On whistling of pipes with a corrugated segment: Experiment and theory

Corrugated pipes are commonly used because of their local rigidity combined with global flexibility. The flow through such a pipe can induce strong whistling tones, which is an environmental nuisance and can be a threat to the mechanical integrity of the system. This paper considers the use of a composite pipe: a shorter corrugated pipe segment embedded between smooth pipe segments. Such a pipe retains some flexibility, while the acoustical damping in the smooth pipe reduces whistling tones. Whistling is the result of coherent vortex shedding at the cavities in the wall. This vortex shedding is synchronized by longitudinal acoustic waves traveling along the pipe. The acoustic waves trigger the vortex shedding, which reinforces the acoustic field for a critical range of the Strouhal number values. A linear theory for plane wave propagation and the sound production is proposed, which allows a prediction of the Mach number at the threshold of whistling in such pipes. A semi-empirical approach is chosen to determine the sound source in this model. This source corresponds to a fluctuating force acting on the fluid as a consequence of the vortex shedding. The functional form of the Strouhal number dependency of the dimensionless sound source amplitude is based on numerical simulations. The magnitude of the source and the Strouhal number range in which it can drive whistling are determined by matching the model to results for a specific corrugated pipe segment length. This semi-empirical source model is then applied to composite pipes with different corrugated segment lengths. In addition, the effect of inlet acoustical convective losses due to flow separation is considered. The Mach number at the threshold of whistling is predicted within a factor 2.     

Whistling of a pipe system with multiple side branches: Comparison with corrugated pipes

Corrugated pipes are widely used because they combine local rigidity with global flexibility. Whistling induced by flow through such pipes can lead to serious environmental and structural problems. The whistling of a multiple side branch system is compared to the whistling behavior of corrugated pipes. The study has been restricted to cavities with sharp edges which are convenient for theoretical modeling. The side branch depth is chosen to be equal to the side branch diameter, which corresponds to cavity geometries in typical corrugated pipes. The low frequency resonance modes of the multiple side branch system have been predicted by means of acoustic models, of which the validity has been tested experimentally. Several experiments have been carried out for characterizing the whistling behavior of the system. While the behavior of a multiple side branch system is interesting on its own it can be compared to that of corrugated pipes. These experiments show that the multiple side branch system is in many aspects a reasonable model for corrugated pipes. Advantage of the multiple side branch system is that it is an experimental setup allowing easy modification of cavity depth. We used this feature to identify the pressure nodes of the acoustic standing wave along the main pipe as the regions where sound is produced. This contradicts recent publications on corrugated pipes. Another interesting aspects is that the system appears to whistle at the second hydrodynamic mode of the cavities rather than at the first hydrodynamic mode. A prediction model for the whistling behavior is proposed, consisting of an energy balance, based on the vortex sound theory. The model predicts the observed Strouhal number but overestimates the acoustic fluctuation amplitude by a factor four.     

Whistling of an orifice in a reverberating duct at low Mach number

An experimental investigation of the parameters controlling the whistling frequency and amplitude of an orifice in a confined turbulent flow is undertaken. A circular single hole orifice with sharp edges, a hole diameter equal to 0.015 m and a thickness equal to 0.005 m, is arranged in an air test rig with an inner diameter equal to 0.03 m. The Mach number ranges around 0.02 and the Reynolds number around 10(4). Variable reflecting boundary conditions are arranged upstream and downstream, and several flow velocities are tested. It is found that the Bode-Nyquist criterion accurately predicts the conditions of self-sustained oscillation and the value of the whistling frequency. Furthermore, it is found that the acoustic velocity in whistling regime varies from 1% to 15% of the steady flow velocity, and that it depends on the overall acoustic reflection of the surrounding pipe and on the Strouhal number.     

Experimental study of the influence of low frequency flow modulation on the whistling behavior of a corrugated pipe

It is well known that airflow in a corrugated pipe can excite whistling at the frequencies of the pipe's longitudinal acoustic modes. This short contribution reports on the results of experiments where a low frequency, oscillating flow with velocity magnitudes of the same order as the airflow has been added. Depending on the oscillation strength, it has been found that this flow may silence the pipe or move the whistling to higher harmonics. It is also shown that the low frequency oscillation itself may excite higher frequency whistling sounds in the pipe.     

