In this investigation, the free vibration analysis of laminated composite rectangular plates with general boundary conditions is performed with a modified Fourier series method. Vibration characteristics of the plates have been obtained via an energy function represented in the general coordinates, in which the displacement and rotation in each direction is described as an improved form of double Fourier cosine series and several closed-form auxiliary functions to eliminate any possible jumps and boundary discontinuities. All the expansion coefficients are then treated as the generalized coordinates and determined by Rayleigh-Ritz method. The convergence and reliability of the current method are verified by comparing with the results in the literature and those of Finite Element Analysis. The effects of boundary conditions and geometric parameters on the frequencies are discussed as well. Finally, numerous new results for laminated composite rectangular plates with different geometric parameters are presented for various boundary conditions, which may serve as benchmark solutions for future research.

This paper investigates the soundscape of a designed urban park in Chennai. Acoustic measurements and subjective evaluations of the soundscape were conducted at various locations in the identified case. The park was divided into nine zones for conducting the primary survey, such as parking area, entrance court, tree court, children’s play, herbal garden, butterfly garden, open-air theatre, duck pond, and mound. Fourteen different sounds were mapped based on the pilot survey that affected the users. Totally thirty responses were collected from each zone. The observed sounds include traffic sound, honking of vehicles, screaming of people & children, chuckling of children, the chirping of birds, ringing of the mobile phone, crowd talking, rustling of leaves, wind sound, play equipment sound, splashing of water, flowing of water, and machine sound. From the subjective survey, it is found that the individual soundscape preference collected from each space have no correlation with the overall soundscape experience of the same space. Based on the measurement data, the individual and the overall experience on soundscape perception were examined.

Two significant causes of noise related to cavities are direct and indirect flow induced turbulence/vortex shedding mechanisms. Examples of induced noise can be found in many applications of both closed-flow and open-flow cavities — some with resonance of acoustic modes. An example is a flow valve with a cavity where flow along the cavity gives pulsations either trapped within the valve or exciting downstream piping acoustic modes. There are passive methods of mitigation besides detuning such as modification of the entrance to the cavity, blockage, and use of Helmholtz resonators. Natural frequencies of cavity acoustic modes can be irregular, but for many such as with circular, square, rectangular or axisymmetric shapes can give symmetry of modes. An example is a cavity at the sides of rotating disks, where transverse symmetrical modes having circular and diametric patterns are similar to structural vibratory modes for bladed disks. In the last decade it has been documented that for centrifugal compressors blade passing acoustic pressure pulsation due to Tyler-Sofrin spinning modes can add to alternating stress from non-uniform flow excitation, such as from stator wakes. Cavity acoustic mode excitation then has been termed “triple coincidence” or “triple crossing”, explaining rare documented impeller fatigue failures and likely a reason, at least partially, for some unexplained failures. A novel method described herein is to treat these and similar cavities as fluid-filled disks, then utilize or add blade-like elements within the cavities. The method described (patent application, PCT US1820880) to reduce response of these cavities is to intentionally mistune the elements as has been documented for bladed disk modes. Other applications of this method are possible for many other mechanisms. These modification(s) can alleviate concern for any mechanism having structural vibration excitation acoustically and/or for environmental noise issues.

Noise presents itself today as the most frequent physical agent in the workplace and its harmfulness is already proven by the literature. Among its main consequences are noise-induced hearing loss that can be caused by any type of excessive sound emission, including those used in Spinning classes. The objective of the present study was to evaluate the sound pressure levels in Spinning classes in Brasília-DF academies, as well as to identify the teacher’s perception of their auditory condition. The Bruel & Kjaer 2250 sound pressure gauge was used to ensure the reliability of measurements. 43 teachers participated and the sound pressure levels were measured at 20 academies. The results indicate that 100% of the academies are working with values above the limits allowed by the current legislation (85dBA). Thus, the higher the weekly workload, the higher the maximum sound pressure level and the equivalent to which the teachers are exposed. Concerning the auditory perception of the teachers, (69.8%) they stated that they did not feel bothered by the loud sound. It is suggested that academies, teachers and students be guided to prevent possible hearing damage and to try to control the noise level in the classroom.

