HUT Acoustics Lab

Analysis and Physical Modeling of Tanbur

Introduction

The acoustics of western string instruments is a widely studied topic. However, some of the more primitive traditional string instruments exhibit many important and unique characteristics, and they can be regarded as interesting acoustical systems by their own right. The tanbur, a Turkish long-necked lute  is a typical example of such a system. A paper describing the properties of the instrument and the preliminary analysis results will be presented in 6th International Congress on Sound and Vibration, Lyngby, Denmark .

The figure on the left  shows the various constituents of the instrument. A quasi-hemispheric body shell resembling the shape of a  halved apple is made of 17, 21 or 23 thin slices of thickness 2.5 to 3 mm. The slices are usually cut from ebony, rosewood, pearwood, walnut or cherry. The soundboard is made of thin (1.5 to 2 mm) spruce  panel.  There is neither a sound hole nor braces, so that the thickness of the soundboard has to be carefully adjusted. It should be thick enough to resist the static forces applied by the bridge, but still thin enough for a good sound quality and loudness. The optimum  thickness causes the soundboard to curve inwards, forming a shallow top plate.  This is a characteristic of the tanbur.
 
The strings are stretched between a raised nut and the bridge. The violin-like bridge is made of rosewood or juniper, and the force is  transmitted to the body via the two legs of the bridge. The long neck (73.5 to 84 cm), which is typically made of ebony or juniper, hosts 52-58 movable frets made of gut or nylon. The tanbur has seven strings, six of them are grouped in pairs, and the lowest-pitched string tuned to A1 (55 Hz) is single. The pairs are tuned to A2, D2 and again A2 (or alternatively A2, E2 and A2). The normal playing style involves the use of just the bottom A2 pair, while the other strings serve as resonators. The two A2 pairs are plain steel strings, whereas the remaining three strings are wounded steel or brass. The plectrum is originally made of tortoise shell, nowadays replaced by synthetic material, and its length varies between 9.5 and 13.5 cm.

Body Vibrations

The analysis of the impulse responses show that the body responds also to the excitations from horizontal and longitudinal directions, besides the usual vertical driving direction.  The prominent body resonances are found at f1=191 Hz and f2=344 Hz. The results of the modal tests to reproduce these low-frequency modal shapes will be available from this page very soon.

Nonlinear String Vibrations

The vibration of the tanbur strings is relatively nonlinear due to the modulation of the tension along the string. The tension modulation exerts a longitudinal force on the bridge which is efficiently radiated as sound from the body. The tension modulation force is nonlinearly related to the vibration of the string. Fundamental frequency variation and coupling of harmonic modes are among the perceptually most important effects of this nonlinearity. In addition, the radiated sound component due to the tension modulation longitudinal force is pronounced and clearly adds to the character of tanbur tones.

Sound Examples for the ICSV'99 Paper

Examples for Figure 6: These examples demonstrate the pitch drift observed in tanbur sounds. The effect is most pronounced for the hard-plucked case, although being still audible for soft- and medium-plucked cases.

A soft-plucked tone (.aif, 44KHz, mono, 323 Kb)
A medium-plucked tone (.aif, 44KHz, mono, 277 Kb)
A hard-plucked tone (.aif, 44KHz, mono, 430 Kb)

Examples for Figure 7: These examples are used to extract the first four harmonics exciting the string in the middle point. Note that the only "real" sound signal is the mic output. The first two signals are signals proportional to string velocity and to the accelerance near the bridge, respectively.

A signal proportional to the string velocity (pick-up output) (.aif, 44KHz, mono, 708 Kb)
A signal proportional to the vibration of the soundboard near the bridge (accelerometer output) (.aif, 44KHz, mono, 516 Kb)
Radiated sound (mic output) (.aif, 44KHz, mono, 430 Kb)

The second harmonics have been extracted from the examples above using band-pass filters. Note the presence and the build-up of the second harmonic in each example, pointing out the tension-modulation nonlinearity.

Second harmonic(pick-up output) (.aif, 44KHz, mono, 708 Kb)
Second harmonic(accelerometer output) (.aif, 44KHz, mono, 516 Kb)
Second harmonic(mic output) (.aif, 44KHz, mono, 430 Kb)

Other Sound Examples

A short example of a tanbur piece (.aif, 22KHz, mono, 463 Kb)
Another piece (.aif, 22KHz, mono, 671 Kb)

A sythesized tone with plectrum scratches (.aif, 22KHz, mono, 156 Kb)
A synthesized Bach prelude on tanbur! (.aif, 22KHz, mono, 2.52 Mb)
 

This URL: http://www.acoustics.hut.fi/~cerkut/tanbur
Last modified: June 5, 1999
Author: Cumhur Erkut