Measurements on time varying systems with the MLS technique

The MLS technique is widely used for measurement in architectural acoustics and in audio. The MLS technique requires that the system under test is time-invariant, and it has been known for years that small changes are a source of error in such measurements, in particular if averaging is used to increase the signal-to-noise ratio. Time variance can, for example, be caused by a gradually changing temperature or motion of the air in a room.
The effect of time-variance on MLS measurements has been studied through computer simulations and experiments. A new indicator analogous to the coherence function in measurements with dual channel FFT analysers has been suggested and examined experimentally in a reverberation room.