Low frequency design of car interior sound using distortion products

Designing a fuselage involves many considerations such as strength and stability, fatigue, damage tolerance, fire and lightning resistance, thermal and acoustic insulation, production, inspection, maintenance and repair. In the background of the application of composite sandwich structures on the aircraft fuselage, the focus of the thesis is to investigate the vibration and acoustic behaviours of sandwich structures. As a preliminary design of aircraft fuselages, a sizing work of sandwich cylinders was conducted with respect to the strength and stability. FE models for the buckling prediction of the sandwich cylinder were validated with the analytical expressions. Under a typical flight loading, the sizing results of a sandwich cylinder and a laminated cylinder were compared and it was found that the mechanical efficiency of the sandwich cylinder is comparable to that of the traditional stiffened cylinder. Subjected to the diffuse acoustic field, the sound transmission loss (TL) of composite sandwich cylinders was investigated using an analytical method and the Statistical Energy Analysis (SEA) method at 100-16000 Hz. The SEA method showed a good agreement with the analytical method. The parameters, including the fibre orientation, facing materials, cylinder geometry, core thickness, sandwich layup and core shear stiffness, were studied for their influences on the TL of cylindrical structures. A uniform laminated, a stiffened and a sandwich cylinder with the equivalent mass were compared for the sound insulation performance. The laminated cylinder had the largest TL below the coincidence frequency and the sandwich cylinder had the largest TL above the coincidence frequency. The structural velocities and noise reductions of laminated and sandwich cylinders were experimentally tested at 1-4000 Hz under a point acoustic excitation, and a mechanical excitation respectively. The wave propagation in the sandwich structure was compared with that in the laminated structure, as an explanation of the noise reduction difference of the two structures. As the coincidence frequency plays an important role on the sound transmission, influence parameters of the coincidence frequency of sandwich structures were also studied. To investigate the vibro-acoustic performance of sandwich structures under different kinds of external excitations, the FEM/BEM numerical method was used to analyze the noise reduction of sandwich cylinders at low frequencies. Under a force excitation, some parameters including the core shear stiffness, sandwich layup, core thickness and facing orientation were studied for their influences on the sound transmission. Results showed that there exist optimal values for these parameters to achieve the best sound insulation performance. Therefore, an efficient optimization technique using the acoustic transfer vector (ATV) and the genetic algorithm (GA) was applied to optimize a typical sandwich cylinder for the best noise insulation. In addition, taking a fuselage section as an example, a multi-objective optimization (weight & noise insulation) was conducted considering the mechanical constraints under flight load cases. The noise control treatment such as the addition of absorption layers is one of the common methods for the noise control of the transport vehicles. Thus the sound transmission of sandwich panels with open-cell foam was studied. The transfer matrix method (TMM) was used for the TL prediction of sandwich panels with porous foams. This method was validated by experimental results. A sensitivity study of the flow resistivity, tortuosity and porosity on the TL of sandwich panels was conducted. Then four kinds of absorption materials were studied for their influences on the TL of sandwich cylinders. Finally the TLs of a stiffened cylinder and a sandwich cylinder were compared in case of addition of absorption layers. As the damping plays an important role on the vibro-acoustic behaviors of sandwich structures, the damping properties of composite sandwich structures were studied using the modal strain energy (MSE) method and experimental measurements. The hysteresis method and the half power method were used for the damping measurement. Compared to the facing, the cores usually have much higher damping and they make the main contribution on the sandwich damping. Therefore the material damping properties of two kinds of foams (PMI & PVC) were measured at low frequencies using the hysteresis method. The measured results have been validated by numerical models. The damping of the PVC foam were also measured using the half power method and results showed a good agreement with those measured using the hysteresis method. For the damping prediction of sandwich structures, the MSE method was verified by the measurements using the half-power method. Finally, the effects of the core thickness and core properties on the damping of sandwich structures were studied.

