Sonically Sculpting a Listening Room – Without TubeTraps

by Art Noxon

One time I was asked to dial in a huge HiFi demo room at a high-end audio show in Newport, CA without using TubeTraps. The room was a large conference room, with a 12’ ceiling and the width and length were both in the 50 to 60’ range. It was a huge room and it would take a truckload of TubeTraps to try to get this room to play. It was running big Magico speakers, Spectral electronics and MIT cables and that package deserved to sound great.

The only thing anyone could get their hands on was show curtains, not the lightweight ones but these were the more classy curtains, velour curtains hanging off of pipe stands 10’ tall. I could have as many of these pipe curtains as I wanted. And the pressure was on because the show was opening the next day.

I studied the curtains. I felt their weight, a reasonable weight, well over one pound per square yard of limp mass. I put the curtain up to my lips held it tight and blew thru a small area of curtain. It was not sealed air-tight, it leaked air under pressure, which also was good. Next I held some curtain material flat, up in front of me and repeated the same old words as always: Check, check, check, testing; one, two, three…. There was a light splash back into my face and ears. And so, here’s what I knew.

1. The curtain had enough weight or mass and it was not particularly acoustically transparent, therefore the curtain could load a sound wave, even in the bass. This means when bass sound wave impacted the curtain it would hold some of the bass pressure back. I could load, steer or channel a sound wave because the curtain could load a sound wave.
2. Because it was heavy enough to load even a bass wave and it also had some resistance to air flow, I figured that if I could get a pressure difference on either side of the curtain, air would flow through the flow resistive curtain and acoustic energy would be absorbed. I could turn the curtain into a bass trap.
3. Finally, the curtain reflected treble, which meant that the curves and folds of the curtain could act as a treble range diffuser.

To absorb bass I figured I would intercept an expanding wavefront with a boat-shaped section of curtain: pointed in the front, close to the speaker and spreading apart as the wave expanded away from the speaker into the room. This would keep a pressure gradient or difference along the hull of the boat, which causes air to flow through the resistance of the curtain and energy would be absorbed. I couldn’t absorb all the wave on the first pass and so what hit the walls and bounced off ran into another boat-shaped set of curtain walls.

The boat-shaped curtains was how I handled the horizontally expanding energy, 2/3rds of the expanding bass wavefront. Next I needed to get rid of the vertical standing wave problem. The speaker was well designed with multiple bass woofers. It put out a coherent bass wavefront horizontally but a broken or staccato wavefront vertically which minimizes the creation of strong tonal vertical standing waves, but still the energy does expand vertically. I also felt we should add some sense of a “room” up front near the speakers. They were built for rooms, not to be played out in the open.

I combined the loading property of the curtains and a recording studio designer trick to weaken the reflection at the ceiling above the speakers. The curtain was placed maybe 4’ behind the speakers, and out 6 to 8’ from the actual room wall. The top of the curtain did not reach the ceiling height of the room. What the curtain did was to load the bass wave expanding towards it, which meant that near the ceiling the loading effect of the curtain suddenly stopped short, leaving just plain air in the two foot space above the curtain. The bass wave felt the unloading and turned into the air space behind the curtain. The opening above the curtain became a vent for the bass wave. It bled pressure away from the ceiling reflection, turning it into a horizontal moving wavefront while weakening the strength of the ceiling bounce.

When people came into the room, they entered through curtain hallways into a curtain listening room that had unusually shaped walls. But the room sounded intimate and yet spacious. It had no small room signature or large room signature. It sounded stable and controlled or linear and at the same time very responsive to dynamics in the bottom end. Its frequency response was devoid of traditional room acoustic peaks and valleys. And in the treble, the room was full of ambience and spaciousness without any kind of room signature. Despite its unusually sculpted shape, people loved the sound.

I have to give special thanks to Maier Shadi, a dedicated audiophile entrepreneur, who trusted my vision, replicated it in every detail and sculpted this high performance sonic space in his big demo room at the Newport Audio Show in 2012.