These words all refer to the absence of visible tension or excitement in persons when such reactions might well be expected by the circumstances. Imperturbable carries the sense of self-control based on temperament or discipline; it is perhaps most often used of a diplomatic or stately detachment and suggests more of a constitutional inability than a conscious refusal to panic: The imperturbable Treasurer responded without asperity to the severe, pointed questioning of the Parliamentary committee members. When applied to attitudes or conduct rather than people, imperturbable means unshakable, and suggests a strong, almost irrational devotion to a particular point of view: an imperturbable optimism that was not to be dampened by misfortune, however great.

Cool and dispassionate suggest a deliberate stifling of emotions in the face of disturbing influences. Where dispassionate emphasizes detachment and disinterestedness, cool points to resistance to excitability: to keep cool even under battle conditions; a dispassionate surgeon; a dispassionate appraisal of the state of our military preparedness.

Calm and collected are often paired in a phrase to suggest the complete intactness of mental resources in the face of difficulty. Calm stresses a quiet approach to a problem, devoid of hysterical actions or utterances, and collected stresses the application of appropriate mental or physical effort to the solution of the problem: to remain calm and collected in the midst of a noisy demonstration.

Composed and sober , in this context, suggest dignified demeanour and conduct in the midst of confusion. Sober suggests a reasoned, quiet and sometimes cautious attitude; composed indicates an even-tempered, self-controlled attitude.

• The speaker remained composed despite the audience’s loud heckling; His sober approach to the crisis averted a catastrophe.