• Frequency: number of cycles in unit of time.
• The unit  is hertz (Hz), 
• Frequency has an inverse relationship to  
  wavelength. 
• Main determinant factor of axial resolution
  of an image.
• Main determinant factor of attenuation.
  Attenuation coefficiency are equal to half
  of operating frequency.
• The propagation speed  of sound wave
  in a medium has nothing to do with
  frequency.
• Active element in a transducer produce a
  pulse with a series of frequencies. The range
  between lowest and highest are called
  bandwidth.
• The frequency of a transducer can not be
  changed by operator.

For more info, click frequency