Aeroacoustic interaction in a corrugated duct

The sound generation by an air flow in a corrugated tube is studied experimentally for different values of the corrugation pitch and different tube lengths. The Strouhal numbers of sound generated in different tubes with different flow velocities lie within 0.4–0.6. As the flow velocity increases, the Strouhal number decreases. The effect of sound absorption by an air flow in a corrugated duct is described: in a corrugated tube with a flow, at frequencies below the generation frequency, the absorption of sound produced by an external source is observed. A semiempirical model of aeroacoustic interaction in a corrugated tube is proposed. The model provides a qualitative agreement with the experiment.     

Nondestructive characterization of musical pillars of Mahamandapam of Vitthala Temple at Hampi, India

This paper presents the first scientific investigation on the musical pillars of the Vitthala Temple at Hampi, India. The solid stone columns in these pillars produce audible sound, when struck with a finger. Systematic investigations on the acoustic characteristics of the musical pillars of mahamandapam (great stage) of the Vitthala Temple have been carried out. The 11 most popular pillars that produce sounds of specific musical instruments are considered for the investigations. The sound produced from these 11 most popular musical pillars was recorded systematically and different nondestructive testing techniques such as low frequency ultrasonic testing, impact echo testing, and in situ metallography were employed on the musical columns of these pillars. The peak frequencies in the amplitude spectrum of the sound produced from various columns in these pillars are correlated with the dimensional measurements and ultrasonic velocity determined using impact echo technique. The peak frequencies obtained experimentally have been found to have excellent correlation with the calculated flexural frequencies based on the dimensional measurements and ultrasonic velocities of the columns.     

Fundamental aspects of pulse phase-locked loop technology-based methods for measurement of ultrasonic velocity.

A new instrument based on a constant frequency pulse phase-locked loop concept has been developed to accurately measure the ultrasonic phase velocity in condensed matter. Measurements of the sound velocity in ultrapure water are reported in which both damped and undamped transducers are used with the instrument together with reflectors of various thicknesses placed in the sound propagation path. An analysis of measurements made with the new instrument and similar measurements, taken under identical experimental conditions, using a popular variable frequency pulsed-phase-locked loop instrument is reported. Uncertainties in both measurement systems are analyzed and discussed. A method for measuring inherent phase shifts, not addressed by previous investigators, within the variable frequency pulsed phase-locked loop system and a derivation of the equations that govern the overall use of variable frequency systems using phase-sensitive comparisons are presented. The effects of a finite pulse length on the measurements of phase velocity in dispersive media are addressed in detail.     

椰胡声功率级的半消声室测试

椰胡是具有地方特色的拉弦乐器之一,多用于演奏广东音乐和广东戏曲、曲艺的伴奏。但对其发声强度一直未进行过科学的测试。本文介绍在华南理工大学半消声室内参照ISO(GB)标准对椰胡声功率级的测量工作。由两位资深乐师分别用两把椰胡在pp,mp,f和ff力度下演奏单音、音阶和乐曲,对每一把椰胡的每一测试内容,由十通道测试设备同步测试中心频率为100～10000 Hz的1/3倍频带声压级谱,通过计算获得每把椰胡在演奏每项内容时的声功率级和动态范围。通过对两把椰胡的测试结果进行平均,获得该乐器在演奏上述内容时的典型声功率级数值及频谱。文中并将半消声室内的测试结果与混响室内的测试结果相对比,探讨测试环境对测试结果的影响。民族乐器发声强度及其频谱特性的获得是开展民族音乐厅堂音质研究的基础。     

