Impulse noise is a short duration transient acoustic event characterized by a sudden rise or spike in sound pressure followed by a uniform or oscillatory decay (depends on type of source equipment) lasting less than ½ second.  Impulse noise usually exhibits a distinct spectral signature across the frequency range without the presence of discrete tones.  Examples of impulse noise include gunshots, pulse cleaning systems, punch presses, etc.

      At birth, the audible frequency range is 20 Hz to 20,000 Hz.  Generally speaking the average audible range in humans is from 30 Hz to 17,000 Hz.  Sound pressure wave forms below and above this range are described as infrasonic and ultrasonic.  Infrasonic sound is experienced as a flutter while ultrasonic sound produces no sensation of hearing.

What Is Diffraction?

      Diffraction of sound is “bending” of the pressure wave around objects, obstacles and walls.  Diffraction is greatest with low frequency sound or where the wavelength is large compared to the object it strikes.  As illustrated above, diffraction of sound results in a less pronounced acoustic shadow zone.