My Mistake

Recently, I posted an entry in the Ultimate Tech blog about a new type of tweeter called the Leading Edge Transducer (LET). In that article, I made some statements to which reader Noah Katz, a structural engineer and, presumably, an audiophile, took exception. At first, I was going to ignore him because he refused to engage in a dialog about it. But, being me, I couldn't resist digging deeper to uncover the truth. After all, one of my guiding principles as a technical journalist is to get the facts straight, and if I don't—which, as a human, is inevitable once in a while—I feel compelled to correct my mistakes.

In the blog entry, I explained that the mass of any speaker driver affects the speed with which it converts the electrical energy in the audio signal to the mechanical energy of vibration due to inertia, which is the tendency of objects to resist changes in their state of motion. The greater the mass, the greater the inertia and the greater the resistance to changes in motion.

I went on to say, "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 with as little mass as possible—the more mass, the more inertia—and a voice-coil motor with as much power as possible."

Katz posted a comment stating that my explanation was "full of errors." When I asked him to provide some evidence of these errors, he declined, saying it wasn't his job to do my homework. (I maintain that it really is his homework, not mine, since he's the one challenging my assertions, but that's a peripheral issue.)

After some back and forth, he did offer one small statement: "Adding mass to any particular driver just reduces efficiency. Its time response (away from the region of its now lower Fs) will be identical, just lower in level." For those who might not know—that is, anyone who's not a speaker designer or audiophile—Fs is the resonant frequency of the driver in free space, not in an enclosure.

To get to the bottom of this, I contacted two preeminent speaker designers—Paul Barton, founder of PSB, and Kevin Voecks, formerly with Snell and now with Harman High Performance AV, which includes Revel among other high-end speaker brands. After conferring with them, I have come to the conclusion that Katz is right and I was wrong. I had promulgated a widespread misconception that I now aim to correct.

As Katz said, increasing the mass of a driver reduces its efficiency, which means its output will be at a lower volume level in response to a given signal. According to Voecks, the mass of tweeters and midrange drivers is generally minimized to increase efficiency (and the tweeter's high-frequency bandwidth), but woofers often have intentionally increased mass in order to lower the free-air resonance (Fs) and compensate for low-frequency rolloff in an overdamped system.

The relationship between mass and volume level can be illustrated with a thought experiment. If you took high-school physics, you might remember Newton's Second Law, which states that F = ma, where F = force, m = mass, and a = acceleration. (To be completely accurate, this is not exactly how Newton formulated the law, nor does it apply to bodies of variable mass, such as a rocket that burns fuel, but it works just fine with speaker drivers.)

Suppose you have two tweeters that are identical in every way except one—they have different masses. Now, suppose you send the same audio signal to both simultaneously. In both cases, the force is the same (identical signal, identical voice-coil magnet); only the mass is different.

With higher mass, the diaphragm's acceleration must be lower so that the equation F = ma remains balanced. Similarly, if the driver has lower mass, the acceleration must be higher. (Remember, the force is the same in both cases.) Thus, the tweeter with more mass undergoes lower acceleration and doesn't travel as far before the audio waveform changes direction and causes it to reverse course. As a result, the amplitude of the more massive tweeter's vibration is lower, so its output is lower in volume.

As I was conducting this thought experiment in my mind, I realized where I made my mistake. As the current begins to flow through the voice coils, both drivers start to move at the same time—the more massive one is not delayed compared with the less massive one. The only difference is that the less massive one moves a greater distance in a given amount of time.

This isn't to say that mass has no effect on leading-edge transients, which are loaded with high frequencies. Barton reminded me that tweeters with greater mass tend to roll off the high frequencies more than low-mass tweeters, primarily because the latter are usually smaller and/or stiffer. Thus, lowering the mass of the tweeter diaphragm improves its reproduction of leading-edge transients. (Of course, real-world driver design is much more complex than this, but that's beyond the scope of this blog.)

Getting back to High Emotion Audio's LET tweeter, its low mass does improve the reproduction of leading-edge transients, though not for the reason I stated, which was based on info given to me by the company that I did not thoroughly think through. And I suspect that the "whiplash" effect on which it is based helps in this regard as well, but I don't know for sure, and the company is unwilling to reveal more about it.

I haven't heard these speakers myself, but those who have—and whose ears I trust—say they really do sound more lifelike than most, so HEA must be doing something right. I remain convinced that the LET tweeter represents a step forward in transducer technology—even though it's based on the 15-year-old Linaeum tweeter—and is worthy of inclusion in the Ultimate Tech blog.

My original post was based on an unsound premise, and I thank Noah Katz for bringing this to my attention. As I now know, speaker drivers do not respond to audio signals faster or slower depending on their mass as I stated originally, and I will update the original blog entry accordingly. Hopefully, my discussion here will serve as an example of how anyone can get it wrong once in a while, and that being open to learning new things—even if they contradict what you think you know—is a worthwhile approach to life.