Sound, Noise and Silence

Sound

If a tree falls in the forest and no one is around to hear it, is there a sound? Strictly speaking, sound is the sensation (brain) created by organs of hearing (ear) stimulated by mechanical waves in the surrounding means (generally air) as a consequence of the vibrations of an object (sound source). Source, means, ear and brain are necessary to have perceived sound that has the following characteristics:

• Carries information from one point to another

• Transports energy without transporting matter

• Travels in a solid, liquid or gas as a wave. In the case of fluids (liquids and gases) always as a longitudinal compression wave.

• Frequency range: 20 Hz – 20,000 Hz

• Intensity between the threshold of hearing and the threshold of pain.

In some texts it is claimed that perceivability is a prerequisite for regarding mechanical waves as sounds. This doesn't mean that it must be heard, merely that it could be heard if an observer was present. This view is actually quite problematic. In its simple form, this approach would claim that sound is defined by the limits of human hearing. What about waves in hostile media (e.g. in deep oceans, lava flows or other planets), outside my particular range of hearing even though perceivable by younger ears? What about "sounds" that are not perceivable to humans but that are perceivable to other species?

To avoid philosophical conflicts, sound may be defined and studied independently from different points of view, according to:

• Its behaviour as a wave in an elastic medium (solid, liquid, or gas). From the physical point of view (physical acoustics), sound is any mechanical vibration in any elastic medium independent of whether it is able to cause a sensation of hearing or not.
• Its behaviour as a sensory stimulus in the ear, including the way in which the brain interprets it (Psychoacoustics)
• The mental processes which extract sound features for purposes of memory storage, intelligibility and problem solving (Sonology).

When we are dealing with speech and musical sounds we generally assume that we are talking on mechanical waves in the range of human hearing being derived into longitudinal compression waves in air at normal temperature and pressure ranges acceptable to humans.

Sound in air

Sound can occur in any medium (solid, liquid or gas). Sound cannot occur in a vacuum as there is no medium.

In fluids (liquids and gases) as air or water, sound is a longitudinal compression wave which distorts the medium by creating moving fronts of high and low particle compression.

Individual particles only move short distances backward and forward in the direction of wave propagation whilst the compression wave front can move considerable distances.

Wave cycles consist of regions of high compression separated by regions of low compression (rarefaction) relative to the ambient or resting state.

Sound in air consists of consecutive regions of higher and lower air pressure relative to ambient air pressure. These fluctuations in air pressure are extremely small relative to ambient air pressure.

The ambient air pressure is typically 1 atmosphere at sea level (1 atm = 1.013 x 10⁵ Pa).

The sound pressure that is only just perceivable is taken to be 2 x 10^-5 Pa (ie. 20 µPa), about 1/5,000,000,000 times atmospheric pressure.

The threshold of pain (ie. the maximum sound pressure that can be perceived without pain) is about 100 Pa or about 1/1000 atm, which is 5,000,000 times the threshold sound pressure.

Silence

Silence is the absence of sound.

From the physical point of view, that means the absence of vibrations while that from the view of hearing, silence means the absence of perception.

Noise

Etymologically the word can be traced back to Old Provençal (France). It has a variety of meanings, the most important of which are the following:

1. Unwanted sound. In general we can define 'noise' as unwanted sound either because is unpleasant or causes any type of disturbance.  This makes noise a subjective term. But it also provides the opportunity for a society to come to a general agreement as to which sounds constitute unwanted intrusions. Particularly, in general usage, noise often refers to any loud sound.

2. Disturbance in any communication system: In electronics and engineering, noise refers to any disturbances which do not represent part of the expected signal, such as the 'snow' on a television screen.

3. Unmusical sound: The 19th century physicist Hermann von Helmholtz employed the term 'noise' to describe sound composed of non-periodic vibrations (e.g. the rustling of leaves), by comparison with musical sounds, which consist of periodic vibrations. Noise is still used in this sense in expressions such as broad band noise, Gaussian noise, narrow band noise, pink noise, random noise, rustle noise or white noise.

Sources of Sound

In the most common case, sound reaches our organs of hearing as a vibration of molecules in the surrounding air, at a frequency we can hear. That air is set into vibration as a consequence of something else that is vibrating as well. This object is the sound source that can be the vibrating parts of a musical instrument, the changing airflow produced by singers; the falling tree, a thunder, etc.

Sound and Music

Music (from Greek μουσική (mousike; "art of the Muses") is an artificial sound that someone creates because people enjoy hearing and respond to. Music creates emotions, or interacts with the emotions we already feel and, sometimes, it makes us want to dance.

Music is an art form whose medium is sound and silence. Its common elements are pitch (which governs melody and harmony), rhythm (and its associated concepts tempo, meter, and articulation), dynamics (intensity), and the sonic qualities of timbre and texture. A music composition is a particular skilful and imaginative combination of sounds.

From the physical point of view, a pleasant sound (and music should be) has a regular wave pattern which is repeated, while the waves of noise are irregular and do not have a repeated pattern.

Compound Sound

At any time, our auditory system receives a mix of simultaneous sounds which must be distinguished from each other. The standard way to study the actual sound is simplifying it by considering that it is composed by a multiplicity of individual sound waves and study the properties and behaviour of those simple waves. Then, we can study how individual sounds interact with each other.

There are a number of ways in which we can study this interaction: Superposition of Sound Waves, Mixing of Sounds, Modulation, Non-linear Combination, Environmental Sound Combination, Musical Pitch Combination