Chapter 2 - Chapter 2 Acoustics Sound is a physical...

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Chapter 2 – Acoustics Sound is a physical phenomenon to which one is exposed incessantly Not easily visualized literally or conceptually Acoustics – science of sound 1. Sound – Its Nature, Dimensions, and Measures Vibratory mechanical motion is required to generate sound but sound is distinct from this mechanical vibration A. THE DISTURBANCE CALLED SOUND Robert Boyle experiment – conclusion that for sound to exist, there must be a medium Sound is an energy form that exists within a medium and that moves through that medium in the form of a wave Sound waves need a vibratory source to occur, they are not spontaneous Sound is a disturbance in the medium which causes movement of adjacent parts of the medium Sound wave represents an orderly change in parts of a medium over space and time caused by the vibratory motion of an object Sound source delivers energy to the medium, and a sound wave carries this energy away from the source The study of sound is the study of waves and their behavior All waves move, propagate, and carry energy Waves provide a means by which energy can be moved from one place to another Attributes of waves are determined by the medium in which they exist o When waves move from one medium to another, their aspects change o When waves encounter obstacles in the same medium, their aspects change B. THE SOUND MEDIUM Solids, liquids, and gases are suitable media for sound o Transmission of sound will depend on the physical properties of the media Medium for sound must be compressible, elastic, and it must have mass o Mass as in the parameter of density, not total mass of the medium Substances are made up of particles
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o The more particles in a given volume or the more mass of each particle in that volume, the greater the density of the substance A force acting on a medium can cause the medium to change o Compressing a gas into a smaller volume increases the density, condensation o Density of air will decrease when forced to expand, rarefaction Compression and expansion of a medium’s particles cause pressure to change o Pressure – force density o (greater pressure exerted by high heels than tennis shoes because heels have the same force exerted onto a smaller surface) Compression and expansion of the medium will be opposed by its elasticity o Not total stiffness measure, but its bulk modulus of elasticity o Bulk modulus – measure of change in pressure of the substance when a force is applied to it or its opposition to a fractional change in its volume A disturbance can be transmitted or propagated to adjacent parts of the medium by means of a chain reaction Elasticity causes the particles to return to original sites after the wave has passed **the medium as a whole does not move, only the energy in the wave moves through the medium C. SOUND PROPAGATION Longitudinal and transverse propagation o Longitudinal – particle motion is along the axis of the
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This note was uploaded on 05/18/2011 for the course CSD 1025 taught by Professor Durrant during the Spring '11 term at Pittsburgh.

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Chapter 2 - Chapter 2 Acoustics Sound is a physical...

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