What is the reason, that the audio on our videos sounds often so much different as we have heard it during the recording of our footage? Without getting too technical, I would like to explain the principles of electronic sound recording. Once you understand how a microphone works, you can use it in ways to improve your audio track.
The two main characteristics, which determine the sound, is the polar pattern or directivity of the microphone and the sensitivity (volume), which is controlled by the electronic circuitry.
Capsule design and directivity
A microphone's directionality or polar pattern indicates, how sensitive it is to sounds arriving at different angles about its central axis. Let's have a look at a simple microphone device for converting sound into electrical energy. Its basic component is a diaphragm, that responds to the pressure or particle velocity of sound waves.
The inner elements of a microphone are the primary source of differences in directivity. A pressure microphone uses a diaphragm between a fixed internal volume of air and the environment, and responds uniformly to pressure from all directions, so it is said to be omnidirectional. A pressure-gradient microphone uses a diaphragm, that is at least partially open on both sides. The pressure difference between the two sides produces its directional characteristics. Other elements such as the external shape of the microphone and external devices such as interference tubes can also alter a microphone's directional response. A pure pressure-gradient microphone is equally sensitive to sounds arriving from front or back, but insensitive to sounds arriving from the side, because sound arriving at the front and back at the same time creates no gradient between the two.
The characteristic directional pattern of a pure pressure-gradient microphone is like a figure-8. Other polar patterns are derived by creating a capsule, that combines these two effects in different ways. The cardioid, for instance, features a partially closed backside, so its response is a combination of pressure and pressure-gradient characteristics.
Polar patterns
An electronic circuitry controls the volume and sensitivity of the microphone and sound.
Below are some terminologies as used by professionals. Please keep in mind that any poor sound captured during production can be salvaged during post production.
AGC (Automatic Gain Control) is a circuit to automatically adjust a microphone's gain (volume) to prevent too high levels, causing distortion or to raise sensitivity to hear quiet noises. It goes as far as an inbuilt camcorder microphone picks up the mechanical noises of a tape or disc drive. In this case it is a good idea to use an external microphone, which is positioned further away from the camera and closer to the action.
Below are some terminologies explained as they are used by professionals.
Audio Jargon
Ambient Sound
Background audio of a given environment. On-camera dialogue might be primary sound; traffic noise and refrigerator hum would be ambient.
Hiss:
Interference in audio recording, result of circuit noise from a playback recorder's amplifiers or from a tape's residual magnetism.
Lavalier:
A small microphone, that can be worn around the neck on a cord or clipped to a shirt.
Noise:
Unwanted sound or static in an audio signal.
Pickup Pattern:
Defines a microphone's response to sounds arriving from various directions or angles.
Sound Bite:
Any short audio segment for use in an edited program - usually a highlight taken from an interview.
On this microphone the pattern can be influenced by turning the knob from "tele to wide".
Signal-to-Noise Ratio (S/N):
Relationship between signal strength and a medium's inherent noise. Audio S/N specifies amount of background tape hiss present with low or no-volume recordings. Higher figures represent a cleaner signal. Usually cited in decibels (dB).
Wild Sound:
Audio recorded independently of picture i.e. rain on roof or clock.
In the 2nd part of this article I will elaborate, what you can do and how to apply several techniques to enhance audio in your video production and how poor sound captured during production can be salvaged in post production.
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