In this paper, we design, in a systematic way, an infinite-dimensional disturbance estimator by the active disturbance rejection control approach. The proposed disturbance estimator can be used to extract real signal from corrupted velocity signal. Its variant form can also be served as a tracking differentiator. The result is applied to stabilization for a multi-dimensional wave equation with position and corrupted velocity measurements. (C) 2015 Elsevier Ltd. All rights reserved.