Vibrational behavior of a soundbox in an atmosphere with a variable speed of sound

This paper describes a semi-quantitative method, suitable for a student laboratory exercise that shows that the acoustic properties of the soundbox of a musical instrument depend on the sound speed of the atmosphere surrounding and filling the instrument. A gas tent was constructed and used to enclose instruments in helium, carbon dioxide and mixtures thereof, allowing the sound speed to be varied from 250 to 1000 m/s. Soundboard admittance data were taken using a guitar and a violin as examples. The data, expressed as contour plots, show clearly the qualitative relationship between air and wood modes, and the guitar data are compared with a simple mechanical model. Experimental details of the construction and operation of gas tent are given, with attention paid to safety issues.     

Direct simulations of trailing-edge noise generation from two-dimensional airfoils at low Reynolds numbers

The aeroacoustic sound generated from the flow around two NACA four-digit airfoils is investigated numerically, at relatively low Reynolds numbers that do not prompt boundary-layer transition. By using high-order finite-difference schemes to discretize compressible Navier–Stokes equations, the sound scattered on airfoil surface is directly resolved as an unsteady pressure fluctuation. As the wavelength of an emitted noise is shortened compared to the airfoil chord, the diffraction effect on non-compact chord length appears more noticeable, developing multiple lobes in directivity. The instability mechanism that produces sound sources, or unsteady vortical motions, is quantitatively examined, also by using a linear stability theory. While the evidence of boundary-layer instability waves is captured in the present result, the most amplified frequency in the boundary shear layer does not necessarily agree with the primary frequency of a trailing-edge noise, when wake instability is dominant in laminar flow. This contradicts the observation of other trailing-edge noise studies at higher Reynolds numbers. However, via acoustic disturbances, the boundary-layer instability may become more significant, through the resonance with the wake instability, excited by increasing a base-flow Mach number. Evidence suggests that this would correspond to the onset of an acoustic feedback loop. The wake-flow frequencies derived by an absolute-instability analysis are compared with the frequencies realized in flow simulations, to clarify the effect of an acoustic feedback mechanism, at a low Reynolds number.     

The effect of the coupling between the top plate and the fingerboard on the acoustic power radiated by a classical guitar (L)

An experimental investigation on the coupling between the fingerboard and the top plate of a classical guitar at low frequencies is presented. The study was carried out using a finished top plate under fixed boundary conditions and a commercial guitar. Radiated sound power was determined in one-third octave bands up to the band of 1 kHz based on measurements of sound intensity. The results provide evidence that the way in which the fingerboard and top plate are coupled is not a relevant factor in the radiated acoustic power of the classical guitar in the studied frequency range.     

Numerical analysis of tonal airfoil self-noise and acoustic feedback-loops

In this study the role of acoustic feedback instabilities in the tonal airfoil self-noise phenomenon is investigated. First, direct numerical simulations are conducted of the flow around a NACA-0012 airfoil at Re=1×10⁵ and four angles of attack. At the two lowest angles of attack considered the airfoil self-noise exhibits a clear tonal contribution, whereas at the two higher angles of attack the tonal contribution becomes less significant in comparison to the broadband noise. Classical linear stability analysis of time-averaged boundary layer profiles shows that the tonal noise occurs at a frequency significantly lower than that of the most convectively amplified instability wave. Two-dimensional linear stability analysis of the time-averaged flowfield is then performed, illustrating the presence of an acoustic feedback loop involving the airfoil trailing edge. The feedback loop is found to be unstable only for the cases where tonal self-noise is prominent, and is found to self-select a frequency almost identical to that of the tonal self-noise. The constituent mechanisms of the acoustic feedback loop are considered, which appear to explain why the preferred frequency is lower than that of the most convectively amplified instability wave.     

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.     

