Austrian physicist active person adopting very quiet line (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

DOER (someone who gets things done = active person), containing (adopting) PP (pp = abbreviation for Italian pianissimo = musical notation for very quiet) + L (abbreviation for line). Austrian physicist Christian Doppler, who described the “Doppler effect” in which the observed frequency of a wave changes with the relative velocity of the source and the observer (for example the perceived change in pitch of an emergency vehicle’s siren as it drives past you).