The Decoupled Acoustic String Instrument: A New Concept for an Acoustic String Instrument
Byungjoo Lee
a b s t r a c t
All existing acoustic string instruments are essentially vibro- acoustic systems that transmit the vibrational energy of strings Traditional Acoustic player for each string part, in turn to a resonator through a bridge. Stringed Instruments connected to several resonators This efficiently converts the within range of the signal, pro- vibrational energy to sound All existing acoustic stringed musical instruments have one radiation energy. The author has vides a fuller sound compared to common factor in their construction: They are essentially proposed an innovative acoustic that of traditional performances. vibro-acoustic systems that transmit the vibration energy of stringed instrument, called In addition, if we record a virtuoso the decoupled acoustic string strings to a resonator (board, sound box and resonator) playing a decoupled acoustic instru- instrument, and has developed through a bridge, efficiently converting the vibration energy ment, we can play back the original a prototype. The most notable to sound radiation energy. In other words, they have physi- feature of this idea is that the acoustic sound from a resonator cally coupled strings and a resonator, and the efficiency of vibrating strings are separated by inputting the recorded signal the energy transfer from the vibration of the strings to the from the resonator and fixed to at any time. It is important to note a distant rigid foundation. sound radiation energy generated by the resonator decreases that the sound field generated by as the impedance of the bridge foundation increases: A small an acoustic resonator and that of impedance means a strong vibro-acoustic coupling between a common electric speaker are the strings and resonator. very different. An acoustic resonator generates a less direc- In recent years, however, there have been great advances tional and more natural sound field than an electric speaker in measurement and excitation techniques. In particular, does. Furthermore, the sound field of an acoustic resona- miniaturized sensors and actuators having a broad frequency tor reveals the nonlinear property of the resonator itself. bandwidth, large dynamic range and improved resolution are available at reasonable prices, including optical piezoelectric (PZT) sensor/exciters and electromagnetic exciters. There- fore, the physical coupling between the strings and resonator is no longer necessary and can be replaced by using a com- Fig. 1. Decoupled acoustic guitar. (© Byungjoo Lee) bination of good sensors to capture the string vibration and actuators that excite the resonator.
Decoupled Acoustic Stringed Instruments I proposed and tested the idea of completely separating the stringed fingerboard from the resonator in order to measure the vibration of the strings using sensors. I then fed the mea- sured signal into the actuator mounted on the resonator to generate the sound. While the existing system consists of two harmonic oscilla- tors that are physically coupled to each other, the decoupled string instrument system has almost no coupling between the strings and resonator. They are physically separated from one another, so the signal generated from the strings is transmitted to the resonator in a one-way path from sensors to actuators. This allows new methods of acoustic performance by eliminat- ing previous requirements such as limiting each player to only one resonator and allowing only a short distance between the player and the instrument body. For example, an orchestra performance with only one
Byungjoo Lee (researcher), Seoul National University, Daehak-dong Gwanak-gu, Seoul 151-742, Korea. Email:
©2014 ISAST doi:10.1162/LMJ_a_00205 LEONARDO MUSIC JOURNAL, Vol. 24, pp. 61–63, 2014 61
Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/LMJ_a_00205 by guest on 30 September 2021 Fig. 2. Sensor part Steel clamp of decoupled acoustic guitar. (© Byungjoo Lee)
Longitudinal force
Longitudinal sensor
Vertical force
String Vertical sensor
Decoupled force exerted by each string. The force The string assembly support should be Acoustic Guitar signals, measured from six independent rigid enough to not influence the vibra- The acoustic guitar is perhaps one of the piezoelectric tiles, are summed into a tion of strings. Therefore, for the base most popular musical instruments in the single common voltage output. support structure I used aluminum 7075 world. Due to its popularity, there has It is clear that complete measurement for its stiffness and light specific weight. been plenty of research about its design of all forces is required to capture the I then attached a manufactured mahog- and acoustic performance. My research information generated by the vibrating any wooden neck to the aluminum base attempts to turn the acoustic guitar into strings. Many researchers have proven (Fig. 2). The end of each string was fixed a decoupled acoustic string instrument that the spectrum of the longitudinal on a small steel jig, with a pair of piezo- with a prototype called the decoupled signal is completely different from other electric sensors embedded around the acoustic guitar. transverse vibrations [1–4]. Thus, I de- jigs and pre-loaded by bolts (see Fig. 2). The acoustic guitar chosen for dem- cided to use two sensor modules along As shown in Fig. 1, I decided to use onstration has a dreadnaught-shaped the vertical and longitudinal directions the moving coil actuator made by NXT in resonator composed of a solid spruce to revive the timbre of vibrating guitar this prototype. This actuator is composed top plate and a laminated mahogany strings. of a light coil and a relatively heavy mag- back and side. It is a mid-quality guitar with a mahogany neck and rosewood Fig. 3. Block diagram of decoupled acoustic guitar system. (© Byungjoo Lee) fingerboard. Other than the resonator, all components have been completely (1-a) removed for the prototype (see Fig. 1).
