The Bowed String As We Know It Today
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ACTA ACUSTICA UNITED WITH ACUSTICA Scientific Papers Vol. 90 (2004) 579 – 589 The Bowed String As We Know It Today J. Woodhouse, P. M. Galluzzo Cambridge University Engineering Department, Trumpington Street, Cambridge CB2 1PZ, UK. [email protected] Summary The vibration of a bowed string has been studied since the 19th century, and today this problem is the only example of vibration excited by friction which can claim to be well understood. Modern theoretical models embody many of the complicating features of real strings, instruments and bows, and detailed comparisons with experimental studies are allowing fine tuning of the models to take place. The models can be used to explore questions directly relevant to instrument makers and players. The main unresolved question at present concerns thedescription of the frictional behaviour of rosin, since recent results have shown that the friction model used in most earlier studies is incorrect. Better frictionmodels are being developed in current research. PACS no. 43.75.De 1. Introduction This paper will review the present state of knowledge about the mechanics of a bowed violin string, as re- Modern research on the physics of the bowed string be- searchers work towards the goal of a detailed, experimen- gan with the discovery by Helmholtz, 140 years ago [1], tally validated model which could be used for the kind of that a string vibrates in a “V-shape” when bowed in the design studies just indicated. (The word “violin” is used normal way. The vertex of the V travels back and forth throughout as a shorthand to include all other members along the string as illustrated in Figure 1. Each time this of the family of bowed-string instruments.) The empha- “Helmholtz corner” passes the bow, it triggers a transition sisofthis paper will be on the current understanding of between sticking andsliding friction: the string sticks to the physics of bowed-string instruments, with the aim of the bow while the corner travels from bow to finger and answering questions raised by players or makers of acous- back, and it slips along the bow hairs while the corner trav- tic instruments. The modelling developed initially for this els to the bridge and back. This “Helmholtz motion” is the purpose has also been applied to real-time synthesis for goal for the vast majority of musical bow-strokes. musical performance purposes, and this has led to a sig- With this in mind, it is possible to distinguish two nificant research literature with rather different goals and types of quality which contribute to the central question priorities: for a recent reviewofthis work see Smith [2]. “What makes one violin better than another?” The first These developments will not be discussed in any detail is the“richness” or “beauty” of the sound produced by here. the violin, and the second is the “playability” of the vio- lin, the ease with which an acceptable note may be pro- 2. The development of theoretical models duced. The first of these relies, elusively, on the subjec- tive opinion of the listener. However, the distinctiveness of 2.1. Early work Helmholtz motion makes the second, “playability”, more Helmholtz wrote that “No complete mechanical theory can readily amenable to quantitative study: Helmholtz motion yet be given for the motion of strings excited by the violin either is or is not produced by a given bowing gesture, and bow, because the mode in which the bow affects the mo- if it is,ittakes a specific amount of time to become es- tion of the string is unknown” [1]. Attempting to rectify tablished. The reduction of “playability” to these simple this situation, Raman [3] was the first to try to describe terms means that theoretical models of the bowed string the dynamics of bowed-string vibration. Working long be- could be used to explore what makes some instruments fore the computer age, Raman made several assumptions easier to play than others. Such knowledge would help in order to obtain a model for the motion of the string suf- makers of violins (or indeed of strings, bows, or rosin) to ficiently simple to solve by hand. Raman’s model is based make more “playable” instruments. on a perfectly flexible string, stretched between termina- tions having reflection coefficients less than unity, excited by a friction force applied at a single point an integer frac- tion of the string length away from the bridge. The coef- Received 17 November 2003, ficient of friction was assumed to be a function of relative accepted 30 May 2004. sliding speed between bow and string. Using this model c S. Hirzel Verlag EAA 579 ACTA ACUSTICA UNITED WITH ACUSTICA Woodhouse, Galluzzo: The bowed string as we know it today