Acoustical properties of petung bamboo for the top plate of guitars

In this paper, the use of petung bamboo (Dendrocalamus asper) as an alternative material for the top plate of acoustic guitars was evaluated. In the first research stage, the analysis was carried out on petung bamboo splits, which were treated with three different preservation conditions, namely unpreserved, preserved by boiling in water and preserved by boiling in a solution of 5% borax and boric acid. The vibration damping ratios and the sound radiation coefficients of these three types of splits were measured. It was found that their vibration damping ratios were not significantly different, whereas the sound radiation coefficient of the petung bamboo splits preserved in borax and boric acid was 40–60% higher than the other two variants. Based on this result, in the second research stage, three acoustic guitars with top plates from the borax–boric acid preserved petung bamboo, spruce, and pine were constructed. The top plate frequency response function of the three guitars was evaluated. Here, the spruce guitar was used as a reference for the generally preferred guitar sound characteristics, whereas the pine guitar was included as a sample of guitar made from local Indonesian wood. The results showed that the frequency response function of the petung bamboo guitar were generally five times lower than that of the spruce guitar, but two times higher than that of the pine guitar. The response amplitudes of the bamboo guitar were significantly higher than those of the spruce and pine guitars for frequencies between 200 and 400 Hz. Based on the results, it is concluded that petung bamboo has the potential to be applied as an alternative material for guitar top plates. However, since the bamboo guitars exhibits different resonances, the produced sound will have distinctive characteristics compared to the sound from guitars with spruce top plates.     

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.     

A reduced-order deterministic model describing an intermittency route to combustion instability

Recently, there has been a growing interest in understanding and characterising intermittent burst oscillations that presage the onset of combustion instability. We construct a deterministic model to capture this intermittency route to instability in a bluff-body stabilised combustor by coupling the equations governing vortex shedding and the acoustic wave propagation in a confinement. A feedback mechanism is developed wherein the sound generated due to unsteady combustion affects the vortex shedding. This feedback leads to a variation in the time of impingement of the vortices with the bluff body causing the system to exhibit chaos, intermittency, and limit cycle oscillations. Experimental validation of the model is provided using various precursor measures that quantify the observed intermittent states.     

Laser driven acoustic instabilities in materials with strain-dependent dielectric constants

A simple analysis of the laser driven acoustic wave instability in a material with strain-dependent dielectric constants is given. The analysis is based on the hydrodynamic model of a plasma in the collision dominated regime. Using coupled mode theory, the acoustic instability in the medium is investigated and the threshold value of the pump electric field and the conditions for the initial growth rates of unstable acoustic waves are deduced. It is found that large values of growth can be achieved for materials having an anomalously large dielectric constant, which is otherwise not achievable with piezoelectric interaction. Because of the non-availability of the relevant experimental data, we were unable to compare our theoretical case with an experimental one.     

Liquid-column sustainment driven by acoustic wave guiding

We report on the formation and sustainment of liquid columns with aspect ratios much larger than the value at the onset of the Rayleigh-Plateau instability. This is achieved by using the passive feedback of the radiation pressure applied on the column surface by an acoustic beam injected at the upper end of the column and guided along it. We develop an analytical model that describes the coupling between the acoustic wave guiding and the balance between acoustic and capillary surface forces exerted on the column surface and find a satisfactory agreement with the experiment.     

Fluid-structure coupling in the guitar box: numerical and experimental comparative study

Resonance boxes are common to many musical instruments and determine the radiated sound to a great extent. The behaviour of the structure and the air inside must be understood as a whole, the complexity of which is increased by the presence of sound holes. In this work, we present a comparative study of the guitar box in which the interior gas is changed both experimentally and numerically. Modal patterns, natural frequencies and quality factors are determined when the box is full of helium, air and krypton, respectively. This allows us to characterise the soundboard-back plate coupling via the cavity fluid, stressing the role of the structural and acoustic uncoupled modes. This could help guitar makers, allowing them to modify the final modes by means of structural modifications. Moreover the methodology, together with the developed finite element model, proves to be valid for studying the dynamic fluid-structure coupling in any arbitrary mechanical system, including cavities connected to the surrounding air.     

Physics-based modeling techniques of a twelve-string Portuguese guitar: A non-linear time-domain computational approach for the multiple-strings/bridge/soundboard coupled dynamics

