Phantom Acoustics Shadow active low-frequency acoustic control

The acoustic environment for music reproduction is easily the most overlooked source of sonic degradation. Many fine playback systems are compromised by room-induced anomalies that severely color the reproduced sound. When we live in a world of directional wire, high-end AC power cords, and $4000 CD transports, paying attention to the listening room's contribution to the musical experience takes on greater urgency.

Audiophilia's underestimation of a room's effect on musical fidelity can be traced to two causes: 1) the science of acoustics is quite mathematical and arcane, and 2) acoustic theory has not been sufficiently popularized to translate into practical information for the audiophile (footnote 1). Although this situation is improving, largely through the work of Peter D'Antonio of RPG Diffusors and Arthur Noxon of Acoustic Sciences Corporation (maker of Tube Traps), we still have a long way to go in improving the listening environment. The listening room should be considered another link in the playback chain, just as any other component. It is here that the acoustic energy produced by the loudspeakers is coupled to our ears.

Before describing the technical aspects and musical effects of the Shadow, let's review the behavior of air in a room.

Any volume of enclosed air will resonate at its natural frequency, determined by the dimensions of the enclosure. The classic example is a bottle that produces a tone when you blow across its opening. The pitch of the tone varies according to the amount of air above the liquid in the bottle (the size of the enclosure). Although the motion of air across the bottle opening remains the same, a different pitch is created because a different resonant frequency within the bottle is excited.

This is also true for the air in a listening room excited by a loudspeaker. Large response peaks are created when the room's natural resonant frequencies (called "modes") are excited. The result is a peak in the frequency response, sometimes as great as 20dB. A room's fundamental resonant frequency can be calculated by dividing the speed of sound in feet per second (1130) by twice the length. For example, a room 21' long will have a fundamental resonance at 27Hz, which represents a wavelength of 42'. Note that the resonance occurs where the room length equals half the acoustic wavelength.

In addition to this fundamental resonance, multiples of the fundamental are also excited at, using our 21' example, 54Hz, 81Hz, 108Hz, 135Hz (1, 1½, 2, 2½ wavelengths respectively), and so on. Further compounding the problem, each room dimension (length, width, height) creates its own resonant series.

If that weren't enough, reflections from the walls interact with the direct sound, creating standing waves. In areas of the room where the reflected wave's compression phase (increased air pressure) meets the direct wave's compression phase, the waves combine constructively, creating a huge increase in level. When the reflected wave's compression meets the direct wave's rarefaction (decreased air pressure), nearly complete cancellation can occur, resulting in almost no sound. You can easily demonstrate this by playing a 60Hz tone on a test CD and walking around the room. The volume will appear to increase and decrease in different areas. If your favorite listening chair happens to be in an area of constructive interference at a certain frequency (reflected compression meeting direct compression), the resultant increased level at that frequency will cause bass to take on a "one-note" characteristic.

A classic problem in typical listening rooms is high-frequency absorption provided by carpets, drapes, and furniture, without low-frequency absorbers to balance this high-frequency softness. Carpets and drapes provide almost no absorption below 300Hz, where most room-induced problems occur. Consequently, high frequencies are damped, while low frequencies run uncontrolled through the room. The result is a short reverberation time at high frequencies and long reverberation time at low frequencies. This condition, along with room resonance modes and standing waves, team up to make bass reproduction tubby, sluggish, and lacking articulation and detail.

In professional applications, such as recording studios, this much-needed low-frequency absorption is usually achieved with "bass traps" or large resonant panels. Where space is a consideration, panel absorbers are preferred to bass traps to conserve floor space. Panel absorbers are made by affixing a thin sheet of 4" by 8" masonite on studs nailed on edge to the wall. When low frequencies strike the panel, sound energy is converted to heat as the panel flexes. Consequently, a portion of the impinging sound is not reflected back into the room, shortening the low-frequency decay time. The panel's resonant frequency is a function of airspace depth and panel mass. I have built these types of resonant absorbers in walls during recording-studio construction, and found them very effective.

However, such structures are usually not practical for the audio enthusiast. In addition to requiring the knowledge to build them, hammer and nails must be taken to one's living room, a prospect that doesn't endear one's spouse to our passionate pursuit of high fidelity. Fortunately, practical, commercially made low-frequency absorbers have become available in recent years. Acoustic Sciences Corporation's Tube Traps put LF absorption into a convenient, easy-to-use package that has been a boon to audiophiles.

The Shadow
Phantom Acoustics has taken a different approach to controlling low frequencies in listening rooms. Instead of using passive devices, the Shadow is an active low-frequency suppression system.

It is well known that two signals of equal amplitude and frequency, but with opposite polarity, will completely cancel. This principle is widely used in industrial noise control: instead of shielding workers from machinery noise, the noise is amplified and reproduced by speakers with inverted phase from the direct sound. The result is greatly reduced SPL.

The Phantom Acoustics Shadow uses this principle to reduce excess bass energy in a listening room. Each Shadow consists of two sets each of pressure-sensing microphones, analysis electronics, power amplifiers, and transducers. Acoustic energy is converted to an electrical signal by the microphone, phase inverted, amplified, and then input to transducers in the Shadow. The transducers' acoustic output is thus the same frequency and proportional in amplitude to the original sound, but 180° out of phase, resulting in reduced low-frequency energy in the room. Since room resonances have greater amplitude than frequencies between resonant modes, the Shadow will work harder to cancel these bothersome peaks.

Footnote 1: One excellent book that attempts to bridge the gap between pages of acoustic formulas and practical room treatments is Acoustic Techniques for Home and Studio, reviewed in detail in the next issue.
Phantom Acoustics
Distributed by InConcert, Auburn, CA (1989)