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Post Index » STAGE TECHNIC » Pro Sound


Media College (Chương 1 : Âm Thanh) (5,6) - Page 4 PDF Print E-mail
Written by tuyenphuc   
Monday, 15 March 2010 17:15
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Audio Limiters

A limiter is a type of compressor designed for a specific purpose — to limit the level of a signal to a certain threshold. Whereas a compressor will begin smoothly reducing the gain above the threshold, a limiter will almost completely prevent any additional gain above the threshold. A limiter is like a compressor set to a very high compression ratio (at least 10:1, more commonly 20:1 or more). The graph below shows a limiting ratio of infinity to one, i.e. there is no gain at all above a the threshold.

Input Level vs Output Level With Limiting Threshold

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Limiters are used as a safeguard against signal peaking (clipping). They prevent occasional signal peaks which would be too loud or distorted. Limiters are often used in conjunction with a compressor — the compressor provides a smooth roll-off of higher levels and the limiter provides a final safety net against very strong peaks.


Audio Expansion

Audio expansion means to expand the dynamic range of a signal. It is basically the opposite of audio compression.

Like compressors and limiters, an audio expander has an adjustable threshold and ratio. Whereas compression and limiting take effect whenever the signal goes above the threshold, expansion effects signal levels below the threshold.

Any signal below the threshold is expanded downwards by the specified ratio. For example, if the ratio is 2:1 and the signal drops 3dB below the threshold, the signal level will be reduced to 6dB below the threshold. The following graph illustrates two different expansion ratios — 2:1 and the more severe 10:1.

Input Level vs Output Level With Expansion

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An extreme form of expander is the noise gate, in which lower signal levels are reduced severely or eliminated altogether. A ratio of 10:1 or higher can be considered a noise gate.

Note: Some people also use the term audio expansion to refer to the process of decompressing previously-compressed audio data.

Audio Effects

This page provides an overview of the most common audio effects used in sound production, with links to more detailed tutorials.

Equalization

Equalization means boosting or reducing (attenuating) the levels of various frequencies in a signal. At it's most basic, equalization can mean turning the bass/treble controls up or down. Advanced equalizers have fine controls for specific frequencies.

Common uses for equalization include correct signals which sound unnatural and reducing feedback.

Compression & Limiting

Compression means reducing the dynamic range of a signal. All signal values above a certain adjustable threshold are reduced in gain relative to lower-level signals. This creates a more even signal level, reducing the level of the loudest parts.

Limiting is an extreme form of compression. Rather than smoothly reducing the gain of successively higher levels, all signal above the threshold is limited to the same gain. This creates a very hard cut-off point, over which there is no increase in level.

Expansion & Noise Gating

Expansion means increasing the dynamic range of a signal. High level signals maintain the same (or nearly the same) levels, low level signals are reduced (attenuated). This creates a greater range between quiet and loud. Expansion is the opposite of compression.

Noise gating is an extreme form of expansion — signals below a certain point are either heavily attenuated or eliminated completely. This leaves only higher level signals and removes background noise when the signal is not present.

Delay / Echo

Delay is a simple concept — the original audio signal is followed closely by a delayed repeat, just like an echo. The delay time can be as short as a few milliseconds or as long as several seconds. A delay effect can include a single echo or multiple echoes, usually reducing quickly in relative level.

Delay also forms the basis of other effects such as reverb, chorus, phasing and flanging.

Reverb (Reverberation)

Reverb is short for reverberation, the effect of many sound reflections occurring in a very short space of time. The familiar sound of clapping in an empty hall is a good example of reverb.

Reverb effects are used to restore the natural ambience to a sound, or to give it more fullness and body.

What is Reverb?

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Reverberation, or reverb for short, refers to the way sound waves reflect off various surfaces before reaching the listener's ear.

The example on the right shows one person (the sound source) speaking to another person in a small room. Although the sound is projected most strongly toward the listener, sound waves also project in other directions and bounce off the walls before reaching the listener. Sound waves can bounce backwards and forwards many times before they die out.

When sound waves reflect off walls, two things happen:

  1. They take longer to reach the listener.
  2. They lose energy (get quieter) with every bounce.

The listener hears the initial sound directly from the source followed by the reflected waves. The reflections are essentially a series of very fast echoes, although to be accurate, the term "echo" usually means a distinct and separate delayed sound. The echoes in reverberation are merged together so that the listener interprets reverb as a single effect.

In most rooms the reflected waves will scatter and be absorbed very quickly. People are seldom consciously aware of reverb, but subconsciously we all know the difference between "inside sound" and "outside sound". Outside locations, of course, have no walls and virtually no reverb unless you happen to be close to reflective surfaces.

Some rooms result in more reverb than others. The obvious example is a hall with large, smooth reflective walls. When the hall is empty, reverb is most pronounced. When the hall is full of people, they absorb a lot of sound waves so reverb is reduced.



 

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