The attenuation coefficientaof longitudinal ultrasonic waves propagating in the 100 direction of an NH4Cl single crystal has been measured as a function of temperature at 1270, 1500, 1700, and 2656 bar. The elastic constantc11corresponding to this wave has also been measured at 1500 and 2656 bar. By combining these data with a variety to other ultrasonic and hypersonic information about NH4Cl it is possible to determine the dynamical behavior near the orderhyphen;disorder transition in terms of the variation in the relaxation strength and the relaxation time. At a given value of the reduced temperaturetequals;verbar;Tminus;Tlgr;verbar;sol;Tlgr;the acoustic velocity and attenuation in the ordered phase are almost identical at all pressures, but the behavior is different in the disordered phase at 1 atm and at the ``tricritical'' pressure of sim; 1500 bar. In particular, the relaxation rate 1sol;tgr; (in units of 108secminus;1) is 414t0.60at 1 atm and is123thinsp;t0.88at 1500 bar.
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