What is sound?
>Sound is defined as "(a) Oscillation in pressure, stress, particle displacement, particle velocity, etc., propagated in a medium with internal forces (e.g., elastic or viscous), or the superposition of such propagated oscillation. (b) Auditory sensation evoked by the oscillation described in (a)."[4] Sound can be viewed as a wave motion in air or other elastic media. In this case, sound is a stimulus. Sound can also be viewed as an excitation of the hearing mechanism that results in the perception of sound. In this case, sound is a sensation.
Sound in brief but remarkeable terms is a vibration, that our ears percieve by the sense of hearing. Most commonly vibrations travel to our ears via the air. The ear then converts these sound waves into nerve impulses that are sent to our brains, where the impulses become sound. To say all that in a more technical language: Sound “is an alternation in pressure, particle displacement, or particle velocity propagated in an elastic material” (Olson 1957). Sound is also a series of mechanical compressions and rarefactions or longitudinal waves that successively propagate through media that are at least a little compressible. What causes sound waves is known as “the source of waves”. Examples of sounds sources is: A violin string that vibrates upon being bowed or plucked.
The four characteristics of sound are frequency, wavelength, amplitude and velocity.
The frequency of sound is the number of air pressure oscillations per second at a fixed point occupied by a sound wave.
The amplitude is the magnitude of sound pressure change within the wave. Basically this is the maximum amount of pressure at any point in the sound wave. A sound wave is caused literally by increases in pressure at certain points causing a “domino effect” outward, the higher pressure points are the crests in a sound wave, and behind them are low pressure points which tail them. These are known as the troughs on a wavelength graph. Sound’s propagation Velocity depends largely on the type, temperature and pressure of the medium through which it propagates. Because air is nearly a perfect gas, the speed of sound does not depend on air pressure.
The frequency range of sound that is audible to humans is approx. between 20 and 20,000 Hz. This range of course varies between individuals, and goes down as are age increases. Sounds will begin to damage our ears at 85 dBSPL and sounds above approximately 130 dBSPL will cause pain, as a result are known as the: “threshold of pain”. Of course again this range will vary among individuals and will change with age.
Pitch
Pitch is perceived as how "low" or "high" a sound is and represents the cyclic, repetitive nature of the vibrations that make up sound. For simple sounds, pitch relates to the frequency of the slowest vibration in the sound (called the fundamental harmonic). In the case of complex sounds, pitch perception can vary. Sometimes individuals identify different pitches for the same sound, based on their personal experience of particular sound patterns. Selection of a particular pitch is determined by pre-conscious examination of vibrations, including their frequencies and the balance between them. Specific attention is given to recognising potential harmonics.[22][23] Every sound is placed on a pitch continuum from low to high. For example: white noise (random noise spread evenly across all frequencies) sounds higher in pitch than pink noise (random noise spread evenly across octaves) as white noise has more high frequency content. Figure 1 shows an example of pitch recognition. During the listening process, each sound is analysed for a repeating pattern (See Figure 1: orange arrows) and the results forwarded to the auditory cortex as a single pitch of a certain height (octave) and chroma (note name).
Duration
Duration is perceived as how "long" or "short" a sound is and relates to onset and offset signals created by nerve responses to sounds. The duration of a sound usually lasts from the time the sound is first noticed until the sound is identified as having changed or ceased.[24] Sometimes this is not directly related to the physical duration of a sound. For example; in a noisy environment, gapped sounds (sounds that stop and start) can sound as if they are continuous because the offset messages are missed owing to disruptions from noises in the same general bandwidth.[25] This can be of great benefit in understanding distorted messages such as radio signals that suffer from interference, as (owing to this effect) the message is heard as if it was continuous. Figure 2 gives an example of duration identification. When a new sound is noticed (see Figure 2, Green arrows), a sound onset message is sent to the auditory cortex. When the repeating pattern is missed, a sound offset messages is sent.
Loudness
Loudness is perceived as how "loud" or "soft" a sound is and relates to the totalled number of auditory nerve stimulations over short cyclic time periods, most likely over the duration of theta wave cycles.[26][27][28] This means that at short durations, a very short sound can sound softer than a longer sound even though they are presented at the same intensity level. Past around 200 ms this is no longer the case and the duration of the sound no longer affects the apparent loudness of the sound. Figure 3 gives an impression of how loudness information is summed over a period of about 200 ms before being sent to the auditory cortex. Louder signals create a greater 'push' on the Basilar membrane and thus stimulate more nerves, creating a stronger loudness signal. A more complex signal also creates more nerve firings and so sounds louder (for the same wave amplitude) than a simpler sound, such as a sine wave.
Timbre
Timbre is perceived as the quality of different sounds (e.g. the thud of a fallen rock, the whir of a drill, the tone of a musical instrument or the quality of a voice) and represents the pre-conscious allocation of a sonic identity to a sound (e.g. “it's an oboe!"). This identity is based on information gained from frequency transients, noisiness, unsteadiness, perceived pitch and the spread and intensity of overtones in the sound over an extended time frame.[9][10][11] The way a sound changes over time (see figure 4) provides most of the information for timbre identification. Even though a small section of the wave form from each instrument looks very similar (see the expanded sections indicated by the orange arrows in figure 4), differences in changes over time between the clarinet and the piano are evident in both loudness and harmonic content. Less noticeable are the different noises heard, such as air hisses for the clarinet and hammer strikes for the piano.
Texture
Sonic texture relates to the number of sound sources and the interaction between them.[29][30] The word 'texture', in this context, relates to the cognitive separation of auditory objects.[31] In music, texture is often referred to as the difference between unison, polyphony and homophony, but it can also relate (for example) to a busy cafe; a sound which might be referred to as 'cacophony'. However texture refers to more than this. The texture of an orchestral piece is very different from the texture of a brass quintet because of the different numbers of players. The texture of a market place is very different from a school hall because of the differences in the various sound sources.
Spatial location
Spatial location (see: Sound localization) represents the cognitive placement of a sound in an environmental context; including the placement of a sound on both the horizontal and vertical plane, the distance from the sound source and the characteristics of the sonic environment.[31][32] In a thick texture, it is possible to identify multiple sound sources using a combination of spatial location and timbre identification. This is the main reason why we can pick the sound of an oboe in an orchestra and the words of a single person at a cocktail party.
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