x EXCITATION x RESONANCE

I

I / I / I / I/

fx fox fxy foy FREQUENCY

.I

y REEONANCE

y EXCITATION

fy

FIG. 1. These resonance curves illustrate the principle of operation for x- and y-axis acoustic modes having nearly the same fundamental resonance frequencies. The x mode is excited at a frequency slightly below that of the x resonance, and the y mode is excited an equal amount above the frequency of the y resonance.

rotation corresponded to about the half-power points of the acoustic-pres- sure-versus-frequency resonance curves.

This work was done by Martin B. Barmatz and James L. Allen of Caltech for NASA's Jet Propulsion Laboratory.

Advanced-degree dissertation in acoustics Editor's note: Abstracts of Doctoral and Master's degree theses will be

welcomed at any time. Please note that they must conform to the usual limit of 200 words, as for archival papers, must be double-spaced, and must bear the appropriate PACS classification numbers. It would be useful to provide the name of the thesis supervisor, and to indicate how a copy of the thesis may be obtained.

Reduction of wind noise Interference in blast noise measurement using a two microphone array [43.28.Ra, 43.50.Gf, 43.50.Nm, 43.88.Hz]mDaniel Francis Marshall, The Graduate College, University of Illinois at Urbana-Champaign, December 1984 (M. S.). The use of micro- phones outdoors is often hampered by wind noise interference, which has traditionally been alleviated through the use of windscreens. In this thesis the effectiveness of an alternative method for reduction of wind noise inter- ference using cross correlation of the outputs of two vertically spaced mi- crophones is examined. The application of this method to sound level mea- surement was investigated with respect to the amount of noise reduction achieved and the effect on the accuracy of the measurement. The method proved capable of reducing wind noise by 10.6 dB for short measurement periods with fiat weighting, and up to 21.7 dB with C-weighting. A study using blast noise signals demonstrated no significant sound level measure- ment error when the two-microphone array was used; thus, the method was shown to be feasible for use in blast noise measurement. Although some windscreens achieve better performance, it should be possible to use the two-microphone method in combination with a windscreen for a greater combined wind noise reduction than either could achieve alone.

Active modification of the acoustic wall impedance for normal in- cidence [43.85.Bh, 43.20.Mv, 43.88.Ew]mK•ithe Karcher, Drittes Phy- sikalisches Institut, Univ. of Gi3ttingen 1982 (Ph.D.). An electroacoustic sys- tem has been developed which allows, for one-dimensional sound propagation, an active control of the acoustic wall impedance. A commer- cial impedance tube is terminated at its open end by a loudspeaker which is fed from the incident wave through a two-microphone system and appropri- ate control electronics. The microphones together with some delay units

I

separate the standing-wave field into incident and reflected waves, thus securing stable feedback operation, and providing additionally a very con- venient means of measuring the complex reflection coefficient. By proper adjustment of the feedback electronics, arbitrary values of the reflection coefficient can be realized between almost 0 and 1.5 with any phase, in the frequency range from 100 through 800 Hz. Broadband operation is realized by inserting a compensation filter equalizing the overall frequency response. Sophisticated feedback electronics have been developed for automatic read- justment of the apparent impedance after performance drift of components or changes in sound propagation properties.

Thesis Advisors: Prof. Dr. M. R. Schroeder and Dr. D. Guicking. A limited number of copies in German are available from Dr. D. Guicking, Drittes Physikalisches Institut, Biirgerstr. 42-44, D-3400 Gfittingen, Fed. Rep. of Germany.

Free-field investigation on coherent-active control of the acoustic wall impedance [43.20.Ye, 43.55.Br, 43.55.Dt]•Mathias Rollwage, Drittes Physikalisches Institut, Univ. of Gi3ttingen 1984 (Ph.D.). In continu- ation of the aforementioned investigation by K. Karcher, active wall linings have been investigated in three-dimensional space by experiments and com- puter simulations. As one step towards the development of active systems capable of modifying the reverberation time or rooms, the potentials of an active test wall 3 X 3 loudspeakers have been studied in an anechoic room. Test signals were sinusoidal sound waves at normal and oblique incidence. The loudspeakers are driven by the incident sound through a directional microphone system and amplifiers with adjustable gain and phase shift. The problem of measuring the effective reflection coefficient has been solved by fitting the parameters of a model function to the experimental sound pres- sure distribution between primary source and active wall. It is possible to realize amplitude reflection coefficients between less than 0.1 and more than 2.0 both for normal and oblique incidence, the upper frequency limit (here: 750 Hz) being determined by the loudspeaker spacing. Computer simulations confirm the experimental results so that the concept of active modification of wall impedance by open-loop control seems to be feasible provided that the systems work in more complex sound fields, too, and the frequency response can be equalized by additional filters.

Thesis Advisors: Prof. Dr. M. R. Schroeder and Dr. D. Guicking A limited number of copies in German are available from Dr. D. Guicking, Drittes Physikalisches Institut, Biirgerstr. 42-44, D-3400 Gtttingen, Fed. Rep. of Germany.

810 J. Acaust. Sac. Am. 78(2), August 1985; 0001-4966/85/08810-01500.80; ̧ 1985 Acaust. Sac. Am.; Notes and Briefs 810

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