MLS (Maximum Length Sequence)

A Maximum Length Sequence (MLS) is a deterministic pseudo-random binary signal used to measure impulse responses in room acoustics. The MLS method works by cross-correlating the known excitation signal with the measured room response to extract the impulse response. Advantages include high SNR through long averaging times and immunity to uncorrelated noise. MLS was widely used from the 1980s through the 2000s and is referenced in older versions of ISO 18233. However, MLS is sensitive to time-variance (moving objects, temperature changes) and non-linear distortion in the measurement chain, which can introduce artifacts into the extracted impulse response. For these reasons, the exponential swept sine technique has largely replaced MLS in modern practice. MLS remains useful in situations requiring very fast measurement times or in systems where harmonic distortion needs to be included in the result.