Music – Instruments – Electrical musical tone generation
Reexamination Certificate
1996-07-18
2001-11-20
Witkowski, Stanley J. (Department: 2837)
Music
Instruments
Electrical musical tone generation
C084SDIG001
Reexamination Certificate
active
06320113
ABSTRACT:
BACKGROUND OF THE INVENTION
In 1990, the “Mendelssohn” Stradivarius violin sold at Christie's in London for $ 1,686,700. A good violin at a typical music store sells for around $ 2,000. What is it about the Stradivarius that makes it cost almost 1,000 times as much? The structure and geometry of the two instruments are very similar, yet subtle differences in the structural dynamics of the two instruments cause them to vibrate differently in response to an excitation by a violinist's bow. This, in turn, causes differences in the sound produced by the two instruments which ultimately determines quality and, to a large extent, price. If it were possible to force the less expensive violin to vibrate like the Stradivarius, the legendary sound would follow.
The relatively new field of smart structural/acoustic control is centered around changing the structural dynamics of an acoustically radiative structure to change, usually to suppress, the sound resulting from vibration of the structure. This is done by connecting actuators that are integrated into the structure in a control loop with sensors that are either in the acoustic field or also integrated in the structure. Smart structural/acoustic control also has the potential to force one acoustically radiative structure to behave like a target acoustically radiative structure, thus replicating its acoustic properties. The less expensive violin might be forced to sound like a Stradivarius. The concept of acoustic replication using smart structures has far reaching implications, from the field of acoustic musical instruments to aircraft cockpits.
To provide a background, a brief review of active acoustics leading to smart structural acoustics is presented. Smart structural acoustics is a relatively recent subset of the broader field of acoustic control wherein an acoustically noisy structure may be controlled at the structure through integrated sensors and actuators. This integration is such that the sensors and actuators are load-carrying parts of the structure as well as control elements. The field of smart structural acoustics has emerged in a natural progression: first, acoustic control by acoustic sources; then, by vibration inputs; and finally, by integrated sensors and actuators or, smart structural acoustic control.
Additionally, a review is given of literature on the acoustic guitar. This instrument has inspired a significant amount of analytical and experimental research from the perspective of acoustics and structural dynamics. As such, there are identified dynamic parameters in the literature that could potentially be further “tuned” using active acoustic control to accomplish desired changes in acoustic parameters.
In most applications, acoustic control is implemented in order to suppress unwanted noise through attenuation or other mechanisms. Sound attenuation is usually implemented through sound-absorbing materials for sounds of medium and high frequencies. Because the thickness of the sound absorption material necessary to produce constant attenuation increases with decreasing frequencies, there is a practical limit on its use at relatively low frequencies. In this low frequency region, active acoustic control has found applications.
The principles underlying active acoustic control have been understood at least since 1802 when Young's principle of interference was introduced. The principle suggests cancellation of a sound wave propagating in space by the addition of an inverse wave. This principle forms the basis of active noise control. Huygen's principle, as applied to acoustics, is an extension of Young's principle for multiple dimensions. Huygen's principle states that the sound field inside a surface that is produced by a source outside the surface can be exactly reproduced by an infinite array of secondary sources distributed along the surface. Since an infinite array of secondary sources are not realizable, in practice, a finite number of secondary sources can be “field-fitted” to achieve an optimum result.
Despite the longevity of the underlying principles of active noise control, one of the first practical implementations was described by Lueg in a German patent in 1933 and in a U.S. patent in 1934 (U.S. Pat. No. 2,043,416). Phase reversal in Lueg's one-dimensional duct was accomplished by considering the electronic system as a transmission line whose length determined the time delay. Lueg also proposed cancellation in a space very near a loudspeaker and in an open space using a microphone and a loudspeaker. It has been found more recently that cancellation at a point is done at the expense of increased noise at other locations in the field. Also, Lueg's approach to control of noise in an open space was probably not viable since successful experiment implementations of this are much more recent and inevitably involve more than one microphone and speaker.
Little was published in the field of active control following Lueg's patent until the 1950's. In 1953, Olson published research on an electronic sound absorber and Conover made early attempts to control transformer noise using a single loudspeaker. Frequency performance range of Olson's devices were limited at low frequencies by loudspeaker performance and at high frequencies by phase errors and electronics. An attenuation is achieved of almost 25 dB in the range of 60 to 80 Hz accompanied by an almost linearly decreasing attenuation up to around 500 Hz where there is an increase of sound pressure of 5 dB. This early work started to map out the frequency range of usefulness of active versus passive noise control, where active is most effective in the range of near DC to 500 Hz and passive is most effective above 500 Hz. This upper limit on active control should continue to increase as theory develops, computing power continues to increase, and computing equipment cost continues to decrease.
Applications in which modern active noise control research continue are plentiful, including approximately one-dimensional problems such as ducts and noise-reducing headsets and multidimensional applications such as cylinder interiors and transformers. Cylinder interiors are of particular interest because of their natural extension to fuselages and launch vehicles.
The idea of noise reducing headsets started as a more advanced version of Lueg's system for controlling duct noise and was implemented by Olson. For low frequencies, sound waves in ducts propagate as approximately one-dimensional plane waves. As the sound frequency increases, the sound propagation becomes multidimensional and much harder to control as the plane wave assumption breaks down and transverse resonances cause pressure fluctuations through a cross section. Active noise control has been applied to fan-induced duct noise in commercial air handlers at low frequencies. The limiting frequency for noise reduction of up to 20 dB for most duct structures is around 500 Hz. This limitation is also imposed by sampling and processing speeds.
Internal cylinder noise can be a pseudo two-dimensional problem or a three-dimensional problem depending on whether the noise sources and secondary sources lie in the same cross-sectional plane and the frequency of the noise. In 1976, Kempton, put forth one of the first illustrations of a multidimensional active acoustic control problem using an array of “anti-sources” to cancel the far-field of a monopole source. Lester and Fuller used four interior monopole control sources to attenuate noise by around 20 dB within a cylindrical cross section caused by 2 exterior monopole noise sources. Later, Fuller, and Jones and Jones and Fuller performed similar studies using a structural control actuator. These will be covered in greater detail in the next section. Elliot et al. determined that as long as secondary sources couple sufficiently with modes that are excited by the primary source, it is possible to achieve noise reduction without locating secondary sources near the primary source. Noise control has also been a
Griffin Steven F.
Hanagud Sathya V.
McColl Chance C
Georgia Tech Research Corporation
Thomas Kayden Horstemeyer & Risley
Witkowski Stanley J.
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