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Media College (Chương 1 : Âm Thanh) (1) PDF Print E-mail
Written by tuyenphuc   
Sunday, 07 March 2010 17:13

Đây là 1 tài liệu về giáo trình AT cơ bản bằng tiếng Anh của Media College khá rõ ràng và dễ hiểu. Từ ngữ cũng đơn giản, không cần trình độ sinh ngữ cao. Bạn nào đọc chưa được, có thể vào http://translate.google.com nhờ dịch cũng tạm được.

Xin cảm ơn bạn Nguyễn Đình Thân đã biên tập và có nhã ý gởi tài liệu này cho trang web của chúng ta.

Phần 1: Giới thiệu Âm Thanh.


Introduction to Audio

This beginner-level tutorial covers the basics of audio production. It is suitable for anyone wanting to learn more about working with sound, in either amateur or professional situations. The tutorial is five pages and takes about 20 minutes to complete.

What is "Audio"?

Audio means "of sound" or "of the reproduction of sound". Specifically, it refers to the range of frequencies detectable by the human ear — approximately 20Hz to 20kHz. It's not a bad idea to memorise those numbers — 20Hz is the lowest-pitched (bassiest) sound we can hear, 20kHz is the highest pitch we can hear.

Audio work involves the production, recording, manipulation and reproduction of sound waves. To understand audio you must have a grasp of two things:

  1. Sound Waves: What they are, how they are produced and how we hear them.
  2. Sound Equipment: What the different components are, what they do, how to choose the correct equipment and use it properly.

Fortunately it's not particularly difficult. Audio theory is simpler than video theory and once you understand the basic path from the sound source through the sound equipment to the ear, it all starts to make sense.

Technical note: In physics, sound is a form of energy known as acoustical energy.

The Field of Audio Work

The field of audio is vast, with many areas of specialty. Hobbyists use audio for all sorts of things, and audio professionals can be found in a huge range of vocations. Some common areas of audio work include:

  • Studio Sound Engineer
  • Live Sound Engineer
  • Musician
  • Music Producer
  • DJ
  • Radio technician
  • Film/Television Sound Recordist
  • Field Sound Engineer
  • Audio Editor
  • Post-Production Audio Creator

In addition, many other professions require a level of audio proficiency. For example, video camera operators should know enough about audio to be able to record good quality sound with their pictures.

Speaking of video-making, it's important to recognise the importance of audio in film and video. A common mistake amongst amateurs is to concentrate only on the vision and assume that as long as the microphone is working the audio will be fine. However, satisfactory audio requires skill and effort. Sound is critical to the flow of the programme — indeed in many situations high quality sound is more important than high quality video.

Most jobs in audio production require some sort of specialist skill set, whether it be micing up a drum kit or creating synthetic sound effects. Before you get too carried away with learning specific tasks, you should make sure you have a general grounding in the principles of sound. Once you have done this homework you will be well placed to begin specialising.

The first thing to tackle is basic sound wave theory...

How Sound Waves Work

Before you learn how sound equipment works it's very important to understand how sound waves work. This knowledge will form the foundation of everything you do in the field of audio.

Sound waves exist as variations of pressure in a medium such as air. They are created by the vibration of an object, which causes the air surrounding it to vibrate. The vibrating air then causes the human eardrum to vibrate, which the brain interprets as sound.


The illustration on the left shows a speaker creating sound waves.

Sound waves travel through air in much the same way as water waves travel through water. In fact, since water waves are easy to see and understand, they are often used as an analogy to  illustrate how sound waves behave.

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Sound waves can also be shown in a standard x vs y graph, as shown here. This allows us to visualise and work with waves from a mathematical point of view. The resulting curves are known as the "waveform" (i.e. the form of the wave.)

The wave shown here represents a constant tone at a set frequency. You will have heard this noise being used as a test or identification signal. This "test tone" creates a nice smooth wave which is ideal for technical purposes. Other sounds create far more erratic waves.

Note that a waveform graph is two-dimensional but in the real world sound waves are three-dimensional. The graph indicates a wave traveling along a path from left to right, but real sound waves travel in an expanding sphere from the source. However the 2-dimensional model works fairly well when thinking about how sound travels from one place to another.

The next thing to consider is what the graph represents; that is, what it means when the wave hits a high or low point. The following explanation is a simplified way of looking at how sound waves work and how they are represented as a waveform. Don't take it too literally — treat it as a useful way to visualise what's going on.

In an electronic signal, high values represent high positive voltage. When this signal is converted to a sound wave, you can think of high values as representing areas of increased air pressure. When the waveform hits a high point, this corresponds to molecules of air being packed together densely. When the wave hits a low point the air molecules are spread more thinly.

