Separating sound from source: sonic transformation of the violin through electrodynamic pickups and acoustic actuation Laurel S. Pardue Kurijn Buys Michael Edinger Design and Media Technology Centre for Digital Music MusikLab.dk / Centre for Digital Music Queen Mary University of Lyngby, Denmark Aalborg University / QMUL London [email protected] Copenhagen / London London, UK [email protected] [email protected] Dan Overholt Andrew P. McPherson Institute for Architecture, Centre for Digital Music Design and Media Technology Queen Mary University of Aalborg University London Copenhagen, Denmark London, UK [email protected] [email protected] ABSTRACT 1. INTRODUCTION When designing an augmented acoustic instrument, it is of- Two common motivations for designing augmented instru- ten of interest to retain an instrument's sound quality and ments are to extend the sonic capabilities of an existing nuanced response while leveraging the richness of digital instrument, or to repurpose the player's existing skill and synthesis. Digital audio has traditionally been generated technique on a traditional instrument. In contrast to tra- through speakers, separating sound generation from the in- ditional acoustic instruments, where sound production is strument itself, or by adding an actuator within the instru- inherently tied to physical construction and energy input ment's resonating body, imparting new sounds along with from the player, it is common with developing digital or the original. We offer a third option, isolating the play- augmented instruments to segment the design into separa- ing interface from the actuated resonating body, allowing ble modules: interface, processing, sound output. us to rewrite the relationship between performance action Most commonly, processing is done on a computer with and sound result while retaining the general form and feel sound then output to a speaker or public address system, of the acoustic instrument. We present a hybrid acoustic- physically separating the sonic output of the instrument electronic violin based on a stick-body electric violin and from the interface. Separating the two not only alters an an electrodynamic polyphonic pick-up capturing individual instrument's directionality and sonic presence, but also re- string displacements. A conventional violin body acts as sults in the loss of immediacy and subtle haptic feedback the resonator, actuated using digitally altered audio of the many performers rely on [22]. string inputs. By attaching the electric violin above the Actuated musical instruments are those instruments ar- body with acoustic isolation, we retain the physical playing tificially excited to create new timbres and sounds [22]. On experience of a normal violin along with some of the acous- string instruments, magnetic actuation of the strings is com- tic filtering and radiation of a traditional build. We propose monly used to create new sounds and means of control. Ex- the use of the hybrid instrument with digitally automated amples include the commercially available Sustainiac [12] pitch and tone correction to make an easy violin for use as and Vo-961, Berdahl's Feedback Resonance Guitar [22, 3] a potential motivational tool for beginning violinists. and McPherson's magnetic resonator piano [17]. Another subset of actuated musical instruments are those using active acoustics with structure-borne sound drivers in- Author Keywords ducing vibration into a resonating body [16]. Active acous- violin, augmented instrument, active acoustics, actuated tics can be used to alter and control normal acoustic be- musical instruments, learning technology haviours, exciting specific modes to modify timbre, or counter naturally occurring phenomena like cello wolf tones [20]. IR- CAM's Smart Instruments project focuses on active instru- CCS Concepts ment control in a variety of instruments including bowed •Applied computing ! Sound and music comput- strings [1], metallophones [4], and woodwinds [19]. ing; Performing arts; •Information systems ! Music Active acoustics can also be used to add a wide range of retrieval; standard audio effects to an otherwise acoustic instrument. This approach is particularly popular for guitars with com- mercial examples such as the Tonewood Amp2, Tonik3, and Yamaha's Transacoustic Guitar4.L¨ahdeoja discusses build- 1 Licensed under a Creative Commons Attribution https://www.kickstarter.com/projects/38513516/ 4.0 International License (CC BY 4.0). Copyright the-vo-96-acoustic-synthesizer 2 remains with the author(s). https://www.tonewoodamp.com/ (accessed: 20.1.2019) 3https://toniksound.com/ (accessed: 20.1.2019) NIME’19, June 3-6, 2019, Federal University of Rio Grande do Sul, 4 Porto Alegre, Brazil. https://usa.yamaha.com/products/musical instruments/ Figure 1: Assembled svampolin with foam separat- ing the stick-violin from the resonating body which is attached upside-down, with the actuator beneath. ing active acoustic guitars in [16, 15]. The Chameleon Gui- tar by Zoran et al. [30] takes the inverse approach, sensing and modelling acoustic soundboards but retaining external speakers