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Understanding SoundCSC121, Introduction to Computer Programming
What is Sound?
• “Sound takes place when bodies strike the air,… by its being moved in a corresponding manner; the air being contracted and expanded and overtaken, and again struck by the impulses of the breath and the strings, for when air falls upon and strikes the air which is next to it, the air is carried forward with an impetus, and that which is contiguous to the first is carried onward; so that the same voice spreads every way as far as the motion of air takes place.”
What is Sound?
• Aristotle described the generation and propagation of sound in De Anima (c. 350 BCE)
Sound Waves
• a typical sound has an measurable average ambient air pressure• but sound pressure is not continuously unchanging• as Aristotle noted, there are fluctuations in pressure that we identify as condensations (increases) and rarefactions(decreases)• these changes form a wave pattern•much like that of dropping a pebble into a pond
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Sound: push and pushback
a) a piston pulse initiates push on the air cells
b) the pressure deforms the air cells that pass the energy to their neighbors
c) darker cells indicate higher pressure, lighter cells lighter pressure
d) the energy is reflected on the rigid surface of the wall
e) reflection and diffraction is evident in the sound propagation
Sound Waves
• Here is an idealized sinusoidal wave with an amplitude A and a wavelength λ
Sound Waves
• the frequency of the wave, measured as the number of repeats or cycles per second
• typically expressed in Hertz or Hz
Sound Wave Properties
• speed & propagation falloff• intensity• superposition & interference• reflection & diffraction
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Sound Waves
• the speed of propagation of a sound wave does not depend on the wavelength or frequency of the sound
Propagation Falloff
• The intensity or amplitude of a sound does decrease over distance
Superposition
• the principle of superposition: sound waves pass through one another with little disturbance
Superposition and interference
• If the sound waves are precisely opposing signals, then they can cancel each other out
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Example: Noise Cancellation Technology
Active noise cancellation is available for audio headsets. It seeks to apply the principle of superposition.
External microphones on the headset capture the sound on its way to the ear. Fast acting electronics attempt to apply an opposing pressure to cancel the noise.
In practice, they reduce the noise by a factor of 100 or 20 dB.
Reflection
• the law of reflection holds true for all radiating energies:• the angle of incidence equals the angle of reflection
Refraction
•when light, for example, falls on a matte-‐painted wall, it reflects diffusively• a similar phenomenon occurs when sound waves encounter surfaces that are rough
Intensity
• the intensity or power of a sound wave can be measured• sound intensity is normally expressed in log form• specifically, the base 10 logarithm• logarithmmeasures the exponent that yields the number• the base of the logarithm is the number we are raising to a power• and the power is the logarithm of the number• example: log10(26) = 1.41497…• i.e., 101.41497… = 26
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decibelsDecibels Pressure (Pascals) Sound
0 0.00002 threshold of human hearing20 0.0002 rustling of leaves40 0.002 residential area at night80 0.2 vacuum cleaner100 2 pneumatic hammer at 2 meters120 20 rock concert130 63.2 threshold of pain150 632 jet engine at 30 meters (hearing severely
damaged)180 20,000 rocket engine at 30 meters (instant death)