An experimental study was conducted to confirm the sound insulation performance of a partition wall that was joined to a steel beam. Fireproofing protections, such as sprayed rock wool, are used to protect steel members from heat in the case of fire. However, the influence of transmitted sound from steel members with fireproofing protections and from joints of the partition wall and steel members has not been quantitatively investigated. Therefore, the sound transmission loss was measured using specimens that consisted of a partition wall and a steel beam, which was coated with sprayed rock wool. The results showed that the sound transmission loss of the specimen was lower than that of the partition wall element. It was inferred that the mass-spring-mass resonance of the fireproofing protections on the steel web reduced the sound insulation performance of the specimen. Moreover, the influence of the transmitted sound from the steel web was greater than that from the joints of the partition wall and the steel beam. In addition, when the gypsum board was added to the specimen as a ceiling, it was confirmed that the sound insulation performance of the specimens was equal to that of the partition wall element.

Considering the effect of teeth surface sliding friction, free vibration of two-stage planetary gears (TPG) is studied theoretically for the first time. The lateral-torsional coupling dynamic model and equation are established with three degrees of freedom: two translations and one rotation. The change rule of natural frequency is discussed with the case of first stage planetary gear’s number 4 and second stage planetary gear’s number 3, 4 and 5. Afterwards three vibration modes are summarized by calculating the free vibration. In order to understand the behavior of friction, the effect of friction on natural frequencies is analyzed for the case of considering friction and not considering friction. Furthermore, the ‘self-coupling’ phenomenon is obtained from the vibration of center component of TPG Meanwhile, the ‘mutual coupling’ is obtained between the first-stage planetary gear (FPG) and the second-stage planetary gear (SPG).

Soundscape of a specific transportation event conveys not only messages for communication purpose but also becomes a soundmark for the riders. This Taipei MRT cabin soundscape study documents the series of acoustic events during the rides between two stations: Shandao Temple and Taipei Main Station. Taxonomy of sounds, technical data analysis and soundwalks were used as the soundscape data collection methods. Social, mechanical, and indicators sounds were identified as the three main sound classifications. Spectrogram showed the distinguished frequencies and orders of the acoustic events. On train announcements used four languages due to the country’s cultural and social history. This fascinating, unique, and repetitive Taipei MRT cabin soundscape adds to the exuberant soundscape and identity of Taipei.

The free vibration characteristics of a rectangular thin plate with series of openings are studied based on the Rayleigh-Ritz method in this paper. Firstly, the strain energy and kinetic energy of the plate are calculated utilizing the modified Fourier series. Then, uniformly distributed transitional and rotational springs are applied to deal with general boundary supports, and the elastic potential energy of the springs can be obtained. Furthermore, the plate is divided into several parts according to its amounts of openings and the energy of each parts are calculated separately and the spring stiffness of cut line between two separated parts are the same. Finally, the governing equation of the plate is obtained with the energy functional variation method. The present method is proved to be accurate by comparing the natural frequencies with those calculated by the finite element method. Besides, the influence of the amounts of openings and the area of total openings are discussed.

Traffic noise is a major noise source in the study of environmental noise. Various noise generation mechanisms depict different spectral features. Some are wide-band noise, such as engine knocks; some have signature frequencies, such as gear transmissions; and some are in a certain frequency region, such as tire/road noise. These spectral features affect the façade design of a building in order to achieve sufficient exterior noise insulation and satisfactory interior noise due to the traffic noise. ISO standard 11819-1 specifies the measurement procedure of statistical pass-by tests. There are three ranges of vehicle speed: slow, medium, and fast. However, it requires that the vehicle must maintain constant speed when passing by the test point. Unfortunately, a vehicle tends to generate higher noise when accelerating, especially at low frequencies. Therefore, it is necessary to distinguish the noise levels at an intersection versus middle-points of the road between two intersections. Presumably, the traffic noise levels at an intersection would be higher. This research measured the traffic noise at various locations of different speed limits. Statistical analyses were conducted to compare the spectra at these locations. This is also an effort to refine the noise map.

This paper explores jet noise control using castellations at the exit of pipe-jets. Far-field acoustic measurements and schlieren visualizations are performed for two configurations of castellations; namely, two and four counts. The results are compared with that of pipe-jet without castellations. The nozzle pressure ratio range of the study is 1.5 to 4.5. With each configuration, the position and strength of vortices vary causing it to interact in different manners. For pipe with two castellations, the screech is amplified and the overall sound pressure level is higher than the reference jet in most of the nozzle pressure ratios under study. For pipe with four castellations, there is no noise variation in the azimuthal direction, and screech is eliminated at all nozzle pressure ratios.

