The task of electroacoustic devices is a transmission of audio signals. The transmitted signal should be distorted as little as possible. Nonlinear distortions are the distortions depending on signal level. The types of nonlinear distortions as well as their measures are presented in the paper. The weakest device in an electroacoustic chain is a loud-speaker. It causes the greatest degradation of the signal. It is usually the most nonlinear part of the electroacoustic system. The nonlinearities in loudspeakers are described in details. Other types of nonlinear distortions as transient intermodulation in power amplifiers and distortions caused by the A/C sampling are also presented.

The reviewed book is an introductory course on engineering acoustics designed for undergraduates with basic knowledge in mathematics. It is not written clearly enough but in my opinion is it a handbook for students of electrical engineering. Some parts of the material require the knowledge of the basics of electricity and magnetism. Particularly, there are chapters about electromechanical and electroacoustical analogies and electroacoustical transducers. It is not clear whether the course is intended for students who plan to specialize in acoustics or for those for whom this will be the only contact with engineering acoustics. In the second case basic information about physiology and psychology of hearing is missing. The book is divided into 15 chapters. The Authors write that each chapter represents material for two hours of lecture. The 15th chapter does not present a material for a lecture. It contains appendices: basic information about complex notation for sinusoidal signals, power and intensity, supplementary bibliography for self-study and exercises. In my opinion the exercises have various levels of difficulty and should be solved under the direction of a teacher. They are a very important part of the entire course.

Zygmunt G. Wąsowicz, PhD emeritus of the Chair of Acoustics and Multimedia, Wrocław University of Technology, passed away on the 8th of January 2014. His whole professional career was associated with the acoustics. Dr. Z. Wąsowicz was born in Nowy Sącz in 1931. In 1956 he graduated from the Faculty of Telecommunications at the Wrocław University of Technology and started to work there in the same year. In 1966 he obtained the PhD title, under supervision of Professor Z. Żyszkowski, for the dissertation concerning the subjective criteria of nonlinear distortions in loudspeakers. His main interests of activity were room acoustics as well as subjective assessment of sound quality. He worked out the subjective method of loudspeaker evaluation for Polish Loudspeaker Company “Tonsil” – this method was based on so-called “the live apparent sound”. He worked also on computer methods of acoustical field modeling in rooms. The works mentioned above were pioneer and modern in Poland. He participated as an acoustician, in various designers groups at for example auditory halls of Faculty of Electronics. Dr. Wąsowicz was the outstanding academical teacher whom students liked very much. He was also a member of Polish Acoustical Society and worked for the Main Board as well as the Wrocław Division of this society. In periods 1979–1983 and 1994–1996 he was the vice-dean of Faculty of Electronics. He received many awards, for example Golden Cross of Merit, Knight’s Cross of the Order of Polonia Restituta, Medal of the National Education Commission and many awards from the Governors of Wrocław University of Technology and Institute of Telecommunications and Acoustics. In 1996 he was retired and beside of this he stayed in contact with Faculty of Electronics for many years. Wrocław acoustical community mourns the loss of Dr. Z. Wąsowicz.

Thermoacoustic converters are devices for direct conversion of acoustic energy into thermal energy in the form of temperature difference, or vice versa – for converting thermal energy into an acoustic wave. In the first case, the device is called a thermoacoustic heat pump, in the second – thermoacoustic engine. Thermoacoustic devices can use (or produce) a standing or travelling acoustic wave. This paper describes the construction and properties of a single-stage thermoacoustic engine with a travelling wave. This kind of engine works using the Stirling cycle. It uses gas as a working medium and does not contain any moving parts. The main component of the engine is a regenerator equipped with two heat exchangers. Most commonly, a porous material or a set of metal grids is used as a regenerator. An acoustic wave is created as a result of the temperature difference between a cold and a hot heat exchanger. The influence of working gas, and such parameters as static pressure and temperature at heat exchanger on the thermoacoustic properties of the engine, primarily its efficiency, was investigated. The achieved efficiency was up to 1.4% for air as the working medium, which coincides with the values obtained in other laboratories. The efficiency for argon as working gas is equal to 0.9%.

The physical phenomena occurring in sound-absorbing and insulating enclosures are subject of the present paper. These phenomena are: absorption in air and by the sound-absorbing material covering the walls and the coincidence effect. The absorption in the air can be neglected in small size enclosures for low ultrasonic frequencies (20-30 kHz). The coincidence plays a role in decrease of the sound insulation, however the main role play the leaks. The boards made of ceramic fibers have been chosen as the optimal sound-absorbing material. They are dense and have deeply porous structures. The enclosure for insulation of 20-kHz noise produced by a welding machine has been designed and manufactured, and reductions of 25 dB of peak and Leq levels have been achieved.