Siegfried Linkwitz, AES San Francisco May 6th 2008
Sixty five members and guests were fortunate to hear Mr. Siegfried Linkwitz speak at the Ex'pression College for Digital Arts, Emeryville.
Mr Linkwitz started by explaining that 'accuracy' refers to perceptional accuracy not any particular measurement. A comparison was made between visual perception and audible perception. One eye conveys as much information as two when studying a photograph of a 3 dimensional scene. The meaning of the photograph develops in the viewer's mind. Similarly, perception of sound is a complex ear/brain process. By the time sound waves reach the ear the wave patterns from the individual sources have been summed. The brain has to analyze the sounds to understand what happened and extract distinct sources again.
This ear/brain processor is still being studied. For example, we can extend our 'acoustic horizon' by focussing on individual aspects of the total sound field. Guenther Theile's Association Model of perception was mentioned. The brain can inverse filter so turning the head ninety degrees the sound is perceived as identical although it must have greatly different spectral characteristics as one's head is now in the way.
Accurate sound reproduction from two channels is about creating a believable illusion of the original auditory scene. In order to achieve this and reinforce the illusion of 'being there', we must minimize misleading auditory cues; otherwise the brain has to work too hard. These misleading cues can come from the loudspeakers themselves and from the interaction with the room
Examples of misleading cues from the room in reverse order of importance would be: low frequency modes and resonances, the reverberant characteristic, acoustic reflections and their spatial symmetry.
Examples of loudspeaker misleading cues, in order of importance are: On-axis frequency response, frequency dependant directivity, resonances, non linear distortion and cabinet diffraction.
Mr. Linkwitz went on to describe a surprising result where two very different speaker designs, one a monopole, the other a dipole. Both measured flat on-axis in free-field but very differently in the room. They happened to sound almost identical. He postulated that in both cases the many room reflections were delayed and attenuated copies of the direct sound that travels from each loudspeaker to the listener. In such cases the brain is able to perceptually separate the listening room from the direct sound. This allows the creation of an illusion of 'being there', at the recording venue.
