Acoustics

Acoustics is a branch of physics that deals with the study, production, transmission, and effects of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound, and infrasound. The ear itself is another biological instrument dedicated to receiving certain wave vibrations and interpreting them as sound.

Studies the properties and characteristics of sound and phenomena related to its propagation in various media. Among its most important phenomena we must remember those that take place during the incidence of the sound wave on a reflecting surface (see reflection) or during its encounter with an obstacle of particular dimensions (see diffraction); the increase in vibration amplitude that occurs when the dimensions of the environment are related by very simple relationships with the wavelength (see resonance); the regular variation in intensity that occurs along a surface where sound waves produced by two sources meet (see interference); the complex of phenomena that occur when a sound wave passes through a body, particularly porous, phenomena related to the density and porosity of the medium, the mass of the skeleton, the density of the air or other fluid that fills the pores, and that ultimately are summarized in the transformation into heat of part of the sound energy (see absorption); the set of phenomena, reflection on the walls and absorption due to materials that cover them (as well as furniture, curtains, etc.. ), which occur in environments due to the effect of internal sound sources, also in relation to the shape and volume of the environment itself (see diffusion).

The frequency range of physical acoustics has been extended beyond the audible frequencies: at lower frequencies we have infrasound, whose field of vibratory phenomena borders on seismology and is of particular interest to building structures; at higher frequencies (studied by ultra-acoustics), we have moved from ultrasound to hypersound, which reaches a frequency of 1010 Hz, which corresponds to a wavelength of 0.35 μm.

The development of aviation with jet engines, at speeds lower and higher than the speed of sound, led to the development of a new field of acoustics called aeroacoustics. This is interested, on one hand, in the sounds created by a continuous flow of air along a surface, or by the incidence of this flow on particular geometric structures, for given values of the Reynolds number (aerodynamic sounds), on the other hand, in the phenomena that occur when the wall of sound is exceeded. Finally, the physical-mathematical interpretation of some phenomena in the field of ultrasound has demonstrated the impossibility, even in the acoustic field, to indefinitely subdivide the energy; it was thus introduced the notion of quantum of sound energy or phonon.

History of acoustics

The history of acoustics begins in the sixth century BC with the studies of Pythagoras and the Pythagoreans, whose philosophy identified the structure of numbers with that of the physical world. They came to establish the existing relations between the length of the vibrating strings and the pitch of the sounds; to them we owe also one of the first musical scales.

With the Pythagoreans are outlined the two trends typical of the first studies of acoustics: the musical one, interested in the physiological-aesthetic aspect, and the physical-mathematical one, interested in the mathematical and experimental aspect. The two trends found expression in two separate treatises (Introduction and Harmonic Section of the Canon), erroneously attributed to Euclid, who later merged in the Harmonics of Ptolemy. It appears from Heron (sec. II BC) that it was known qualitatively to the Greeks that sound is due to the vibratory motion and collisions of air particles.

At least a rough knowledge of the phenomenon of reflection results from the form of the Greek theater and the later Roman theater, which was little different: the central orchestra for the chorus, the scene for the performance and the tiers for the audience, surrounding a large part of the orchestra. From the orchestra to the audience came both direct sound and reflected sound once, together still considered necessary and sufficient for a pleasant listening.

The Romans, however, were also aware of the phenomena of echo, interference and reverberation. After the Romans, it is necessary to arrive at Galilei and one of his contemporaries, the Frenchman Marin Mersenne, who experimentally determined the mathematical relationships between frequency, length, tension and mass of vibrating strings.

Subsequent researches can be summarized in the following phases: birth and first development of the wave theory, according to which sound propagates by longitudinal vibration of the medium (Newton and more precisely Huygens in Traité de la lumière) and does not propagate in vacuum; theoretical researches on strings and other vibrating bodies, such as plates (J. B. D’Alembert, D. Bernoulli, L. Euler, T. Young, E. F. Chladni); study of interference and resonance phenomena and acquisition of the concept of sound analysis (J.- B.-J. Fourier); the work of lord Rayleigh, essentially concerning the measurement and study of physical quantities related to sound, such as sound pressure and speed of vibration. Schematically, until Lord Rayleigh, acoustics was essentially interested in the study of physical phenomena and, to a lesser extent, of physiological aspects.

At the beginning of the twentieth century acoustics was affected in a decisive way by the introduction of electronic tubes, the use of which has made possible the construction of speakers, microphones, amplifiers, recorders for acoustic and ultraacoustic frequencies. Electroacoustics, ultraacoustics, phonometry and environmental acoustics were added to the previous fields. The new techniques have made it possible to extend the study from pure and complex sounds to sound phenomena of any kind, such as impulsive or otherwise non-coherent.

