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Paul Faure

September 24, 2013 Psych 3A03: Audition Fourier Analysis Near Field versus Far Field - Too close: near field dominates - Free field: boundary of near field far field and where you start to get reverberation - Acoustic near field  Molecular displacements are greater than normal  Usually extends about 1-2 wavelengths from source - Acoustic far field  Pressure displacement travels as a wave away from source  Pressure dominates measurement - Cross in the graph indicates where near field because far field - Pressure more dominant in far field; distance more dominant in near field Excess Attenuation by the Atmosphere - Causes sound energy to dorp more quickly then otherwise expected from geometric spreading - Increases with temperature and square of frequency - Nonlinear with humidity - Stepper curve then spherical spreading is excess attenuation, gerater than 6dB - Attenuation increases with frequency - Larger absorption coefficient with increases in humidity - Peaks with humidity then non linear - Sound attenuates then would be expected from geometric spreading - Humidity shifts peak with higher frequency Deviations from the Inverse Square Law - Inverse square law holds true when sound waves propagate through a free, unbounded medium - A free, unbounded medium is a theoretical construct - In reality, transmission media have obstacles and other conditions that interfere with sound transmission - Under these conditions, sound may be:  Reflected – sounds encounter an obstacle and bounce back toward the source (echos)  Diffracted – sounds scatter about the obstacle  Absorbed – sound energy penetrates the obstacle – depends upon relative humidity and density of the medium which effects absorption  Refracted – conditions of the media cause the sound waves to bend and change direction (e.g. air to water, moving from one medium to another changes the speed of the sound) Acoustic Impedance (Z) - Can be thought of as the degree to which a medium is compressible (Zair water
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