CH16 Sound and Hearing
 

Standing sound waves and normal modes

Sound waves in a fluid may be described either in terms of the displacement or in terms of the pressure variation of the fluid.
In a standing wave, a pressure node is always a displacement antinode, and a pressure antinode is always a displacement node.
 
Sound wave resonance depends on the instrument
Open pipe: both ends are open
 
Sound wave resonance depends on the instrument
Stopped pipe: one end open and the other closed
 
Resonance and sound
Resonance occurs when a periodically varying force is applied to a system with many normal modes.
 
Wave interference ˇ­ destructive or constructive
Constructive interference occurs wherever the distances traveled by the two waves differ by a whole number of wavelengths, 0, , , , ˇ­
Destructive interference occurs wherever the distance differ by any half-integer number of wavelengths,, , , ˇ­
 
Slightly mismatched frequencies cause audible ˇ°beatsˇ±
 
Beats
Angular beat frequency:     
Beat frequency:      
Group velocity:      

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