for the final sound. Active acoustic instruments benefit from retaining a fa- miliar sound and feel, but a significant drawback is that Figure 2: Component parts of the hybrid violin; the they are additive: the sound of the acoustic instrument is stick-violin with mounted pick-up system is on top retained. This limits any transformation that seeks to re- with foam, Bela, actuator, and power amplifier in tain the technique and feel of a traditional instrument but the middle, and the violin body at bottom. alter its original sound. This paper presents an alternative hybrid acoustic-electronic approach similar to active acoustics, but where the physical The stick violin is made by Slawomir Ganiec6. It provides inputs of the instrument are acoustically separated from the the fingerboard, bridge and strings to support the core play- resonating body. Overholt's Overtone Fiddle [21] demon- ing interactions7, while the actuated acoustic body (with no strates normal violin actuation exciting a separated carbon- fingerboard or bridge attached) provides the sonic output. fibre acoustic body hung beneath. Similar to more recent revisions of L¨ahdeoja's guitar [10], we take Overholt's design 2.1 Electrodynamic Pickup a step closer to an active acoustic violin, using a traditional Our instrument features a pickup whose design is based on violin body as resonating chamber. Note that if there is no the StringAmpTM system by Michael Edinger of MusikLab goal of restoring acoustic sound characteristics, one could Denmark.8 The principle of operation is electrodynamic, simply attach small loudspeakers to an electric violin, as 5 where according to Faraday's Law a wire moving in a mag- seen in [23] or the Fusion guitar . netic field leads to an induced voltage proportional to the We have built a new violin which we informally call the velocity and the strength of the magnetic field. However, svampolin designed to maintain similar sound and feel to the rather than the traditional arrangement used in electric gui- traditional violin, but with the capability of sonic substitu- tars, where the string modulates the magnetic field which tion. In this paper we will present: the svampolin's physical is then picked up by a stationery coil of wire, in our ar- build; a custom-designed electrodynamic pickup capturing rangement the string itself acts as the wire carrying the the velocity of each string with minimal cross-talk or body induced voltage. All strings are grounded at the scroll end, coloration; and the actuation of the physical violin body. and the voltage on each string at the tailpiece is passed into We then present an example application of the svampolin a low-noise preamplifier described in the next section. as a simplified violin for beginners, where we use automated To provide the ambient magnetic field, neodymium mag- real-time pitch correction to improve player intonation and nets are placed underneath each string near the bridge (Fig- real-time bowed-string feature extraction to regulate the ure 3). We found it convenient to place them close to the timbre. The resulting instrument retains a similar response bridge, where they could be installed on a laser-cut platform to the player as an acoustic violin, and use of a traditional without permanent modification to the violin. Since the sig- violin body as resonator restores many of the acoustic sound nal level is proportional to the magnetic field strength, the characteristics, making it viable as a potential motivational magnets should be placed as close to the strings as possible, tool in violin learning. making placement near the bridge more convenient as the string will have a smaller range of vertical displacement due 2. INSTRUMENT DESIGN to bow pressure. This pickup arrangement has several advantages. First, The svampolin (Figures 1 and 2) comprises an electric\stick" the signal captures the motion of the string in the horizon- violin with a custom pick-up, an acoustic violin body with no strings, a vibration actuator, an embedded computer, 6http://www.urbgan.cba.pl (accessed: 27.1.2019) and foam and string to clamp the two violins together. 7The pictured violin is a five string violin, however the pres- ence of a fifth string is not core to the implementation, and guitars basses/ac guitars/ta series/ (accessed: 20.1.2019) any stick-violin would suffice. 5https://fusionguitars.com/ (accessed: 27.1.2019) 8http://www.stringamp.com (accessed: 20.1.2019) Actuator location Magnets (bottom) x4 x4 LF-noise Figure 4: Low-noise preamplifier schematic for elec- Integration prevention trodynamic string pickups (one per string). HP Amp LP 15~50Hz velocity (clean) displacement ner frequency of 1=(2πR3C3) ≈ 21:9kHz. A bipolar power duty cycle supply is required for the first stage. C2 and R5 allow the $duty cycle(magnet \approx positon) (magnet⇡ /(string length) <latexit sha1_base64="(null)">(null)</latexit> second stage to be referenced to an offset voltage Vcom, ) positon)/(string length)$ 0.05 Stick Slip which in our application is set to 2V to centre the signal within the input range of an analog-to-digital converter.