Certain operating conditions such as fluctuation of the external torque to planetary gear sets can cause additional sidebands. In this paper, a mathematical model is proposed to investigate the modulation mechanisms due to a fluctuated external torque (FET), and the combined influence of such an external torque and manufacturing errors (ME) on modulation sidebands. Gear mesh interface excitations, namely gear static transmission error excitations and time-varying gear mesh stiffness, are defined in Fourier series forms. Amplitude and frequency modulations are demonstrated separately. The predicted dynamic gear mesh force spectra and radial acceleration spectra at a fixed position on ring gear are both shown to exhibit well-defined modulation sidebands. Comparing with sidebands caused by ME, more complex sidebands appear when taking both FET and ME into account. An obvious intermodulation is found around the fundamental gear mesh frequency between the FET and ME in the form of frequency modulations, however, no intermodulation in the form of amplitude modulations. Additionally, the results indicate that some of the sidebands are cancelled out in radial acceleration spectra mainly due to the effect of planet mesh phasing, especially when only amplitude modulations are present.

The weak dynamic stiffness of thin plate is one of the important factors that limit the use of thin plate. Improving the dynamic stiffness of thin plate is one of the effective methods for the vibration control of thin plate. In this paper, the influence of pre-stress on the vibration characteristics of thin plate is studied. A vibration control method of thin plate based on pre-stress is proposed. The vibration differential equation of quadrate thin plate under pre-stressing is established. Using the Galerkin principle, the natural frequencies corresponding to the shape functions of the quadrate thin plates under pre-stressing in different distribution forms are obtained. By comparison, it is found that pre-stressing on the thin plate can change the dynamic stiffness of thin plate. In particular, tensile stress can increase the dynamic stiffness of thin plate while compressive stress can reduce the dynamic stiffness of the thin plate. The greater the pre-stress, the more obvious the effect. In the end, the requirements of the pre-stress distribution which can improve the dynamic stiffness of thin plate effectively are derived.

Household appliances and their sound quality are important for our daily life quality. However the appropriate characterization of their sound is a difficult task. Not only product users but also manufacturers can profit from a sound label which characterize the perception of the customers. The purchase decision-making process according to acoustic criteria will be supported by such kind of label. In addition, a label and its components give orientation to the manufacturers during their product development process. Essential aspect for such kind of label is that it should represent the perception of the customers. Therefore psychoacoustical properties, e.g., loudness, sharpness, roughness, tonality, etc., are advantageous for characterization purposes. It would be beneficial to combine these psychoacoustical descriptors into a sound quality label, which is easy to understand. The authors have developed several sound labels for household appliances based on psychoacoustic properties. These sound labels are the result of the listening experiments which were conducted with potential customers. In this paper, various aspects of these investigations are summarized, extended and discussed.

The acoustic radiation analysis of a fully-submerged infinitely long half-filled cylindrical shell coupling with fluid field is a typical acoustic-structure problem in the infinite domain, the solution of which is currently mainly based on numerical method. The analytic or semi-analytical method is indispensable for the numerical method and the mechanism to reveal the acoustic-structure coupling characteristics. In this paper, an analytic solution is presented that can calculate the acoustic radiation of infinitely long half-filled cylindrical shell. The displacement of the shell, the fluid load and the excitation force are expressed as the combination of trigonometric series and Fourier series, and displacements of the other two directions are removed by orthogonalizing, only the radial displacement is retained. The control equation of the fluid-structure interaction can be obtained from the relationship between the amplitude of fluid load and the amplitude of radial displacement which can be established by orthogonalizing the continuous conditions of the fluid-structure coupled contact surface and the free surface boundary condition. Solving the control equation, the vibration and acoustic radiation of the coupling system can be determined. Compared with the finite element software Comsol, the results of forced vibration and underwater radiated noise show that the presented method is accurate and reliable. A new way to solve acoustic-vibration problem with partial coupling of elastic structure and sound field is provided in this study.

This paper aims to identify the attributes that describe aircraft interior noise, determine most important psychoacoustic models that characterize cabin sounds, and construct a prediction model that can be utilized for VIP and business jets to evaluate subjective perception. In the first part, paired comparison listening tests and free verbalization are conducted with expert subjects who experienced VIP and business aircraft flight. The study generated a list of adjective pairs that describe perception of cabin sounds to be used for semantic differential listening tests. Multi-dimensional scaling is performed on paired comparison data. Results showed that subjects’ decisions can be categorized in loudness and annoyance dimensions which are not necessarily linearly associated. The second part of the study is the development of a sound quality prediction model for aircraft cabin. Semantic differential tests are conducted with potential customers. Objective sound quality metrics are correlated to subjective test responses using principal components regression. This model is found to be most effective explaining pleasantness, comfort, and loudness perception. It is intended to be utilized to modify/redesign noise control treatments and sound signature of an aircraft. All listening tests were conducted inside an aircraft cabin simulator considering the influence of visual content.

