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PhD Defence Farnaz Tajdari

energy-efficient low-frequency flat acoustic source - analysis, design and experimental validation

Farnaz Tajdari is a PhD student in the research group Applied Mechanics. Her supervisor is prof.dr.ir. A. de Boer from the Faculty Engineering Technology.

There is a need for compact acoustic sources that operate at low frequencies. These acoustic sources can be employed to produce good quality sound in the low frequency range, for instance in the limited space that is available in the flat-screen televisions and in airplane cabin walls. Additionally, low-frequency acoustic sources can be applied to eliminate transmission of unwanted noise using active noise control techniques, for example, in homes, hotel rooms or airplane cabins.

A problem arising when the acoustic sources operate at low frequencies is that the acoustical power is not always enough to allow for sound to be emitted in the form of propagating waves. As a rule of thumb, in the low frequency range the larger the enclosure volume of acoustic sources, the greater the radiated acoustical power. Therefore, in the applications with limited build space it is difficult to compensate for the sufficient acoustic power of the radiators.

A thin sandwich acoustic source with a large surface area and a relatively small thickness that is integrated with an internal cavity can fulfill the need for both large enclosure volume and limited build space. The sandwich structure makes the thin acoustic source light-weight and stiff. Therefore, the resulting acoustic source has a reasonably high fundamental resonance frequency.

The use of voice coil actuators as the driver of the thin acoustic source results in dissipation of a large portion of the input electric power due to Joule heating of the coil. This makes the thin acoustic source system inefficient. An inefficient acoustic source consumes extra input electric energy, which causes energy loss and high energy expenses. Piezoelectric actuators are efficient replacements to ensure an energy-efficient operation of the thin acoustic source. However, due to the capacitive nature of the piezoelectric devices, they cannot result in a higher efficiency of the acoustic source alone, especially at low frequencies. The existence of an appropriate electrical amplifier in combination with the piezoelectric devices is crucial to achieving an energy-efficient thin acoustic source. An appropriate combination of the actuators and amplifiers is investigated in this research. Taking the electrical considerations into account, the appropriate combination results in an energy-efficient operation of the thin acoustic source. Due to the limited build space in the low-frequency applications, a need for a compact design of the piezoelectric stack actuators arises. An appropriate auxiliary flexural mechanism is investigated for use as an appropriate driver in the limited space of the thin acoustic source. The resulting acoustic source is thin and energy-efficient and can be used in the low frequency operations. The motivation of the present research is to design a compact energy-efficient actuation mechanism for the thin acoustic source that operates in the low-frequency range.

In this research, the combined electrical, mechanical and acoustical aspects of the thin acoustic source are investigated. An energy-efficient performance of the thin acoustic source is evaluated when it is connected to multiple amplifiers and actuators. A flexural mechanism is proposed to be integrated with piezoelectric stack actuators and to be used as the driver of the thin acoustic source. A fully-coupled multiphysics Finite Element model is developed to analyze the thin acoustic source system that is actuated by the integrated flexural mechanism. The resulting acoustic source system is manufactured and studied experimentally to validate the FE analysis. The study shows that the developed FE model of the thin acoustic source is in very good agreement with the experimental result. As a result of this research, an energy-efficient thin acoustic source is successfully designed and manufactured. It is excited by a flexural mechanism that is integrated with energy-efficient piezoelectric actuators. The constructed energy-efficient acoustic source meets the requirements and is an excellent choice to be employed in the low-frequency applications. It is sufficiently thin to be employed in applications with limited build space. Its sandwich structure is combined with its large surface area to provide sufficient enclosure volume to generate propagating sound waves in the low frequency range.