  # Sound

## Look at This:Watch the development of shockwave from an F-14 jet.

 Q:    In some movies scenes involving explosives actors are asked to keep their mouth open and warned that if they don't their eardrums could implode.  Why would a blast from explosives require the actors to keep their mouths open? A:    When the explosion goes off there is a pressure wave caused by the rapid burring of materials and thus the rapid change in volume of the explosive.  If the actor's mouth is closed, air at the pre-explosion pressure is trapped in the actor's sinus area.  The shock wave causes an area of high pressure to form around the actor.  When the actor's mouth is closed, the difference of pressure between the sinus area and the outside air can cause the ear drums to burst inward.  However, if the actor's mouth is open, the pressure wave will be the same both inside and outside the actor's mouth and the actor should be safe.  This same effect happens (although with less severity) when you go diving or when you go from high to low altitudes.

## References

### Equations

Speed Waves
 Speed of Sound Longitudinal Displacement of a mass s = sm cos (kx - t) Pressure change p = pm sin (kx - t) Pressure amplitude pm =(v  ) sm
Interference = 2 ( L / )
 Condition Corresponds to Constructive interference = m 2 m = 0, 1, 2, ... L = m m = 0, 1, 2, ... Destructive Interference = (m + ) 2 m = 0, 1, 2, ... L = (m + ) m = 0, 1, 2, ...
Sound Intensity
 Insensate I = P/A Intensity related to displacement amplitude (sm) Intensity at a distance r from a sound emitting point source Sound Level in Decibels = (10 dB) log ( I / I0 ) where I0 = 10-12 W/m2

Standing Wave Pattern in Pipes
 Pipe, two open ends Pipe, one open end Beats

f beat = f1 - f2

The Doppler Effect Shock Wave
 Mach cone angle sin = v / vs

# Mechanics List of Topics

 Measurements Newton's Laws Potential Energy and Conservation of Energy Rotation of Rigid Bodies Elasticity Vectors Forces and Fields Linear Momentum Angular Momentum Mechanical Oscillations Motion of Point-Mass Objects in One Dimension The Gravitational Field Collisions Torque Mechanical Waves Motion of Point-Mass Objects in Two and Three Dimensions Kinetic Energy and Work Circular Motion of Point-Mass Objects Equilibrium Sound  ครั้งที่

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