BackstageAn Engineer's Perspective

Accoustical Sense

by William Dudleston

Every day at noon I head down to the corner coffee shop to restore my faith in humanity and get me some common sense. Yep. You heard me right. The 'Coffee Cup' is the joint with the 11 pickups and and my minivan parked out front. Inside you'll find the heart of middle America. Farmers, construction workers and small time mechanics with one thing in common: they've all got real dirt on their clothes and life's wisdom in their eyes. They have the kinds of faces that show up to fix your broken-down car on rainy nights.

I try to blend in, but I know they're picturing Joe College back at the office with a pocket protector and a calculator on his belt. I do best at the 'Cup' by sipping my coffee slowly and just listening. Besides, I'm there to get a free education. You just can't get common sense like this by reading journals. After a quarter of a century of working with loudspeakers, I might be able to share some common sense with you about acoustics. Don't mind the sawdust on my clothes. The stereo sound field created by a pair of loudspeakers s a fragile 3-D illusion built upon auditory clues. But it must be understood that the reproductive task of each individual playback channel is merely two dimensional: amplitude vs. time. It doesn't matter where you are in the chain, whether it's pressure at the microphone, voltage at the mixing console, magnetic flux at the tape, or displacement of a speaker diaphragm; it all comes down to amplitude fluctuating over time. Visualize the reels turning on a tape deck as marking time, and the meters monitoring amplitude. Our brain does the 3-D work, not the playback system. Loudness is varying over time. It's that simple.

Speaker Problems:

Speakers typically have 30 dB less dynamic range capability  in the deep bass range than they do in the midrange.
Speakers typically offer the listener less than 2 dB of channel separation in the far field, compared to the 6-7 dB that a listener would naturally experience in a free field.
Speaker clarity is related to the percentage of direct to diffuse sound arriving at the listener. If we consider the direct sound from a speaker as signal and reflected sound as noise, then a typical loudspeaker’s signal could be considered 6 dB into the noise floor at listener position!

Like microphones, loudspeakers suffer from proximity effects (sometimes called diffraction loss). This is particularly the case with smaller speakers. All speakers sound somewhat different as the distance from the listener changes. Microphones often have some adjustment controls to counter this effect. Speakers typically do not. Speakers also receive differing amounts of boundary reinforcement as room sizes change.
Since transducers are of a mechanical nature, speakers typically introduce amplitude distortions of a magnitude greater than amplifiers.
Listening rooms are so positionally sensitive at low frequencies, that moving a loudspeaker (or listener) a mere 18” will typically introduce power response fluctuations of +/10 dB below 100 Hz.

Legacy Solutions:

1. By incorporating more piston area, FOCUS offers 9-12 dB more low frequency dynamic capability than its competitors.
2. WhisperÕs differential technology provides 3-4 dB more separation than conventional designs.
3. Our controlled directivity design improves this ratio by nearly 4 dB over conventional designs.
4. Our Steradian environmental processor allows adjustment for separation, proximity and room gain.
5. Increasing piston area prevents rising distortion as frequency decreases. Legacy speakers provide enough piston area to lower distortion levels to typically half that of the competition.
6. Whisper is 5 dB less sensitive to room placement than conventional designs.

The level of signal integrity that a loudspeaker delivers to the listener is dependent on how well the speaker tracks these changes in signal amplitude and how little it engages the listening room surfaces in the process.

Any decent speaker today has relatively smooth frequency response on axis. Therefore, the most defining characteristics of a loudspeaker's "sound" are its directivity pattern, bandwidth and dynamic range capabilities.  These areas are where loudspeaker designs tend to deviate the most from ideal linearity. Below, you'll find a list of problem areas remaining in loudspeaker designs. Next to this list you'll see examples of how Legacy has addressed these problem areas.

Acoustics is by far the simplest science I've studied. There are no chemical reactions of concern, no significant thermal considerations in the working medium (air), no depletion of process elements An Engineer's Perspective (unless you neglected to pay your electric bill), and no physical phase changes such as coalescence, vaporization, sublimation, or condensation. The wavelengths we are dealing with can be easily visualized and measured in feet or inches. Angstroms, microns or parsecs are not required.

Yet, loudspeakers remain as strange and mysterious things to most folks - particularlyloudspeaker designers. While we've managed to gather little blocks of knowledge about how loudspeakers should function, rarely do speaker "designers" stack these blocks in a structured manner to take full advantage of what has been learned. Common sense tells me that loudspeakers with adequate piston area, high efficiency, and controlled radiation patterns directed at the listener will deliver better performance. Why should we wait for the other guy to catch on. Sometimes you have to swim upstream to get the job done. We'll continue to do just that at Legacy. You can count on it.

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