Lexicon 960L owner manual Using The Reverb Program Lexicon, Inc

Using The Reverb Program Lexicon, Inc.

Reverberation and Reality, Continued

phrases. Reflections alter our sense of direction, distance, and hall shape in ways that depend on the duration and complexity of the notes, and on the properties of the spaces between notes.

But human perception is more complex than just direction, distance, and room shape. We are reacting to the music on quite a different plane. Directional localization is not an object in itself – it is a tool that helps us separate one musical line from another, and thus helps us grasp the intellectual and emotional content of the music. The sense of space is also not particularly interesting in itself. It is the emotional effect of the space that makes the perception worth the trouble. It is hard to separate the awe inspired by the sound of a Gothic cathedral from Gregorian chant – chant was developed to work alongside this awe, and to enhance it. Listen to the cellos and basses play a pizzicato bass line in a dead opera house, and feel how

each note falls lifeless. Add an enveloping reverberation and each note takes a life of its own – swirling around us as the notes decay and pull on our heart with sadness or joy. Are we aware of these effects? A trained listener can be, but most listeners can only describe the emotional impact. This impact can be very real.

The sense of distance has a similar emotional effect. When the direct sound is too strong voices and instruments seem to be stuck to the speaker – up front and in-your-face. This perception is particularly strong when a engineer tries a 5.1 mix for the first time. Put the vocals in the center speaker and Bam! they whack you in the face. The up-front perception is psychologically important. But you want to use it with great caution. Sometimes it is just the effect you want, but after a sustained period the listener can tire, and psychologically back away. This is probably one of the reasons many engineers raised on two channel stereo prefer a phantom center to a hard center. The phantom may be just as up-front, but there is no speaker in that position, so it seems more acceptable.

There is another solution. Humans perceive distance in large part through the strength and time behavior of reflected sound. In a typical room the direct sound arrives uncorrupted by reflections. If the sound has a fast rise-time the ear is able to determine the direction of the sound by using the interaural time delay and the interaural level differences as cues. After 10ms or more reflections start to arrive – contaminating the time and level differences and making localization difficult. Our brains are able to use this reduction in localizability of the sound as a distance cue – the degree to which

precise localization is reduced after the onset of a note is a cue to both the size and the furnishings in a room.

Reflections from almost any direction EXCEPT the direction of the sound source can create this sense of distance, and they can come at a great variety of times after the direct sound. However, there are optimal directions and optimal times. The optimal direction is frequency dependent. For frequencies below 700Hz the optimal direction is from the side (90 degrees from the front). At about 1500Hz the optimal direction is +/- 30 degrees from the front (the standard front speaker angle.) Reflections that arrive earlier than about 15ms from the direct sound begin to interfere with localization, and can cause comb filtering and timber alteration. Reflections that arrive more than 50ms after the direct sound can be heard as separate sound events, and can cause serious problems with intelligibility.

Thus if we add reflections to the left and right front speakers and the left and right rear speakers, and adjust the time delay so these reflections occur between 15ms and 50ms, we can move the vocals out of our center speaker. The vocals are pushed back into a space behind the front speaker array. It seems magical – we are increasing the perceived distance to a source in the center speaker by adding reflections to all the other speakers EXCEPT the center speaker – yet this is the result of some very simple psychophysics.

Remember that we are not perceiving the individual reflections separately – we are only perceiving their ability to mess up the localization of the direct sound. Because there is no actual perception of the reflections themselves the distance perception (perhaps the "room" perception would be a better description) is bound to the direction of the direct sound. If the direct sound comes from the center speaker, the "room" impression seems concentrated in the front of the listening space, even though the reflections that produce it are coming equally from the front and the rear. Switch the direct sound to the left rear, and the "room" impression also switches to the left rear, even without altering the reflection patterns at all.

Thus for producing the perception of "room" or "depth" it is not necessary to have reflection patterns that mimic a particular source position in a particular room. It is only necessary that the reflections be primarily in loudspeakers NOT in the same direction as the sound source, and that the time delays of the reflections in each loudspeaker should be different. This makes our life much simpler – we can use a two channel echo

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