ChatterBox! intercom system (more info):

Up to a 5-mile range on wireless systems.  Affordable in-car intercom setup.  The ChatterBox! line of wired and wireless intercom systems are a tremendous benefit to both driver and instructor/coach.  Most obviously, an intercom reduces fatigue and dramatically improves communications.  Have you ever misunderstood your student, or the your instructor?  Has the misunderstanding resulted in a challenging situation?  Here's the tool you need to all but eliminate communication issues in a loud environment.

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