Page 62 - WINTER2019

Second Century of Electroacoustics
Figure 4. The Eigenmike® spherical microphone array is the first to provide higher order ambisonic beam patterns to record the spatial characteristics of a sound field. From mh acoustics, with permission.
array. The sensor outputs were processed to form an omni- directional beam and the three dipole patterns along three orthogonal axes. The intent was to record these four signals and use them to reconstruct the sound field in a room using speakers positioned for the room.
Methods for reconstructing the sound field are many, but the fundamental strategy for recording has been retained. Ambisonic recording now includes not just the dipole pat- terns but allows for the inclusion of quadrupole patterns and higher order multipoles when they are available. In fact, higher order ambisonics has conceptually come to include all of the spherical harmonics (Tarzan et al., 2019). However, there are practical difficulties in forming the higher order beam patterns. For the nth order multipole patterns, the sensitivity varies as the nth power of frequency. Thus, for low frequencies, the sensitivity is low and the signal-to-noise ratio becomes problematic. After the initial patents (Elko et al., 2009), commercial products now exist, with spherical harmonic patterns up to fourth order (mh acoustics, 2019).
Figure 4 shows one of these products.
What Next?
It is easy to predict that evolutionary changes to the present devices and systems are likely to provide small improvements to those systems. It is also easy to say that larger changes
are likely to arise from new materials, new manufacturing processes, and advances in engineering analysis. That state- ment has always been true. But where are those advances to be found?
I cannot claim clairvoyance in this (or any other) area. How- ever, it is true in the past that most of the material advances that enabled major improvements in acoustical devices were not primarily motivated by the market for acoustical devices. Advances in permanent magnet materials in the 1930s pre- sumably had much larger markets in the manufacture of electric motors and electric-power generation equipment. Neodymium magnets are used in electric vehicles and in wind turbine generators. Improvements in other magnetic materials may be possible, and cost reductions may continue as sales volumes increase. These seem likely but will probably not result in revolutionary system improvements.
One application area that may lead to changes is the improve- ment in analyzing and reproducing the effects of directional sound. Work in ambisonics may aid our understanding of the details of the complex wave fields that are judged to provide a superior listening experience. The requirement to provide a realistic virtual reality experience may generate new require- ments for sound recording and reproduction equipment.
Improvements during the first century started slowly as vacuum tube electronics took hold. We may expect the rate of change in the next decades to be much more rapid because advances in electronics, computational capabilities, and advanced materials are all in progress.
References
Benthien, G. W., and Hobbs, S. L. (2005). Modeling of Sonar Transducers and Arrays. Document ADM001666, Defense Technical Information Center. Available at https://gbenthien.net/assets/docs/modeling.pdf. Accessed August 21, 2019.
Beranek, L. L. (1954). Acoustics. McGraw-Hill, New York.
Berlincourt, D A., Curran, D. R., and Jaffe, H. (1964). Piezoelectric and piezomagnetic materials and their function in transducers. In W. P. Mason,
Physical Acoustics, Vol. 1 Pt. A, Chap. 3. Academic Press, New York. Brock-Nannestad, G. (2016). Against all odds: Commercial sound record- ing and reproduction in analog rimes. Acoustics Today 12(3), 12-20.
https://doi.org/10.1121/AT.2016.12.3.12.
Carson, D. L. (1962). Diagnosis and cure of erratic velocity distributions in sonar projector arrays. The Journal of the Acoustical Society of America 34, 1191-1196. https://doi.org/10.1121/1.1918297.
Decarpigny, J. N., Debus, J. C., Tocquet, B., and Boucher, D. (1985). In-air analysis of piezoelectric Tonpilz transducers in a wide frequency band using a mixed finite element-plane wave method. The Journal of the Acoustical Society of America 78, 1499-1507. https://doi.org/10.1121/1.392785.
62 | Acoustics Today | Winter 2019