Hh v h Implementing the Sensor and Actuator I used a piezoelectric sensor to measure Player a×G Hact the multidirectional vibration at the end of the string, because the sensor generally has a very wide bandwidth and relatively Hv v v large sensitivity, with little effect on the string vibration itself. The commercial piezoelectric sensor module, by ARTEC, is composed of six small piezoelectric perception sound tiles fixed in a thin slot. Each ceramic Hair Hresonator tile measures the unidirectional normal (loudness) pressure
62 Lee, Decoupled Acoustic String Instrument
Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/LMJ_a_00205 by guest on 30 September 2021 net, where the light coil is fixed on the 11 Fig. 4. The resulting acoustic guitar body so that the electro- nasality and clarity (the dynamic force between the coil and mag- same signed mixing). 10 net directly excites the attached point. (©Byungjoo Lee)
The frequency response is relatively flat 9 in the wide frequency range, except when
the resonance region is at f=50Hz. The 8
3dB bandwidth is about 8 kHz beyond Clarity the resonance peak, and the resonance 7 is not critical to the performance of the actuator, as the lowest frequency of typi- cal acoustic guitars is around 80Hz. The 6 phase characteristics are also very flat in 5 the frequency range of interest. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 The selection of optimal location for a (mixing weight) the actuator becomes an important is- 2 sue in the decoupled resonator, because any location can be chosen. I adopted a 1.95 modal energy function [5] to find the op- 1.9 timal position at which the actuator can
excite the low frequency normal modes 1.85 (up to 400Hz) equally and powerfully y (see Fig. 1). 1.8 Nasalit
1.75 System Assembly Figure 3 shows a block diagram repre- 1.7 senting the dynamics of this prototype 1.65 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 system. In this diagram, Hh represents the Frequency Response Function (FRF) of a (mixing weight) the strings and horizontal sensor system, while H represents the FRF of the string v ments, which represent the arbitrary 3. B. Bank and L. Sujbert, “Generation of Longitu- and vertical sensor system. By assuming steady state playing signal. I calculated dinal Vibrations in Piano Strings: From Physics to the input made by finger picking to be Sound Synthesis,” Journal of the Acoustical Society of the nasality and clarity using the fre- the impulse signal and the very begin- America 117, No. 4, Pt. 1, 2268–2278 (2005). quency domain approach [7] and plot- ning part (~0.5 second) of the response as ted them as shown in Fig. 4, with varying 4. H.A. Conklin Jr., “Generation of Partials Due to the steady state response, these two FRFs Nonlinear Mixing in a Stringed Instrument,” Journal mixing weights for the same measured can be obtained by interpolation. The of the Acoustical Society of America 105, No. 1, 536–545 signals. From the results, the clearest (1999). location of excitation was maintained and least nasal sound was obtained at at the ¾ point from the bridge dur- 5. A. Hac and L. Liu, “Sensor and Actuator Location the same signed mixing weight of a=0.67, ing the experiments. H , H and in Motion Control of Flexible Structures,” Journal of act resonator while the harshest and most nasal sound Sound and Vibration 167, No. 2, 239–261 (1993). H build up the FRF of the actuator sys- air was obtained at the same signed mixing tem. To modify the sound spectrum for 6.
Lee, Decoupled Acoustic String Instrument 63
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