Hearing Science

1Sound Propagation in Different EnvironmentsWhat is Sound?Free FieldSound FieldRoomsSound in Motion12What is Sound?Alternating condensation and rarefaction of molecules that are propagated across a medium.

23Properties of a Sound SourceMassElasticityE.g., tuning fork. As you vary the mass and elasticity you will change its natural frequency.All sound sources have mass & elasticity.

34Properties of a MediumMediums (e.g., air molecules) also have mass and stiffness.Medium is not displaced over an appreciable distance.Sound needs a medium, it cannot propagate in a vacuum.45Effect of Medium on Speed of Sound

56Inverse Square Law

67Inverse Square Lawhttp://hyperphysics.phy-astr.gsu.edu/hbase/acoustic/invsqs.html

78Inverse Square LawBottom Line:In a free field, the sound pressure will decrease 6.02 dB for every doubling of distance.In a sound field, the inverse square law doesnt hold because of reflection and diffraction of sound waves.89Free FieldAny acoustic field that is free of reflective surfaces.Anechoic Chamber

910Sound FieldAny field that has reflective surfaces.Virtually all acoustic fields are sound fields.1011Sound Fields

1112Sound Fields

1213Sound Fields

1314Sound Fields

1415Sound Fields

1516Constructive and Destructive InterferenceInvolves addition of two or more waveforms. Waves can be from different sources or one source where sound has been reflected.Constructive Interference - 0 to 120 degree phase differenceDestructive Interference - 120 to 180 degree phase differenceIf waves are 0 degrees phase and are of the same amplitude you will see a 6 dB increase in sound pressure.If waves are 180 degrees phase and are of the same amplitude you will have silence (Remember: silence 0 dB).

1617Constructive and Destructive InterferenceIn this example, the reflected (secondary) wave arrives out of phase from primary wave.

1718Standing WavesReflection of waveforms (i.e., standing waves) Results in areas of constructive and destructive interference, and occurs in a system that is enclosed at both ends e.g., ear canal with hearing aid or earphones, or sound treated room.

1819Standing Waves

1920Doppler EffectAs object approaches, the frequency is higher than expected. (e.g., 1000 Hz source might be perceived 1500 Hz). When object recedes, the frequency decreases.The effect is directly related to speed of the object. 2021Doppler Effect

2122Summary22