Chapter 1
Introduction
Human beings are usually considered to predominantly perceive their environment through the visual sense—in other words, humans are conceived as visual beings. However, this is certainly not true for inter-individual communication.
It is audition and not vision that is the most relevant social sense of human beings. The auditory system is their most prominent communication organ—particularly in speech communication. Take as proof that it is much easier to educate blind people than deaf ones. Also, when watching TV, an interruption of the sound is much more distracting than an interruption of the picture. Particular attributes of audition compared to vision are the following.
–In audition, communication is compulsory. The ears cannot close by reflex like the eyes can.
–The field of hearing extends to regions all around the listener—in contrast to the visual field. Further, it is possible to listen behind optical barriers and in darkness.
These special features, among other things, lead many engineers and physicists, especially those in the field of communication technology, to a particular interest in acoustics. A further reason for the affinity of engineers and physicists to acoustics is based on the fact that many physical and mathematical foundations of acoustics are usually well known to them, such as mechanics, electrodynamics, vibrations, waves, and fields.
1.1 Definition of Three Basic Terms
When working your way into acoustics, you will usually start with the phenomenon of hearing. The term acoustics is derived from the Greek verb ακoυ´ ιν [akúIn], which means to hear. We thus start with the following definition.
© Springer-Verlag GmbH Germany, part of Springer Nature 2021 |
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N. Xiang and J. Blauert, Acoustics for Engineers, https://doi.org/10.1007/978-3-662-63342-7_1
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Auditory event … An auditory event is something that exists as heard. It becomes actual in the act of hearing. Frequently used synonyms are auditory object, auditory percept, and auditory sensation.
Consequently, the question arises of when auditory events appear? As a rule, we hear something when our auditory system interacts via the ears with a medium that moves mechanically in the form of vibrations and/or waves. Such a medium may be a fluid like air or water, or a solid like steel or wood. The phenomenon of hearing usually requires the presence of mechanic vibration and/or waves. The following definition follows this line of reasoning.
Sound … Sound is mechanic vibration and/or mechanic waves in elastic media.
According to this definition, sound is a purely physical phenomenon. Be warned, however, that the term sound is also sometimes used for auditory events, particularly in sound engineering and sound design. Such an ambiguous usage of the term is avoided in this book.
It it worthy noting that vibrations and waves are often mathematically expressible as differential equations—see Chap. 2. Vibrations require common differential equations with the dependent variable being a function of time, while waves require partial ones because the dependent variable is a function of both time and space. Further, note that, although rare, auditory events may happen without sound being present, such as with tinnitus. In turn, there may be no auditory events in the presence of sound, for example, for deaf people, or when the frequency range of the sound is not in the reach of hearing. Sounds are categorized in terms of their frequency ranges—listed in Table 1.1.
The interrelation of auditory events and sound is captured by the following definition of acoustics.
Acoustics … Acoustics is the science of sound and of its accompanying auditory events.
This book deals with engineering acoustics. Synonyms for engineering acoustics are applied acoustics and technical acoustics.
Table 1.1 Sound categories by frequency range
Sound category |
Frequency range |
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Audible sound |
≈16 Hz–16 kHz |
Ultrasound |
>16 kHz |
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|
Infrasound |
<16 Hz |
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Hypersound |
>1 GHz |
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1.2 Specialized Areas within Acoustics |
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1.2 Specialized Areas within Acoustics
Figure 1.1a presents a schematic of a transmission system as is often used in communication technology. A source renders information that is fed into a sender in coded form and transmitted over a channel. At the receiving end, a receiver picks up the transmitted signals, decodes them, and delivers the information to its final destination, that is, the information sink.
Figure 1.1b depicts a modified schematic to describe the receiving end of a transmission chain with acoustics involved. This delineation helps to distinguish major areas within engineering acoustics. The transmission channel delivers signals that are essentially chunks of electric energy. The receiver picks up these signals and feeds them into an energy transducer which transforms the electric energy into mechanic (acoustic) energy. The acoustic signals are then sent out into a sound field where they propagate to the listener. The listener receives them, decodes them, and processes the information. Note that undesired noise may enter the system at different points in addition to the desired signals.
The main areas of acoustics are as follows. The field that deals with the transduction of acoustic energy into electric energy, and vice versa, is called electroacoustics. The field that deals with the radiation, the propagation, and the reception of acoustic energy is called physical acoustics. The fields that deal with sound reception and auditory information processing by human listeners are called psychoacoustics and physiological acoustics. The first of these two fields focuses on the relationship between the sound and the auditory events associated with it, and the second one deals with sound-induced physiological processes in the auditory system and brain.
Acoustics as a discipline is usually further differentiated due to practical considerations. The following cover labels of the sessions at a major acoustics conference illustrate the broadness of the field.
Fig. 1.1 Schematic of a transmission system. (a) General form. (b) Electroacoustic transmission system—receiving end
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Acoustic and auditory signal processing, acoustic-measurement engineering, active acoustic systems, aircraft noise, architectural acoustics, artificial intelligence in acoustics, audio technology, audiological acoustics, bioacoustics, cognitive systems in acoustics, effects of noise, electroacoustics, evaluation of noise, flow acoustics, hydro-acoustics, medical acoustics, musical acoustics, noise propagation, noise protection, numerical acoustics, philosophy in acoustics, physical acoustics, physiological acoustics, psychoacoustics, room acoustics, speech-and-language processing, structure-borne sound, ultrasound, vehicle acoustics, vibration technology, virtual auditory reality.
Accordingly, there is a wide variety of professions dealing with acoustics, including diverse engineers, such as audio, biomedical, civil, electrical, environmental, and mechanical engineers. Further, for example, administrators, architects, audiologists, designers, ear-nose-and-throat-doctors, lawyers, managers, musicians, computer scientists, patent attorneys, physicists, physiologists, psychologists, sociologists and linguists.
1.3 About the History of Acoustics
Acoustics is an ancient science. Pythagoras already knew, around 500 BC, of the quantitative relationship between the length of a string and the pitch of its accompanying auditory event. In 1643, Torricelli demonstrated the vacuum experimentally and showed that there is no sound propagation in it. In 1802, Chladni, in his Germanlanguage book “Die Akustik” introduced acoustics as a separate branch into physics. At the end of the 19th century, classical physical acoustics had matured. The book “The Theory of Sound” by Rayleigh 1896, is considered a substantial reference even today.
At about the same time, basic inventions in acoustical communication technology were made, including the telephone (Reis 1867), television (Nipkow 1884) and tape recording (Ruhmer 1901). It was only after the independent invention of the vacuum triode by von Lieben and de Forest in 1910, which made amplification of weak currents possible, that modern acoustics enjoyed a real up-swing due to applications such as radio broadcast since 1920, sound-on-film since 1928, and public-address systems with loudspeakers since 1924.
From about 1965 on, computers made their way into acoustics, making effective signal processing and interpretation possible and leading to advanced applications such as acoustic tomography, speech-and-language technology, surround sound, binaural technology, auditory displays, mobile phones, and many others. Acoustics in the context of the information and communication technologies and sciences is nowadays called communication acoustics, including psychoacoustics and sound-related cognitive psychology.
However, the current book concentrates on the classical aspects of engineering acoustics, particularly on physical acoustics and electroacoustics. To this end, this book employs the following theoretical tools: the theory of electric and magnetic processes, the theory of signals, vibrations and systems, and the theory of waves and fields.