The sound quality in a room is of fundamental importance for both recording and reproducing processes. Because of the room modes, the distributions in space and frequency of the sound field are largely altered. Excessive rise and decay times caused by the resonances might even mask some details at higher frequencies, and these irregularities may be heard as a coloration of the sound. To address this problem, passive absorbers are bulky and too inefficient to significantly improve the listening conditions. On the other hand, the active equalization methods may be complicated and costly, and the sound field might not be well controlled, because of the added sound energy in the room. Another approach is the active absorption, which consists in varying the impedance of a part of the enclosure boundaries, so as to balance the sound field thanks to the absorbed sound power into the active boundary elements. The thesis deals with the design and optimization of electroacoustic absorbers intended to specifically reduce the effect of the unwanted room modes. These active absorbers are closed box electrodynamic loudspeaker systems, whose acoustic impedance at the diaphragms is judiciously adjusted with passive or active components to maximize their absorption performance in the domain in which it is located. Several topologies merging sensor- and shunt-based methods are proposed resulting in an efficient and broadband sound absorption at low frequencies. A multiple degree-of-freedom target impedance that is assigned at the transducer diaphragms is then optimized to lower the modal decay times at best. The performance of the electroacoustic absorbers for the modal equalization is investigated in actual listening rooms, and their audible effect is subjectively evaluated. The overall combination of concepts and developments proposed in this thesis paves the way towards new active absorbers that may improve the listening experience at low frequencies in rooms.

Dedicated sequences for auditory fMRI

ecent decades; consequently the Bending WaveLoudspeaker (BWL) and Distributed Mode Loudspeaker(DML) have been of great interests of research forloudspeaker producers and have been developed formany different applications. Both of BWL and DMLapplies the theory of bending waves, thus often beconfused by public. The main difference is that theformer uses infinite plates approach while the latterutilizes finite plates approach, or preferably, the modalapproach, which is exactly why the latter one was namedas Distributed Mode Loudspeaker, which is the mainconcern of this paper.As the beginning of this paper, an introduction to thetechnologies of DML is given, and then following themode theories, which says how one uses modes todescribe the vibration patterns of a limit panel. Afterward,the properties of DML are shown, as well as theapproaches to these properties.In order to evaluate the performance of DML, asample from NXT was used for the measurement, as wellas reference, another conventional electro-dynamicloudspeaker. These two speakers were gone through aseries of measurements, containing the impulse responses- which also imply the frequency responses - , thedirectivities, the sound powers, the efficiencies, thesensitivities, and the distortions. In addition, the soundpressure levels were measured at 30 spots in a regularlistening room for the plotting of distribution contours.In the end of the project, a blind listening test was alsoperformed to investigate the audiences’ responses andacceptance to DML and the preference between DMLand piston radiator.With the result of the experiments mentioned above, itis conclude that DML has lower sensitivity and efficiencythan the electro-dynamic loudspeaker. Also DMLs lackof low frequency due to low modal density and highfrequency because of transition of bending waves totransverse waves at higher frequencies. The directivitiesof DML do not outdo electro-dynamic type from top totoe; it is even weaker in middle frequency.

An effective design is proposed to solve the design of acoustical treatemnts in enclosures at low frequencies, especially below the Schroeder frequency. In this paper, the boundary element method is used to predict the sound field of acoustical systems. The Sequential Quadratic Programming is selected as the continuous design variable optimizer, and the Branch and Bound method is used to treat the non-continuous design variable. A two-dimensional rectangular cavity is demonstrated. The general agreement between this paper and previous work [Berhard and Takeo, J. Acoust. Soc. Am. 83 (6)] is considered good enough for the proposed tool to be used for preliminary design studies of acoustical treatments in enclosures.

