Sound is a periodic pressure change that can be perceived by the hearing organs of living things. It is produced by the vibration of molecules in a substance. Physically, sound can be seen as a simple mechanical distortion that occurs in solid, liquid or gaseous media, and also as a stimulus arising from the hearing mechanism that enables hearing. Hearing, the perception of sound stimulation, is one of the five basic senses that enable people to communicate with their environment.
In addition to the sounds that humans or animals make to communicate for various reasons, inanimate objects can also cause sound generation. The sound produced by a person pushing a table is caused by the friction of the table with the floor and is the result of a living thing hitting an inanimate thing. The sound produced by the wind blowing over fallen leaves is an example of sound caused by the interaction of two inanimate objects.
Sound, as a type of energy produced by vibrations, requires a material medium for its transmission and propagates in the form of wave movements in air and other elastic media. Sound waves in air and any liquid medium are longitudinal waves, because the particles of the medium through which the sound travels vibrate parallel to the direction in which the sound wave moves. Sound waves have five main physical properties: wavelength, amplitude, frequency, period and velocity. Wavelength is the distance a sound wave travels to complete one cycle — the distance between two successive identical points on the wave, usually measured between two successive peaks or troughs. Wavelengths, measured in metres, micrometres and nanometres, are denoted by the letter λ (lambda). The amplitude of a wave describes the maximum displacement of particles disturbed by a sound wave as it travels through a medium. As the amplitude of sound waves increases, the energy and intensity of the sound also increase. The unit of sound level is the decibel (dB). The frequency of a sound wave indicates the number of sound waves produced in one second. In the same time period, low frequency sounds produce fewer waves than high frequency sounds. The time it takes for a complete wavelength to pass through a given point in space as the sound wave travels through the air is called a period; it is denoted by T and is usually measured in fractions of a second. Finally, the speed of a sound wave refers to the distance travelled by the wave in a unit of time and is usually measured in m/s. The density of the medium through which sound propagates affects the speed of the sound wave — the denser the medium, the faster the sound wave travels in it.
As with other physical phenomena that exhibit wave properties, the Doppler effect is also observed in sound: the situation where the frequency and wavelength of a sound are perceived differently by a moving observer at different times or locations. Sounds emitted from the source travel through a carrier medium to reach the observer. The time it takes for the sound emitted from the source to reach the observer varies based on the distance between them and the speed of sound in the medium. If the source and observer are not moving relative to each other, the frequency of the sound perceived by the observer will be the same as the frequency of the sound emitted from the source. However, when the source and observer begin to move, the situation changes. If the source and observer are moving away from each other, the frequency of the sound perceived by the observer decreases, because each peak of the sound wave produced by the source travels a greater distance than the previous peaks before reaching the observer. The observer therefore begins to perceive the sounds more sparsely — the frequency of the perceived sounds decreases, and the wavelength increases. In contrast, when the source and observer approach each other, the opposite occurs: each peak of the sound emitted from the source travels a shorter distance to reach the observer than the previous ones, so the observer begins to perceive the sounds more frequently — the frequency of the observed sounds increases, and the wavelength shortens. This effect can be observed in everyday life. For example, while waiting by the side of the road, we hear the frequency of an approaching ambulance's siren increasing, making the sound feel sharper as it approaches; once the vehicle passes and begins to move away, the frequency starts to decrease.
The sound of a sonic boom — sometimes heard as an explosion or a clap of thunder, and startling or disturbing to hear — is associated with the shock wave produced when an object moves faster than the speed of sound in air. Sonic booms produce enormous amounts of sound energy. The sound of a fired bullet or the crack of a whip can be considered small sonic booms. Large supersonic aircraft can produce particularly loud and frightening sonic booms, waking people up and causing minor damage in some structures.
Throughout history, people have produced music through various musical instruments to express their emotions and thoughts. Musical instruments create sound clusters made up of many simple sounds called musical sounds, not merely simple sounds. Musical sounds are those that have a specific repeating period and frequency. As the frequencies, amplitudes and relative phases of the combined sounds change, the quality of the musical sound also changes. Many musical instruments are used worldwide, and although there are so many varieties, the principles they depend on are fundamentally the same. Their shapes vary, they are made from various materials, and they are played in various ways, but they all produce sound through either a string, an air column, a membrane, or a plate.
Humans produce various sounds to communicate and express their emotions and thoughts. For sound to form in a person, there must be a vibration: when a person exhales, the air from the lungs vibrates the vocal cords (vocal folds) located in the larynx to produce sound. The larynx, or voice box, consists of three sections: the middle section where the vocal cords are located (glottis), the section above the vocal cords called the supraglottic area, and the section below the glottis called the subglottic area. At birth, a baby's larynx is 2 cm long and 2 cm wide, about one-third the size of an adult's larynx, and is much softer and more flexible. The vocal cords do not have the layers that exist in adults, and there is no difference in the larynx at birth between the sexes. At birth, the newborn's larynx is positioned higher than at any other time in life; in infants, the larynx is so high that the epiglottis touches the soft palate, allowing for both breathing and drinking milk simultaneously. The descent of the larynx begins right after birth. Laryngeal descent lengthens the vocal tract, lowers the resonance frequency, and creates the perception that the body size is larger. The same situation has been shown in studies conducted in chimpanzees and deer. Research indicates that low male voices are perceived by women as an indicator of increased body size, and most men's vocal frequency is lower than that of most women. There is a developmental relationship between the descent of the larynx (as the pharyngeal cavity elongates, lower fundamental frequencies achieve more resonance) and the average pitch of the voice. Laryngeal descent is evolutionarily significant for both swallowing and speech.
Every human being has two vocal cords in the centre of the neck, just below the pharynx (throat) and just above the trachea (windpipe). In the course of evolutionary development, the primary function of the vocal cords and the larynx (voice box) in general is to protect the airway and lungs from foreign particles entering from outside and from food getting into them. The vocal cords, which are open in a V-shape during breathing, come together and close completely when eating and producing sound. The vocal cords consist of two separate layers: the upper layer, which contacts on both vocal cords, is made up of mucosa, epithelium and lamina propria, and the layer below the mucosa is muscle tissue. The human vocal cords vibrate and contact each other on average 120 times per second in men and 200 times per second in women during speech and voice production. The fundamental frequency (F0) of a newborn baby's vocal cords is between 450-520 Hz — when the baby cries, the vocal cords vibrate at this rate per second. You can find basic information about the human voice here.
