83RD MEETING ß ACOUSTICAL SOCIETY OF AMERICA

2:30 2:45

T6. Spectrum Development with Playing Level in Wind In- struments. W. E. WORMAN, Case Western Reserve University, Cleveland, Ohio 44106.--Starting from the theory of musical wind instruments proposed by Benade and Gans and further developed by the author, it is possible to draw very general conclusions about the development of the spectrum in the mouthpiece as a function of loudness. A threshold'blowing pressure exists below which no oscillations are possible. Near threshold, the pressure amplitude of the fundamental is pro- portional to the square root of the excess of the blowing pres- sure over its threshold value. The pressure amplitudes of the higher harmonics can be written as Pn = (P•/E,O '• IZ,• [, where P• is the pressure amplitude of the nth harmonic, IZ•[ is the magnitude of the input impedance of the bore at the frequency of the nth harmonic, and En is a constant that can be evalu- ated experimentally and calculated for simple systems. Ex- periments by Benade on many types of wind instruments (including clarinet, bassoon, trumpet, and saxophone) show that this form of spectrum development takes place over a surprisingly large range. In many instruments, the range is about 40 dB in the pressure amplitude of the fundamental (pianissimo to mezzoforte). Observed departures from this behavior may be understood in terms of the same basic theory.

T7. Real-Time Music Analysis for the Performer. INGO R. TITZE AND WILLIAM J. STRONG, Department of Physics, Brigham Young University, Provo, Utah 84601.--Waveform, pitch, dynamics, and 32 harmonic components of a tone with a specified nominal pitch are displayed on a computer-driven CRT. The rate of display is approximately 10/sec, thus pre- senting essentially real-time analysis. The nominal pitch and the expected pitch deviation are specified by the performer. The pitch deviation can range from a quarter step to a whole step above and below the nominal pitch. The performer can visually adjust his average pitch, monitor his vibrato rate and range, check his dynamics, and observe his spectrum while he sings or plays into the microphone. As an additional feature, the spectrum may be averaged over any specified time inter- val. The spectrum is obtained by taking a 64 point FFT over a single period of the waveform. The period is determined by sweeping the pitch over the specified range and selecting the best value on the basis of the minimum of a one-cycle average of the waveform and a "fractional period correlation" of adjacent cycles. The present system is comprised of a 30-kHz A/D converter and a PDP-15 computer system with associ- ated graphics.

WEDNESDAY, 19 AVRIL 1972 EMma•: Sxax•: RooM, 1:30 v.M.

Session U. Noise I: Propagation of Sound in Ducts

ISTV.KN VP-R, Chairman

Bolt Beranek and Newman Inc., Cambridge, Massachusetts 02138

Contributed Papers (12 minutes)

1:30

U1. Effects of High-Intensity Sound on Muffler Element Performance. M.P. SACKS AND D. L. ALLEN, Department of Mechanical Engineering, University of Toronto, Toronto, Ontario.--This paper presents the results of an experimental investigation designed to study the effect of high-intensity sound on muffler element performance. Five prototype mufflers were built and tested over a wide range of frequency and intensity. The mufflers were chosen as being representa- tive of basic single element and combination reactive mufflers. The test facility consisted of an acoustic transmission line driven by an electromagnetic sound source and terminated anechoically. Attenuation data is presented superimposed on curves derived from linear acoustic theory. Conclusions are stated for expansion chamber mufflers at higher SPLs than could be produced by the test equipment by using constric- tion-tube data and acoustic similarity between these elements. It is shown that the expansion chamber muffler will operate as a linear element when subject to incident SPLs of 165-180 dB depending on muffler attenuation. Mufflers containing resonator elements show no deviation from a linear theory when subject to SPLs of the order of 160 dB except at points near a resonant frequency.

1:45

U2. Acoustic Impedance of Perforated Plate Liners with Two- Frequency Excitation. A. N. ABDELHAMID, Faculty of Engineer-

ing, Carleton University, Ottawa, Ontario, K1S 5B6, Canada.- The acoustic impedance of a lining material has been previ- ously shown to be dependent on the noise spectrum impressed upon the liner and several analytical models for its determina- tion have been recently proposed. The results of these models, however, seem to be in very good agreement with experi- mental data for single-frequency excitation, but not as good with the limited available data for two-frequency excitation. In this paper, an impedance tube with two sound drivers at one end is used to investigate the performance of perforated plate liners under two-frequency excitation. The two fre- quencies varied from 800 to 2000 Hz with SPLs at the liner up to 145 dB. The plate thickness varied from 0.05 to 0.20 in. with open area ratio ranging from 0.05 to 0.10. The experi- mental results indicate that, for the same SPL at the liner, the effect of excitation at the liner resonant frequency on the acoustic resistance at other frequencies is considerably higher than the reverse effect. Based on the experimental data ob- tained, modifications to the previously proposed impedance models are discussed.

2:00

U3. Sound Attenuation in Ducts Lined with Nonisotropic Material. U. J. KURZE AND I. L. V•R, Bolt Beranek and Newman Inc., Cambridge, Massachusetts 02138.--This paper deals with the sound attenuation in rectangular ducts with nonisotropic lining. The nonisotropy may be due to the basic structure of the fibrous lining material or it may be deliber-

148 Volume 52 Number 1 (Part 1) 1972

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