The Art & Science of Audio & Video

Siegfried Linkwitz

Siegfried Linkwitz

By Shannon Dickson • April, 1996

Siegfried Linkwitz was born in Germany in 1935. He received his electrical engineering degree from Darmstadt Technical University prior to moving to California in 1961 to work for Hewlett-Packard. During his early years in the USA, he did postgraduate work at Stanford University. For over 30 years Mr. Linkwitz has developed electronic test equipment ranging from signal generators, to network and spectrum analyzers, to microwave sweepers and instrumentation for evaluating electromagnetic compatibility.

He has also had a long and distinguished second career as an audio engineering visionary. Along with Russ Riley he developed the famed, and widely used, Linkwitz-Riley crossover filter in the mid-1970s. Since then, he has contributed several important technical papers covering a variety of measurement and speaker issues to such publications as the Journal of the Audio Engineering Society, Electronics (Wireless) World, and Speaker Builder.

Most recently, he has joined forces with fellow HP engineer Marshall Kay, CAD (computer-aided design) specialist Kurt Pasquale, and marketing consultant Tom Hoffman to form Audio Artistry. This three-year-old, North Carolina-based company is dedicated to developing and crafting speakers based on the accumulated insights and wisdom Mr. Linkwitz has gained over three decades of loudspeaker research. I spoke with Siegfried about some of these insights and experiences during the course of evaluating the Audio Artistry Dvorak, the review of which is found elsewhere in this issue. My first question concerned what had motivated Linkwitz to get involved in audio.

Siegfried Linkwitz: I grew up in a family in which music was very much appreciated. My father and brother played the piano, and although circumstances during World War Two prevented me from learning an instrument, I’ve always had a love for music. After graduating from university and joining Hewlett-Packard to design electronics, it was only natural that I wanted to build audio equipment I could use at home, so I got very involved in building power amplifiers, FM tuners, preamps and you name it—anything electronic I needed to reproduce music. Then I had the fortune of meeting some other engineers at HP who were similarly involved in audio, particularly Lyman Miller and Russ Riley. Lyman was very much into electronic design and making recordings while Russ built amplifiers and had a keen interest in speaker development. They really turned me on to investigating things even deeper, and loudspeakers, to us, were the most interesting and challenging area since so little was really understood about them. The speakers then on the market could certainly be improved so we saw a real chance to make a genuine contribution.

Shannon Dickson: Could you share with us some of the fundamental problems you and your colleagues encountered during the early attempts to improve speaker performance?

Linkwitz: One of the problems at the time was that good test equipment wasn’t available to us. Russ Riley developed his real-time 1/3-octave analyzer and a pink noise source which we used to make in-room measurements. I bought an early Advent speaker, measured it using the real-time analyzer, and consequently developed an equalizer to flatten-out its frequency response. That was a first attempt on my part. I then experienced a real surprise after we went to some local stores and heard the Electrostatic Sound System’s ESS-7. It just sounded great, much better than the Advent. Naturally, I bought the speaker and took it home, but after measuring it, I was astonished—it measured very poorly! That led to a whole investigation into why it sounded so good but tested so badly.

We found out rather quickly how important driver quality was, as well as the distortion contributions of cabinet resonances. We began experimenting with wool stuffing in the box and with various bracing and panel damping techniques. We found that wool could be a very effective loading material. A number of commercial designs sounded much better when we replaced whatever they had inside with natural wool fiber.

In my early designs, we tried two basic concepts built around rather small enclosures, both of which worked quite well. For instance, we made some very rigid, heavily braced small monitors; then we went the other way, using very limp, thin panels for the box construction. These were very easy to damp by applying roofing tar with sand mixed in. As you can imagine, this was a real messy operation—it smelled pretty bad too, particularly if you placed the speaker in the sun. It would out-gas for several weeks before you could tolerate the smell!

While it damped box resonances quite effectively, this approach was not really practical from a commercial point of view, nor would it have been a very welcome addition to most people’s living rooms. But it did demonstrate how important minimizing box resonances is and just how difficult it is to really control this form of resonant behavior.

Dickson: You’ve worked with some of the most respected engineers in audio over the years. Who had the greatest impact on your thinking regarding speaker development?
Linkwitz: I mentioned Lyman and Russ already. Lyman was really into the recording side of things so he did a lot of recordings on a semiprofessional basis and was particularly interested in capturing sounds as close to their natural origin as possible. So we had some great reference material to guide our evaluation. I learned a lot about recording from Lyman and continue to make many of my own reference recordings, which I used extensively during the development of these new speakers.