电子琴频谱的实验研究

介绍一个学生实验，用最基本的频谱分析方法，以ＣＡＳＩＯ ＭＴ－１８电子琴作声源，对其中四种模拟乐器乐音信号的频谱进行测量及分析。     

The metre

A musical wind instrument transforms a constant pressure input from the player's mouth into a fluctuating pressure output in the form of a radiating sound wave. In reed woodwind and brass instruments, this transformation is achieved through a nonlinear coupling between two vibrating systems: the flow control valve formed by the mechanical reed or the lips of the player, and the air column contained by the pipe. Although the basic physics of reed wind instruments was developed by Helmholtz in the nineteenth century, the application of ideas from the modern theory of nonlinear dynamics has led to recent advances in our understanding of some musically important features of wind instrument behaviour. As a first step, the nonlinear aspects of the musical oscillator can be considered to be concentrated in the flow control valve; the air column can be treated as a linear vibrating system, with a set of natural modes of vibration corresponding to the standing waves in the pipe. Recent models based on these assumptions have had reasonable success in predicting the threshold blowing pressure and sounding frequency of a clarinet, as well as explaining at least qualitatively the way in which the timbre of the sound varies with blowing pressure. The situation is more complicated for brass instruments, in which the player's lips provide the flow valve. Experiments using artificial lips have been important in permitting systematic studies of the coupling between lips and air column; the detailed nature of this coupling is still not fully understood. In addition, the assumption of linearity in the air column vibratory system sometimes breaks down for brass instruments. Nonlinear effects in the propagation of high amplitude sound waves can lead to the development of shock waves in trumpets and trombones, with important musical consequences.     

Some characteristics of the concert harp's acoustic radiation

The way a musical instrument radiates plays an important part in determining the instrument's sound quality. For the concert harp, the soundboard has to radiate the string's vibration over a range of 7 octaves. Despite the effort of instrument makers, this radiation is not uniform throughout this range. In a recent paper, Waltham and Kotlicki [J. Acoust. Soc. Am. 124, 1774-1780 (2008)] proposed an interesting approach for the study of the string-to-string variance based on the relationship between the string attachment position and the operating deflection shapes of the soundboard. Although the soundboard vibrational characteristics determine a large part of the instrument's radiation, it is also important to study directly its radiation to conclude on the origins of the string-to-string variation in the sound production. This is done by computing the equivalent acoustical sources on the soundboard from the far field sound radiation measured around the harp, using the acoustic imaging technique inverse frequency response function. Results show that the radiated sound depends on the correlation between these sources, and the played string's frequency and location. These equivalent sources thus determine the magnitude and directivity of each string's partial in the far field, which have consequences on the spectral balance of the perceived sound for each string.     

Vibrational frequencies and tuning of the African mbira

The acoustic spectrum of the mbira, a musical instrument from Africa that produces sound by the vibration of cantilevered metal rods, has been measured. It is found that the most prominent overtones present in the spectrum have frequencies that are approximately 5 and 14 times the lowest frequency. A finite-element model of the vibration of the key that takes into account the acoustic radiation efficiency of the various normal modes reveals that the far-field power spectrum is dominated by modes involving predominately transverse motion of the key. Modes involving longitudinal motion do not radiate efficiently, and therefore contribute little to the sound produced. The high frequencies of the dominant overtones relative to the fundamental make it unlikely that the tunings of the mbira that are used by expert musicians are determined by matching the fundamental frequencies of the upper keys with the overtones of the lower keys.     

Roughness of organ pipe sound due to frequency comb

In the sound spectrum of flue organ pipes in addition to the usual harmonic partials, sometimes a series of equidistant but not harmonic lines can be found. This phenomenon has been observed in the recorded sound of pipes from different pipe ranks. The second set of spectral lines is similar to "frequency combs" used in optics for accurate measurement of optical frequencies. Analysis of measured sound spectra with and without frequency comb and simulations are presented and discussed in the paper. The appearance of frequency combs in the sound spectrum is explained by a model that assumes the presence of a mouth tone in addition to the pipe sound. Mouth tone bursts are generated when the oscillating air jet passes the upper lip. The burst repetition frequency is locked to the fundamental frequency of the pipe and the bursts are coherent with a pulse-to-pulse phase shift. The phase shift explains the observed frequency offset of the frequency comb to the harmonic frequencies. The simulations also show that weak and fluctuating mouth tones cannot generate frequency comb due to a lack of coherence.     

