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Media College (Chương 1 : Âm Thanh) (7,8) PDF Print E-mail
Written by tuyenphuc   
Thursday, 18 March 2010 10:57
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Chương 7: Màu sắc âm thanh. Noise, Colours & Types.

Noise Colours & Types

Certain noises are described by their colour, for example, the term "white noise" is common in audio production and other situations. Some of these names are official and technical, others have more loose definitions. These terms generally refer to random noise which may contain a bias towards a certain range of frequencies.

Black Noise

A term with numerous conflicting definitions, but most commonly refers to silence with occasional spikes.

Blue Noise

Contains more energy as the frequency increases.

Brown Noise

Mimics the signal noise produced by brownian motion.

Gray Noise

Similar to white noise, but has been filtered to make the sound level appear constant at all frequencies to the human ear.

Green Noise

An unofficial term which can mean the mid-frequencies of white noise, or the "background noise of the world".

Orange Noise

An unofficial term describing noise which has been stripped of harmonious frequencies.

Pink Noise

Contains an equal sound pressure level in each octave band. Energy decreases as frequency increases.

Purple Noise

Contains more energy as the frequency increases.

Red Noise

An oceanographic term which describes ambient underwater noise from distant sources. Also another name for brown noise.

White Noise

Contains an equal amount of energy in all frequency bands.

Note: Some of these definitions refer to "all frequencies". This is only theoretical — in practice this means "all frequencies in a finite range".

Black Noise

Black noise has various definitions — as far as we are aware none of them are official. Some common definitions are listed below:

(1) Silence, no noise at all.

(2) Noise with a 1/fβ spectrum, where β > 2.

(3) Noise which has zero energy at most frequencies but contains occasional random spikes.

(4) The noise created by active noise control systems, designed to cancel existing noises.

(5) Ultrasonic white noise, i.e. white noise which is at a frequencies too high to hear but which can still affect the environment.

Blue Noise

Blue noise, AKA azure noise, is similar to pink noise except the power density increases 3 dB per octave as the frequency increases. In technical terms the density is proportional to f (frequency).

Brown Noise


Brown noise is a random noise which mimics the signal noise produced by brownian motion. Technically speaking, the spectral density is proportional to 1/f2, which basically means it has more energy at lower frequencies (decreasing by around 6dB per octave).

To the human ear, brown noise is similar to white noise but at a lower frequency. Examples in nature include waves on the beach and some wind noise.

Note: Some people use the term brown noise as a synonym for brown note, a controversial and unproven sound which causes the listener to lose control of their bowels.

Gray Noise

Gray noise is a random noise which sounds the same at all frequencies to the human ear. This is not the same as white noise, which has the same energy at all frequencies. Rather, gray noise is subjected to a "psycho acoustic equal loudness curve" which compensates for the bias of the human ear so that it sounds the same at all frequencies.

Green Noise

Green noise is not an officially recognised term. There are several unofficial definitions in use — these appear to be the two most common:

(1) The mid-frequency component of white noise.

(2) "The background noise of the world", a sort of new-age description of ambient noise averaged from several different outdoors locations. Similar in sound to pink noise with an emphasis on the range around 500Hz.

Orange Noise

The semi-official definition of orange noise is "a quasi-stationary noise with a finite power spectrum with a finite number of small bands of zero energy dispersed throughout a continuous spectrum." We have not been able to determine where this definition originated but it is commonly used in reference material.

Orange noise relates to musical scales. The bands of zero energy coincide with the notes in the scale. In effect this means that the in-tune notes of a scale are removed, leaving only the out-of-tune frequencies. This creates a clashing, displeasing noise.

Pink Noise


Pink noise (AKA 1/f noise or flicker noise) is similar to white noise except that it contains an equal amount of energy in each octave band. To put it technically, the power spectral density is proportional to the reciprocal of the frequency.

Sound engineers use pink noise to test whether a system has a flat frequency response.

Pink noise can be generated by putting white noise through a pinking filter which removes more energy as the frequency increases (approximately 3 dB per octave).

As white noise is anagous to white light (representing all frequencies equally), pink noise is anagous to light which tends towards the lower end of the visible light spectrum (red light).

Purple Noise

Purple noise is similar to brown noise except that the power density increases 6 dB per octave as the frequency increases. In technical terms the density is proportional to f2.

Purple noise is also known as also known as violet noise or differentiated white noise.

Red Noise

Red noise has two common definitions:

(1) Another name for brown noise.

(2) An oceanographic term which describes the ambient noise of distant underwater objects.

The "red" name reportedly refers to the loss of higher frequencies and the emphasis on lower frequencies (this is from the white noise / white light analogy). This would apply to either of the above definitions.

Red noise is not nearly as clearly defined as white or pink noise. Some definitions found on the web conflict with each other, for example, some sources define red noise as a synonym for pink noise.

White Noise


White noise is a random noise that contains an equal amount of energy in all frequency bands.

White noise is the equivalent of white light, in fact this is how it gets it's name. White light is made up of all light frequencies (colours), while white noise is made up of all audio frequencies.

White noise is used in electronic music, either directly as a sound effect or as the basis to create synthesized sounds. For example, many percussion instruments have a high component of white noise.

White noise is also used to mask other sounds. This process takes advantage of the way the human brain works — the brain is able to single out simple frequency ranges but has trouble when too many frequencies are heard at once. When white noise is present, other noises appear diminished.

White noise is available on CDs etc, marketed as a noise reducer or sleeping aid.

Audio Test Tone


Audio test tones are a special class of artificially-created sounds. An example is the sine-wave tone you sometimes hear at the end of a video, or when a television station goes off the air.

There are two things test tones are usually used for:

  1. Testing the quality of an audio signal.
  2. Testing the quality of audio hardware systems and identifying faults.

DTMF (Dual-Tone Multi-Frequency

DTMF, better known as touch-tone, is a system of signal tones used in telecommunications. Applications include voice mail, help desks, telephone banking, etc.

There are twelve DTMF signals, each of which is made up of two tones from the following selection: 697 Hz, 770 Hz, 852 Hz, 941 Hz, 1209 Hz, 1336 Hz, 1477Hz.

The tones are divided into two groups (low and high), and each DTMF signal uses one from each group. This prevents any harmonics from being misinterpreted as part of the signal.

The following table shows the frequencies used for each signal:


1209 Hz

1336 Hz

1477 Hz

697 Hz




770 Hz




852 Hz




941 Hz





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