The Portuguese guitar is a pear-shaped instrument with twelve metal strings which is widely used in Portuguese traditional music. Unlike most common guitars, it has a curved top-plate and a specific violin-like bridge which is not rigidly fixed to the soundboard of the instrument. From the dynamical point of view, if the bridge transmits the strings vibrations to the instrument body in order to maximize the radiated energy, it also couples all the component parts of the instrument which therefore interact by structural coupling. This can originate various audible effects such as beating behavior and the excitation of numerous sympathetic resonances enhanced by the large number of strings of the instrument, and this is certainly why the Portuguese guitar has such distinct sound compared to other guitars. In this paper, a fully coupled time-domain model of the Portuguese guitar is developed and a series of simulations are presented to emphasize the various coupling phenomena involved in sound production. To reproduce the main musical features, the model includes the coupled dynamics of the twelve strings supported by a bridge which interact with the body of the instrument, described through Finite-Element modeling of the soundboard of a typical Portuguese guitar. Further simple models have been devised for the string/fret interaction and the pluck excitation. Since nonlinear effects are quite apparent in the behavior of string musical instruments, the string dynamics is modeled by the Kirchhoff–Carrier equations which describe large-amplitude string vibrations, and includes the coupling between both polarizations of string motion. The coupling between the strings and the soundboard at the bridge is provided by a model of the bridge kinematics, built on the basis of simple geometrical rationale, so that the two perpendicular string motions can exchange energy back and forth. By a close examination of the energy transfers between the various subsystems of the model, we first assess the correct behavior of the physical model and then examine the respective influence of the string nonlinearity and the bridge on the nonplanar motion of the string. The fully coupled model which pertains to the restricted group of studies which deals with the complete physical-based modeling of a multi-stringed instrument, captures many important phenomena observed in practice, among which the pitch glide effect and the mutual excitation of sympathetic vibrations.     

冲击射流激波振荡与抑制

欠膨胀冲击射流具有复杂的激波结构,并伴随产生高幅值的离散频率单音.通过高速摄像获取的纹影图像并结合噪声测量,对欠膨胀冲击射流激波振荡过程、剪切层不稳定波的模态和离散频率单音的产生进行了系列研究.给出了冲击距离为5倍喷嘴出口直径的复杂流动实验结果分析,射流剪切层不稳定波有对称和非对称两种模态,发现不同模态下的离散频率单音...     

Experimental study of whistle noise generated by insert edge and oil separator model of refrigeration cycle

The compressed air experiments are conducted to investigate the whistle noise radiated from the oil separator component of refrigerant cycle system. Two types of insert edge and a height-adjustable oil separator model are adopted. The acoustic characteristic of flow through plain top insert edge is more likely to be broadband. Flow through ramp top insert edge would induce whistle peaks at Mach number higher than 0.1197 and the oil separator model works like an acoustic amplifier. Moreover, the Strouhal number jumps are captured inside oil separator model and two mechanisms are presented to clarify the phenomena. First, the whistle noise at Modes A1, A3, B1 and B3 is regarded as coupling of shear layer instability with resonance acoustic modes of oil separator model. Second, the whistle noise at Modes A2 and B2 is regarded to be the feedback loop of flow-acoustic interaction in fluctuated shear layer and defined as jet-cavity interaction tones.     

Radiation efficiency of a guitar top plate linked with edge or corner modes and intercell cancellation

This paper was based on a theoretical framework to determine strong and weak radiation by a guitar top plate, vibrating through deflections hard to analyze: multipolar mode shapes. The air-structure interaction was examined in terms of edge modes or corner modes, and considering even or odd number modes. A numerical model was implemented and experimentally calibrated, exhibiting several advantages exploring the coupling between vibratory and acoustic waves in a top plate. Two analyses were applied detecting high or low radiation efficiency for the structure. First, the addition of volume velocity for odd numbers of poles and cancellation for even numbers were examined. In fact, both behaviors can happen at the same time, as it was shown for a corner radiator case used as an example. Second, the ratio between bending and acoustic wavenumbers was explored. To illustrate the importance of this ratio, some theoretical features of a more efficient radiator than the corner mode were exposed in an edge mode example. Labeling multipolar mode shapes as efficient or inefficient radiators showed to be a useful alternative analyzing the top plate behavior. It can be applied knowing the nodal lines of the vibration pattern and estimating the bending and acoustic wavelengths.     

Improvement of rotary encoders in human-machine-interfaces through optimized acoustic feedback

This work investigates the effect of the acoustic feedback of rotary encoders that are common in current human-machine-interfaces. The results are based on subjective trials in which the probands had to complete generic tasks using an encoder with programmable electro acoustic feedback. The tasks had to be performed with individually optimized feedback and two reference conditions. The results of the investigation showed the advantage of well-defined acoustic feedback on both accuracy and speed of task fulfillment.     

