On the playability of stringed instruments

As first noticed by Helmholtz, strings vibrate in a “V-shape” when they are bowed correctly and a full tone is produced, where the vertex of the “V” shuttles back and forth along the visible envelope of the string’s motion. If the instrument is bowed incorrectly, i.e. the instrument does not “speak”, then this “Helmholtz motion” is not produced, and the shape of the string as it vibrates will be quite different. The goal of this research is to gather experimental data from a stringed instrument and use it in the on-going development of a theoretical model of the mechanics of the bowed string, which can be used to investigate which aspects of the violin, strings or bow influence the ease with which this “Helmholtz motion” can be produced. The design, testing and application of a robotic bowing machine are described, which has allowed the speed and force of a bow as it plays a cello to be controlled. Extensive measurements of various aspects of the motion of a cello string being bowed by the bowing machine are presented, and compared with predictions from nominally similar theoretical models. Although certain models do reflect the qualitative behaviour seen in experiment under some conditions, all show vast room for improvement. Aspects of theoretical predictions that are at odds with experimental results, and would therefore impede efforts to use theoretical modelling in the design of a more “playable” violin, are subsequently described. Shortcomings of each model are attributed to physical defects of the theories underpinning them, and various modifications are discussed and tested.This project was financially supported by the Engineering and Physical Sciences Research Council

Discriminating music performers by timbre: On the relation between instrumental gesture, tone quality and perception in classical cello performance

Classical music performers use instruments to transform the symbolic notationof the score into sound which is ultimately perceived by a listener. For acoustic instruments, the timbre of the resulting sound is assumed to be strongly linked to the physical and acoustical properties of the instrument itself. However, rather little is known about how much influence the player has over the timbre of the sound — is it possible to discriminate music performers by timbre? This thesis explores player-dependent aspects of timbre, serving as an individual means of musical expression. With a research scope narrowed to analysis of solo cello recordings, the differences in tone quality of six performers who played the same musical excerpts on the same cello are investigated from three different perspectives: perceptual, acoustical and gestural. In order to understand how the physical actions that a performer exerts on an instrument affect spectro-temporal features of the sound produced, which then can be perceived as the player’s unique tone quality, a series of experiments are conducted, starting with the creation of dedicated multi-modal cello recordings extended by performance gesture information (bowing control parameters). In the first study, selected tone samples of six cellists are perceptually evaluated across various musical contexts via timbre dissimilarity and verbal attribute ratings. The spectro-temporal analysis follows in the second experiment, with the aim to identify acoustic features which best describe varying timbral characteristics of the players. Finally, in the third study, individual combinationsof bowing controls are examined in search for bowing patterns which might characterise each cellist regardless of the music being performed. The results show that the different players can be discriminated perceptually, by timbre, and that this perceptual discrimination can be projected back through the acoustical and gestural domains. By extending current understanding of human-instrument dependencies for qualitative tone production, this research may have further applications in computer-aided musical training and performer-informed instrumental sound synthesis.This work was supported by a UK EPSRC DTA studentship EP/P505054/1 and the EPSRC funded OMRAS2 project EP/E017614/1

Driving the Bow

Publisher PD

The acoustics of the violin: a review.

To understand the design and function of the violin requires investigation of a range of scientific questions. This paper presents a review: the relevant physics covers the nonlinear vibration of a bowed string, the vibration of the instrument body, and the consequent sound radiation. Questions of discrimination and preference by listeners and players require additional studies using the techniques of experimental psychology, and these are also touched on in the paper. To address the concerns of players and makers of instruments requires study of the interaction of all these factors, coming together in the concept of 'playability' of an instrument.This is the author accepted manuscript. The final version is available from IOP Science at http://iopscience.iop.org/0034-4885/77/11/115901

Physical modelling meets machine learning: performing music with a virtual string ensemble

