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.
The reviewed book is a new, expanded and modernized edition of the classical position of the book “Acoustics” published in 1954 and reprinted in 1986. The subtitle: “Sound Fields and Transducers” reflects well the nature of the changes in relation to the original. The chapters concerning sound fields and transducers have been added or significantly expanded and chapters on other subjects, such as noise control, hearing, and speech, have been removed. The chapter about acoustic measurements has been also removed, although some parts of it concerning reciprocity calibration of transducers would be compatible with the concept of the present version of the book. The book consists of 14 chapters. The chapters are divided into parts which are numbered consecutively, independently on numeration of chapters, and sections numbered within chapters. The book contains also three appendices. The scientific level of the book is high and it is designed for advanced readers, e.g., graduate students, and professionals who want to expand their knowledge.
The paper presents results of research on an influence of listening fatigue on the detection of changes in spectrum and envelope of musical samples. The experiment was carried out under conditions which normally exist in a studio or on the stage when sound material is recorded and/or mixed. The equivalent level of presented sound samples is usually 90 dB and this is an average value of sound level existing in control room at various recording activities. Such musical material may be treated as a noise so Temporary Threshold Shift phenomenon may occur after several sessions and this may lead to a listening fatigue effect. Fourteen subjects participated in the first part of the experiment and all of them have the normal hearing thresholds. The stimuli contained the musical material with introduced changes in sound spectrum up to ±6 dB in low (100 Hz), middle (1 kHz) and high frequency (10 kHz) octave bands. In the second part of research five subjects listened to musical samples with introduced envelope changes up to ±6 dB in interval of 1 s. The time of loud music exposure was 60, 90 and 120 minutes and this material was completely different from the tested samples. It turned out that listening to the music with an Leq = 90 dB for 1 hour influences the hearing thresholds for middle frequency region (about 1-2 kHz) and this has been reflected in a perception of spectral changes. The perceived peaks/notches of 3 dB have the detection ability at 70% and the changes of low and high ranges of spectrum were perceived at the similar level. After the longer exposure, the thresholds shifted up to 4.5 dB for the all investigated stimuli. It has been also found that hearing fatigue after 1 hour of a listening influences the perception of envelope which gets worse of 2 dB in comparison to the fresh-ear listening. When time of listening to the loud music increases, the changes in envelopes which can be detected rise to the value of 6 dB after 90-minutes exposure and it does not increase with further prolongation of listening time.
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.
The paper presents results of hearing loss measurements provided for 81 young people (from 16 to 25 years old). The main aim of the work was to find the influence of headphones of the types used (closed, semi-open, open and in-ear) on the hearing losses. The first part of the research was to answer questions about the influence of: time of listening, loudness of music, other noise exposures as well as the type of the headphones used. It turned out that all factors mentioned above influence thresholds of hearing but the found dependencies are not explicit. The greatest hearing losses were observed for people who work as sound reinforcement engineers and, moreover, no influence of the headphone types was found for them. It turned out that the use of in-ear headphones causes the greatest hearing losses for some subjects (thresholds shifted up to about 20 dB HL at 4 kHz). The daily time of a listening also affected the hearing thresholds. It was found that for users of in-ear and close headphones, an average time of musical exposure of three hours causes the hearing loss of 10-15 dB HL at higher frequencies. The use of open as well as semi-open headphones has no influence on the hearing damage. Thus it would be stated that these kinds are safety in use. Almost 15% of the investigated young people have their thresholds shifted up at higher frequencies, particularly at 4 kHz, which means that they have the first symptoms of a permanent hearing damage.