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Sound can be described as an oral sensation created by vibrations in air or water. It's a mechanical motion in matter within a frequency range that a human ear can perceive. Sound is an auditory sensation that happens when there is a change in air or water pressure. It travels in the form of waves. There are two types of waves - longitudinal and transverse waves. Longitudinal waves are sound waves in air and the direction of motion of waves, changes according to the directional change of particle movement. Whereas, transverse waves are sound waves in water and the direction of the wave is perpendicular to the energy. Sound waves occur when something vibrates. It may be the string of a guitar, a person's vocal chords or a truck's tires on the pavement.

The sound envelope is a term used to describe what happens to a sound during its duration. The first phase is called the attack. How quickly does the sound reach its highest volume? Some sounds, like a note played on a flute, have a rather slow attack. Other sounds, like a guitar note played with a pick, have a very fast or sharp attack. The second phase is often described as the internal dynamics or sustain. Following the attack, the sound may settle for a period of time at a different decibel level. How long does it sustain that level? The third phase is the decay. This describes how quickly the sound diminishes back to silence. A large church bell may have a fairly sharp attack, followed by a long period of sustain and a very slow decay. A snare drum, on the other hand, may have a very sharp attack, almost no sustain, and a very rapid decay. Sounds that rise to full volume very quickly and decay very quickly are called transients.

To study sound we need to know the characteristics of sound. There are three characteristics of sound:

1. Frequency.
2. Wavelength.
3. Amplitude.

1. Frequency - How quickly a sound source vibrates is it's frequency. Frequency is perceived by our ears as pitch. We measure the vibrations in cycles per second. One cycle per second equals one hertz (Hz). Most humans can hear between 16 Hz and 16 kHz (kHz stands for kilohertz or thousands of cycles per second), but the ability to hear higher frequencies varies greatly from person to person. Younger people can usually perceive high frequencies better than older people and, in general, women's high frequency perception is better than men's. The range of frequencies that a piece of equipment can reproduce is said to be it's frequency response.

2. Wavelength - The distance between two corresponding or successive crest, points or troughs is called wavelength. It is denoted by lambda. Higher the frequency the shorter will be the wavelength and vice versa. 17 meters is the wavelength for 20 Hz and 1cm is the wavelength for 20 KHz frequency.

3. Amplitude - Measure of extend to which a wave deviates from its normal position is called as its amplitude. It depends on the maximum displacement of the particle from the mean position. It is same as the volume or level of sound. It refers to the strength, loudness, intensity and volume of a particular sound.

PROXIMITY EFFECTS:

Why is it that even with your eyes closed you can tell if a person speaking to you is 20 centimeters or 5 meters (6 inches or 16 feet) away? The first thought might be that the voice of a person 20cm away would be louder than if the person were 5 meters away. That's just a part of the answer. But if you think about it, there's more to it than that. You might want to say that the voice of a person that's close to you just sounds different than a person who is farther away. This just sounds different element becomes highly significant when you try to start editing scenes together. Getting the audio in scenes to flow together without noticeable changes, takes an understanding of how sound is altered with distance. Sound traveling over a distance loses low frequencies (bass) and, to a lesser extent, the higher frequencies (treble). Conversely, microphones used at a close distance normally create what is called a proximity effect-exaggerated low-frequency response. Some microphones have low cut filters to reduce unnatural low frequencies when the microphones are used at close distances.

FACTORS CONTRIBUTING TO THE QUALITY OF SOUND:

1. Frequency Response: Every electronic system has an effect on a particular sound. Whenever a sound passes through a system each audio signal gets affected and makes the original sound different. This degree of change caused by a system is its Frequency Response. A flat frequency response is when the produced sounds, recorded sounds, and reproduced sounds all are the same.
   
2. Distortion: Disturbance or change in frequency or pitch because of system limitation is called Distortion or Noise. This happens because when sound is transferred from one tape to another or one system to another, the frequency response may vary. Hence the sounds are interpreted and executed differently by each system.
   
3. Signal to Noise Ratio: Signal is any wave that carries information. Whereas an signal carrying sound is called noise. This is not noise in the ambience but noise generated by an electronic system. This may happen to several reasons like overused system, too low sound level while recording, etc. In analog and digital communications, signal-to-noise ratio, often written S/N or SNR, is a measure of signal strength relative to background noise. The ratio is usually measured in decibels (dB).
   
4. Wow and Flutter: These are terms used for speed instability while playback of a recorded sound. In a recorder or player while in action, the belt that reads the audio signals might be loose. Hence the recorded speed is different from playback. Wow is a slow cyclic variation, aurally it is a wobbling sound. Flutter is a rattling cyclic variation. It is a trembling sound.