This thesis describes the mechanisms with which tip vortex cavitation is responsible for broadband pressure fluctuations on ship propellers. Hypotheses for these are described in detail by Bosschers (2009). Validation is provided by three main cavitation-tunnel experiments, one on a model propeller and two on a stationary wing. These have resulted in a model that can quantify the resonance frequency of a tip vortex cavity based on a limited number of propeller related parameters. Simultaneous measurement of sound and high-speed video recordings of propeller tip-vortex cavitation were performed, in the presence and absence of an upstream wake inflow. In uniform inflow no significant sound production was observed. For conditions of wake inflow a strong tonal sound was measured that decreases in frequency as the cavitation number decreases. In the frequency domain there was a 30 dB increase over a broadband range surrounding the tonal frequency. This tonal sound was directly related to the tip-vortex cavity-diameter oscillations downstream of the wake. The model described in chapter 2, based on a resonance frequency of a tip vortex cavity, accurately describes the dominant sound frequencies. The basis for the model are the dispersion relations of three deformation modes. The relations were found experimentally in the frequency and wave number domain of cavity-diameter fluctuations obtained from high-speed video on a fixed wing. Resonance of the tip vortex cavity occurs at zero group velocity of the volume variation mode (n=0-). This resonance frequency was obtained experimentally while a significant sound source was absent. The quantitative model input for the cavity angular velocity was the single fitted parameter and required validation. Validation was performed by measurement of the flow field of a tip vortex in presence as well as in absence of cavitation. This was achieved by stereo particle image velocimetry in combination with a correlation averaging method. It provided sufficient spatial resolution and accuracy, to show the effect of a tip vortex cavity on the flow field. The tip vortex cavity is surrounded by a region of retarded azimuthal velocity, similar to the viscous core of a vortex without cavitation. The tip-vortex cavity-resonance frequency is underestimated when the measured cavity angular velocity is used. This showed the limits of the dispersion relation model that is based on a potential flow vortex. An empirical closure was proposed to serve as input for the cavity angular velocity. A Proctor vortex model was used to describe the flow field of the tip vortex without cavitation. This model required the vortex circulation, the propeller diameter and an empirical roll-up parameter beta. This model was able to provide the cavity diameter as function of cavitation number. The angular velocity without cavitation at a radius equal to the cavity radius, was used as model input for the cavity angular velocity. This closure of the dispersion relation model was able to describe the dominant sound frequencies as found in the model propeller experiment in a wake inflow.

In this thesis we use quartz tuning fork resonators to probe properties of normal and superfluid ⁴He and ³He. Our main goal is to study both quantum turbulence and acoustic emission of tuning forks in liquid helium. By employing a multi-frequency lock-in amplifier we contrast single and multi- frequency methods of measuring tuning forks in the linear regime. In the non-linear response of tuning forks during turbulence we create multi-frequency excitations called intermodulation products which are used to find the non-linear forces that created them. We apply this technique to quantum turbulence in superfluid ⁴He-II and find that the retarding in-phase force on the fork increases at a critical velocity for turbulence nucleation. We also observe that the out-of-phase non-linear force increases, which we attribute to energy loss via vortex ring emission by the fork. Superfluid ³He is a fermionic condensate of Cooper pairs of ³He atoms. At ultra-low temperatures of 120 μK thermally excited unpaired quasiparticles travel ballistically through the condensate. We beam quasiparticles from a black body source towards a 5 × 5-pixel camera and observe that the excitations follow photonic-like trajectories. We apply the source-camera configuration to non-invasively detect and even image quantum vortices, that are topological defects in the superfluid. Lastly, we explore the frequency dependent damping of quartz tuning forks in liquid ³He. We find that at high frequencies the fork damping is governed by acoustic emission. Furthermore, we show that existing models developed for sound emission in ⁴He can be used to predict observed acoustic damping in ³He. The results also suggest that devices for ³He experiments should be placed in cavities or designed to operate at low frequencies.

As a result of increasing the demand of power converters in electric aircrafts and electric vehicles, the power converter must be designed to deliver better, faster and cheaper performance. Finding the appropriate LC filter is the first step of the power converter design. Considering the desired losses, weight and cost, the optimization method is key in LC filter design. The value of the filter components has been optimized to satisfy the design of specific objective function such as maximizing the quality factor of the filter or minimizing the attenuation of harmonics distortion. Controllers help reduce harmonics distortion in filters. The proportional-integral controller as the first control method, has been used to control the power converters (buck converter, single and three phase inverter). This type of the controller can maintain the bandwidth and the stability margin of the controlled system within desired range. The PI controllers in different systems have been tuned by root locos, bode plot and pole placement methods. Then the sliding mode controller, which does not need the average model of power converter and has a high level of stability in different performance condition (such as load variations), has been applied to the single phase inverter. As the last control step, the three phase inverter with low switching frequency has been controlled in stationary frame with multiloop controller which provides sufficient bandwidth to eliminate low level voltage harmonics even in low switching frequency. This type of the controller doesn’t need park transformation which reduces the calculation time. The optimized value of the controller gains has been investigated by minimizing the integral of time-weighted absolute error or reducing the multiplication of raise time, settling time and the overshoot of the output voltage. The Matlab simulation and optimization toolbox has been used to verify the performance of the proposed controllers and also find the optimized value of filters components and controllers gains.

