Fast Bass, Slow Bass - Myth vs. Fact

People are always talking about how fast or slow the bass is of loudspeakers, amps, preamps, even source components. You especially hear how "slow" dynamic woofers are compared to the high-speed of electrostatic panels, especially when the two drivers are paired in a hybrid loudspeaker. Comments often heard go something like this: "Well, sure there is a speed difference between the panel and the woofer. Nothing is as fast as a good electrostatic panel. Expecting the woofer to keep up just isn’t realistic." Well, this month I will attempt to explain just how wrong these snapshots of reality are.

The first thing we must know is that bass itself is not particularly fast. Virtually any woofer, even those with heavy cones can easily reproduce bass frequencies with every scintilla of speed present in that bass. So don’t buy a bunch of baloney about low-mass woofer cones leading to "high-speed bass" -- it just isn’t going to happen. If the woofer can reproduce 40Hz with low distortion, how fast the woofer starts is almost irrelevant (within reason of course). It only needs to accelerate fast enough to match the rise time of 40Hz at the fastest point along a 40Hz sine wave. If the woofer can do that, it is going as fast as it needs to in order to be as fast as fast can be -- at 40Hz. The woofer cone does not need to be able to accelerate at 20kHz velocities in order to produce instantaneous 40Hz energy and if you could build a woofer that "fast," 40Hz would sound exactly the same through a "slow" woofer.

Does this mean that there is not such thing as fast bass and slow bass? Absolutely not. It exists, just not for the reasons and explanations you have been hearing for years, and certainly not for the attributions you’ve read in the high-end press. There are reasons to use lighter, lower-mass woofer cones. They just happen to be different reasons than the ones you’ve read in print. Smaller woofers don’t make faster bass, but they do reproduce higher frequencies than larger woofers can reproduce, and this is all important when it comes to speaker design. You want the midrange driver and the woofer to integrate with sublime symmetry, with perfection and with nary a single problematic interaction throughout their overlap zone. This is why you want smaller, lighter, "faster" woofer cones -- not because they lead to faster bass. That overlap zone is so amazingly critical to your perception of bass speed that there is little or no tolerance for error. The null tolerance for integration error extends to phase, amplitude, frequency, and time. Introduce even slight variations between any part of the woofer and midrange (or panel) overlap zone and you get audible effects in the bass or midbass. This is where all of your perception of bass speed comes from.

In fact, bass speed is virtually 100% a function of how ideally the midrange and woofer are integrated. Bass linearity is greatly involved also; you may see a flat frequency-response curve, but the speaker can still sound like it has lumpy bass response because of less-than-ideal phase (or other) relationships between the midrange driver and woofer. Phase can often change with frequency. The woofer and midrange drivers can actually veer off in different directions, phase-wise. This is especially possible when you mix driver types like panels and dynamic drivers. But large dynamic drivers (woofers) operating at the top of their range and medium-sized dynamic drivers operating at the bottom of their range can often diverge significantly in their phase response. When phase (or other) errors happen, you get comb-filtering effects. This comb filtering results in the complex response of the loudspeaker (to music) being quite different than the response of the speaker when the input is something simple like the sine-wave sweep used to measure "frequency response."

To avoid comb-filtering effects that cause "beating" (reinforcement) and "cancellation" effects in the sound (both are usually partial effects), it is imperative for the phase, time domain, amplitude and frequency performance of the woofer and midrange driver to be "aligned." Get the midrange or woofer a little ahead of or behind the other driver, and comb filtering starts. You can do things to minimize it, but you can’t stop it with certain combinations of driver and crossover. It is fearfully hard to integrate a dynamic woofer with an electrostatic panel because the two drivers are so different from one another. Your absolute best shot is using an active crossover with infinitely variable phase/frequency, polarity, time domain and amplitude adjustments. Play with it long enough and you could dial in the response of the dynamic woofer and electrostatic panel to achieve perfection in their integration. Achieving the same thing using a passive crossover is incredibly difficult. Some designers are getting better as they learn from years of trying, but it is still one of the hardest things to do in audio that I can imagine. Just getting a dynamic midrange and dynamic woofer to integrate perfectly is enough of a challenge. You can hear even small errors show up as speed problems in the bass or midbass. These are the kinds of "character" that will remain with the speaker no matter where it is used.

But want about amps, preamps or source components that sound like they make the bass faster or slower? There are many ways for this to happen, but they all involve changing some relationship between the midrange driver and woofer so that the integration changes in some way that manifests itself as "faster" or "slower" bass. The component itself is not producing faster or slower bass; it is creating an interaction with the speaker crossover that results in you hearing that. Some of those interactions will be consistent from system to system, while others will be chameleon-like, changing from "fast" bass in one system to "slow" bass in another. Yet if you measure the component electrically, there is nothing in its measurements that indicate that it is anything but perfect, speed-wise, at bass frequencies. It only takes a small loudspeaker-dependent phase shift to occur when using a particular amp to make it chameleon-like. This would be harder to have happen in a preamp or source component -- their signatures will tend to be more consistent from system to system.

What about bass detail? How come one speaker has so much more bass detail than another? This too is strictly driver integration and NOT the quality of the woofer itself, as you may have heard. The fact is, bass detail comes from the midrange driver. But your ear/brain is so completely fooled by this complex interaction of midrange and bass sound that you believe that it is strictly a bass-related thing. It isn’t, and you can prove it by listening to something very boring but also very instructive. Listen to a subwoofer all by itself for a while. You won’t hear anything vaguely resembling speed coming from that slow, soggy-sounding, plodding subwoofer. It has no detail and no speed whatsoever when heard all by itself. Integrate it carefully with a nice set of main speakers, however, and suddenly the subwoofer has scads of detail, and if the integration is off a little, the bass will sound fast or slow too. All of that sense of speed and detail is coming from the main speakers, but from the midrange, not the woofer. That is why the integration of the woofer and the midrange drivers is so critical to getting a good-sounding speaker.

Another thing to bear in mind: live bass does not sound fast or slow; it just sounds like bass associated with whatever instrument or other source is creating it. The concept of "fast" or "slow" bass is a loudspeaker and audio-system-related thing. Oh, I suppose you could devise a live demonstration to show how the midrange of a string bass can affect the perceived quality of the bottom end of its range (and for all I know, the best musicians may use this to further extend their emotional reach in their playing). But in day-to-day listening situations when you hear live music, I doubt you’ve ever thought about the "fast" or "slow" bass that you were hearing. No, that’s something that happens at home in the reproduction chain, and it’s an artifact of integration errors. Remove the integration errors and the bass loses all sense of being fast or slow, just like live bass.

For you this means something profound. If you hear a system (hopefully not yours) that sounds "fast" or "slow" in the bass, enough that you have noticed anyway, that system has a problem. It might be fixable if the bass is coming from a subwoofer with lots of adjustments. But most of the time, it will take some minor or major change to remove the fast or slow character. A different footer can affect apparent bass speed because it changes the midrange of the electronic component or loudspeaker it is used under, not because it couples (or isolates) to the floor or shelf better. The different foot simply changes the character of the midrange a little bit, and because the midrange and bass quality are so tightly intertwined, the quality of the bass changes too, even though nothing specifically changed in the bass itself.

So there you have it, the symbiotic existence of bass and midrange, which are more tightly interwoven and interdependent that you may have thought.

...Doug Blackburn
db@soundstage.com

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