A self-sustained sound, more usually known as a whistle, refers to a distinct tonal noise created due to the interaction between the sound and flow field. When a positive feedback loop is formed between the two fields, the energy in the mean flow will be transferred into the sound wave, thus giving rise to a whistle. In engineering practice, whistles are destructive as they can produce high sound and vibration levels and may result in risk for mechanical failures. In this work, a flow-related high level tonal noise was found during a measurement on a particle agglomeration pipe, which is a quasi-periodic corrugated structure designed for the exhaust system of heavy-duty trucks. The purpose of the pipe is to enhance particle agglomeration to increase the size of exhaust gas particles. To investigate the origin of the detected tonal noise additional measurements were carried out. Based on the measurement result, the aero-acoustic coupling in the agglomeration pipe was analyzed, revealing that the pipe has a large potentiality to amplify the incident sound power in the presence of a mean flow. Furthermore, the Nyquist stability criterion was applied to confirm the existence of exponentially growing modes in the system at certain conditions.

Mitigating the propagation of low frequency noise sources in ducted flows represents a challenging task since wall treatments have often a limited area and thickness. Loading the periphery of a duct with a periodic distribution of side-branch Helmholtz resonators broadens the bandwidth of the noise attenuated with respect to a single resonator and generates stop bands that inhibit wave propagation. However, significant flow pressure drop may occur along the duct axis that could be reduced using micro-perforated patches at the duct-neck junctions. In this study, a transfer matrix formulation is derived to determine the sound attenuation properties of a periodic distribution of MPPs backed by Helmholtz resonators along the walls of a duct in the plane wave regime. In the no-flow case, it is shown that an optimal choice of the MPP parameters and resonators separation distance lowers the frequencies of maximal attenuation while maintaining broad stopping bands. As observed in the no-flow and low-speed flow cases, these frequencies can be further decreased by coiling the acoustic path length in the resonators cavity, albeit at the expense of narrower bands of low pressure transmission. The achieved effective wall impedances are compared against Cremer optimal impedance at the first attenuation peak.

Random excitations can result from various types of non-deterministic loads such as wind loads, terrain loads, and other types of white noise loads. In this paper, an overview is presented of the modal method to obtain the random response of a coupled structural-acoustic system subjected to random excitations. When the structural system is coupled with an enclosed cavity, the structural-acoustic frequency response functions (FRFs) can be obtained using the uncoupled structural modes and the uncoupled acoustic modes, with structural-acoustic coupling as well as modal damping included in the formulation. The random response of the coupled structural-acoustic system is then obtained by summation of the structural-acoustic FRFs with the applied auto- and cross-spectral random loadings at the excitation locations. The theoretical formulation of the coupled structural-acoustic system is described. An example of a rectangular cavity coupled with flexible panels exposed to external random white noise load is presented. The methodology is then applied to an automotive vehicle travelling over a randomly rough road to predict the interior sound pressure response in the vehicle.

The acoustic conditions in school spaces are fundamental for the success of the learning process. This article aims to present the results of acoustic comfort by the analytical method of 9 classrooms with different geometric characteristics of the University of Brasília. In classrooms the background noise limits are 40 to 50dBA and considering that the teacher’s voice reaches a certain 65 decibels we can highlight that it can be heard clearly by the students. However in an environment with a lot of reverberation of sounds the teacher will have to exert more effort to be understood. The sound perception in a room depends on the intensity and temporal relationship between the direct sound and the indirect sound reflected by the walls of the room, therefore, in the present study were verified two parameters namely the reverberation time and speech intelligibility. The results obtained were compared with the normative parameters of ANSI S12.60: 2010 and it was verified that all rooms are not suitable for teaching-learning activity. To guarantee an environment with better conditions of concentration and learning for the students, acoustic coverings were suggested in order to fit the normative limits.

Arrays of dry-coupled thickness-shear transducers are often employed in the guided wave sector to inspect pipelines and plate-like structure. The dry coupling permits to dismiss any coupling material between the transducer and the waveguide, but as a drawback a preload must be applied on the transducers to guarantee an effective coupling between the two surfaces. Although the influence of the preload on the natural frequencies is studied in the literature, the frequency response function of a transducer relating the input voltage to the displacement output is not present in the literature. Moreover, the distribution of force on the backing mass and the effect of the preload on the uniformity of vibration of the transducers are still missing. A natural frequency analysis and a forced analysis are then computed numerically with finite element analysis to quantify the influence of the preload on a thickness-shear transducer. Furthermore, these results are compared with experimental results obtained with a Laser Vibrometer. It is then shown how the geometrical layout of the transducer coupled with the preload influences the vibration of the transducer.