Performance above 20kHz matters
Fact or fiction?
Does amplifier performance above 20kHz matter?
This question came first in our list of Fact or Fiction because it’s a fairly easy one to answer but the answer’s going to piss off a whole bunch of objectivists. Let’s try and fix that so everyone wins.
To an objectivist, 20kHz and above does not matter for one plain and simple reason: humans cannot hear above 20kHz. Ergo, if we cannot hear something then it doesn’t matter. Case closed.
While the base statement is accurate, it ignores the consequences of implementation.
If we instead look towards the practical application of high bandwidth amplifiers we see that we could ask a very different question and get a very different answer. The right question to be asking is this:
Does phase accuracy matter within the pass band of human hearing?
Now we have a proper question that circumvents all the narrow focus arguments that plague us.
The answer to question 2 is an unequivocal yes! Human hearing is very sensitive to phase differences within the audio pass band.
Circling back to the first question, we can now answer it with another unequivocal yes! because there are no practical means of bandwidth limiting an analog amplifier to 20kHz without affecting the phase. It’s typically important to have a bandwidth approaching 100kHz to get phase accurate performance in the audio pass band.
See? That wasn’t so hard.
Not necessary for good subwoofer performance, but there are many good subs using this technology.
Sensing the right thing
A servo system is a generic name that has many meanings depending on who is using it. To me, a servo woofer system involves a motional feedback element capable of measuring the woofer’s acceleration and position. To quite a number of others, a servo woofer can be as simple as another coil of wire on the woofer. Both definitions are correct but the results of each are radically different.
Of the few servo woofer systems available today most rely upon a second woofer voice coil. The first voice coil acts as the magnetic motor that drives the woofer’s cone back and forth in response to the amplifier’s output. The second voice coil generates a small voltage in proportion to the woofer’s movement. If the sensing voltage is compared to the driving voltage an approximate difference signal can be derived that can be used to correct for the box enclosure’s restrictions. What you get is a flatter output from the woofer, thus we have a servo system.
In the servo systems I prefer, an accelerometer or positional sensor is mounted to the woofer that can not only measure the woofer’s movement but additionally offers precise information about its rate of acceleration and exact position. When this signal is compared to the original amplifier’s input far more information is available. Because we can now trace the exact position of the woofer cone in real time our difference signal can be used for more benefits than a simple second coil. An accelerometer-based system flattens frequency response but then goes beyond what a second coil offers: lower distortion, reduced overhang, improved transient response.
And here’s where you can tell if a system is really giving you what you assume to be advanced technology. Can it be achieved by other, simpler methods? The simple servo can be replicated by EQing a particular woofer to a specific box—but it’s far sexier to call it a servo system.
The next time you hear a fancy technology label applied to a product, take a peek under the covers to discover what the claimed benefits of that system are.