Application of acoustic feedback to target detection in a waveguide: experimental demonstration at the ultrasonic scale

People are familiar with the acoustic feedback phenomenon, which results in a loud sound that is heard when a musician plays an electric instrument directly into a speaker. Acoustic feedback occurs when a source and a receiver are connected both acoustically through the propagation medium and electrically through an amplifier, such that the amplified received signal is continuously re-emitted by the source. The acoustic feedback can be initiated from a continuous sine wave. When the emitter and the receiver are in phase, resonance is obtained, which appears to be highly sensitive to any fluctuation of the propagation medium. Another procedure consists in initiating the acoustic feedback from a continuous loop of ambient noise. It then generates an unstable self-sustained feedback oscillator (SFO) that is tested here as a method for monitoring temperature fluctuations of a shallow-water oceanic environment. The goal of the present study is to reproduce and study the SFO at the laboratory scale in an ultrasonic waveguide. The experimental results demonstrate the potential applications of the SFO for the detection of a target in the framework of the acoustic-barrier problem in shallow-water acoustics.     

基于LabVIEW的频率-音分转换设计

当前已有大量研究人员使用基于Lab VIEW的虚拟声频信号分析系统开展研究工作,Lab VIEW正在成为声频信号测量分析的主要研究手段,但是现有成果均使用了以频率为代表的物理声学单位,没有同时提供在音乐实践常用的音分标记法,影响了研究成果的普适性。本文通过分析频率与音分的转换方法,使用Lab VIEW为平台编写频率与音分转换应用,结果表明,基于Lab VIEW的频率与音分转换应用,应用运算稳定,适用性好。该设计方便了物理声学单位转换为音乐声学单位,便于研究成果能够推广到更广泛的声学研究中去。     

中国筝的声功率级测试

筝是中国古老的弹弦乐器。但迄今为止,对其声功率级一直未进行科学的测定。本文在一混响室内根据ISO及GB标准,对二十一弦筝的声功率级进行了首次测定。两位资深乐师在混响室内分别演奏各自的乐器,通过围绕乐师和乐器布置的四通道测试设备,对筝所辐射的声功率级和动态范围进行测定。测试结果表明,中国筝在以不同力度演奏单音、音阶和乐曲时所辐射的声功率级及其频率特性均有所不同。考虑到乐器演奏音阶时所辐射的声功率级及其动态范围与演奏乐曲时的声功率级接近,并且,音乐的空间感也大都在乐器以f力度演奏乐曲的强音标志乐段时最为显著,故此我们建议中国筝所辐射的声功率级用其以f力度演奏音阶时的平均声功率级表示。本文测试的两架中国筝以f力度演奏音阶时的平均声功率级为85.9dB。文中不仅首次公布了中国筝声功率级的测试结果,并且所介绍的测试方法对其它乐器声功率级测试也具有借鉴意义。民族乐器所辐射的声音性能的确定是民族音乐厅堂音质研究的基础。     

The physics exhibition, 1971

There has been a considerable upsurge of interest in musical acoustics over the last 10 to 15 years and this article is an attempt to summarize the present position in this multidisciplinary field. It first looks at the essential physics involved, reduced to its simplest terms, goes on to consider the relationships between real musical instruments and the resonance tubes and sonometers of the old text-books on ‘sound’ and outlines the history of the subject up to the Second World War. The main section then discusses areas of current study and the relevant measurement techniques in relation to three of the main groups of musical instruments. The article concludes with a discussion of some aspects of the difficult, but all-important, subject of psycho-physics and a brief attempt to predict the future prospects for research in musical acoustics     

Study of the brightness of trumpet tones

This study focuses on a particular attribute of trumpet tones, the brightness, and on the physical characteristics of the instrument thought to govern its magnitude. On the one hand, an objective study was carried out with input impedance measurements, and, on the other hand, a subjective study with hearing tests and a panel of subjects. To create a set of different trumpets a variable depth mouthpiece was developed whose depth can be easily and continuously adjusted from "deep" to "shallow." Using this mouthpiece and the same trumpet, several instruments were generated which may be played in three ways: (i) by a musician, (ii) by an artificial mouth, and (iii) using physical modeling simulations. The influence of the depth of the mouthpiece on the perception of the trumpet's tones was investigated, and the ability of a musician, the artificial mouth, or physical modeling simulations to demonstrate perceptively noticeable differences was assessed. Physical characteristics extracted from the impedance curves are finally proposed to explain the brightness of trumpet tones. As a result, the physical modeling simulations now seem to be mature enough to exhibit coherent and subtle perceptual differences between tones. This opens the door to virtual acoustics for instrument makers.     

