Wall Reflections
But what happens to this hifi footprint is that it gets set up inside a listening room. Surrounded by a floor, ceiling, two sidewalls, a front wall located behind the speakers and a rear wall behind the listener. These rooms are about 16’ wide and 21’ long, sometimes larger or smaller but the width to length ratio of about 3 : 4 is retained.
Speakers are about 4’ off the side wall and 5’ off the front wall. The side and front wall speaker setbacks are never equal, always offset. The distance between listener and rear wall is 6 to 7’.
Clearly, some of the early off-axis sound paths will bounce off some spots on the walls, floor and ceiling in the direction of the listener. These are early reflections and they mess with the delicate loudness and delay signal adjustments used to create and separate images on the sound stage. Room reflections tend to ruin the sound stage.
The first of the off-axis signals that reflect off various surfaces of the room directly back towards the listener are “early-reflections”. These early reflected signals mix with and blend into the direct signal to create a composite new signal which still sounds like the music in the direct signal, except they change the sound stage.
The early reflections are not heard as separate reflections, like echoes are. Echoes arrive 40 ms or more following the direct signal. Early reflections are those off-axis signal paths that arrive within 25 to 30 ms following the arrival of the direct signal.
One of the earliest reflections bounce off the ceiling and floor. They follow the arrival of the direct signal by about 4ms. Usually, hifi setups include sound absorbing (Persian) carpet and the floor bounce is so weak it isn’t perceived but the ceiling bounce is and it mixes with the direct to change the vertical part of the sound stage.
The ceiling bounce raises the sound stage but otherwise does not change its overall wrap around shape. Most hifi listeners don’t mind the raising of the sound stage, it seems natural, more authentic than “seeing” the image of a sound stage that lies at ear level straight ahead.
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