Effects
Automation: This is the effect of creating events that will happen automatically when you play back the audio. For instance, automation can be used on the volume of a sound while it is playing so it automatically gets louder or softer. It can also be used in combination with panning to create the effect of sound sources moving around the speaker array and any other effects.
Delay: An effect which stores a sound and then replays it over a period of time.
Equalisation: This is an effect used to alter the frequency range of a sound. It allows you to either cut or boost a set of frequencies, for example, if a sound contains too much "bass" or lower frequencies then they can be reduced using equalisation.
Limiting: Is an extreme form of compression which limits all signals above the threshold to the same gain, rather than smoothly reducing it.
Optimum Gain Signal: This is not an effect but rather how to record sounds at an optimum level. If a gain structure is too low then noise begins to be introduced to the sound. If a gain signal is too high then the sound will begin to distort. An optimum gain signal for recording audio is around 0dB, this is high enough to not introduce noise and allows enough headroom so the sound will not distort.
Panning: Is the spread of a sound signal into a multichannel or stereo sound field. In other words panning allows you to place sounds around the speaker array, whether it be directly coming from one speaker or between speakers creating a phantom image.
Reverb: is an effect which simulates a component of sound that consists of the reflections of the sound bouncing of the walls or objects in a room. In other words it is a room simulator. The reverberation of a room depends on many different factors:
Early Reflections - The first reflection of a sound off an object/wall that reaches the listener's ear milliseconds after the direct sound does. Early reflections enable the listener to quickly determine room size.
Reverb time - A larger enclosed space produces a larger reverb time. However a larger reverb time requires a louder source sound to allow the sound enough energy to be reflected around a large space.
Absorption - When a sound reflects of a wall/object some of the sound is absorbed into the material that the wall/object is made of. Different materials absorb different ranges of frequencies as high frequency have a smaller wavelength than lower frequencies. The greater the absorption in the room equals a smaller reverb time.
Human Hearing
Localisation and Direction:
Inter-aural Time Differences (ITD) - This is one of the cues that helps humans perceive direction of sound, it is the difference in the time of arrival of a sound between the two ears. If a sound is coming from a source on the left then it will arrive at the left ear before the right and it is this difference in time that helps the brain determine the direction of a sound.
Inter-aural Intensity Differences (IID) - This is another cue that helps humans perceive direction of sound, it is the difference in intensity of a sound between the two ears. If a sound is coming from a source on the left then it will be louder in the left ear than the right and it is this difference in intensity that helps towards the localisation of sound.
Head Related Transfer Function (HRTF) - Is the way in which sound waves physically interact with the human ear. The shape of the outer ear, including parts such as the pinna, helps to direct sound into the ear canal. The differences in the physical interaction of a sound wave between the ears alongside IID and ITD is how humans localise sound sources.
Physical Factors of Sound
Amplitude - the maximum deviation of the pressure waveform from the average, unit N/m squared.
Frequency and Pitch - Frequency is the number of cycles that a sound waves make per second and is measured in Hertz. The higher the number of cycles (frequency) equals a higher pitch. The human hearing is capable of hearing from 20Hz to 20,000 Hz, however damage to hearing may cause this range of frequencies to be smaller.
Phase - Time or angular difference between one signal an another.
Resonance - The vibration of an object or medium at a specific frequency.
Velocity - The speed of time in air under normal conditions is 340m/s.
Wavelength - The length of the physical wave in metres.
Measurement of Sound:
Intensity - The "concentration" of sound, unit N/m squared or Watts/m squared.
Loudness - The subjective "measure" of SPL and has the unit phon.
Sound Pressure Level - Sound can be described as variations in air pressure, SPL uses the unit dB. SPL is a ratio between the actual sound pressure and a reference level usually the threshold of human hearing.
Propagation of Sound:
Diffraction - Sound bends easily around obstacles that are of a similar size or smaller than the wavelength of the sound. Lower frequencies bend/diffract more freely around obstacles because of their longer wavelength.
Humidity - This has the decreasing the velocity of a sound. Dry air absorbs more energy than moist air, thus attenuates the sound to a greater level.
Pressure - Air pressure has little effect on the velocity of sound.
Refraction - When sound travels from a less dense medium to a more dense medium it bends in towards the normal. When it travels from a more dense medium to a less dense medium it bends away from the normal.
Temperature - The speed of a sound increases at around 0.6m/s for every degree rise in temperature.
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