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Fig. 11 (a). Spectrum of x-axis; (b). Spectrum of y-axis; (c). Spectrum of z-axis.
adjusting the mix, a variation in tonal balance can be achieved with natural sound reproduction. The extended upper harmonics are still missing due to the bandwidth lim- itation of the current accelerometers but the sound repro- duction was still surprisingly true.
Conclusion
We are essentially reporting on a work-in-progress. Low-g MEMS accelerometers demonstrate clear potential as high quality acoustic pickups for musical instruments and they do not suffer from traditional feedback problems. A three-axis accelerometer mounted on a Fender Strat acoustic guitar achieved promising sound reproduction. The three axes clearly have different tonal characteristics related to the vibration modes of the instrument in the different directions of the body. The three output channels can be mixed to gen- erate realistic sound reproduction. In addition, these chan- nels can be mixed in different ways resulting in creative tonal effects.
While the performance of the accelerometer in this experiment is very promising, there are a few drawbacks. The noise floor of the sensor is audible, a problem that can be minimized using noise gating or other techniques, but the ideal sensor must have a noise floor comparable to conven- tional microphones. The high frequency response of the sen- sor must be extended, ideally up to 20 kHz, to capture the full tonal range of the instrument.
MEMS accelerometer technology has clear potential for acoustic pickup applications in musical instruments especial- ly in live performances where acoustic feedback is often a problem. A very small, low power MEMS device can be mounted unobtrusively anywhere on the instrument without affecting the instrument’s natural vibration characteristics. In fact, multiple sensors can be mounted at different points around the instrument providing additional flexibility to the sound engineer to reproduce the natural character of the instrument without fear of acoustic feedback in live sound application— one step closer to “Sonic Nirvana.”AT
References
1 F. Goodenough, “Airbags Boom When IC Accelerometer Sees 50 G,” Electronic Design, August 8, 1991, pp. 45-56.
2 Rai-Choudhury, MEMS and MOEMS Technology and Application (SPIE Press, Bellingham, WA, 2000).
3 B. Hopkin, Getting a bigger sound: Pickups and Microphones for your Musical Instrument (Sharp Press, Tucson, 2002).
4 H. Olsen, “A History of High Quality Studio Microphones,” J. Audio Eng. Soc. 24, 798-807 (1976).
5 R. Fontenot, “I Feel Fine: The history of this classic Beatles song,” www.About.com (http://oldies.about.com/od/thebeatlessongs/ a/ifeelfine.htm)
6 A. Freed and O. Isvan, “Musical Applications of New, Multi-axis Guitar String Sensors,” International Computer Music Conference, pp 543-546 (2000).
7 H. Olsen, Acoustical Engineering (Professional Audio Journals Inc., Philadelphia, 1991).
The output from the MEMS accelerometer is very inter- esting. The immediate weak points are that the noise floor was too high and audible at the beginning and end of the track and the bandwidth of the z-axis was clearly limited to lower frequencies. The sound reproduction from each axis was noticeably different.
The x and y-axis sounded bright and articulate and had clearly discernable differences in tonality. As expected the z- axis obviously sounded bass dominated. Figure 11(a) shows the x-axis spectrum, Fig. 11(b) the y-axis and Fig. 11(c) the z-axis.
The x, y and z axes mixed together produced a fair rep- resentation of the instrument with some brightness. By
32 Acoustics Today, July 2008