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Stage depth is created by adding treble range early reflections off the front wall.  Speaker boxes with rounder corners, no speaker grill frame and that get narrow towards the top are particularly suited for illuminating the front wall with full range treble.

Note that bass waves also reflect off the front wall and this can cause a “phase add and cancel” effect that colors, that EQ’s the front wall reflection. This is why TubeTraps are used in this position. It positions a bass trap where the bass wave hits the front wall which greatly reduces the phase add/cancel effect while providing a curved treble range reflector to help scatter the front wall reflection.

The wall spaces between the TubeTraps now become strip reflectors which produce a natural diffractive diffusion effect so as to soften the reflection strength. The direct signal fuses with the early reflections off the front wall to create a stage depth effect that can literally locate the apparent sound stage on the other side of the front wall. Notice the different delay paths compared to the single bounce off a flat front wall. Speaker illumiates all the poly reflectors inside of TubeTraps, lots of different time delays. The diffractive diffusion reflections occur behind the speaker. There are many time delayed reflections off the front wall.

Notice the different delay paths compared to the single bounce off a flat front wall.  The speaker illuminates all the poly reflectors inside of TubeTraps with lots of different time delays.  The diffractive diffusion reflections occur behind the speaker.  There are many time delayed reflections off the front wall.

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Dead Front Wall

HiFi show rooms often have a curtain on the front wall which can be drawn over the stage depth traps, eliminate the front wall reflection and place the image between the speakers. But there is more we can do.

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Acoustic Zoom Lens

Early in the days of hifi, and surely still today audio nuts gather together to listen to each other’s sound systems.  It was during one of these gatherings where the front wall was setup to bring out the stage depth effect.  16” TubeTraps were located in each of the front corners and in the middle of the front wall. Smaller traps were spaced between these 3 big traps to develop the diffractive diffusion effect of wall strip reflections and minimize low end coloration reflections.

The center trap was always referred to as the image trap. It seemed to illuminate the central stage position which is usually where the lead singer or instrument is located on stage. Reverse the trap position, rotate the reflector to the wall and the center stage images darken down a bit so they are illuminated equally as elsewhere.

Someone suggested that if instead of rotating the reflector toward the wall we could slide the center image trap a little forward, it might brighten up the center stage a bit. Instead of brightening the center stage image, the center stage image took a step forward, moving closer to the listener.

Naturally, the center trap was slid a little further forward and the image accordingly moved forward some more. This zoom lens effect continued as the center trap was moved up through the plane of the speakers. The center image was floating somewhere between the listener and the plane of the speakers.

This was an astonishing effect we were witnessing. As the center trap was moved forward, ahead of the plane of the speaker, the center image kept moving forward but at the same time faded and died out. The center reflector was no longer reflecting the wave from the speaker towards the listener.

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Later we made a custom image zoom trap with 2 reflectors, one on each side, so the reflectors always faced the speakers and tried the zoom lens again. It worked perfectly behind and in front of the plane of the speakers. And we retried the experiment. It worked as before and it seemed that when the zoom image trap was about half-way between the plane of the speakers and the listener, the acoustic image seemed to be right in front of the listener. We were zooming Acid Queen by Tina Turner.

About halfway between the plane of the speakers and the listening position the image had grown so big and moved so close it was literally in our lap, she was screaming singing right into our face. It was as if we had become the microphone that she was screaming singing into.

Pure sonic imaging, no drugs, alcohol or mind altering anything — just sound. It was so intense it became an unbearable experience for any length of time. We’d jump out of the listening chain, taking turns standing the intensity of it as long as possible, usually not more than 10 seconds. We just had to slide that double sided image zoom trap back to the front wall to calm things back down and get back to listening to music.