Week 2 Sound Propagation

Sound Propagation

• To transmit through a medium/movement through a medium

• To travel through space or a material

What does the moving?

Air molecules and exchange of energy

How are waves of energy classified?

based on how the molecules displace compared to the direction of the movement of energy (transverse, longitudinal)

Wavelength

The distance it takes for one cycle to complete

Transverse Waves

Molecules displace up and down, Energy moves left to right, Perpendicular

Longitudinal Waves

Energy moves left to right, Molecules displace back and forth, parallel

How does Sound propagation occur?

Forces (F of inertia and F of elasticity) and energy (Kinetic Energy and Potential Energy)

F of inertia (Fi)

• Magnitude of applied force

• direction of applied force

F of elasticity (Fe)

• Dependent of stiffness

• Force of the object to return its origin

Kinetic Energy (KE)

• Work exerted as an object moves from rest to a certain velocity

• Dependent on acceleration

• Joule: Work = Fx d

Potential Energy (PE)

• Energy held by an object that is not in motion

• Dependent on the work put into the object

Compression/condensation

• increased pressure

• increased density

• decreased volume

Rarefaction

• decreased pressure

• decreased density

• increased volume

Wavelength Vs period

Length of one cycle in meters while the period is the length of time in seconds

Wavelength equation

Speed of sound/sec

Speed of sound in meters

343/sec

Speed of sound in cm

34,300/sec

Speed of sound in mm

343,000/sec

Frequency and wavelength relationship

Inverse relationship, when one goes up the other goes down

Impedance (Z)

the opposition to flow of energy

Why does impedance matter?

Sound dies down over time. When sounds encounter other objects/mediums they can 1) continue 2) be absorbed or 3) be reflected

Absorption

conversion of thermal energy

Reverberations

Multiple continuous reflections

Echoes

single distinct reflection

Inverse Square Law

S/4πr² = Intensity

Reverberation Time

Time it takes for a sound to fade to 60 dB after it is turned off. can be affected by room size, room additions like rugs, curtains, and furniture, people

Critical Distance

Distance at which the level of a direct (incident) sound meets the level of reverberant sounds

What makes up Impedance?

Resistance, Reactance, Resonant/Natural Frequency

Resistance and Damping

reducing vibrations

Determining the damping factor?

In(A1/A2)

Resonant Frequency

Natural Frequency an object will vibrate at based on mass and stiffness properties

Impedance spring velocity and force

Spring velocity (stiffness) leads force by 90 degrees,

Impedance mass velocity and force

lags force by 90 degrees

Impedance spring velocity (stiffness) and mass

are 180 degrees out of phase

Impedance Xm - Xs

Doesn’t contribute to impedance

R < 1

Small, Minimal damping, Takes longer for oscillations to stop

Large >1

High Damping, Quicker for oscillations

R = 0

No damping, Continuous sine wave

Critical Damping 

No oscillations