Audio Basics:
PROPAGATION
Nyk Cowham for Introduction to Music Production at Coursera.org - Spring 2014
WEEK ONE: Lesson One
Who are you?
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My name is Nyk, pronounced "Nick".
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I Live in Northern Thailand, just south of Myanmar.
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I play acoustic guitar as my principle instrument.
- I have been know to make a nuisance of myself with keyboards.
But enough about me. On to the lesson! ...
What is propagation?
Propagation is the movement of sound through a medium.
Even though a medium is involved, propagation does NOT mean hearing dead people!
Propagation through stuff
Sound waves can propagate through any kind of material, in any physical state:
- GASEOUS: atmospheric air, etc.
- LIQUID: water, oil, etc.
- SOLID: wood, metal, stone, etc.
The sound of a railroad train can be heard from a couple of miles away just by listening to the track, because sound can propagate through steel. Did you know that sound can be such a badass?
The need for speed!
One of the most important factors of propagation that effects our perception of sound is its speed. It takes time for sound waves to travel any distance along the medium.
The speed of sound can vary depending on what kind of material the medium is made of.
Usually when we talk about the speed of sound we are talking about the speed of sound through the air in the atmosphere.
What affects the speed of sound through the atmosphere?
The speed of sound through the atmosphere can vary depending on:
- Air pressure / Elevation above sea level,
- Air temperature,
- Humidity in the air,
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Etc.
However, the speed of sound in air only varies by a very slight amount and in most cases we can speak informally about the speed of sound as a specific approximate value.
What is the approximate speed of sound through the air?
- 340 m / sec
- 1 ft / msec
- 1 km / 3 sec
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1 mile / 5 sec
These numbers are only rough guidelines, they are not precise values.
The human ear can detect very minute variations in the amount of time that it takes a sound source to reach each of our ears (arrival time).
How does propagation and speed of sound shape our perception of sound?
- Gives a sense of direction to sound
- Defines a sense of space and depth
- Suggests surfaces and sound reflections
- Creates a sense of physical presence
A Gothic cathedral has very different acoustic properties to other spaces. Human hearing is very sensitive to minute differences in the arrival times of sounds reflected from surfaces.
What if I don't have a cathedral or concert hall in which to record?
Modern music producers have a number of different tools and effects that can be used to model different types of spaces and to give a recording a sense of place, presence and depth.
These effects create subjective perceptions of physical space and depth of field by manipulating parameters related to propagation.
What effects can music producers use to manipulate sound parameters related to propagation?
- Delays
- Reverbs
- Phasers
- Flangers
- Etc.
Summary
- Propagation is the movement of sound through a medium.
- It takes time for sound waves to pass through a medium, so sound has a speed, and it can be delayed.
- We can perceive minute differences in the arrival of sound reflected from different surfaces in an environment.
- Differences in the arrival of sounds give us an auditory experience of space and depth.
- We can use various sound effects like delay and reverb to recreate the auditory experience of listening to sounds in a large cathedral, concert hall, or some other acoustically rich environment.
Audio Basics: Propagation
By nyk
Audio Basics: Propagation
Week One teaching assignment for Introduction to Music Production at Coursera.org . Spring 2014