Effects of using scalloped shape braces on the natural frequencies of a brace-soundboard system

Many prominent musical instrument makers shape their braces into a scalloped profile. Although reasons for this are not well known scientifically, many of these instrument makers attest that scalloped braces can produce superior sounding wooden musical instruments in certain situations. The aim of this paper is to determine a possible reason behind scalloped shaped braces. A simple analytical model consisting of a soundboard section and a scalloped brace is analyzed in order to see the effects that changes in the shape of the brace have on the frequency spectrum of the brace-soundboard system. The results are used to verify the feasibility of adjusting the brace thickness in order to compensate for soundboards having different stiffness in the direction perpendicular to the wood grain. It is shown that scalloping the brace allows an instrument maker to independently control the value of two natural frequencies of a combined brace-soundboard system. This is done by adjusting the brace’s base thickness in order to modify the 1st natural frequency and by adjusting the scalloped peak heights to modify the 3rd natural frequency, both of which are considered along the length of the brace. By scalloping their braces, and thus controlling the value of certain natural frequencies, musical instrument makers can improve the acoustic consistency of their instruments.     

基于线性状态反馈的混沌系统全局控制

针对一类混沌系统,通过分析其非线性向量场的结构特征,提出了一种全局快速控制混沌的新方法.利用线性状态反馈,分离出一个稳定的独立状态变量方程,使受控的混沌系统逐渐退化为稳定的线性系统,进而完全消除非线性抖振现象.此方法可应用到Lorenz系统族或具有类似结构的新混沌系统的控制中,相应设计出一种结构简单的控制器,保证闭环系统在原点全局渐近稳定.数值算例验证了所提出方法的快捷性和正确有效性. 关键词： 混沌系统 线性状态反馈 全局稳定     

Warm string tone in acoustics

The result of a basic warm string tone in a concert hall is that the whole orchestra not only sounds extremely well balanced, but the individual instruments outside the string family sound warmer and richer, and no attention is drawn to their extraneous noises. On the basis of orchestral conducting, performing, recording, and listening experience, some conclusions are reached, and suggestions made, concerning the physical conditions that are likely to be necessary, or conductive, to attainment of warm string tone. Among the physical factors considered are the acoustic and vibratory properties of the stage itself, hall volume, shape and reverberation time, and surface materials. Emphasis is placed, in discussion of the physical and musical factors, on the ways in which the musicians themselves, during performance, interact with “the acoustics”.     

Broadband performance of an active headrest.

This paper presents a study of the attenuation of broadband random acoustic disturbances, when using a feedback active headrest system, as originally suggested by Olson and May. Previous studies showed that a practical active headrest can be designed for tonal disturbances using feedforward controllers. However, many applications, such as jet aircraft and cars, require feedback systems to control random disturbances over a wide frequency bandwidth. In this work, robust feedback controllers are designed to control broadband random disturbances in the low-frequency range based on measured data from a laboratory headrest system. The results show that a practically useful performance can be achieved, but only if the controller is designed to minimize the pressure at a "virtual microphone" close to the listener's ears, and that the performance is maintained reasonably well with movements of the listener's head. The paper emphasizes the importance of both the acoustics and the control in the design of broadband active headrest systems.     

Life and work of E.F.F. Chladni

Ernst Florens Friedrich Chladni, the son of the rector of the University of Wittenberg, studied law and philosophy at the Universities of Wittenberg and Leipzig, wrote a dissertation in each of these disciplines and became an experimentalist concerned with sound and vibrations. Chladni's name is associated with the Chladni figures, and this is the first real effort to experimentally investigate the nature of sound. In the lecture the publications of Chladni are presented and, cursorily, his new instruments, designed and built by the physicist. Chladni began to tour half Europe in his own coach with sufficient space for his musical instruments. He gave lectures on the physics of plate and rod vibrations and included demonstrations of the figures and his musical instruments. Chladni's work has had a profound influence on the experimental advances in acoustics, room acoustics, in the verification of theories of superposition of waves, in elasticity, vibrational modes, sound velocities in various media, and much more. The lecture presents a portrait of the scientist Chladni, the times, and his contributions to experimental acoustics which influenced research for many decades.     