Mixing layer resonance under high-speed stream forcing

In the majority of fluid–structure interaction problems, the biggest challenge lies in the fundamental understanding of the flow physics. Forced mixing layers is an important phenomenon found in many cases of flow-induced vibrations and acoustics. The response of a mixing layer to high-speed stream acoustic forcing is investigated with a theoretical and experimental approach. Two different experiments demonstrating the fluid mechanic phenomenon are presented. The first experiment consists of a circular jet impinging on a vibrating plate. The second experiment demonstrates the mixing layer resonance in the context of a fluidelastic instability causing high-amplitude vibrations in gas turbine high-pressure compressor rotor blades. Both the plate and the adjacent blade vibration induce an acoustic feedback that propagates within the jet and blade tip clearance flow, respectively. The resonance was found to occur when the feedback wavelength matched either the jet-to-plate or the inter-blade distance. In both experimental cases, the resonance condition has been simply modeled by the coincidence of a 1D feedback wave, which propagates upstream at reduced velocity by the high-speed flow. The coupling between the jet induced mixing layer and the feedback wave is assumed to naturally occur when one of the wave crests reaches the separation edge. The objective of this study is to improve the understanding of the coupling mechanism between an emanating shear layer and the acoustic forcing originating within a fast flow stream. The study is based on a simplified analytical model in order to enlarge the current understanding of the mixing layer receptivity to the more specific case of its response to high-speed stream forcing. To identify the mixing layer resonant modes, an analytical resonance condition is proposed. It is found that the mixing layer response becomes spatially resonant for specific source locations downstream in the high-speed flow. The study also provides an analytical mean to capture the critical source location periodicity that has been experimentally observed. The resulting theoretical prediction of the resonant source locations is in good agreement with the experimental data. Therefore, it supports the stream forced mixing layer analytical model and the proposed spatial resonance condition. The simple 1D reduced speed feedback wave model, which has been used to identify the experimental resonance conditions, is also in good agreement, and thus validated, with the results of this study.     

Modulation instability of the stimulated Brillouin scattering in fibers in a presence of acoustic diffraction

An influence of acoustic wave diffraction on the modulation instability of the backward stimulated Brillouin scattering in fibers is numerically investigated, when a small feedback for the Stokes wave is present. The cases of acoustic waveguide and anti-waveguide fibers are considered. It is shown that a presence of acoustic diffraction may expand the region of occurrence of modulation instability and may lead to 2–5 times increasing the peak amplitude of output Stokes pulses. In this case, a stable train of short powerful Stokes pulses of nanosecond duration is produced at the output.     

Suppression of Helmholtz resonance using inside acoustic liner

When a Helmholtz resonator is exposed to grazing flow, an unstable shear layer at the opening can cause the occurrence of acoustic resonance under appropriate conditions. In this paper, in order to suppress the flow-induced resonance, the effects of inside acoustic liners placed on the side wall or the bottom of a Helmholtz resonator are investigated. Based on the one-dimensional sound propagation theory, the time domain impedance model of a Helmholtz resonator with inside acoustic liner is derived, and then combined with a discrete vortex model the resonant behavior of the resonator under grazing flow is simulated. Besides, an experiment is conducted to validate the present model, showing significant reduction of the peak sound pressure level achieved by the use of the side-wall liners. And the simulation results match reasonably well with the experimental data. The present results reveal that the inside acoustic liner can not only absorb the resonant sound pressure, but also suppress the fluctuation motion of the shear layer over the opening of the resonator. In all, the impact of the acoustic liners is to dampen the instability of the flow-acoustic coupled system. This demonstrates that it is a convenient and effective method for suppressing Helmholtz resonance by using inside acoustic liner.     

Optimal frequency-response sensitivity of compressible flow over roughness elements*

Compressible flow over a flat plate with two localised and well-separated roughness elements is analysed by global frequency-response analysis. This analysis reveals a sustained feedback loop consisting of a convectively unstable shear-layer instability, triggered at the upstream roughness, and an upstream-propagating acoustic wave, originating at the downstream roughness and regenerating the shear-layer instability at the upstream protrusion. A typical multi-peaked frequency response is recovered from the numerical simulations. In addition, the optimal forcing and response clearly extract the components of this feedback loop and isolate flow regions of pronounced sensitivity and amplification. An efficient parametric-sensitivity framework is introduced and applied to the reference case which shows that first-order increases in Reynolds number and roughness height act destabilising on the flow, while changes in Mach number or roughness separation cause corresponding shifts in the peak frequencies. This information is gained with negligible effort beyond the reference case and can easily be applied to more complex flows.