Russ Riley is a very ingenious design engineer and, on top of it, a superb listener. I was always impressed by how easily he could identify just what the problems were in a speaker and in what frequency range and what one needed to do about them. He had absolutely superb hearing. While not as well-known as some of the other engineers, both Lyman and Russ had a big impact on my early audio career

Through my work in developing test equipment for Hewlett-Packard, I met Laurie Fincham [then with KEF, now with Lucasfilm THX after a stint at Infinity] and we became good friends. We’ve shared a vast amount of information with each other over the years, have met frequently, and consequently had some very positive mutual influence on one another. Through Laurie, I was also introduced to a number of distinguished engineers such as Floyd Toole, Stan Lipshitz, John Vanderkooy, and Peter Walker from Quad. I had been following all of these people’s writings very intensely all along, so it was a joy to meet them.

Most of these folks have been at my house at one time or another to listen to various ideas I had been working on. In addition, I have been an avid reader of the JAES throughout the years as well as Wireless World from the UK [now Electronics World—Ed.]. Wireless World used to carry a great deal of high-quality information about audio and speakers; it still does, in fact, though it’s not as easy to find these days. Actually, my first publication appeared in 1978 as a lengthy three-part article in Wireless World in which I described the construction of a three-way active speaker system consisting of small satellites and a subwoofer.

In summary, the various influences on my thinking have led to a general approach that is really a blend of the analytical—meaning the measurement of things—and the subjective listening experience, to try to find out what is really going on. If there is an hypothesis of why something works—this way or that—I’ll set up an experiment to see if I can prove it or disprove it. In this way, I’ve always attempted to correlate what we hear with objective measurements, not always successfully mind you, but at least making the connection where possible. This method will give you a lot of insight into which measurements or artifacts are important and which are not so important. Occasionally, I’ve found results that look very significant on paper but are barely perceptible, if at all, while on the other hand, some extremely slight irregularities can be very important sonically.

Dickson: Can you tell us what your priorities are in making and evaluating specific measurements.

Linkwitz: I’ve learned there is a whole battery of measurements one needs to use—and interpret correctly—in order to get a better picture of any given speaker. No one measurement will tell you the whole story. At the top of the list is definitely a loudspeaker’s on-axis anechoic frequency response measurement because this represents the direct sound you hear. However, of similar importance are the vertical and horizontal anechoic off-axis responses. So in my designs, I try to achieve a very well-behaved off-axis response which duplicates the shape of that on-axis, but steadily decreases in level the farther you move off-axis. This is so important in determining the reverberant field and the reflected sound in the listening room.

Another key factor, I learned during the development of my crossover design, is that when two drivers are combined in the crossover region, their summed output should be at its maximum on-axis. In other words, the radiation pattern remains stable at the crossover region and doesn’t shift. For example, I’ve found through experimentation that it is definitely audible if you go some distance above-axis and all of a sudden have a maximum peak or sharp dip in the crossover region.

This problem is similar to what happens with many large-panel dipole designs. As they produce higher frequencies, their off-axis response becomes more irregular, with peaks and valleys that can color the overall sound and make speaker placement in a given room very critical. If the crossover on any speaker doesn’t blend together you can get this kind of off-axis peak.
Another measurement I look at is the overall frequency response on a half-octave or octave basis, just to see the general trend, whether the treble is rising or sloping, etc. When you look at any response in detail, you never get a flat picture, you always have little ups and downs, but I’ve found you don’t really gain anything by trying to smooth out these small ripple effects in the response. However, how smooth the response is over a third- or half-octave basis is important. I’m essentially looking for an averaged-flat anechoic response.
I do my quasi-anechoic measurements outdoors, with the speakers mounted on a 50″ turntable so that the speaker is as far away from any reflecting surfaces as possible, yet still manageable. I try to get 10 milliseconds of undisturbed sound between the initial impulse response and the arrival of the first reflection, which will give me a frequency resolution of 100Hz and useful data for all frequencies above a couple of hundred Hertz. I also try to minimize the first reflection off the floor or ground with acoustic absorbers (footnote 1). But as you can see, this method really doesn’t tell you much about the bass.

After my series of anechoic tests, I perform in-room measurements over a 50ms time window. This gives me a frequency resolution of 20Hz and since 50ms is a pretty long time in a room, it does takes into account the room reflections. I also use 50ms because that is about the maximum time span [during which] the human brain can process the characteristics of a sonic event. Basically, I use these in-room measurements as confirmation of the anechoic results, not to correct for all the reflection anomalies or peaks and dips that show up in the response. I do, however, make these in-room tests from several different locations, and with our new dipole designs, even these in-room measurements over a long time window are surprisingly smooth and flat.

Footnote 1: Color me jealous. An anechoic time window of 10ms is excellent. Performing speaker measurements in the Stereophile listening room, with its 9′ ceiling height and a microphone distance of 50″, results in about a 3ms-4ms anechoic time window, this with a very thick pile of absorbing material on the floor. Siegfried must live in a very quiet neighborhood; there is too much background noise in Santa Fe to perform measurements outdoors and get usable waterfall plots.—JA


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