Towards Active Transducers

One of the trends within consumer audio systems is the requirement for surround sound systems, based on 5-7 or even more audio channels, resulting in the same number of power amplifier channels and loudspeakers for each system. Most systems on the market today are based on linear amplifiers techniques developed in the middle of last century, which is surprising when the last few decades’ development within the audio source material, that is CD, DVD and SACD to mention the most popular, are taken into account. Audio performance of linear amplifiers has reached a level suitable for high quality audio reproduction many product generations ago, but the biggest disadvantages are still left untouched; power efficiency, size and cost. With today’s technology, high efficient switch mode, or class D audio amplifiers based on pulse width modulation, PWM, are realizable. With the class D technology, consumer audio systems can benefit significantly from the highly increased power efficiency of class D amplifiers as well as their reduced size without need for bulky heat sinks, and also very important, low cost. The topic of this project is a total integration of switch mode audio amplifiers and loudspeakers into one single unit using the voice coil of the loudspeaker as output filter for the amplifier, with a perspective of highly reduced system power losses, system size and cost. Standard switch mode audio amplifiers and loudspeakers on the market are designed for use in traditional audio systems, and cannot without severe modifications be used for the integrated system without sacrifice of power efficiency. For this reason techniques for dedication of amplifier and loudspeaker for the specific purpose of the integration has been of major importance in this project. This thesis is a fundamental study of the loss mechanisms in loudspeakers and amplifiers and suggestions for optimizations are made to reduce the system power losses and cost without compromising the audio performance. Some of the results obtained in the project are redesign of and optimization of the parts in a loudspeaker, so the function of output filter for the amplifier can be obtained without significant power losses. Guidelines for dedication of speaker and amplifier to the integration process with significantly lower system power losses are also given. Furthermore, the work done in the project has resulted in new switch mode amplifier topologies, with very high audio performance realizable at a very low cost

Development of the Carbon Nanotube Thermoacoustic Loudspeaker

Traditional speakers make sound by attaching a coil to a cone and moving that coil back and forth in a magnetic field (aka moving coil loudspeakers). The physics behind how to generate sound via this velocity boundary condition has largely been unchanged for over a hundred years. Interestingly, around the time moving coil loudspeakers were first investigated the idea of using heat to generate sound was also known. These thermoacoustic speakers heat and cool a thin material at acoustic frequencies to generate the pressure wave (i.e. they use a thermal boundary condition). Unfortunately, when the thermoacoustic principle was initially discovered there was no material with the right properties to heat and cool fast enough. Carbon nanotube (CNT) loudspeakers first generated sound early in the 21st century. At that time there were many questions unanswered about their place in the sound generation toolbox of an engineer. The main goal of this dissertation was to continue the development of the CNT loudspeaker with focus on practical usage for an acoustic engineer. Prior to 2014, when this effort began, most of the published development work was from material scientists with objective acoustic performance data presented that was not useful beyond the scope of that particular publication. For example, low sound pressure levels in the nearfield at low power inputs was a common metric. Therefore, this effort had three main objectives with emphasis placed on acquiring data at levels and in nomenclature that would be useful to acoustic engineers so they could bring the technology to market, if adequate. Investigation into the true power efficiency of CNT loudspeakers Investigation into alternative methods to linearize the pressure response of CNT loudspeakers Investigation into the sound quality of CNT loudspeakers Overall, it was found that CNT loudspeakers are approximately four orders of magnitude less power efficient than traditional moving coil loudspeakers. The non-linear pressure output of the CNT loudspeakers can be linearized with a variety of drive signal processing methods, but the selection of which method to use depends on a variety of factors (e.g. amplification architecture available). In general, all methods studied are on the same order of magnitude power efficiency, but the direct current offset and amplitude modulation drive signal processing methods are superior in terms of sound quality

A High-Efficiency 4x45W Car Audio Power Amplifier using Load Current Sharing

A 4x45W (EIAJ) monolithic car audio power amplifier is presented that achieves a power dissipation decrease of nearly 2x over standard class AB operation by sharing load currents between loudspeakers. Output signals are conditioned using a common-mode control loop to allow switch placement between loads with minimal THD increase. A prototype is realized in a SOI bipolar-CMOS-DMOS process with 0.5μm feature size. Die area is 7.5x4.6mm2. THD+N @(1kHz,10W) is 0.05%