Lectures on sound - Introduction to Sound Waves • Sound...

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Unformatted text preview: Introduction to Sound Waves • Sound waves are waves of density and pressure propagating in a medium • Sound waves are mechanical waves • Sound waves are longitudinal waves • They travel through any material medium Notes • The speed of the wave depends on the properties of the medium • The mathematical description of sinusoidal sound waves is very similar to sinusoidal waves on a string Categories of Sound Waves • There are three categories of sound waves: infrasonic, audible and ultrasonic • There is no real qualitative difference between these three categories, they just designate three wave frequency ranges • Audible waves are within the sensitivity of the human ear ange is approximately 0 Hz to 20 kHz Notes – Range is approximately 20 Hz to 20 kHz • Infrasonic waves have frequencies below the audible range • Ultrasonic waves have frequencies above the audible range (dogs can hear some ultrasonic waves) Speed of Sound Waves • Let us determine the speed of sound waves • Consider gas in a cylinder with a piston • Before the piston is moved, the gas has uniform density (equilibrium situation) Notes • When the piston is suddenly moved to the right, the gas just in front of it is compressed – Darker region in the diagram – Both pressure and density of gas near the piston increase Speed of Sound Waves, cont • When the piston comes to rest, the compression region of the gas continues to move – This corresponds to a longitudinal pulse traveling through the tube with speed v – The speed of the pulse is typically much faster than the speed of the piston – The speed of the pulse is much slower than e speed of the molecules of gas Notes the speed of the molecules of gas . – This means that the pulse is not a group of molecules of gas flying from one end of the cylinder to the other ! – The pulse propagates due to intermolecular collisions and associated energy transfer Bulk Modulus • The speed of sound depends on the bulk modulus B of the propagation medium • Bulk modulus B characterizes the degree of compression of an object when uniform pressure is applied perpendicularly over the entire surface of the object his type of compression is typically Notes • This type of compression is typically experienced by objects submerged in fluids (if the object size is small compared to its depth in the fluid) • The object will undergo a change in volume , but not in shape Bulk Modulus • The relative change of the volume of the object in response to the uniform pressure is called volume strain : Δ θ Δ θ Δ θ Δ θ = Δ V/V • Bulk modulus B is negative of the ratio of pressure variation Δ P to volume strain Δ θ Δ θ Δ θ Δ θ B = - Δ P/ Δ θ Notes • Bulk modulus is independent on the shape of the object and on the pressure (for low enough pressure) • The equation above is analogous to k = - Δ F/ Δ x for a spring • Hard materials such as diamond have large bulk modulus The inverse of bulk modulus is called compressibility Speed of Sound in Liquid or Gas...
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This note was uploaded on 11/22/2010 for the course PHYSICS Physics 7C taught by Professor Ilyakrivorotov during the Winter '10 term at UC Irvine.

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Lectures on sound - Introduction to Sound Waves • Sound...

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