To establish a reference baseline for the following discussion of Hi-fi listening, it is helpful to skim an article published in Stereophile:  The View into the Soundstage by John Atkinson

It is a commentary about listening to audio equipment and the perception of what is known as a HiFi soundstage. It confines itself to the recording technique, the playback equipment and the effect created as the sound signals from the left and right speakers are received by the listeners left and right ears, otherwise known as the “direct signal”. Although instructive, it is a foreshortened presentation of the actual HiFi listening experience because it excludes the inescapable: Room reflections and the influence they have on the listening experience as they mix with the direct signal.

Introduction

When one listens to a set of loudspeakers, over 99% of the sound wave emitted from the speaker expand in all directions other than toward the listener and is called “off-axis” sound. There is a very small portion of sound that does pass by the listener and it is called the “direct signal”. One might wonder what the direct signal sounds like. It’s a similar sound to what we hear with headphones, except the sound comes from the speakers in front of us, while headphone sound comes from either side of our head.

The direct signal is what we would hear if we could play the sound system in an area where there are no reflections. Set up the hifi system outside on a huge lawn area far from any buildings and we’d have a reflection free playback experience. More formally we might rent an anechoic chamber, an acoustic test room that is 100% sound absorbing. But in the real world, a reflection free environment does not exist.

In a hifi listening room, the other 99% of the sound, the off-axis part of the sound, can’t escape, it is reflected back into the room over and over again by the walls, floor and ceiling. It is essentially captured by the room and remains captured by bouncing around the room until it dies out.

With continuous music, there exists an ongoing plethora of reflections being created, replacing those which have died out, in the listening room, many of which cross and recross the listener’s ears which augments our perception of the direct signal. The effect of the reflections in the listening room is so powerful compared to the direct signal that the listening room (not the speakers or the direct signal) is referred to as the “last link in the audio chain”.

The listening room is where the direct signal is acoustically mixed with an ever-changing variety of previously created off-axis reflections. It is this sonic assembly of sounds built out of acoustic space and time that is impinging on the listener’s head that produce the gestalt of the stereophonic hifi listening experience. The last link in the audio chain is no small and simple thing.

The Listening Room Effect

In the simple view of audio playback, the speaker/listener setup and resulting soundstage exists independent of the listening room. This presumption leads to misunderstandings in discussions with seasoned audiophiles, who have well developed high power listening rooms, which include considerations for sound reflection containment and management:

Room Gain

To the right is shown an impulse response, sound level dB vs time in milliseconds. If the “direct sound” is a distinct quick snap then all the reflections; “early” and “late” reflections are also distinct quick snaps, as shown on the right. The “Room Effect” or Room Gain is the sum of all reflections which is usually 5 to 10 dB louder than the “Direct Sound” alone. “Room Gain” which is what makes sound seem louder inside a room compared to when setup outdoors. In a typical room, under steady state conditions, a speaker plays 10 dB louder than if set up outside. Since the reflections (room gain) in the room are significantly louder than the direct signal, the audiophile has acclimated to a different concept. they don’t imagine that they are really listening to the gear and the direct signal, they have accepted that they are, for the most part, listening to the room as it is being played by the speakers. In other words, the room is essentially an acoustic instrument that is being played, plucked and bowed, struck and blown by the loudspeakers.

Although each reflection is less loud than the direct, the sheer number of them add up to deliver 5 to 10dB more acoustic energy to the ear-brain system than the direct signal alone. Again, the reflections of the room overpower the strength of the direct signal and literally change what the direct signal sounds like.

Sound Fusion

In addition to the steady state condition, where the sound being heard is 10 dB louder than the sound from the speakers is heard, we have the image shift version of room gain, called the sound fusion or Haas effect. This is where all early reflections that sound just like the direct signal are fused into and become one with the direct signal.

This is not a consequence of room reflections per se, it is a consequence of the reaction time in the process of hearing. Similar to sight, variations in the sound that is heard within the sonic reaction time of 30ms are perceived as one sound. There are dozens of early or primary reflections within the first 30ms following the arrival of the direct signal.

Music Rooms

Some rooms are designed and arranged to support the playing of a particular musical instrument or ensemble in the room. The music room is voiced to support the performance of the instrument. The nature of the flooring, the size and shape of the room, its features, sound diffusing columns, partially draped and partially reflective windows, high coffered and corniced ceilings combine to provide an ambience that compliments the music and the instrument being played.

The music room is “voiced” to compliment the instrument. By this is meant that early reflections are considered and adjusted as they join with and help to define the sound of the instrument. Late reflections, also known as stage shell sound, compete with the direct and early reflections introducing echoes and coloration of the music. Lastly, there is the reverberation, the ambience that seems to fill the room and provide a background glow that is ever-changing with the music.

Audiophile Music Rooms

Like an instrumental music room, the HiFi listening room is voiced for the sound producing instruments, the loudspeakers and type of music being played in the room. Here is where the art of sculpting the enveloping acoustics of the room is introduced. The last link in the audio chain is inescapably the acoustic compliment brought to the music by the listening room.