To achieve comfortable noise levels inside the passenger cabin, sound damping measures have to be taken to improve the sound insulation properties of the bare airframe. Usually the sound insulation requirements of a passenger cabin are met after the mechanical design of the fuselage structure is already finished, by adding damping materials. To be effective in the low frequency range, a lot of passive damping material has to be used resulting in considerable added weight. In this frequency range active noise control can be a competitive alternative to passive damping materials. To be able to find an optimal layout of an active noise control system a Design & Engineering Engine (DEE), a design support tool, has been developed. The DEE contains a parametric model initiator starting from user-defined requirements. From the parametric model an input file is generated for analysis with the FEM package ABAQUS. The DEE is capable of investigating active noise control systems comprising piezo electric sensors and actuators. Until now only parts of the fuselage section are investigated (panel level). The next step would be to analyse complete fuselage models. This research is mainly focussed on the influence of aspects like panel stiffness, panel curvature, stiffeners and the positioning of the sensors and actuators on the efficiency of the active noise control system. The efficiency of the active noise control system is expressed by the achieved transmission loss. The transmission loss for the different panel configurations is found using the transmission loss predicting algorithms developed at TNO TPD (Berkhoff [l]). These algorithms are based on transfer functions, which are the responses of the sensors to predefined input signals, and are determined with the DEE. Because the DEE can run automatically, many configurations can be investigated and the optimum layout for highest transmission loss can be found.

This paper reports results of the authors’ studies on the virtual design method used in the development of low noise intake system of I.C. engine. The resulting high pass-by noise at level above the legislative target at full throttle when engine speed was around 5200 r/min necessitated a BEM-aided redesign task, following the typical process of design and development of an intake system. During the initial design, based on the acoustic theory and the requirements (1. The air flux of the redesigned should equal to or exceed the value of the original flux; 2. The filtering area must not be degraded), and considering the constraint of space in the engine compartment, total volume and rough internal dimensions were determined. During the detailed design, the exact internal dimensions of the air cleaner were determined, and an effective method was applied to improve the acoustic performance at low frequency. The predicted sound power of the intake system indicated that the objective of reducing the overall engine noise by minimizing intake system noise was achieved.

Objective To study the effect of distortion product otoacoustic emission (DPOAE) on guinea pigs exposed to narrow band noise of different intensity. Methods Sixty healthy male guinea pigs were randomly divided into 3 groups: the 105 dBA group, the 110 dBA group and the 115 dBA group. The duration of exposure for all the groups was 3 hours and noise spectrum was narrow - band, with a mid - frequency of 4 000Hz and band-width of 1000 Hz. Two days before and 15 minutes after noise exposure, DPOAE of guinea pigs was measured by means of Smart OAE Hearing Test System. Results With increased intensity in noise exposure, DPOAE amplitude and throughput of guinea pigs at all the distortion frequencies showed a decreased tendency. For the 105 dBA group and the 110 dBA group at the distortion frequency of 1 409 Hz, and for the 105 dBA group and the 115 dBA group at frequencies of 499 Hz,704 Hz,1 003 Hz, and 1 409 Hz, the difference in DPOAE amplitude and throughput after exposure all had statistically significance (P<0.05). Conclusions Following exposure to 4 000Hz narrow band noise, DPOAE began to change at high frequencies and heating damage extended from higher frequency to lower frequencies as noise level increased. Key words: Guinea pig; Narrow band noise; Exposure; Distortion product otoacoustic emission

The effects of fluid flow and surface stabilisation on sonoluminescence and sonochemistry.