In the diagram below, the black dots represent air molecules. As the loudspeaker vibrates, it causes the surrounding molecules to vibrate in a particular pattern represented by the waveform. The vibrating air then causes the listener's eardrum to vibrate in the same pattern. Viola — Sound!

Note that air molecules do not actually travel from the loudspeaker to the ear (that would be wind). Each individual molecule only moves a small distance as it vibrates, but it causes the adjacent molecules to vibrate in a rippling effect all the way to the ear.

Now here's the thing: All audio work is about manipulating sound waves. The end result of your work is this series of high and low pressure zones. That's why it's so important to understand how they work - they are the "material" of your art. 

Sound Wave Properties

All waves have certain properties. The three most important ones for audio work are shown here:

Wavelength: The distance between any point on a wave and the equivalent point on the next phase. Literally, the length of the wave.


Amplitude: The strength or power of a wave signal. The "height" of a wave when viewed as a graph.

Higher amplitudes are interpreted as a higher volume, hence the name "amplifier" for a device which increases amplitude.


Frequency: The number of times the wavelength occurs in one second. Measured in kilohertz (Khz), or cycles per second. The faster the sound source vibrates, the higher the frequency.

Higher frequencies are interpreted as a higher pitch. For example, when you sing in a high-pitched voice you are forcing your vocal chords to vibrate quickly.


How Sound Waves Interact with Each Other

When different waves collide (e.g. sound from different sources) they interfere with each other. This is called, unsurprisingly, wave interference.


The following table illustrates how sound waves (or any other waves) interfere with each other depending on their phase relationship:

  • Sound waves which are exactly in phase add together to produce a stronger wave.
  • Sound waves which are exactly 180 degrees out of phase cancel each other out and produce silence (this is how many noise-cancellation devices work).
  • Sound waves which have varying phase relationships produce differing sound effects.

Sound Systems

Working with audio means working with sound systems. Naturally, the range of systems available for different applications is enormous. However, all electronic audio systems are based around one very simple concept: To take sound waves, convert them into an electric current and manipulate them as desired, then convert them back into sound waves.

A very simple sound system is shown in the diagram below. It is made up of two types of component:

  • Transducer - A device which converts energy from one form into another. The two types of transducers we will deal with are microphones (which convert acoustical energy into electrical energy) and speakers (which convert electrical energy into acoustical energy).
  • Amplifier - A device which takes a signal and increases it's power (i.e. it increases the amplitude).

  1. The process begins with a sound source (such as a human voice), which creates waves of sound (acoustical energy).
  2. These waves are detected by a transducer (microphone), which converts them to electrical energy.
  3. The electrical signal from the microphone is very weak, and must be fed to an amplifier before anything serious can be done with it.
  4. The loudspeaker converts the electrical signal back into sound waves, which are heard by human ears.

The next diagram shows a slightly more elaborate system, which includes:

  • Signal processors - devices and software which allow the manipulation of the signal in various ways. The most common processors are tonal adjusters such as bass and treble controls.
  • Record and playback section - devices which convert a signal to a storage format for later reproduction. Recorders are available in many different forms, including magnetic tape, optical CD, computer hard drive, etc.

  1. The audio signal from the transducer (microphone) is passed through one or more processing units, which prepare it for recording (or directly for amplification).
  2. The signal is fed to a recording device for storage.
  3. The stored signal is played back and fed to more processors.
  4. The signal is amplified and fed to a loudspeaker.

The 3-part audio model

One simple way of visualising any audio system is by dividing it up into three sections: the source(s), processor(s) and output(s).

  • The source is where the electronic audio signal is generated. This could be a "live" source such as a microphone or electric musical instrument, or a "playback" source such as a tape deck, CD, etc.
  • The processing section is where the signal is manipulated. For our purposes, we will include the amplifiers in this section.
  • The output section is where the signal is converted into sound waves (by loudspeakers), so that it can be heard by humans.

This portable stereo is a good example of a simple system.

Sources: There are three sources - two tape machines and one radio aerial (technically the radio source is actually at the radio station).

Processors: Includes a graphic equaliser, left/right stereo balance, and amplifiers.

Outputs: There are two speaker cabinets (one at each end), each containing two speakers. Note that there are also two alternative outputs: A headphone socket (which drives the small speakers inside a headphone set) and twin "line out" sockets (which supply a feed for an external audio system).

Now imagine a multi-kilowatt sound system used for stadium concerts. Although this is a complex system, at it's heart are the same three sections: Sources (microphones, instruments, etc), processors and speakers.

Whatever the scale of the project, the same underlying principles of sound reproduction apply.

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