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Acoustic book

The sound

The field of study of acoustics

Acoustics studies the sound in general (sound sources, propagation mean and the receiver apparatus, ear). The vastness of the subject necessitated the division of acoustics fields in various specializations depending on its practical use: among the main there are physical acoustics, environmental and architecture acoustics, hydroacoustics, music electroacoustics, psychoacoustics and physioacoustic.
In particular, the last two mainly concern our hearing organ.
The oscillations of an elastic body transmitted through a solid, liquid or gaseous mean, if strong enough and within a certain range, are translated by our ear as acoustic sensations called sound. The sound does not propagate in lacking of element space regions. These oscillations are known as mechanical waves capable of transporting energy without mass. There are, however, besides mechanical vibrations electromagnetic waves too. They are of other physical nature but still equally important to man life as light, revealed to eyes as colour, infrared radiation felt by skin, X-rays, gamma rays, radar and radio waves.


The sound derives from sensorial impulses, which in our case are the acoustic ones that reach our ears thanks to the air pressure variations produced by a sound source. These variations convert themselves into nerve impulses and finally produce specific perceptions that derive form sensations processing.
The sounds are therefore perceptive processing of nervous impulses. Other perceptive processing related to our senses are images, colours, tastes, smells, and the perception of shapes thanks to the touch.
The most immediate effect of perceptive processing seems to be the understanding of the cause (or causes) that has or have produced the impulse in terms of physical source or action or mechanical event or sound gesture that has (or have) determined the sound event. Examples are the string of the violin, the friction of the bow, the membrane of the drum and the drum with your hand.
The auditory perception is binaural since there is a perceptual difference between the two ears (called stereo) that allows the source localization and may be associated with a more immediate knowledge of perception of the visual representation. For example, lip-reading can help for a better understanding of the text, when spoken by our interlocutor.
Many perturbations of atmospheric pressure, which may have one or more causes and one or more different locations, impact on the tympanic membrane to make a general disturbance that through the auditory perception is then processed in order to separate and reconstruct in a distinct way the sound world that surrounds us.
There are numerous studies that have analysed how the perceptive organization of sound happens. These include the investigation of the McGill Auditory Research Laboratory, on whose web site you can download an audio CD in mp3 format, with numerous examples and listening experiences. Please, check here for the link:


The sound source must be able, through a stirrer, to transmit energy to the air or to other elastic mean in the form of rapid wavelike disturbances. The most common sources are:

  1. Solid bodies oscillating as a steel cord (or other elastic body) stretched between two fixed points, set in vibration through percussion, rubbing or pizzicato (e.g. piano, violin or harpsichord), or as the membrane of a beaten drum, or even as a loudspeaker membrane set in vibration by alternated electrical impulses;
  2. Air columns put in vibration by different stirrers (wind instruments and organ);
  3. The human voice that is the combination of the two mentioned above mechanisms;
  4. Solid bodies in rapid motion (propellers, whips);
  5. Gas or liquid violent leakages and violent pressure shocks (hecklings and fizzles of rockets, explosions, thunder, explosions etc.).

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