Acoustics

Acoustics is a branch of physics that deals with the study, generation, 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. Among the most important phenomena are:

• those that occur when the sound wave hits a reflecting surface (see reflection) or an obstacle of a certain size (see diffraction);
• the increase in amplitude that occurs when the dimensions of the environment are related to the wavelength by very simple relationships (see resonance);
• the regular variation in intensity that occurs along a surface where sound waves from two sources meet (see interference);
• the set of phenomena that occur when a sound wave passes through a body, especially a porous one, phenomena related to the density and porosity of the medium, the mass of the skeleton, the density of the air or other fluid filling the pores, and which ultimately result in the conversion of part of the sound energy into heat (see absorption);
• all the phenomena of reflection on the walls and absorption by the materials covering them (as well as by the furniture, curtains, etc.) that occur in rooms as a result of internal sound sources, also in relation to the shape and volume of the room itself (see diffusion).

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

The development of aeronautics with jet engines, at speeds higher and lower than the speed of sound, then led to the development of a new field of acoustics called aeroacoustics. It is interested, on the one hand, in the sounds generated by a continuous flow of air along a surface or by the incidence of this flow on certain geometric structures, for given values of the Reynolds number (aerodynamic sounds) and, on the other hand, in the phenomena that occur when the sound barrier is exceeded.

Finally, the physical-mathematical interpretation of certain phenomena in the field of ultrasound demonstrated the impossibility, even in the acoustic field, of an infinite subdivision of energy; thus the notion of a quantum of sound energy, or phonon, was introduced.

Applications of acoustics

The study of sound waves also leads to physical principles that can be applied to the study of all waves. Applications of acoustics include music and the study of geological, atmospheric, and underwater phenomena. Its origins began with the study of mechanical vibrations and the propagation of these vibrations as mechanical waves, and continue to this day. Research has been conducted to explore the many aspects of the fundamental physical processes involved in waves and sound, and the possible applications of these processes in modern life. From an application point of view, acoustics can be divided into numerous fields:

• Architectural acoustics: which is concerned with the acoustic quality of buildings, theaters, and other spaces that have pleasing sound quality and safe sound levels; includes architectural acoustics, engineering acoustics, physical acoustics, structural acoustics, and vibration;
• Musical Instrument Acoustics: which deals with the properties and characteristics of how music is made, how it travels, and how it is heard; includes Musical Acoustics, Psychological and Physiological Acoustics, and Noise;
• Noise and Environmental Acoustics: which deals with problems related to outdoor noise (natural and man-made); includes Noise, Structural Acoustics and Vibration, Speech Communication;
• Architectural Acoustics: which aims to isolate rooms from disturbing noise;
• Underwater Acoustics: which deals with the propagation of waves and their perception in marine environments; includes Underwater Acoustics, Acoustic Oceanography, Animal Bioacoustics, Physical Acoustics;
• Medical Acoustics, which deals with the development of methods and tools in the therapeutic and diagnostic field based on the propagation of acoustic waves in the human body; includes Biomedical Acoustics, Engineering Acoustics, Speech Communication, Noise.
• Animal Bioacoustics: the study of how animals make, use, and hear sound; includes Acoustical Oceanography, Animal Bioacoustics, Underwater Acoustics;
• Speech and hearing: the study of how our ears perceive sounds, what types of sounds can damage our ears, and how speech is produced, transmitted, and heard; includes Speech Communication, Physiological and Psychological Acoustics, Noise
• Intensimetric diagnostics such as imaging acoustics is one of the latest application frontiers.

The perceptual and biological aspects of acoustics are then the subject of specific fields of study, 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 the measurement of hearing ability.

