The material in this section introduces concepts relating to the nature of sound and the use of sounds to produce music. Its reading can be omitted if these notions are already possessed, or postponed to the point where it is indicated later. In a very basic approach, you can consider music as the combination of sounds that produce a pleasant aesthetic impression on the listener. To understand why this effect occurs, the sound nature must first be considered and then the way in which particular sounds can be combined to produce music. This section describes very briefly the first aspect and makes a brief introduction to the second aspect, which is part of the theory of music. An excellent introduction to this theory is the work "Theory of Music" by Adolphe-Leopold Danhauser [Danhauser].
Sounds
Sounds are produced by altering the position of ambient particles by using a sound source. Clicking the "Display" button below will show that the vibration of the gray object on the left is transmitted to the particle set on its right, giving rise to areas where they are more grouped and others where they are more separated and that these areas move continuously to the right. When reaching the ear of a listener, these variations produce tympanic membrane movements and the capture of this sound.
Since the sinusoidal oscillations are the most important from the theoretical point of view, a sinusoidal movement around its resting position has been printed to the sound source, which transmits to the rest of the particles; however, as this alteration propagates with finite velocity, around 340 m / s in the air, there is a shift between the positions they occupy. It is important to emphasize that the sound is not propagated by the displacement of the particles but by the propagation of the pressure changes produced by the vibration of these particles.
In the following figure the waveform of the particle pressure has been superimposed on the particle diagram for a particular moment, which is sinusoidal with maximums corresponding to the points of greatest concentration of particles and minimums corresponding to the points of lower concentration of particles. A horizontal dashed reference line has also been drawn that corresponds to the environmental pressure that existed before being affected by the sound wave. The pressure wave varies around this reference line.
From the musical point of view, our main interest is to know how this pressure wave on the eardrum of a listener located at certain distance from the source varies; therefore in the previous figure a vertical line of dashes has been drawn at this distance, and in the following figure the variation of this sound pressure is represented, with respect to the normal pressure:
In this graph p is the amplitude of the pressure increase with respect to the pressure at rest and T is the period of this pressure wave. As we will see throughout these pages, the fundamental characteristic of these sound waves, from the musical point of view, is their frequency, given by
Sound perception
Sounds are perceived in the ear due to vibrations that are generated in the air particles present in front of the eardrum; these vibrations are characterized by their amplitude and frequency; both, but especially the last feature, are intimately linked to the musical aspect of the sounds. If a string is pressed and allowed to vibrate freely it will produce simultaneous sounds of many different frequencies. A single-frequency sound is called a pitch, its frequency determines its height; the higher this frequency is said to have a higher pitch, the lower the frequency it is said to have a lower pitch.
Sounds produced by musical instruments consist of sound waves with a fundamental frequency and multiples, or harmonics, of this fundamental frequency; the timbre of such a sound is determined by the number of harmonics present and their relative amplitudes, and gives each instrument its characteristic sound. A good timbre is associated with a quality instrument and is the essential quality that is sought when buying it.