Objective assessment of coloration in active acoustic systems: Real-room measurements with time-invariant filter equalization

Active acoustic systems are designed to enhance or modify a room’s natural acoustics, using a matrix-based processing of loudspeakers and microphones. Feedback loops in these systems cause a maximum amplification level, known as gain before instability (GBI). A well-adjusted gain limit is essential to avoid unwanted acoustic feedback. A safety margin of 2 dB or more is recommended, with some sources advising a margin of 5–7 dB for time-invariant systems. Even below the GBI, uneven energy distribution may cause unwanted coloration. State-of-the-art methods improve the GBI in active acoustic systems; however, time-variant filters, often used in these approaches, can introduce additional artifacts. Therefore, time-variant filters are excluded from consideration in this paper. In order to control room parameters like clarity, proximity, or lateral fraction, it might become necessary to deviate from an even energy distribution to achieve the desired results. This stability aspect has not been previously addressed and requires a separate examination. In this paper, a spectral-flatness-based method to measure the coloration of an active acoustic systemin an objective way is introduced. By applying this method across different system equalization stages, we provide a comparison of coloration effects, using measurements taken in real rooms with a functional active acoustic system.