Acoustics

(Omnidirectional sound source in anechoic chamber (Czech Technical Univerzity, Prague). )

Acoustics is the science of sound (physics). The discipline has extended its field to the study of mechanical waves within gases, liquids, but also within solids, whatever the frequency range (infrasound, sound and ultrasound). We talk about vibroacoustics when the study focuses on the interaction between solids and fluids.

The acoustics include many ramifications among which we meet electroacoustics (microphones, speakers), hearing, musical acoustics, architectural acoustics, etc.

Acoustics has applications in the fields of Earth and Atmospheric Sciences, Engineering Sciences, Life Sciences and Health Sciences, as well as in the Humanities and Social Sciences.

History

(The principles of acoustics are applied since antiquity: Roman Theater of Bosra, Syria.)

Science whose foundations go back to antiquity, Pythagoras studied in the sixth century BC the musical acoustics, in particular the intervals. The theater of Epidaurus testifies that from the fourth century BC the Greeks mastered the sound properties of materials to build amphitheatres: the periodic arrangement of the rows of seats of the theater of Epidaurus allows to filter the low frequencies (below 500 Hz) of the background noise (rustling of trees, audience).

Empirical acoustics

The origin of acoustics is attributed to Pythagoras (6th century BC), who studied the functioning of vibrating strings producing pleasant musical intervals in the ear. These intervals are at the origin of the Pythagorean tuning bearing its name today. Aristotle (4th century BC) correctly anticipated that sound was generated by the movement of air by a source “pushing the contiguous air forward in such a way that the sound travels”. His hypothesis was based on philosophy rather than experimental physics. Moreover, he mistakenly suggested that the high frequencies propagated faster than the low frequencies, a mistake that will last several centuries.

The speculation that sound is an wave phenomenon owes its origin to the observation of waves on the surface of water. Indeed, the notion of wave can be considered, in a rudimentary way, as an oscillating disturbance which propagates from a source and does not transport matter over the long distances of propagation. The possibility that sound behaves in a similar way was particularly emphasized by the Greek philosopher Chrysippo (3rd century BC) and the Roman architect and engineer Vitruvius (around 25 BC), who contributed in addition to the design of the acoustics of ancient theaters. The Roman philosopher Boethius (470-525 AD) also hypothesized similar behavior, also he suggested that the human perception of pitch was related to the physical property of frequency.

Experimental acoustics, acoustic measurements and instrumentation

A first important experimental result was obtained at the beginning of the 17th century, whose discovery is mainly due to Marin Mersenne and Galileo Galilei: the movement of the air generated by a vibrating body at a certain frequency is also a vibratory movement of identical frequency at the vibration frequency of the vibrating body. In Universal Harmony (1637), Mersenne described the first absolute determination of the frequency of an audible sound (at 84 Hz). This description implied that Mersenne had already demonstrated that the ratio of absolute frequencies of two vibrating strings, one creating a first musical note and the other the same note an octave above, was 1/2. The harmonic consonance that was perceived by the ear when listening to these two notes could only be explained if the ratio of the oscillation frequencies of the air was also 1/2. The history of this discovery, which is the result of previous reflections on the subject, some of which date back to Pythagoras (550 BC), thus intertwines with the development of natural frequency laws of vibrating strings. and the physical interpretation of musical consonances. Galilei, in his mathematical speeches concerning two new sciences (1638), unveils the most lucid discussions and explanations given until now on the notion of frequency.

Domains of acoustics

Physical acoustics

Physical acoustics (also called fundamental acoustics or theoretical acoustics) determine the principles of the generation and propagation of sounds and develop the mathematical formalism. His domain is not necessarily limited by human perception; she is also interested in ultrasound and infrasound, which obey the same physical laws.

Theoretical acoustics has many specialized areas of application:

• Architectural acoustics study the propagation of sounds in rooms and buildings including sound insulation.
• Electroacoustics specializes in electroacoustic transducers (microphones, speakers, hydrophones).
• Vibroacoustics (also called acoustic structures) studies mechanical waves in structures and how they interact and radiate in surrounding fluids.
• Thermoacoustics, based on the thermoacoustic effect, studies the conversion of heat into acoustic energy and vice versa.
• Medical imaging uses ultrasound, Doppler ultrasound, high intensity focused ultrasound surgical techniques, acoustic ultrasound applications.
• Nonlinear acoustics study the cases where the deviations from the linearity postulated in the general acoustic equations are too important to be able, as in the general case, to neglect them.
• Non-destructive testing uses the results of nonlinear acoustics to characterize the state of integrity and “health” of structures or materials, without degrading them, either during production or in use, either in the maintenance frame.
• Underwater acoustics studies the propagation of sound in water and the interaction of sound waves with water and boundaries with other media.
• Aeroacoustics studies the generation of a noise by a turbulent flow (ex: turbulence of a free jet), or interacting with a surface (wing profile, rotor blades of a helicopter, compressor or turbine wheels , cavity, …).

Human acoustics

• The hearing details the physiology of the ear (outer, middle and inner ear) and explains the mechanisms of sound perception by hearing, and measures the acoustic sensitivity of individuals.
• Psychoacoustics studies how sounds captured by the auditory system are interpreted by the human brain.
• Acoustic phonetics is devoted in particular to the physical aspect of the sounds produced by the human phonatory apparatus, leading to the systems of automatic speech recognition and speech synthesis.

Cross-cutting domains

(Automotive acoustics study. )

• Musical acoustics is concerned with the production and perception of musical sounds.
• Acoustic instrumentation and metrology.
• Environmental acoustics is concerned about noise pollution.
  • Transportation acoustics (maritime, rail, air and automobile) focus on issues related to the interior acoustics of vehicles and the reduction of external noise due to traffic.
  • Industrial acoustics encompasses all the techniques used to modify the production and transmission of industry-specific sounds and noises, as well as techniques that use sound and ultrasonic vibrations for mechanical purposes.
  • Noise control focuses on active or passive solutions to prevent noise propagation.