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Tech Page: Let There Be LET

January 1, 2008
image of a speaker

FIG. 1: The S7 includes two LET tweeters facing front and back and one 6.5-inch midrange that can reproduce frequencies down to 45 Hz. The crossover frequency is about 3 kHz, and the input impedance is flat (±1Ω) across all frequencies. Notice how the LET diaphragm on top of the speaker is actually two hemicylindrical membranes.

Transducers — devices that convert one form of energy into another — are among the most mature technologies in the electronic musician's toolbox. The most common musical transducers are microphones, which convert the mechanical energy of acoustic sound waves into electrical signals, and speakers, which do exactly the opposite. Both have been around for a century or so, and despite a few innovations and variations, they haven't changed much in all that time.

One of the biggest problems faced by transducers of all types is the time it takes them to convert mechanical to electrical energy and vice versa. This is especially challenging for relatively massive speaker drivers, which must overcome a lot of inertia (the tendency for objects to resist changes in their motion) and move in response to the electrical signals they are fed. The faster the driver can respond, the more accurately the attacks, or leading-edge transients, will be reproduced, which improves the perceived sound quality immensely. As a result, most speaker drivers use a diaphragm having as little mass as possible (the more mass, the more inertia) and a voice-coil motor with as much power as possible.

Still, getting any diaphragm to respond quickly isn't easy, so a company called High Emotion Audio ( has taken a different approach with its new tweeter, the Leading Edge Transducer (LET). Instead of moving the entire diaphragm in and out, the voice coil induces the waveform to propagate through the diaphragm's special plastic material, absorbing it at the outer edge to prevent reflections within the material.

The inertia that must be overcome in this process is far less than conventional pistonic vibration, and the voice-coil motor is more powerful than most, allowing the LET to reproduce attack transients much faster than conventional tweeters. In addition, the LET is extremely efficient — about 100 dB/W/m — and the diaphragm's surface area is about five square inches (compared with less than one square inch for most tweeters), which allows it to produce very high sound-pressure levels with relatively little power. An analogy drawn by the company is cracking a whip: a small motion in the handle can create a loud pop once the waveform reaches the tip.

The first products using this technology are dubbed the S5 and S7, which are consumer-oriented bookshelf speakers that combine two LETs firing forward and backward with a proprietary pistonic midrange/bass driver and custom-designed crossover in an optimized cabinet (see Fig. 1). Among the advantages of this design is an exceptionally wide dispersion — the company claims 120 degrees at 20 kHz — and ruler-flat impedance at all frequencies, allowing the speakers to sound roughly the same being driven by many different amplifiers in a wide variety of rooms. Another claimed benefit is extreme clarity, allowing you to hear deeper into the music, which is invaluable to recording engineers.

Even more important, the LET's superfast transient response and other design factors have been clinically demonstrated to affect the human limbic system (which regulates emotion) more than conventional speakers, resulting in a more deeply moving experience. In fact, the company's ultimate goal is to approach the emotional impact of live performance, which has heretofore eluded virtually all electronic audio systems.

Prototypes of the S7 have already made their way into some studios in Nashville, where the company is based, and the response has been uniform praise. In addition to consumer products, High Emotion Audio intends to use the LET and its other innovations in professional studio monitors and sound-reinforcement applications. I have every confidence that we'll be seeing — and hearing — much more from this innovative company in the future.

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