We present { NH _ { 3 } } observations of the B5 region in Perseus obtained with the Green Bank Telescope ( GBT ) . The map covers a region large enough ( \sim 11′ \times 14′ ) that it contains the entire dense core observed in previous dust continuum surveys . The dense gas traced by { NH _ { 3 } } ( 1,1 ) covers a much larger area than the dust continuum features found in bolometer observations . The velocity dispersion in the central region of the core is small , presenting subsonic non-thermal motions which are independent of scale . However , it is thanks to the coverage and high sensitivity of the observations that we present the detection , for the first time , of the transition between the coherent core and the dense but more turbulent gas surrounding it . This transition is sharp , increasing the velocity dispersion by a factor of 2 in less than 0.04 pc ( the 31 \arcsec beam size at the distance of Perseus , \sim 250 pc ) . The change in velocity dispersion at the transition is \approx 3 ~ { } { km s ^ { -1 } pc ^ { -1 } } . The existence of the transition provides a natural definition of dense core : the region with nearly-constant subsonic non-thermal velocity dispersion . From the analysis presented here we can not confirm nor rule out a corresponding sharp density transition .