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District of Columbia - May 17, 2008

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Summary

The Art and Science of Loudspeakers in Rooms
A seminar by Frederick J. Ampel
by Fred Geil (AES-DC Secretary) and David J. Weinberg (AES-DC Chair)

On 17 May 2008 the DC sections of the Audio Engineering Society (AES), the Acoustical Society of America (ASA), and the Society of Motion Picture and Television Engineers (SMPTE) enjoyed a presentation by Frederick J. Ampel, whose consultancy serves the professional and consumer audio/video industries worldwide.

Everyone listens to sound systems, whether for production/post-production work or for pleasure. Ampel made clear that the goal of a surround installation is entertainment, not science. A properly set up and calibrated audio system will produce a higher Reality Creation Quotient™, enabling the sound designer to create a more accurate and involving sound space, which will more effectively place the listener, with his own properly set up and calibrated audio system, in that space. The more effectively the system places the listener in the space the soundtrack represents, the higher the entertainment value — the better the system, the stronger the emotional power of the sound.
System set-up based solely on measurements (the science) can only get you so far.

Ampel explained a methodology that combines:
• Recognized and respected instrumentation-produced information, with
• The sound you hear
to give you a more powerful analytical tool to use in ascertaining a system's sound quality.

He demonstrated how to use soundtracks to evaluate multi-channel audio system performance and guide you in making final adjustments.

This bimodal approach to system setup is necessary because equipment doesn't hear as we do, and your customers' ears pass final judgment on your effort. This is true whether the audio is music or a soundtrack, and whether the customer is a consumer, a mastering engineer, or a movie sound designer.

Every time you listen to an audio system (especially a multi-channel one) you are, without formally recognizing the process, receiving a huge amount of data on that system's attributes and capabilities. Unfortunately, you process only a small portion of what you hear, primarily because you're not listening for the rest of the information.

Loudspeaker specifications and measurements are generally based on anechoic analysis, yet speakers are used in rooms, where acoustics always modifies expected performance and often dramatically changes sound quality. Ampel gave a short refresher on room acoustics, speaker/listener placement issues, and the three-dimensional nature of room modes. He fed a 50Hz tone through the system while we walked around to find the horizontal and vertical locations of sound energy peaks and dips.

He considers the most critical parameter to be the precisely matched timing of the signal from each speaker to the prime listening location (the 'money' seat, where the person with the checkbook sits). His experiments have revealed that the measured distance almost always differs from the acoustic distance, sometimes by as much as 1m, because room reflections within the first 30msec (the ear's approximate fusion time) affect the perception of sound arrival. He uses the Audio Control Iasys, an analyzer he assisted in the design of, to determine the relative acoustic distances of the speakers from the microphone placed at the prime listening spot.

The Iasys also detects each speaker's level, signal polarity, and detailed frequency response at the listening position, using fuzzy logic to suggest equalizable corrections. The algorithm never suggests trying to fill a null, since it isn't possible and will severely stress the system, violating one of his tenets: distortion is distortion, regardless of the explanation or name, and is not to be tolerated. Based on the Iasys data and his judgment, he enters appropriate corrections and crossover frequencies (between each main speaker and the subwoofer) into the Audio Control Diva, which electronically fits between the Maestro (or any other) surround processor and the power amplifiers. He emphasized that these steps are undertaken using a combination of measurements and ear-checking.

To illustrate the importance of his bimodal approach, Ampel played a series of movie clips, beginning with the cornfield scene from "Field of Dreams" ("If you build it, he will come"). His selections were chosen to highlight the subtleties in the sound, not explosions, because they more strongly affect our sense of reality. Each was played three ways:
1. First, with the system in "out-of-the-box" mode — unpack, place and interconnect the electronics and speakers, with no further adjustments;
2. Second, with added timing correction and level balancing; and
3. Third, with added EQ.

He suggested listening to:
• Dialog (actors must sound "real", not thin or boxy).
• Space (ambience of the physical places represented on the screen; especially the small, quiet details sound designers spend hours creating and placing — footsteps, corn stalks rustling in the wind, birds, a creaking screen door, etc.).
• Accuracy (a smooth, even sound field with no artifacts; nothing that isn't in the source signal).
• Colored/Uncolored (no harmonics or resonances produced by the speakers or the room).
• Naturalness (voices, music and ambience must sound "real" and not forced).

The results could be summarized as "Fair" (because good equipment and speakers were used in a reasonably decent room), "Better", and "Best" — the latter version effectively transporting the audience to that place and time on the screen. It was easy to imagine actually being there; the sound reflected our mental sense of the space, and no audible artifacts degraded or distracted from the experience. The attendees were invited to visit the "sweet spot" occupied by the measurement microphone to get the maximum effect.

Ampel said that when a system is working properly, there will often be a physical 'jerk' or snap in the listener's body language when the stop button is pressed and all of a sudden their location shifts instantly from the picture back to the reality of the room they are in. This tells him that he has successfully re-created the acoustical space intended, and that the details are coming through to make everything seem 'real'.

Additional points Ampel made:
• Rooms can't be "tuned", only the signal feeding the room can be altered to improve the result in the room.
• Bad loudspeakers are bad loudspeakers; they can't be fixed with equalization.
• Precise arrival time control is the key to accurate surround sound reproduction. Accuracy in terms of milliseconds, even microseconds, is important.
• Measure a lot, but trust your ears.
• The only good system is one that the customer is willing to pay for.

Although this was a lengthy program, many stayed behind to ask questions and discuss various surround sound issues.

Appreciation must be expressed to Greg Lukens of Washington Professional Systems (Wheaton, MD; www.WPSWorld.com) for hosting the event and providing the five Genelec 8050 monitors, the Meyer Sound subwoofer, and two Sony DVD players, plus the Christie projector with Stewart Studiotek 130 screen for the PowerPoint slides and movie clip images.

Thanks also to Audio Control (www.AudioControl.com), which loaned and shipped a Maestro surround sound processor, a Diva DSP, and an Iasys measurement system especially for this event.

The attendees, plus Fred Ampel and Greg Lukens, expressed their pleasure at the results.

Funding for this event was shared among the DC sections of the AES, the ASA and the SMPTE, with help from the AES' Distinguished Speaker Program.

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AES - Audio Engineering Society