All this has radically changed the research tools of the acoustician, who has transformed from a physicist to a certain extent, in electronic engineer. This has also caused a profound conceptual change, with the introduction of the notion of circuit in the propagation of sound energy. The next step was to extend these methods to the study of devices comprising transducers of sound energy into electrical energy, i.e., transformers, in which the transformation ratio contains a constant that expresses the change in form of energy.

The contemporary development of the science of communications and electronics was meanwhile highlighting the importance, in telecommunications, of the signal-to-noise ratio, spectral bandwidth and distortions.

The notion of “propagation of sound energy” has therefore replaced that of “transmission of a signal” that can be manifested in various forms (sound signal, magnetic, electrical, electromagnetic) all expressible in circuit form and having in common some characters (intensity, bandwidth, fidelity, amount of information transmitted in a given time). In this sense to the transmission of a signal that in a certain phase is sound, the principles of communication science and information theory should be applied: the medium in which the signal is transmitted has become a support, or rather a channel of information.

The last step in the evolution of acoustics was made when it was realized that the previous principles, albeit in a more complex form, could also be applied to phenomena that take place in the inner ear, stimulated by an external sound or noise; in other words, the external sound signal is transformed into a mechanical signal in the middle ear, enters the inner ear through the window, where it is translated into a certain vibratory configuration of the basilar membrane.

To a first approximation, this configuration translates into nerve discharges along the auditory nerve, which, appropriately encoded, carry the information to higher brain centers. It was also found that during the transmission of signals from the periphery to the higher brain centers is valid the principle of duality and symmetry of the temporal and frequency analysis of a signal of any form (in our case nerve). In the case of sound stimuli of short duration, in the auditory system there are phenomena of spatial and temporal integration that translate the previous duality (see hearing). From the set of these correspondences arose the possibility of simulating nerve networks with circuits; thus were born the mechanical, electrical and electronic models, not only of the middle and inner ear, but also of the nerve network.

The problems of acoustics have begun to interest cybernetics and then bionics. The acoustics, understood in the broadest sense and modern, can therefore be considered an interdiscipline, which converge contributions from physics, electronics, biology, physiology, computer science.

Application sectors of acoustics

The study of sound waves also leads to physical principles that can be applied to the study of all waves. Applications of acoustic technology include music and the study of geologic, atmospheric, and underwater phenomena. Its origins began with the study of mechanical vibrations and the radiation of these vibrations through mechanical waves and still continues today. The research was done to look into the many aspects of the fundamental physical processes involved in waves and sound and into possible applications of these processes in modern life. From an application point of view, acoustics can be divided into numerous sectors:

  • architectural acoustics: which deals with the acoustic quality of buildings, theaters, and other spaces that have a pleasing sound quality and safe sound levels; includes Architectural Acoustics, Engineering Acoustics, Physical Acoustics, Structural Acoustics, and Vibration;
  • the acoustics of musical instruments: which deals with properties and characteristics of how music is made, travels and is heard; includes Musical Acoustics, Psychological and Physiological Acoustics, Noise;
  • noise and environmental acoustics: which deals with problems related to outdoor noise (natural and man-made noise); include Noise, Structural Acoustics, and Vibration, Speech Communication;
  • building acoustics: which aims to isolate rooms from disturbing noises;
  • underwater acoustics: which deals with the propagation of waves and their perception in marine environments; includes Underwater Acoustics, Acoustical Oceanography, Animal Bioacoustics, Physical Acoustics;
  • medical acoustics that deals with developing methods and tools in the therapeutic and diagnostic field based on the propagation of acoustic waves within the human body; includes Biomedical Acoustics, Engineering Acoustics, Speech Communication, Noise
  • animal bioacoustics: study of how animals make, use and hear sounds; includes Acoustical Oceanography, Animal Bioacoustics, Underwater Acoustics;
  • speech and hearing: study of how our ears sense sounds, what types of sounds can damage our ears and how speech is made, travels and is heard; includes Speech Communication, Physiological and Psychological Acoustics, Noise
  • intensimetric diagnostics such as imaging acoustics are one of the latest application frontiers.

The perceptual and biological aspects of acoustics are then the subject of specific study areas such as psychoacoustics, which studies the psychology of sound perception in humans, and audiometry, which deals with the evaluation of the physiological characteristics of the ear and hearing ability measurement.

Subdisciplines

  • Archaeoacoustics
  • Aeroacoustics
  • Acoustic signal processing
  • Architectural acoustics
  • Bioacoustics
  • Electroacoustics
  • Environmental noise and soundscapes
  • Musical acoustics
  • Noise
  • Psychoacoustics
  • Speech
  • Structural Vibration and Dynamics
  • Ultrasonics
  • Underwater acoustics
  • Acoustician

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