Subwoofer “Speed”
Dear Mr. Hardesty: I feel there is a special matching problem with metal drivers that you don’t seem to have addressed thus far in your review of subwoofers. I spent three intensive days of listening at the Hi-Fi ’97 San Francisco show and reconfirmed my strong appreciation for the sound of metal drivers. I have AE1s (Acoustic Energy) 2-way mini-monitors with 4-inch metal drivers and have been seeking a perfectly matched subwoofer. In my perception, the metal drivers have a clarity, transparency and accuracy that is due, as I understand it, to their speed and rigidity. They, as a class, seem to have a distinctly different sound from speakers with conventional drivers, and as a result, demand a subwoofer that complements that difference. A subwoofer with a metal driver would seem like the obvious starting point. Since my priority is musical reproduction, your personal choice of the Vandersteens would seem like a good choice except for your comment that they don’t integrate well with mini-monitors. (The AE1’s lower range is in the mid-30s). The Bag End review was also very compelling, but would a huge 18-inch paper cone really match the speed of small metal drivers? N.E.A.R. Speakers emphasize in their literature the importance of matching the speed of their metal drivers with a subwoofer that also has a fast metal driver. Their latest subwoofer is the PS 2. At this point, I am most interested in the Genesis 900 with a 15-inch metal driver. Please include this subwoofer in an upcoming review. I would appreciate any insight you have into matching fast metal drivers with the inherently slower nature of subwoofers. Another issue I don’t believe you’ve covered is the need for a crossover bypass switch on subs so you can use the digital crossovers in the new pre-amplifier/processors. Your coverage of the subject is simply the best I’ve read and is very much appreciated.
Clark Haneline, Hermitage, Tennessee
Audio Equipment Review Editor Richard Hardesty Comments:
“Speed” is a word that is frequently used to hype hi-fi components, but what are we really talking about when we call something “fast?” Are we talking about the ability to accelerate or start up quickly, which is usually referred to as rise time? Rise time is a corollary of bandwidth, nothing more. It is really not relevant to midrange or bass frequencies. One driver can’t reproduce 1,000Hz faster than another driver. Frequency and speed are not separate entities. Metal cones (or domes) aren’t necessarily faster, even in terms of rise time, than diaphragms made from other materials. In fact, since metal usually weighs more than paper or plastic materials, rise times of metal cones (all other things being equal) could actually be “slower.” Metal diaphragms have positive and negative performance aspects just like all the other materials that can be used to make loudspeakers. Metal can be stiffer than some other materials, raising the frequency at which the first break-up mode occurs. The down side is that metal offers little internal damping so that when it does break up, it rings like a cow bell, and this problem has to be dealt with by the designer. How a driver is implemented in the overall speaker design is far more important than the material from which its diaphragm is made. The low frequencies reproduced by subwoofers are by definition, slow. If they were fast, they would be higher frequencies. The leading edge of a bass transient (if there is one) will be reproduced by the main speakers, not the subwoofer, because a fast-rising leading edge on a waveform will be blocked by the low-pass filter. That’s how low pass filters work—on time constants. It’s got to be slow or it just ain’t low. All the subwoofers in the review have sufficient rise time speed to reproduce the bass frequencies that they cover. Subwoofers that sound slow don’t sound that way because they can’t start up fast enough, it’s because they don’t stop quickly enough when the signal ceases (high-Q sealed boxes or misaligned vented enclosures), or they delay the signal in the time domain, or they store and release side-band energy after the signal stops. (Read the intro articles in the subwoofer series for discussions of all these factors, and particularly the individual reviews of the Vandersteen and Bag End subwoofers which talk about why subwoofers sound slow or disconnected.) The biggest problem that you will encounter in trying to achieve a perfect blend between your tiny satellite speakers and a good subwoofer is the limited bass response of your main speakers. If this blend could be accomplished without compromise, Acoustic Energy wouldn’t bother to make larger speakers with extended bass response (which they do). Your AE1s may indeed have measurable response “down to the mid 30s, but at what level? It would be optimistic to expect output levels of 100dB at frequencies as low as 80Hz from these speakers. (See the intro article of the subwoofer review in Issue 26 for a discussion of flat response verses audible response). You can get an acceptable blend between your AE1s and a number of good subwoofers, but you must be willing to accept some compromise, and some of that compromise will be to the midrange purity that probably attracted you to your speakers in the first place. The Bag End subwoofers will work well. You’ll get superb, extended bass with excellent pace, but the signal going to your AE1s will pass through the Bag End high-pass filter and I suspect that you will hear the effects. The single-pole, passive filter used with the Vandersteen system is more transparent, and the use of three smaller drivers, a much “deader” cabinet and slot loading will reduce side-band signal output, but lots of bass energy will still go to your main speakers, limiting maximum sound pressure levels and increasing midrange distortion on louder passages that contain low frequency energy. You will also probably have a slight “hole” or “saddle” in response between 60Hz and 80Hz where the SPL capability of your speakers is limited. For a true “purist” approach, I’d start by trading your AE1s in for a larger Acoustic Energy model that will extend low frequency response down to an octave below the subwoofer crossover point while retaining the midrange quality that you like. This will make it possible to achieve a much better blend with a quality subwoofer. If that is unacceptable, then find a good dealer who will either set up your speakers with various subwoofers so that you can listen in the store, or will allow you to try the subwoofers in your home with your speakers. Pick the set of compromises that you find most acceptable by listening. Crossovers could be the subject of another series of articles, but here are a few comments regarding your final question. Almost all the subwoofers in the review have a direct input that bypasses the internal low-pass filter. This is a requirement for THX® certification, and THX-certified processors have specified build-in filters. The built-in crossovers in most processors (THX or not) are steep slope and fixed in frequency (usually fourth-order at 80Hz). This is an acceptable situation for systems with modest performance capability and where the main speakers and the subwoofer make no claims regarding time domain accuracy—in other words, the vast majority of home systems. Outstanding products like the Vandersteen and Bag End subwoofers must be used with their own crossover systems which are an integral part of their designs and an important factor in their high performance. Re-read the reviews of these products for complete explanations.
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