Evaluating the suitability of acoustical measurement techniques and psychophysical testing for studying the consistency of musical wind instrument manufacturing

Musicians often claim to be able to discern differences in the playing properties of musical wind instruments that have been manufactured in exactly the same way. These differences are most likely due to disparities in bore profile or in the positioning and sealing of any valves or side holes. In this paper, the suitability of acoustic pulse reflectometry and a capillary-based impedance measurement technique for detecting differences between instruments of the same model is explored through measurements on two low-cost, mass-produced trumpets. Differences in the measured bore profiles of the two instruments are reported, with the largest deviation caused by the presence of a leak in the third valve of one of the trumpets. Differences in input impedance measurements made on the two instruments are also noted, with the main cause shown to be the leaky valve. Controlled playing tests are carried out using the same two trumpets in order to evaluate the effectiveness of psychophysical testing in establishing whether there are perceptible differences in the playing properties of nominally identical wind instruments. A semi-professional musician is proved to be able to discriminate between the trumpets whereas an amateur player is shown to be unable to do the same.     

Active control of double-glazed windows. Part II: Feedback control

This paper describes an experimental investigation of an actively controlled double-glazed window. It is the second of two companion papers of which the first treated results obtained employing adaptive feedforward control. Herein, the outcome using adaptive feedback control is presented. This adaptive feedback controller has been tested in different configurations, i.e. fully and partially connected controllers. The differences between fully connected controllers with few filter coefficients and partially connected controllers with many filter coefficients are discussed. Additionally, tests with different traffic noise examples have been performed showing the ability of the actively controlled window to enhance protection against traffic noise.     

A theoretical study of the vibration and acoustics of ancient Chinese bells

In this paper, the acoustics of an ancient Chinese bell, which was made some 3000 years B.C., is studied theoretically. In ancient times, a set of the bells was used as a musical instrument. Unlike a western church bell and an ancient Indian bell, an ancient Chinese bell has two interesting acoustics. First, two tones can be heard separately as the bell is struck at two special points. The interval between the two pitches is always a minor or major third. Second, tones of the bell attenuate quickly, which is necessary for a musical instrument. So, an ancient Chinese bell is sometimes called a two-tone bell or a music bell. Although a three-dimensional model should be used to simulate the acoustics of the bell, a simplified model proposed in this paper does give some insight. Based on the lens-shaped cross section of an ancient Chinese bell, two tones of an ancient Chinese bell can be simulated by the vibration of a double-circular arch and the quick attenuation of tones can be simulated by acoustics of a cylinder with the lens-shaped cross section like a double-circular arch. Numerical results on the vibration and acoustics of the models are presented.     

Oscillation threshold of a clarinet model: a numerical continuation approach

This paper focuses on the oscillation threshold of single reed instruments. Several characteristics such as blowing pressure at threshold, regime selection, and playing frequency are known to change radically when taking into account the reed dynamics and the flow induced by the reed motion. Previous works have shown interesting tendencies, using analytical expressions with simplified models. In the present study, a more elaborated physical model is considered. The influence of several parameters, depending on the reed properties, the design of the instrument or the control operated by the player, are studied. Previous results on the influence of the reed resonance frequency are confirmed. New results concerning the simultaneous influence of two model parameters on oscillation threshold, regime selection and playing frequency are presented and discussed. The authors use a numerical continuation approach. Numerical continuation consists in following a given solution of a set of equations when a parameter varies. Considering the instrument as a dynamical system, the oscillation threshold problem is formulated as a path following of Hopf bifurcations, generalizing the usual approach of the characteristic equation, as used in previous works. The proposed numerical approach proves to be useful for the study of musical instruments. It is complementary to analytical analysis and direct time-domain or frequency-domain simulations since it allows to derive information that is hardly reachable through simulation, without the approximations needed for analytical approach.     

Acoustic characterization of on-stage performers in performing spaces

The influence of on-stage performers on the acoustic characteristics of performing spaces is significant because the musicians are absorptive and close to the sound source. However, the acoustic information of the musicians and their effect on absorption, scattering, and diffusion remain unclear. The acoustic characteristics of musicians were measured in a reverberation chamber and a semi-anechoic chamber while varying the type of clothes, the instruments, and seating density. The clothing worn showed a larger impact on the absorption per person, whereas the addition of cellos resulted in low-frequency absorption per person. The addition of cellos also increased the scattering and diffusion characteristics. Finally, the total absorption by the musicians under various conditions was analyzed in a concert hall using the simple Sabine equation where the influence of the musician’s instruments resulted in reverberation time of more than 0.1 s decrease with about 90 musicians on stage. However, the influences of additional musical instruments resulted in no significant difference in the reverberation time compared to the people alone. In addition, scattering due to the various conditions of orchestra such as the seating density and the addition of musical instrument was not a significant predictor of reverberation time.     

