension: Its translucent blue surface illuminates with dynamic animations that reflect the musical changes. For both performers and viewers, the reactable offers an experience that is not only enjoyable to listen to but captivating to watch.

Reactable Basics

While its exterior is flashy and vibrant, the reactable's core function is that of a music synthesizer. It electronically manipulates notes and rhythms, warping and shaping the sound waves. This process generates unique sounds and variations that are difficult to achieve with traditional acoustic instruments like pianos or guitars. For example, by twisting a knob on a keyboard synthesizer while holding a note, the user can make the note’s pitch fluctuate, creating eerie electronic effects that ebb and flow.

When four students from the Universitat Pompeu Fabra—Sergi Jordà, Martin Kaltenbrunner, Günter Geiger, and Marcos Alonso—set out to design the reactable, they kept several key principles in mind. They aimed for the instrument to be collaborative: While one person can play alone, the round table design encourages multiple people to gather and engage with the various blocks. Additionally, the reactable should be intuitive and easy to learn, yet challenging in terms of both listening and playing. The goal was for anyone, whether a beginner or an experienced DJ, to quickly grasp the basics and create impressive sounds and visuals without needing instructions, according to its creators.

As described by the team, the reactable is a "novel multi-user electro-acoustic musical instrument with a tabletop tangible user interface" [source: Universitat Pompeu Fabra]. While the term might sound complex, it accurately captures the essence of the instrument. The reactable is a multi-user device, where people gather around and create music by manipulating blocks with their hands. Mastery takes practice, but with time, playing it becomes effortless. The term "electro-acoustic" refers to music produced using electrical components, and in the case of the reactable, all sounds are electronically generated, making it electro-acoustic. The term tabletop refers to the device's flat surface, and the "tangible user interface" means that players interact with physical objects, moving and rotating blocks just as they would turn knobs on a traditional synthesizer.

After the blocks are placed on the surface, what exactly do they do? How can you tell each block apart from the others?

Reactable Objects

When you place one side of an object on the reactable's surface, it's akin to pressing a button on a synthesizer, causing a sound or beat to loop continuously. However, simply placing a block doesn’t capture the essence of the reactable—there are various block types, each with different shapes and sides. The placement of one block in relation to another determines the resulting music.

There are six types of blocks, each with a distinct shape and function. Square blocks are sound generators. Rotating a generator alters its frequency, and dragging your finger around an animated circle can adjust its amplitude (the loudness or softness of the sound), similar to controlling the volume on an old television. Additionally, a sound can be cut off by making a "cutting" gesture along the line connecting the block to the center of the table, with the sound returning when the animated circle is touched again.

Squares with rounded edges act as sound filters, modifying sounds by adding various effects. These filters work similarly to a guitar pedal, adding flange, fuzz, or feedback-like resonance to the sound. For example, if a sound generator produces a steady tone, placing a sound filter next to it will distort the sound, making it more dynamic.

Circular blocks serve as controllers, transmitting control data to nearby objects, altering the frequency of the sound wave. This allows for different effects—whether maintaining a smooth, continuous sound or varying the frequency to produce a wah-wah effect.

Control filters (octagonal or eight-sided) and audio mixers (pentagonal or five-sided) are more geometrically intricate and serve more complex functions. These two types act as samplers and mixers, enabling musicians to craft elaborate melody loops and progressions that harmonize, shift shape, and change key.

Global objects, which have a hemispherical shape, stand out because they possess their own unique field, also circular, that influences every object within that field. Typically, they act as a metronome, keeping time for affected objects, or they serve as tonalizers, adjusting notes produced by the sound generators and filters.

So, how does the system's software transform all this interaction into music? We'll explore that in the next section.

Under the reactable Table: reacTIVision

To determine the positions of the blocks in relation to one another, the reactable utilizes a computer vision (CV) system hidden beneath the table's surface. The CV setup includes two main components: a camera and a projector.

The camera and projector both face upward towards the underside of the tabletop, but they each have distinct roles. The camera, powered by a specialized vision engine called reacTIVision, observes the blocks and examines several key factors:

The reacTIVision engine processes all of these positions, analyzes the layout, and sends the data to a connection manager. This manager performs two tasks at once: First, it forwards the information to an audio synthesizer, which generates the sounds the musicians are attempting to play and outputs them to a sound source. Simultaneously, the connection manager sends the same data to the projector, which also faces upward at the underside of the table. Unlike the camera, the projector doesn't gather data, but instead projects animations onto the translucent blue tabletop, offering players and observers visual cues that correspond to the music being produced.