Acoustics in small rooms suffer from resonances in low frequencies, resulting in the well-known sound colouration problem. In the present work, the finite element method was used to investigate specific case studies of proposed treatments of this problem. The treatments are based on techniques that make use of three basic mechanisms: wavelength, boundary impedance, and active control. The quality of the treatments is evaluated using two spatial statistical measures of the sound pressure level frequency responses regularly sampled in the listening area. The results demonstrate the advantage of the impedance-and active control mechanism-based techniques. Resonant panels and multiple source excitation treatments improve the sound field flatness.

A system for low-frequency noise control in small, enclosed sound fields is proposed, using compound sound sources optimized by a genetic algorithm (GA). It is the integration of the developed low-Bl driver compound sources with a GA computer program in the Python language, aiming to control the modal field. The lack of appropriate free space in small rooms is critical for positioning the secondary sound sources; therefore, the proposed system has been designed to adapt to any available position. Two quadrupole topologies of the secondary compound source are applied and examined in a room. The convergence of the algorithm to the optimal solutions is attained through parametric configuration. The spatial radiation of the compound source at a single fixed position is adapted to couple with the modal noise field and attenuate it. The experimental results indicate that the proposed system can successfully control resonances of different low frequencies down to 50 Hz at multiple positions. The tonal noise attenuation reaches up to 32 dB at 100 Hz, confirming the applicability of the small subwoofer loudspeaker configurations for low-frequency control. This new method offers a practical and effective alternative to the typical abatement techniques that use distributed monopole sources in limited spaces.

The proposed compound sound sources for low-frequency noise control applications are composed of dipole sources. Their spatial radiation, which is critical in the modal field of small, closed spaces, is intended to be controlled with independent driving signals of each dipole. The need for small and efficient low-frequency elementary monopole sources led to the proposed vented sub-woofer loudspeaker design with low force factor (low-Bl) drivers. The investigated sources are set up in quadrupole configurations and measured in terms of polar near field response patterns to verify the theoretical predictions. The measurement results consist of the validation of the proposed compound sound sources on the implementation of active noise control problems in the low-frequency range. Also, their small size and modular construction make them interesting for use in other applications.

A novel method of active noise control using adaptive radiation sound sources is investigated. A finite element model of a modal enclosed sound field is excited harmonically, representing a noise field in the low-frequency range. The control sources are comprised of elementary dipole sources for which the driving signals are adjusted by an optimization method. Two set-up cases of the proposed compound sources are investigated. The coupling of the control sources with the modal sound field is discussed. The simulated performance of the proposed method is compared with that of a system with distributed simple sources and the results show the effectiveness of the sources with adaptive radiation for active noise control in small enclosures.

Software for estimation and processing simulation of microphone line arrays was developed, based on the fundamentals of sensor array theory and the basic concepts governing the array beamforming. Periodical, nonperiodical, and logarithmic microphone line arrays can be designed in a convenient way. Spatial structures, directivity patterns, and other properties of the microphone arrays are shown and examined diagrammatically. Optimization of the microphone array directivity index through the entire audio spectrum, using the logarithmic design and the apropriate filters, is shown as well. All properties can be easily examined and dynamically changed via a useful user interface. The designed microphone arrays can also be simulated using sound recordings of a single microphone or WAV files in PCM format. The program’s output is an audio processing simulation of the acoustic field which can be accessed through WAV file registering. This stand-alone, fully adjustable simulation software can be a user-friendly tool in understanding the basic concepts of the microphone array design theory.