BackstageAn Engineer's Perspective

How to Catch a Wave

More than a decade ago, as Legacy began a tour of road shows debuting new audio and home theater products, we would have to set up in different venues each week.  Rooms varied from ballrooms to hotel conference rooms and suites, and acoustics varied considerably. The real challenge was to generate a sweet spot that would serve more than a dozen listeners at a time.

Typically, we would set up the Legacy Whispers spread quite wide with a strong toe-in, then position the Focus speakers to the inside of them in the same plane. This allowed listeners to comparatively experience the systems without relocating themselves or the speakers.

While we could dictate the seating positions and speaker locations, we certainly could not dictate the distribution (position, frequency, or amplitude) of standing waves that would develop in a given room*. Whisper with its narrower and more controlled low frequency radiation generated and propagated a more useful pattern over the length of the room and offered an extra degree of room independence.  It also benefited from the Whisper analog processor which could be set for listener proximity. So helpful was this control, that within a year the STEP One (Stereo Environment Processor) was born to assist the Focus system. This unit was factory adjusted for the typically floor to ceiling axial resonance (71 Hz) for those with 8 ft. ceiling heights.

In the decade that has passed**, Legacy has continued to pioneer the methods of treating boundaries and reflections in the listening room (the reader may elect to read further about what I refer to as the Three Rs of listening room acoustics; REINFORCEMENT, RESONANCE, and REVERBERATION***.)

Invention and diligence allowed us to develop a method of terminating the low frequency wave-launch just as it reached the end of the listening room. Utilizing DSP and two subwoofers (placed strategically against the wall behind the listeners) we introduced a terminating signal from the subs.  Delayed to precisely synchronize with the initial launch, an anti-phase signal was generated at the rear boundary, reducing pressure buildup ( nodal formation or fold-back).  Ideally, one could add additional piston area to launch from one end of the room a cylindrical wave-front, and ‘retire’ energy with a synched mirror image setup that could dominate the room’s behavior.

Additionally, our studies found that typical listening rooms suffer from a greatly reduced amount of L-R component relative to L+R component below 400 Hz compared to open half space environments (the outdoors, concert halls, etc.).  The stereo information is masked or destroyed by near reflections and early crosstalk.  Utilizing DSP, we are able to implement modest amounts of correction to these components, restoring natural spaciousness.

A decade later, we are proud to state that Legacy is implementing our method of Acoustic Wave Termination in many of our installations.  The process is as follows:

  1. Initiate the low frequency launch ahead of higher frequencies to account for startup inertial differences.
  2. Apply dynamic braking to the woofer motor system to counter back EMF, increase efficiency, and prevent overshoot.
  3. Shape the low frequency launch into a cardioid pattern by combining a dipole (figure of eight) with an omni radiator.
  4. Increase the ratio of stereo difference  (L-R)  component to mono (L+R) component below 400 Hz.
  5. Terminate the initial launch with an anti-signal (typically timed to approximate the length of the room).  A reduction of 3 dB in reflected energy is required to provide a benefit.

 

Sometimes you have to think “inside the box” to get a handle on things.  Thanks for listening.

Regards,

Bill Dudleston
Founder, Legacy Audio

*Ironically, most audio publications are still measuring loudspeakers at a distance of barely a meter or more.  This does not include the critical initial floor bounce to the listener, which is the most dominant of all room reflections, as it is within the ‘fusion time’, meaning it directly affects tonal balance.
**Recently an on-line publication completed an evaluation of our Helix loudspeaker system. This speaker was conceived more than eight years ago, and is now being recognized as cutting edge by the press.  Kudos is awarded to the publication which took the time to carefully investigate our technological claims, pried into every aspect of the design, and ultimately weighed the concepts of the design in terms of musical relevance.  http://www.dagogo.com/View-Article.asp?hArticle=60

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