Siegfried Linkwitz Page 2
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