南海北部负跃层环境下海底参数声学反演

利用2015年南海北部声学实验的实验数据,对海底声学参数进行反演。在综合分析声速起伏及海底模型对海底反演影响的基础上,通过选择等效的海水声速剖面和海底模型,改进了多参量联合地声反演方法,使得其不仅能解决反演中的多值性问题,还能适用于负跃层起伏环境下的声学反演。即当用匹配场反演海底声速和用传播损失反演海底衰减时,如果温跃层内有内波等随机起伏,可使用传播路径上平均声速剖面和单层等效海底进行声场计算。反演得到的海底声速和密度结果与海底采样测量符合较好,拟合给出了海底衰减系数随频率的非线性经验关系式。反演结果可为南海北部声传播规律研究与应用提供海底参数。      

抑制风噪装置的实验

本文基于风噪原理提出了一种抗风噪结构设计,来抑制传声器拾取风噪。该设计由渐变管道和填充泡棉的腔体两部分组成。渐变管道用来降低气流流速,填充泡棉的腔体用来充当声屏障。COMSOL有限元仿真验证了渐变管道降低流速的能力,风噪实验证明了该设计能有效地抑制风噪。讨论了关键因素对该结构抗风噪能力的影响：大的腔体,小的渐变管道等效直径和高的泡棉密度,都有利于抗风噪效果。最后,基于声传输线和等效电路理论,建立了声波经过该抗风噪结构的传递损失模型。基于该模型和高频不失真条件,提出了抗风噪结构的尺寸设计指标。     

Characterization of acoustic streaming in water and aluminum melt during ultrasonic irradiation

It is well known that ultrasonic cavitation causes a steady flow termed acoustic streaming. In the present study, the velocity of acoustic streaming in water and molten aluminum is measured. The method is based on the measurement of oscillation frequency of Karman vortices around a cylinder immersed into liquid. For the case of acoustic streaming in molten metal, such measurements were performed for the first time. Four types of experiments were conducted in the present study: (1) Particle Image Velocimetry (PIV) measurement in a water bath to measure the acoustic streaming velocity visually, (2) frequency measurement of Karman vortices generated around a cylinder in water, and (3) in aluminum melt, and (4) cavitation intensity measurements in molten aluminum. Based on the measurement results (1) and (2), the Strouhal number for acoustic streaming was determined. Then, using the same Strouhal number and measuring oscillation frequency of Karman vortices in aluminum melt, the acoustic streaming velocity was measured. The velocity of acoustic streaming was found to be independent of amplitude of sonotrode tip oscillation both in water and aluminum melt. This can be explained by the effect of acoustic shielding and liquid density.     

Inline ultrasonic rheometry by pulsed Doppler

This will be a discussion of the non-invasive determination of the viscosity of a non-Newtonian fluid in laminar pipe flow over the range of shear rates present in the pipe. The procedure used requires knowledge of the flow profile in and the pressure drop along a long straight run of pipe. The profile is determined by using a pulsed ultrasonic Doppler velocimeter. This approach is ideal for making non-invasive, real-time measurements for monitoring and control. Rheograms of a shear thinning gel will be presented. The operating parameters and limitations of the Doppler-based instrument will be discussed. The most significant limitation is velocity gradient broadening of the Doppler spectra near the walls of the pipe. This limitation can be significant for strongly shear thinning fluids (depending also on the ratio of beam to pipe diameter and the transducer's insertion angle).     

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。     

Acoustical analysis of pipes with flow using invariant field functions

An analysis of the pressure field within a pipe is carried out using simplified formulations of pipe acoustics. The fluid contained within the pipe is considered non-viscous, while the flow velocity of the fluid is assumed to be smaller than the speed of sound.The analysis is limited to frequencies which are well below the pipe ring frequency, i.e., at which only simple waves can propagate. Expressions and diagrams are given which specify the applicable frequency range in each particular case.Three invariant functions of the internal pressure field are evaluated. These functions allow for the determination of the following quantities: base pressure spectrum (spatial mean r.m.s. value), lower and upper bounds of the pressure spectrum for the entire pipe, pressure spectrum at an arbitrary position, speed of sound in the contained fluid and fluid flow velocity.Experimental identification of these quantities requires simultaneous measurement at three points. A few measurements carried out on one air-filled and one water-filled pipe have demonstrated the potential of pipe invariant functions for acoustical analysis.