Siegfried Linkwitz Page 3

Another test I perform looks for resonances and stored energy in various locations—using a Shaped Tone Burst stimulus which is particularly well-suited for this. This is a tremendous test signal. I measure the impedance curve of the drivers themselves to reveal driver anomalies, and I also use complex multi-tone signals to test for nonlinear intermodulation distortion artifacts.

Dickson: This is essentially the spectral contamination distortion measurement you are speaking of?

Linkwitz: Yes exactly, the same concept, in order to find nonlinear problems. Interestingly, in the old days when we used pink noise as the stimulus to try to equalize a speaker to be flat at the listening position in a real room, it typically turned out too bright-sounding. This is an approach that may be useful in a PA set-up, but in a listening room it doesn't lead to a correct result (footnote 2).

Dickson: Tell us more about the Shaped Tone Burst test you just referred to. I found your article in the April 1980 issue of the JAES (Vol.28 No.2), discussing the benefits of using this stimulus in speaker evaluation, very interesting.

Linkwitz: From a practical standpoint, the advantage of using a shaped tone burst (one that rises and decays gradually in a sinusoidal envelope) is that all of the burst energy is concentrated into a very narrow frequency band. This is quite different from tone bursts used in the past, where you had a rectangular burst covering a fairly wide frequency band. I chose a spectrum width of a third of an octave for this stimulus—which is a 5-cycle burst—because this corresponds closely to how we hear. A third-octave is about the width of the critical band of hearing. Also, because the burst is so short in duration, you mask out the effect of reflections, so it becomes a sort of poor man's approach to anechoic measurements. As long as you measure the peak of the burst before the first reflection, you've essentially captured an anechoic-like response giving you some of the benefits of Time Delay Spectrometry or Maximum Length Sequence (MLSSA) techniques without the expense.

Now, the shaped tone burst can be used in several ways. For instance, one can just use a microphone to measure the peak amplitude that the burst reaches after you apply it to a speaker, which will give you an approximation of the frequency response. Likewise, after the decay of the 5-cycle burst, there shouldn't be any output from the speaker. In reality, however, if there is stored energy in the drivers or cabinet, the speaker keeps on ringing. Therefore, the shaped tone burst is very useful for identifying the sources of resonant storage. In any event, I do get extremely good correlation between the frequency response measurements derived from the shaped tone burst test and what we hear, as well as specific information about cabinet and driver resonances.

The real benefit of this type of test is that it concentrates the energy into a constant narrow frequency band so that it is a third-octave in width at 100Hz or 1kHz or 10kHz. Therefore, it is much narrower on an absolute basis at 100Hz than at, say, 10kHz. In other words, the tone burst test has a constant resolution on an octave basis. This is important when you compare it to FFT analysis, where you get good resolution at high frequencies but very little information at low frequencies. The shaped tone burst test works on a logarithmic scale so we can get good resolution all the way down to the lowest frequencies. I use this type of test signal to look at the decay of the burst, which gives me the same type of information that you would be looking for in a spectral decay or waterfall plot that MLSSA can generate.

I also have MLSSA, so I do generate the spectral-decay plots as well, but I have to say, I have not found the waterfall plots very useful except for maybe above 1kHz. Below 1kHz there are so many artifacts in the typical spectral-decay waterfall plot that it is useless. Anyway, it's simply a lot easier to get the same, and even much more, information out of the shaped tone burst response. Extending the time record for the FFT in order to get useful low frequency data is generally not practical; using a narrow burst signal makes it so direct and easy. Plus, you can change the frequency of the tone burst on the fly, while you watch the dynamic changes on an oscilloscope, as the tail of the burst stretches out—in effect allowing you to see directly when you're close to a resonance!

I guess I'm beginning to sound a little like a missionary for the shaped tone burst test, but I really do believe it is an extremely powerful technique that is too infrequently employed. Many people are just not aware of how it differs from traditional tone-burst stimuli. Today it is particularly easy to generate the required burst signals since you can buy an arbitrary waveform generator fairly inexpensively. Also, it would be very easy to include a series of 5-cycle-wide bursts at various frequencies on a test CD; then with an oscilloscope, or perhaps one of the PC-based software test systems, the audiophile would be equipped with a powerful tool for evaluating his system and speakers.

One final attribute of the shaped tone burst that I find very important is that it's a particularly safe signal with which to test the maximum output of components. For instance, if you use a burst rate of 1Hz with a 5-cycle burst you'll have a very low duty-cycle, so even if you require 100 watts to clip your tweeter, the short duration of the burst—it's essentially like a frequency specific pulse—will prevent you from overheating the voice-coil and damaging the driver.

Dickson: You're most widely known as the developer of the Linkwitz-Riley crossover. Could you explain a few of the characteristics of this crossover?

Linkwitz: To answer your question, we need to go back to when I started out exploring the whole speaker issue in the early '70s. Then you could take the grille cloth off many of the available speakers and see a strange, almost haphazard arrangement of the drivers on the baffle. It really puzzled me and I wondered what was going on. So I asked some of the designers why they were doing this and they said; "Because we've found it sounds better."

Footnote 2: This is because you are equalizing the loudspeaker's power response, which includes the full contribution in-room of its off-axis behavior. As the power response tends to slope down with increasing frequency with conventional speakers, such equalization will boost the highs on-axis. As a result, unless you are sitting a very long way away from the speakers, the perceived balance will have a strong contribution from the speaker's direct sound which, after equalization, will tend to be too bright.—JA
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