9th December 2003 - The Loudspeaker-Room Interface - Controlling Excitation of Room Modes

The lecture started with Rhonda introducing the aim of the paper: to identify the strongest room modes excited by each loudspeaker, and to pre-filter the signal fed to each loudspeaker in such a way that the adverse effects of dominant room modes are significantly reduced. Such a system must be simple to set up and must be spatially robust. Improvements at one listening position should create improvements, and not degradations, at other listening positions.

Typically customer room specifications constitute small room acoustics. The reverberant field under these circumstances is very important. Various methods of correction were discussed. Methods to avoid are full bandwidth magnitude response equalisastion and full bandwidth impulse response equalisation. These methods are very sensitive to position effects. Also audio delay may be necessary, which is not appropriate for DVD video playback.

Modal ranges cover 20Hz to 200Hz. Room effects include emphasis of some frequencies, ringing at resonant frequencies, pitch changes during decay, beat and echo effects. The room resonant modes of rigid walled rooms of certain dimensions can be calculated. Usually, both fundamental and harmonic modes are set up. Non-rigid walls causes the modes to decay exponentially. Plots showing the decay of room resonances, beat frequency effects, and echo effects were shown.The magnitude of a room mode varies with listening position, however the decay time of a room mode is the same at different listening positions. Also, there is no requirement to use a special microphone. Hence there is a focus on decay time.

The filter to control decay time uses original Q, target Q and centre frequency. The resulting notch filter has a gain and bandwidth which can be calculated based on target decay time and room mode decay time. Plots of level against delay time after correction showed some sensitivity to pole frequency errors. 32-bit coefficients and double precision implementations are also necessary to avoid errors. It is not desired to create an anechoic environment. Previous work has measured the reverberation time of living rooms. Plots of reverberation time against frequency showed reverberation times of 0.4 to 1 seconds, quite long compared to studios.

Some work was done to make sure that the direct response has not been corrupted too much. Plots showed that the direct response changes phase when the exciting signal is switched off. Music is much more complex than tones. All pre-filtering is only done below 250Hz. The trade-off is to pre-filter enough to reduce the reverberation without affecting the direct response too much. Notches up to 6dB deep are OK, larger is more problematic.

Michael then continued the presentation. A standard process is used to characterise the decay time in the range 500Hz to 2kHz and then to set a target decay time for modes below 250Hz which increases as frequency decreases. A third octave bandpass filter is used for measuring the decay time, followed by Schroeder integration. A more accurate process is required to determine decay of individual room modes. Waterfall FFT plots give a better resolution. The Schroeder integration is prone to noise, so special attention is given to starting from the peak impulse and ending just above the noise floor. Example waterfall plots were presented. Least squares regression is used to give the optimum decay time.

To identify the dominant room modes, all the peaks in the magnitude response are measured. The decay time at each room mode is found, and then the dominant modes are identified using the decay time. A more sophisticated method is to calculate many decay profiles from a waterfall plot. All the decay profiles are then summed and the largest peaks represent the dominant modes. Example plots clearly showed the dominant modes. The system implemented includes a microphone or SPL meter in the centre of the room, a PC and a surround decoder. An MLS test signal is used for each loudspeaker, from which the impulse response for each loudspeaker is calculated. An FFT and decay time analysis is performed, followed by the design of the filters for each loudspeaker.

Meridian has developed PC software driven by a user friendly wizard to perform the measurements and correction. An example room was discussed. Magnitude responses for different listening positions were shown before correction. Post-correction magnitude and decay responses showed clear improvement. The filters generated for one listening position improved the responses for many listening positions.

In conclusion, a spatially robust filter design technique has been developed which uses only one microphone position, but controls the decay time at multiple positions. The filter design is automated and does not require specialist knowledge. Initial feedback from dealers and customers has been positive.

A lively question and answer session followed. The lecture concluded with a hearty round of applause.

More information is available in the paper: The Loudspeaker-Room Interface - Controlling Excitation of Room Modes, Rhonda J. Wilson, Michael D. Capp, and J. Robert Stuart, Meridian Audio, AES 23rd Int'l Conference, Copenhagen, 2003.

Steven Harris