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Periodicity (in time or space) is a part and parcel of every living being: one can see, hear and feel it. Everyday examples are locomotion, respiration and heart beat. The reinforced N-dimensional periodicity over two or more crystalline solids results in the so-called phononic band gap crystals. These can have dramatic consequences on the propagation of phonons, vibrations and sound. The fundamental physics of cleverly fabricated phononic crystals can offer a systematic route to realize the Anderson localization of sound and vibrations. As to the applications, the phononic crystals are envisaged to find ways in the architecture, acoustic waveguides, designing transducers, elastic/acoustic filters, noise control, ultrasonics, medical imaging and acoustic cloaking, to mention a few. This review focuses on the brief sketch of the progress made in the field that seems to have prospered even more than was originally imagined in the early nineties.

Application of the piezoelectric patch to mitigate the train wheel squeal is presented in this paper. A complete model for the wheel, rail and foundation is employed in the frequency domain. The wheel squeal model with the shunted piezoelectric element is presented and the consequent added damping is calculated for the critical frequencies. Two different shunt circuits including the resistor ($R)$ and resistor–inductor (RL) are applied and the consequent performances in the noise mitigation are evaluated for different frequencies. The effect of contact point lateral displacement on occurrence of the wheel squeal is investigated. In the parametric study, the effects of shunting circuit and its parameters, size, position and number of piezoelectric elements on the control performance are evaluated. It is found that the resistance shunt circuit has optimal performance when the added damping over a broadband frequency range is implemented. In addition, both the added damping and added damping per piezoelectric volume become saturated by increasing the number of piezoelectric patches further over a specific limiting value.

Perforated panels are one of the structures that are widely used nowadays. The sound absorption behavior of materials is studied by solving the equations governing the wave propagation in these materials. In this paper, the finite element method (FEM) was used to predict the absorption performances of few different perforated composite panels. Also, the studied structures were examined by two-microphone impedance tube to validate the results of the numerical method. The relative consistency of the results of the current method with the results of the impedance tube suggests the accuracy of this method in simulating the absorption rate of perforated composites. In addition, the results of evaluating various layouts (arrangements) showed that the use of absorber materials with higher flow resistivity at the back of the perforated panel and at the beginning of the sound wave entry increases the absorption performance by 1.6 times than that of the inverse layout ratio.

The noise created when a firearm is fired has a lot of adverse effects on humans and the environment, so analyzing and attenuating this noise is essential. This study aimed to examine propellant flow and the sound generated by this flow using a hybrid computational fluid dynamics and computational aeroacoustics method. The compatibility of this numerical study was also validated by comparing it with the experimental result. The impact of critical parameters, such as the shape and number of baffles of the suppressor, was studied. Finally, the overpressure and acoustics results of different suppressors were compared with each other and with the unsuppressed condition. According to the result, the suppressor with curved baffles shows a better performance. When the exit pressure at the tip of suppressor compared to the initial inlet pressure to the suppressor the percentage drop in overpressure was 76.01%, 78.79% and 81.3% for the suppressor with one, three and five curved baffles, respectively. For condition without suppressor, 169.49dB of SPL was recorded. When using a suppressor without a baffle, this value was reduced to 162.13dB. For the suppressor with one, three and five curved baffles, the SPL value was 160.234dB, 159.43dB and 158.11dB, respectively. Generally, this study shows that the suppressor’s efficiency is directly proportional to its shape and number of baffles.

The aim of this paper is to provide an overview of the existing industrial practices used for cabin noise control in various industries such as automotive, marine, aerospace, and defense. However, emphasis is placed on automobiles and armored vehicles. Generally, automobile cabins usually constitute of thin structural panels, where the fundamental frequency typically falls below 200Hz. If a specific structural mode couples with a specific acoustic mode of the cabin, booming noise occurs. As such, discomfort may be felt by the occupants. Fundamentally, vibroacoustics problems may be minimized if the acoustic modes and the structural modes are decoupled, which is achieved usually by structural modifications or acoustical treatments. However, if excessively performed, the weight limitation of an automobile design will be exceeded; not to mention the adverse effect of increased weight on several factors such as fuel efficiency, mileage life of tires and acceleration of the vehicle. Moreover, current solutions have several drawbacks in low frequency noise control. In light of this, it is of great interest to explore the feasibility of acoustic metamaterials as an alternative with hope to improve cabin noise.

Indium–zinc in situ composites were fabricated and their viscoelastic properties studied over 8.5 decades of frequency. Material with 5% indium by weight was found to have a stiffness damping product (the figure of merit for damping layers) of 1.9 GPa at 10 Hz; 3 times better than the peak of polymer damping layers and over a wider frequency range. Material with 15% indium had a stiffness damping product of 1.8 GPa. The indium segregated in a platelet morphology, particularly favorable for attaining high damping from a small concentration, as predicted by viscoelastic composite theory.

This paper presents a methodology that enables one to trace back the forced-vibroacoustic components (F-VAC) that are responsible for acoustic radiation from an arbitrarily shaped structure. This methodology relies on a modified Helmholtz equation least-square (HELS) method that takes partial normal surface velocities obtained by using a laser vibrometer and partial acoustic pressures measured using a small array of microphones as input to reconstruct the distributions of the normal surface velocity and acoustic pressure over the entire source surface. Next, the normal component of the time-averaged acoustic intensity distribution is reconstructed and linked to the field acoustic pressure. Finally, the singular value decomposition (SVD) technique is used to reveal the most critical components of the structural vibrations that are responsible for acoustic radiation. The advantages of using partial normal surface velocities and acoustic pressures as input data are that: (1) it enables one to acquire the rich near-field information embedded in the normal surface velocities; (2) it is a noncontact and noninvasive measurement approach using a laser vibrometer or scanning laser and a small microphone array at a remote location; (3) it enables one to obtain a comprehensive picture of the vibroacoustic fields that traditional near-field acoustic holography (NAH) cannot offer; (4) it allows for correlating the near-field effects to the far-field acoustic pressures and revealing the root causes of structure-borne sound radiation; (5) it provides an insight into developing the most cost-effective noise reduction measures; and (6) it is suited for engineering applications since in practice there are always areas that are inaccessible for measurements. Finally, it is emphasized that although sound may be produced by structural vibrations, not all structural vibrations may produce sound. Therefore, all one needs to do is to suppress the most critical components of structural vibrations that can emit sound. Experimental validations of using F-VAC analyses to reduce sound radiation from an arbitrarily shaped structure are demonstrated.

A modified Helmholtz equation least-square (HELS) method is developed to reconstruct vibroacoustic quantities on an arbitrarily shaped vibrating structure. Unlike the traditional nearfield acoustical holography that relies on the acoustic pressures collected on a hologram surface at a short stand-off distance to a target structure, this modified HELS method takes the partial normal surface velocities and partial acoustic pressures as the input data. The advantages of this approach include but not limited to: (1) The normal surface velocities that represent the nearfield effects are collected directly, which lead to a more accurate reconstruction of the normal surface velocity distribution; (2) The field acoustic pressures are also measured, which leads to a more accurate reconstruction of the acoustic pressure on the source surface as well as in the field; and (3) There is no need to measure the normal surface velocities over the entire surface, which makes this approach quite appealing in practice because most vibrating structures do not allow for measuring the normal surface velocities over the entire source surface as there are always obstacles or constrains around a target structure. Needless to say, regularization is necessary in reconstruction process since all inverse problems are mathematically ill-posed. To validate this approach, both numerical simulations and experimental results are presented. An optimal reconstruction scheme is developed via numerical simulations to achieve the most cost-effective reconstruction results for practical applications.