Historical notes

The history of acoustics begins in the 6th century BC with the studies of Pythagoras and the Pythagoreans, whose philosophy identified the structure of numbers with that of the physical world. They established the relationship between the length of vibrating strings and the pitch of sounds; they are also credited with one of the first musical scales. With the Pythagoreans, the two tendencies typical of early acoustic studies emerged: the musical one, interested in the physiological-aesthetic aspect, and the physical-mathematical one, interested in the mathematical and experimental aspect. The two tendencies found expression in two separate treatises (Harmonic Introduction and Section of the Canon), erroneously attributed to Euclid, which later converged in Ptolemy’s Harmonics.

According to Heron (2nd century B.C.), the Greeks knew qualitatively that sound was due to the oscillation and collision of air particles. At least a rough knowledge of the phenomenon of reflection is derived from the form of the Greek and later Roman theater, which was little different: the central orchestra for the chorus, the stage for the performance, and the tiers for the audience surrounding much of the orchestra. From the orchestra to the audience came both direct and reflected sound, which together were still considered necessary and sufficient for pleasant listening. The Romans were also familiar with the phenomena of echo, interference and reverberation.

After the Romans, we must move on to 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 research can be summarized in the following stages: birth and early development of the wave theory, according to which sound propagates by longitudinal vibration of the medium (Newton and, more specifically, Huygens in Traité de la lumière) and does not propagate in a vacuum; theoretical research on strings and other vibrating bodies, such as plates (J. B. D’Alembert, D. Bernoulli, L. Euler, T. Young, E. F. F. Chladni); study of interference and resonance phenomena and acquisition of the concept of sound analysis (J.- B.-J. Fourier). B.-J. Fourier); the work of Lord Rayleigh, essentially concerned with 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 concerned with the study of physical phenomena and, to a lesser extent, physiological aspects.

At the beginning of the twentieth century, acoustics was decisively influenced by the introduction of the vacuum tube, the use of which made possible the construction of loudspeakers, microphones, amplifiers and recorders for acoustic and ultra-acoustic frequencies. Electroacoustics, ultraacoustics, phonometry and room acoustics were added to the previous fields. The new techniques made it possible to extend the study of pure and complex sounds to sound phenomena of all kinds, such as impulsive or otherwise non-coherent. All this radically changed the research tools of the acoustician, who was transformed from a physicist to an electronic engineer. This also led to a profound conceptual change, with the introduction of the notion of a circuit in the propagation of sound energy. The next step was the extension of such methods to the study of devices containing transducers of sound energy into electrical energy, i.e. transformers, in which the transformation ratio contains a constant expressing the change of form of the energy.

Meanwhile, the simultaneous development of communications science and electronics emphasized the importance of signal-to-noise ratio, spectral bandwidth, and distortion in telecommunications. The notion of “propagation of sound energy” was thus replaced by that of “transmission of a signal”, which can manifest itself in various forms (sound, magnetic, electric, electromagnetic signal), all of which can be expressed in terms of circuits and have certain characteristics in common (intensity, bandwidth, fidelity, amount of information transmitted in a given time). In this sense, the principles of communication science and information theory should be applied to the transmission of a signal that is sound at a certain stage: the medium in which the signal is transmitted has become a medium, or rather a channel of information.

The final step in the evolution of acoustics was taken when it was realized that the previous principles, albeit in a more complex form, could also be applied to phenomena taking place inside the 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 results in nerve discharges along the auditory nerve, which, appropriately coded, carry the information to the higher brain centers. It has also been established that in the transmission of signals from the periphery to the higher brain centers, the principle of duality and symmetry of the temporal and frequency analysis of a signal of any form (in our case, nerve) is valid.

In the case of sound stimuli of short duration, spatial and temporal integration phenomena take place in the auditory system, which translate the previous duality (see hearing). From the set of such correspondences arose the possibility of simulating nerve networks with circuits; thus mechanical, electrical and electronic models were born, not only of the middle and inner ear, but also of the nerve network. The problems of acoustics began to interest cybernetics and then bionics. Acoustics, in its broadest and most modern sense, can thus be considered an interdisciplinary field, bringing together contributions from physics, electronics, biology, physiology, and computer science.

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

Related keywords

• Acoustician