This dissertation describes a new method of computer performance of bowed string instruments (violin, viola, cello) using physical simulations and intelligent feedback control. Computer synthesis of music performed by bowed string instruments is a challenging problem. Unlike instruments whose notes originate with a single discrete excitation (e.g., piano, guitar, drum), bowed string instruments are controlled with a continuous stream of excitations (i.e. the bow scraping against the string). Most existing synthesis methods utilize recorded audio samples, which perform quite well for single-excitation instruments but not continuous-excitation instruments. This work improves the realism of synthesis of violin, viola, and cello sound by generating audio through modelling the physical behaviour of the instruments. A string's wave equation is decomposed into 40 modes of vibration, which can be acted upon by three forms of external force: A bow scraping against the string, a left-hand finger pressing down, and/or a right-hand finger plucking. The vibration of each string exerts force against the instrument bridge; these forces are summed and convolved with the instrument body impulse response to create the final audio output. In addition, right-hand haptic output is created from the force of the bow against the string. Physical constants from ten real instruments (five violins, two violas, and three cellos) were measured and used in these simulations. The physical modelling was implemented in a high-performance library capable of simulating audio on a desktop computer one hundred times faster than real-time. The program also generates animated video of the instruments being performed. To perform music with the physical models, a virtual musician interprets the musical score and generates actions which are then fed into the physical model. The resulting audio and haptic signals are examined with a support vector machine, which adjusts the bow force in order to establish and maintain a good timbre. This intelligent feedback control is trained with human input, but after the initial training is completed the virtual musician performs autonomously. A PID controller is used to adjust the position of the left-hand finger to correct any flaws in the pitch. Some performance parameters (initial bow force, force correction, and lifting factors) require an initial value for each string and musical dynamic; these are calibrated automatically using the previously-trained support vector machines. The timbre judgements are retained after each performance and are used to pre-emptively adjust bowing parameters to avoid or mitigate problematic timbre for future performances of the same music. The system is capable of playing sheet music with approximately the same ability level as a human music student after two years of training. Due to the number of instruments measured and the generality of the machine learning, music can be performed with ensembles of up to ten stringed instruments, each with a distinct timbre. This provides a baseline for future work in computer control and expressive music performance of virtual bowed string instruments

Modelling loudness: Acoustic and perceptual correlates in the context of hypophonia in Parkinson’s disease

Hypophonia (quiet speech) is a common speech symptom associated with Parkinson’s disease (PD), and is associated with reduced intelligibility, communicative effectiveness, and communicative participation. Studies of hypophonia commonly employ average speech intensity as the primary dependent measure, which may not entirely capture loudness deficits. Loudness may also be affected by the frequency components of speech (i.e. spectral balance) and speech level variability. The present investigation examined relationships between perceived loudness and intelligibility with acoustic measures of loudness, speech intensity, and spectral distribution in individuals with hypophonia secondary to Parkinson’s disease (IWPDs) and neurologically healthy older adults (HOAs). Samples of sentence reading and conversational speech from 56 IWPDs and 46 HOAs were presented to listeners for ratings of perceived loudness and intelligibility. Listeners provided ratings of loudness using visual analogue scales (VAS) and direct magnitude estimation (DME). Acoustic measures of speech level (e.g. mean intensity), spectral balance (e.g. spectral tilt), and speech level variability (e.g. standard deviation of intensity) were obtained for comparison with perceived characteristics. In a spectral manipulation experiment, a gain adjustment altered the spectral balance of sentence samples while maintaining equal mean intensity. Listeners provided VAS ratings of perceived loudness of these manipulated samples. IWPDs were quieter, less intelligible, and had a relatively greater concentration of low-frequency energy than HOAs. Speech samples with weaker contributions of mid- (2-5 kHz) and high-frequency (5-8 kHz) energy were perceived as quieter. Results of the spectral manipulation experiment indicated that increases in the relative contribution of 2-10 kHz energy were associated with increases in perceived loudness. The acoustic time-varying loudness model (TVL) demonstrated stronger associations with perceived loudness and larger differences between IWPDs and HOAs, and successfully identified differences in loudness in the spectral manipulation experiment. Loudness ratings provided with VAS and DME were consistent, both providing excellent reliability. Findings of this investigation indicate that perceived loudness, acoustic loudness, and spectral balance are important components of hypophonia evaluation. Incorporating spectral manipulation in amplification by increasing high-frequency energy may improve efficacy of amplification devices for hypophonia management

Gesture and listening: towards a social and eco-systemic hyperinstrument composition

The research implements interactive music processes involving sound synthesis and symbolic treatments within a single environment. The algorithms are driven by classical instrumental performance through hybrid systems called hyperinstruments, in which the sensing of the performance gestures leads to open and goal-oriented generative music forms. The interactions are composed with MAX/Msp, designing contexts and relationships between real-time instrumental timbre analysis (sometimes with added inertial motion tracking) with a gesture-based idea of form shaping. Physical classical instruments are treated as interfaces, giving rise to the need to develop unconventional mapping strategies on account of the multi-dimensional and interconnecting quality of timbre. Performance and sound gestures are viewed as salient energies, phrasings and articulations carrying information about human intentions, in this way becoming able to change the musical behaviour of a composition inside a coded dramaturgy. The interactive networks are designed in order to integrate traditional music practices and “languages” with computational systems designed to be self-regulating, through the mediation of timbre space and performance gestural descriptions. Following its classic definition, technology aims to be mainly related not to mechanical practices but rather to rhetorical approaches: for this reason the software often foresees interactive scores, and must be performed in accordance with a set of external verbal (and video) explanations, whose technical detail should nevertheless not impair the most intuitive approach to music making