Real-time synthesis of clarinet-like instruments using digital impedance models

A real-time synthesis model of wind instruments sounds, based upon a classical physical model, is presented. The physical model describes the nonlinear coupling between the resonator and the excitor through the Bernoulli equation. While most synthesis methods use wave variables and their sampled equivalent in order to describe the resonator of the instrument, the synthesis model presented here uses sampled versions of the physical variables all along the synthesis process, and hence constitutes a straightforward digital transposition of each part of the physical model. Moreover, the resolution scheme of the problem (i.e., the synthesis algorithm) is explicit and all the parameters of the algorithm are expressed analytically as functions of the physical and the control parameters.     

Speaker compensation for local perturbation of fricative acoustic feedback

Feedback perturbation studies of speech acoustics have revealed a great deal about how speakers monitor and control their productions of segmental (e.g., formant frequencies) and non-segmental (e.g., pitch) linguistic elements. The majority of previous work, however, overlooks the role of acoustic feedback in consonant production and makes use of acoustic manipulations that effect either entire utterances or the entire acoustic signal, rather than more temporally and phonetically restricted alterations. This study, therefore, seeks to expand the feedback perturbation literature by examining perturbation of consonant acoustics that is applied in a time-restricted and phonetically specific manner. The spectral center of the alveopalatal fricative [∫] produced in vowel-fricative-vowel nonwords was incrementally raised until it reached the potential for [s]-like frequencies, but the characteristics of high-frequency energy outside the target fricative remained unaltered. An "offline," more widely accessible signal processing method was developed to perform this manipulation. The local feedback perturbation resulted in changes to speakers' fricative production that were more variable, idiosyncratic, and restricted than the compensation seen in more global acoustic manipulations reported in the literature. Implications and interpretations of the results, as well as future directions for research based on the findings, are discussed.     

A platform for manipulation and examination of the acoustic guitar: The Chameleon Guitar

A platform for manipulation and examination the acoustic guitar is presented, based on a novel guitar design – the Chameleon Guitar – featuring a replaceable acoustic resonator functioning as the soundboard of the instrument. The goal of the design process is to create a tone as sonically close to that of a traditional guitar as possible, while maintaining an easily replaceable soundboard. An iterative, data driven approach was used, each design step coming under examination from one or more measurement tools: finite-element method, acoustic impulse testing, and laser vibrometry. Ideal resonator geometry, bridge location, and piezoelectric sensor positions were determined. The finished instrument was then examined with laser vibrometry to confirm earlier results, evaluate the behavior and chosen sensor positions for various tonewoods, and examine the acoustic effects of adding sensors and wax finish. The conclusions drawn are diverse and point to the significance of attention to detail in each step of instrument construction. For example, when changing instrument material from one softwood to another, ideal locations for piezoelectric sensors are subject to change. We conclude that detailed acoustic analysis can significantly aid in the construction of new instruments by quantifying the impact of instrument geometry and material properties.     

基于部分变量反馈的混沌系统控制

提出了混沌系统部分变量逆序反馈控制器的定义，并以陈氏系统为例对该类控制器进行了研究，得出了部分变量正序和逆序控制器存在的条件，应用该条件扩展了有关文献的研究成果，得到了混沌系统的多种形式的反馈控制器.仿真结果显示，基于部分变量的混沌系统的控制是简单而有效的. 关键词： 部分变量反馈控制 逆序控制器 混沌系统     

Applied topology optimization of vibro-acoustic hearing instrument models

Designing hearing instruments remains an acoustic challenge as users request small designs for comfortable wear and cosmetic appeal and at the same time require sufficient amplification from the device. First, to ensure proper amplification in the device, a critical design challenge in the hearing instrument is to minimize the feedback between the outputs (generated sound and vibrations) from the receiver looping back into the microphones. Secondly, the feedback signal is minimized using time consuming trial-and-error design procedures for physical prototypes and virtual models using finite element analysis. In the present work it is demonstrated that structural topology optimization of vibro-acoustic finite element models can be used to both sufficiently minimize the feedback signal and to reduce the time consuming trial-and-error design approach. The structural topology optimization of a vibro-acoustic finite element model is shown for an industrial full scale model hearing instrument.