1. Speaker position, directivity and off-axis reflection control
2. Redistribution of early, late, echo and parallel surface paired reflections
3. Cultivation of reflection density and direction toward enhancing ambience
4. Development of direction, timing and strength of spaciousness reflections
5. Room resonance control, minimize speaker/mode coupling and add mode damping
6. Stage acoustics, controlled reflection development in the front 1/3rd of the listening room
7. Venue acoustics, controlled reflections development in the back 2/3rds of the listening room

There are 3 levels in the evolution of upgrading the audiophile listening room. These upgrades are developed by judicious placing of TubeTraps along the walls and corners of the room. The TubeTrap is an acoustic product specifically developed for the voicing voice or musical instrument recording and playback rooms.

Music

The first level of “dialing in a room” improves the musicality of the playback system. By reducing the coupling between the speakers and the natural acoustics of the listening room, the musicality of the sound system is greatly improved.

In level one the TubeTraps are placed in the 4 corners of the room. Bass collects in the corners of the room and adding bass traps to the corners help control the buildup of unwanted bass reverberation, tightens the bass line providing it with more musical dynamics. Room modes are rooted in the corners of the room and are effectively controlled with corner bass traps.

Soundstage

The second level of listening room development unveils a sound stage in the front third of the room, in and around the speakers. Curiously, the loudspeakers seem to sonically disappear, replaced by the sound stage. The speakers become silent sentinels that somehow are secretly weaving wonderous fabrics of sonic imagery throughout the front of the room.

The sound stage has width, depth and height punctuated with a distribution of distinct separate sound sources, typically musical instruments, positioned at various locations across the front of the room. The sound stage is an illuminated and active participant in the music. It appears to actually be the source from which the music is emanating. But the remaining two thirds of the room, where the listener is located remains quiet and unassuming, it’s just the back of the listening room.

At level two, TubeTraps added across the front wall and along the two front side walls. The diffusing panels along the front wall are set facing into the room but along the two front side walls they tend to be facing the front wall, essentially helping to keep the ambience localized in the front of the room.

Venue

The third level of listening room development introduces the concept and experience of venue into the audio performance of the listening room. The venue effect is located in the back two thirds of the listening room, the part of the room that surrounds the listener. It immerses the listener into the internal volume of the performance hall, or more specifically the acoustic space in which the performance on the sound stage is taking place. With venue development, the sound stage is no longer viewed from a seat at the back of a non-descript room. Instead, the listener is engulphed in ambience and spatial detailing of the environment in which the sound stage is performing.

This third level of listening room development transforms viewing the soundstage into an unforgettable immersive experience. This level of listening is often referred to as the magic carpet ride, silently powered by the sound machine through the clouds of sound in the sky. A literal sonic time machine, that leaves the listening room behind to set the listener back down in a new, wholly engaging sonic venue, a manifestation of the original venue. As the performance ends, you are gently returned into the familiar silence of your own listening room. A thoroughly transformative experience you know you can never forget, incomparable where all other rooms and systems pale.

At level three, TubeTraps continue to be distributed along the middle and rear side walls and across the back wall, spaced around the rear two thirds of the room. The music becomes superb, the speakers remain sonically invisible and the sound stage becomes more vivid, stable and detailed. Surprisingly, the listening room completely disappears, replaced by the palpable in-filling that replicates the concert hall, acoustic space or sonic environment in which the original performance takes place. The listener is unknowingly engaged into a truly immersive experience.

2C3D

This third performance level has been coined 2C3D, short for a 2-Channel 3-Dimension listening room. Different audio manufactures were upgrading their factory reference listening rooms om the early 1990’s and ended up with very similar rooms, which they coined a 2C3D listening room: Avalon Acoustics, Spectral Audio, MIT Cables and ASC TubeTraps. Overture Audio with Terry Menaker in Wilmington, Delaware was the earliest high end audio dealer to setup 2C3D. Overture is running Pass Lab amps and Magico speakers in this setup. Many combinations of gear can produce the same 2C3D effect because the 2C3D effect is derived from the acoustic setup, not the electronics.

A 2C3D setup for high end audio reviewer’s room, Mike Malinowski of 6 Moons, project by Overture and ASC. This project includes structural isolation and damping of walls and ceiling surfaces as does the Overture demo room. You can read about it here

But before any of these developments took place the pioneering work of J Peter Moncrieff, premier audio equipment reviewer whose International Audio Review IAR publication review of TubeTraps initially took place in 1985 and a follow-up work 1989. Starting with Level 1 he empirically developed Levels 2 and 3, effectively the 2C3D system.

He called his research and development work Perimeter Control. The first article was in 1985 and only used 9×3 and 11×3 TubeTraps stacked 3 high around the perimeter of the room on about 3’ centers.

IAR: Your Room: The Final Link. Optimizing ASC TubeTraps, Issue 39, 1985 ; A follow-up article was done in 1989, after replacing many 9×3 and 11×3 with 16×3 TubeTraps. These articles are excellent reading and can be found on the ASC website: IAR: Optimizing ASC TubeTraps, Issue 56. 1989

Room Development:

Each level of listening room performance is achieved by adding a complementary set of acoustic devices. Level 2 builds on the existence of Level 1 by adding on the needed compliment to the front third of the room sufficient to achieve level 2. Level 3 builds on level 2 by adding a specific acoustic compliment to the remaining two thirds of the room, which takes it to level 3. There is no lost investment when the upgrade sequence is a progression in the number and arrangement of basic building blocks.

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