  We present a detection scheme for diffusing wave spectroscopy (DWS) based on
a two cell geometry that allows efficient ensemble averaging. This is achieved
by putting a fast rotating diffuser in the optical path between laser and
sample. We show that the recorded (multi-speckle) correlation echoes provide an
ensemble averaged signal that does not require additional time averaging. We
find the performance of our experimental scheme comparable or even superior to
camera based multi-speckle techniques that rely on direct spatial averaging.
Furthermore, combined with traditional two-cell DWS, the full intensity
autocorrelation function can be measured with a single experimental setup
covering more than 10 decades in correlation time.
