On Reverberation Enhancement in Auditoria

Doktorsavhandling, 1994

The analysis of electroacoustic systems for reverberation enhancement in auditoria is the topic of this thesis. Such systems have been developed since the 50's and been commercially available since then as well. During the 80's several new system ideas have been presented but there are still some open questions that are addressed here. The development of assessment methods for rooms with such active systems is of primary importance. Active systems can generate unnatural sound in terms of factors that rarely occur in "passive" rooms, e. g. pronounced coloration, modulation, distorted spatial distribution, aural/visual incompatibility, and noise problems. The study of ordinary acoustic parameters is necessary but not sufficient in such situations. Subjective testing, based on binaural recordings and computer simulations, should thus be used. The binaural recording and reproduction technique has some limitations, however, when it comes to the perception of spatial factors. It can be improved upon by extracting the electroacoustic system's own impulse response from measurements of a hall's impulse response with and without the system. Without the masking of the natural sound field in the room, the electroacoustic sound field is emphasized. Methods are presented for computer simulations of multi-channel feedback systems with complex electronic signal processing, based on frequency-domain analyses. For time-invariant filters, a method is used that gives the total impulse response explicitly. Systems employing linear, time-varying (LTV) filters can be studied as well with an iterative method. With both methods auralization, acoustic rendering, is possible for subjective assessment. For less detailed analyses, classical diffuse field relations are refined to demonstrate electroacoustic system's capabilities to change the early part of a room's impulse response to any significant extent. The performance of some basic LTV filters for feedback control has been investigated in an experimental four-channel system. Objective and subjective assessment demonstrated that, when close to instability, one has to choose between coloration (without LTV filters) and modulation artifacts (with LTV filters). Both can be kept low by using phase or delay modulation, with slow modulation frequencies, around 1 Hz for reverberation times around 1.5 s. It is argued that delay modulation is optimal above 500 - 700 Hz, whereas other methods should be used below.