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HearTomorrow
Acoustics
Sound is part of the natural environment of mankind. It is first of all a perceived phenomenon, a sensation, raw material provided by the sense of hearing. It is then a cry, a voice, a word, thus spoken communication, a privileged bond between people. It is also music, another form of expression ad communication, and noise, a byproduct of the industrial society.
Our daily hearing experience leads to 3 observations:
- The sound perceived is associated with an oscillatory movement of a vibrating system. To convince yourself of this, it is enough just to touch your larynx lightly while talking loudly;
- Sound is not an immediate and simultaneous phenomenon in space: 2 people standing in different places perceive lightning simultaneously, but not the ensuing thunder;
- When the distance covered is greater than 20 to 30 meters, the time of propagation is perceptible as shown by echo phenomena: the speed of propagation of sound is slow.
Historically, such rough data are at the origin of acoustics, the branch of science which endeavors to explain nature, treating the production, the propagation, and the perception of sound in order to discover its mechanisms and laws.
The science of acoustics shows that the physical nature of sound consists of a propagation of an oscillatory movement of the particles of a fluid or solid medium. Certain media favor propagation, other hinder it.
Depending on the frequency of the oscillatory movement, the sound is audible or not. For humans, the field of audible sound spreads roughly from 16Hz to 16kHz.
By definition, sound, or acoustic vibration, is "a movement of particles around the equilibrium position in an elastic medium (as the air, for instance)". Corresponding to the natural hearing experience of humans, audible sound has a double meaning, conveying 2 points of view:
- Acoustic oscillation of such a character as to be capable of exciting the sensation of hearing;
- Sensation of hearing excited by an acoustic vibration.
Physical acoustics deals with sound as an audible or inaudible oscillation: its central interest is the physical phenomenon of production and propagation of oscillations in an elastic medium, and by extension in any sort of medium, irrespective of the perceived phenomenon.
The acoustics of hearing, which includes psychoacoustics and physioacoustics, deals with the audible sound according to the second point: it is a question of establishing the hearing mechanisms and the relationships between the characters perceived and the physical characteristics of the sounds.
The audible sounds are the audiofrequencies. An ultrasound is a sound having a frequency that is above the high-frequency limit of audible sound. The frequency of an infrasound is below the low-frequency limit of audible sound.
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