During auditory stimulation with several frequency components, distortion products (DPs) are generated as byproduct of nonlinear cochlear amplification. After generated, DP energy is reemitted into the ear channel where it can be measured as DP otoacoustic emission (DPOAE), and it also induces an excitatory response at cochlear places related to the DP frequencies. We measured responses of 91 inferior colliculus (IC) neurons in the gerbil during two-tone stimulation with frequencies well above the unit's receptive field but adequate to generate a distinct distortion product (f2-f1 or 2f1-f2) at the unit's characteristic frequency (CF). Neuronal responses to DPs could be accounted for by the simultaneously measured DPOAEs for DP frequencies >1.3 kHz. For DP frequencies <1.3 kHz (n = 25), there was a discrepancy between intracochlear DP magnitude and DPOAE level, and most neurons responded as if the intracochlear DP level was significantly higher than the DPOAE level in the ear channel. In 12% of those low-frequency neurons, responses to the DPs could be elicited even if the stimulus tone levels were below the threshold level of the neuron at CF. High intracochlear f2-f1 and 2f1-f2 DP-levels were verified by cancellation of the neuronal DP response with a third phase-adjusted tone stimulus at the DP frequency. A frequency-specific reduction of middle ear gain at low frequencies is possibly involved in the reduction of DPOAE level. The results indicate that pitch-related properties of complex stimuli may be produced partially by high intracochlear f2-f1 distortion levels.

The Phase Vocoder plays a central role in sound analysis and synthesis, allowing us to represent a sound signal in both time and frequency, similar to a music score – but possibly at much finer time and frequency scales – describing the evolution of sound events. According to the uncertainty principle, time and frequency are not independent variables so that any time-frequency representation is the result of a compromise between time and frequency resolutions, the product of which cannot be smaller than a given constant. Therefore, finer frequency resolution can only be achieved with coarser time resolution and, similarly, finer time resolution results in coarser frequency resolution.While most of the conventional methods for time-frequency representations are based on uniform time and uniform frequency resolutions, perception and physical characteristics of sound signals suggest the need for nonuniform analysis and synthesis. As the results of psycho-acoustic research show, human hearing is naturally organized in nonuniform frequency bands. On the physical side, the sounds of percussive instruments as well as piano in the low register, show partials whose frequencies are not uniformly spaced, as opposed to the uniformly spaced partial frequencies found in harmonic sounds. Moreover, the different characteristics of sound signals at the onset transients with respect to stationary segments suggest the need for nonuniform time resolution. In the effort to exploit the time-frequency resolution compromise at its best, a tight time-frequency suit should be tailored to snuggly fit the sound body.In this paper we overview flexible design methods for phase vocoders with nonuniform resolutions. The methods are based on remapping the time or the frequency axis, or both, by employing suitable functions acting as warping maps, which locally change the characteristics of the time-frequency plane. As a result, the sliding windows may have time dependent duration and/or frequency dependent bandwidth. As an example, in a constant Q frequency band allocation, the ratios of center band frequencies over bandwidth remains constant, so that the frequency bands become wider and wider as center frequency increases, similarly to the frequency distance of 12-tone scale notes or of octaves.While time-frequency allocation can be performed in an arbitrary way, the ability to reconstruct the original signal from Vocoder analysis data is essential in sound processing and transformation applications. Moreover, even the analysis or the production of spectrograms benefits from the perfect reconstruction property if one needs to be confident that no important information is hidden, which serves to completely describe the signal.

Aims: In this study, the amount of the reduction in absorption silencer sound filled with Iranian absorbing materials and influence of thickness and density of adsorbent material in silence was investigated. Materials and Methods: We used galvanized channel 0.6 mm with 30 × 30 cm 2 dimension and axial fan. Length of absorptive silencer was 50 cm, and there used the mineral wool absorbent. Absorptive silencer as channel was designed to cross a section of channel and silencers after embedded the absorber be identical together. This study deals with a number of variables that affect performance of absorptive silencer in the channel and their impact on reducing the sound pressure level be examined. Results: In the frequencies 125 and 250 Hz a reduction of sound pressure level was found. While in frequency 500 Hz minimal reduction is achieved. In this study, an increase in the thickness of the absorbent material of absorption silencer, there was a little change in the volume decreasing but with increasing density from 80 to 100 Kg/m 3 better results were seen to reduce noise levels and it caused to increase NR rate. Conclusion: According to data from the present study we can used silencer with 5 cm thick and 100 Kg/m 3 density in the reduction of sound pressure level at frequencies lower than 250 Hz in the ventilation system as an effective device.