Vol. 90 (2004) Bridge Finger Bow Figure 1. The Helmholtz motion ofabowedstring. Thedashed v v line shows the envelope of the motion, and the solid lines show h 0 b three different snapshots of the string position at different stages Friction force in the cycle. The “Helmholtz corner” circulates as indicated by the arrows. Raman was able to find various possible periodic string motions, including Helmholtz motion. Sliding velocity Some decades later, Friedlander [4] and Keller [5] stud- 1.2 ied the same model as Raman but with rigid string termi- nations, and found the surprising result that all periodic 1 waveforms are unstable under those conditions. This obvi- m ously conflicts with the experience of playing a real violin. 0.8 The source of the instability was later tracked down to the absence of dissipation in their model: if an ideal Helmholtz 0.6 motion is initiated on the string and then a small perturba- 0.4 tion is made to it, the perturbation grows with time in the Coefficient of friction form of a subharmonic modulation of the Helmholtz mo- tion [6, 7, 8]. Although traces of these subharmonics can 0.2 occasionally be observed in a real bowed string [9], under normal circumstances the energy losses in the string and 0 -0.15 -0.1 -0.05 0 0.05 violin body are sufficiently great that stable periodic mo- Slider velocity (m/s) tion is indeed possible. The need to allow for losses in a realistic way led to the next major development in mod- Figure 2. Friction force versus string velocity. (a) The relation elling. assumed by the “friction curve model” (solid line), with a bow v speed b .The dashed line corresponds to equation (1) as de- 2.2. Rounded corners and the digital waveguide scribed in the text. (b) The results measured in a dynamic experi- model ment (solid line), showing a hysteresis loop which is traversed in the anticlockwise direction. The dashed line shows the measured In practice, the perfectly sharp-cornered waves which are a result from a steady-sliding test; the bow speed was 0.042 m/s, feature of ideal Helmholtz motion, and of Raman’s theory, as indicated by the vertical dashed line. The “sticking” portion are unlikely to occur. The idea of modifying Helmholtz of the dynamically-measured curve shows “loops” because the motion by “smoothing out” the sharp corner was first ex- measuring sensor was not exactly at the bow-string contact point plored by Cremer and Lazarus [10]. Cremer then studied [25]. thechange in shape undergone by a rounded corner as it passes underneath the bowing point, and developed an ap- proximate theory of periodic Helmholtz-like motion which showed that the “corner” is significantly rounded when the taining for the moment the assumption that the “bow” acts normal force exerted by the bow on the string is small, but at a single point on the string, two dynamical quantities t becomes sharper when the force is increased [11, 12, 13]. enter the digital waveguide model: the velocity v of the t Asharper Helmholtz corner implies more high-frequency string at the bowed point, and the friction force f acting content in the sound, so this gave a first indication of how at that point. These two quantities can be related to each theplayer can exercise some control over the sound spec- other in two different ways, which when combined give a trum of the note: ideal Helmholtz motion is completely in- complete model. v dependent of the player’s actions, except that its amplitude The first relation between f and is via the constitutive is determined by the bow speed and position. law governing the friction force. The simplest law, and the Cremer’s approach was extended to cope with transient one used in all work on the subject until recently, is to as- v motion of the string by McIntyre et al. [14, 15], to give sume a nonlinear functional relationship between f and , what is now usually called the “digital waveguide model” of the general form shown in Figure 2a. When the string’s v of bowed-string motion. (Similar models were also devel- velocity matches thatofthebow ( b )thetwoaresticking, oped for other self-sustained musical instruments, such as and the force can take any value on the vertical portion wind instruments [15]. For a recent overview, see [2].) Re- of the curve, between the limits of static friction. When 580 Woodhouse, Galluzzo: The bowed string as we know it today ACTA ACUSTICA UNITED WITH ACUSTICA Vol. 90 (2004) there is relative sliding between string and bow, the fric- motion, this method only requires the motion from the last tion force falls with increasing sliding speed. period or so, since a wave returning to the bow is subse- The second relation comes from the vibration dynam- quently replaced by the next outgoing wave travelling in t ics of the string.