Numerical And Experimental Investigations On High Power And High Intensity Air-Modulated Speaker

Air-Modulated Speaker (AMS) is one of the most popular high power and high intensity acoustic sources due to its high sound pressure level (SPL) output and wide frequency responses. Most of the previous works on AMS are concerned with its sound field. However, it is well understood that acoustic characteristics of AMS are closely related to the unsteady flow and energy conversion process inside the source, which are essential to the research of sound generation mechanism and optimal design of the AMS device. In this paper, a numerical simulation model of AMS was developed and an experimental system was established for recording the internal unsteady flow and near field sound pressure. In order to understand the flow and sound characteristics, numerical simulations and experimental tests of the flow and sound fields were carried out for various working conditions. For the first time, supersonic flow modulated speaker (SFMS) was designed and implemented. Considerable SPL gain of SFMS was obtained after introducing a special time-delay modulation function. Meanwhile, the role of vocal tract playing in the sound generation was studied by the comparison of unsteady flow simulation results in different vocal tract designs.The main works are summarized as follows:The numerical model of AMS was established for the internal transient flow and near field nonlinear acoustic field. Without the quasi-steady hypothesis on derivation of model equations, a model of the internal flow was developed by utilizing a finite volume compressible CFD solver and dynamic mesh technique. The intensive sound field was calculated based on a hybrid method with Reynolds Averaged Navier-Stokes (RANS) and FW-H analogy. As a result, the model of AMS can be applied to handle complex geometric structure of the actual device for various working conditions. Agreements in steady flows were obtained between simulation results and flow field measurements. The hybrid method was proved to be feasible for performance prediction of AMS and better understanding the sound generation mechanism.To understand the energy conversion process, several experimental methods in fluid dynamics were used. The steady velocity field of a two dimensional AMS model was captured by the particle image velocimetry (PIV) technique. Both flow field static pressure and near field sound pressure were recorded by a flow-induced sound monitoring system.Experimental results of the internal flow and acoustic field near the source were obtained for various chamber pressures, geometric parameters or driving signals. Velocity field of the PIV results reveals that there are two dominant characteristics in the steady flow. One is the pressure recovery process with flow separation on the outer wall, and the other is the shear layer with vortex formation between main flow and reversed flow. According to the sound pressure data, frequency responses of the source are related with the chamber pressure. It is implied that the voice coil oscillation is coupled with the high speed jet flow. Variation of the static pressure in vocal tract increases with the increment of the chamber pressure and the driving signal amplitude. The fundamental and second harmonic frequency components in the disturbing pressure were obvious in the source generation zone during a monochromatic modulation. Spectral composition of the internal static pressure is coincident with that of the near field sound signal under the same working conditions.Based on the numerical model, the performance and energy conversion process of AMS were compared for different chamber pressures, vocal tract inlet sizes, modulation frequencies and gas types. Simulation results under various working conditions have demonstrated that SPL output becomes saturated at high chamber pressure conditions, which is consistent with the quasi-steady theory result. When the size of vocal tract inlet is changed, disturbing pressure in the acoustic zone and the true modulation area ratio are almost the same. Variations of the static pressure in the transient flow are closely related to the modulation frequency. In a low frequency modulation, amplitude of negative disturbing pressure decreases along the main flow direction. In the case of high frequency modulation, a positive pressure zone was formed. The classical nonlinear effects were also found in this case, such as waveform distortion, shock wave and additional attenuations. When the modulation frequency increased, vortex flow was also enhanced in the shear layer.Numerical and experimental studies were carried out for the SFMS. It was considered as a feasible way to overcome sound saturation at high chamber pressure conditions. Experimental results have shown that, compared with the sonic flow case, SPL output gain of the SFMS is obtained only when the following two requirements are fulfilled. Firstly, a high chamber pressure and a high amplitude driving signal are required. Secondly, a specified frequency band is needed to obtain large displacement for voice coil oscillation, such as a frequency band around the peak value in the frequency response curve. Experimental results show that, SPL gain for SFMS at the position of 1 meter from the source was 8dB at chamber pressure 0.6MPa, driving current 10A and modulation frequency 500Hz. In order to explain the frequency dependence of SPL gain, numerical simulations employing a mach-3 nozzle were carried out for a fixed modulation frequency 500Hz and different chamber pressures. SPL gain of the SFMS is also confirmed by the simulation results. However, they have demonstrated that supersonic jet is not established near the nozzle exit even for chamber pressure 1.2MPa. This is because that time needed for supersonic flow development inside the nozzle is similar to the modulation period. In order to realize the true supersonic flow modulation, a time-delay modulation function was designed to replace the original sine function. Simulation results of the time-delay modulation have validated that the flow speed near the nozzle exit has been greatly added to supersonic. Both of the mass flow rate and SPL output remarkably increase in the time-delay case.The function of vocal tract on the energy conversion in AMS was studied by the comparison of unsteady flow simulation results for different vocal tract designs under the same working condition. There are considerable differences in the average flow and sound generation mechanism among results for different designs. Development of flow disturbance and spectral composition of the transient pressure are also influenced by the vocal tract contour. It is pointed out that two factors related with the contour design affect the energy conversion process. One is the impedance mismatch on the vocal tract exit; the other is the interaction between high speed main flow and disturbance propagation. They are also responsible for the internal transient flow changes. In order to reduce the wave distortion of the flow disturbance inside the vocal tract, a novel external jet design was proposed. The advantage of this new design was validated by the simulation of the energy conversion process in AMS.