Comparative analysis of modulation types used in ultrasonic speaker systems

Frequencies above 20 kHz, the upper limit of the human hearing threshold, are referred to as ultrasound. Due to their small wavelengths, ultrasound is well suited for applications where directional sound transmission is targeted. Many studies in recent years have focused on the use of ultrasound in loudspeaker systems. Thanks to the technology called ultrasonic loudspeaker, sound can be transmitted to the desired point at an acute angle, just like a laser. Ultrasonic loudspeaker systems exploit the nonlinear behavior of air to achieve directional sound. These systems usually consist of an analog audio input, analog-to-digital converter, modulator, amplifier and ultrasonic speaker array. This study focuses on the modulator element used in the signal chain of ultrasonic loudspeaker systems. The study was conducted with a bimodal design, which is one of the quantitative research methods. The aim of the study is to determine the most efficient modulation type that can be used in ultrasonic sound transmission. In most of the studies in the literature, there is no information on how the modulator element is designed and whether any pre-processing is applied. It was considered that this situation would make it difficult to make sense of the problems that may be encountered in the research process in a cause-effect relationship. For this reason, the study started with the design of the modulator software that can apply the modulation types in the literature. After the preparation of the modulator application, an ultrasonic speaker system was created, sound samples were prepared, and a series of measurements were recorded for each modulation type, near-field and far-field. The audio recordings were converted into numerical and visual data. The data obtained for each modulation type were compared with each other in terms of a number of criteria such as frequency analysis, total harmonic distortion, signal-to-noise ratio, signal-to-distortion ratio, peak level and intermodulation distortion. As a result of analyzing the numerical and visual data, efficient and inefficient modulations were identified. After all these procedures, the recommended modulation type for ultrasonic loudspeaker systems was determined as RRSBAM 3. As a result of this study, important information has been obtained that can be used in the design of ultrasonic loudspeaker systems. Unlike most of the studies on ultrasonic loudspeaker design in the literature, the research process and the data obtained are explained in full detail. This study is a step towards approaching the goal of using ultrasonic loudspeakers in music reproduction.