Technology & Engineering

Feed Forward Distortion Suppression Technology

Geek Alert! Using Math to Squelch Damaging Distortion in Your Subwoofer.

At Polk, we want our cake and we want to eat it, too. And by cake, we mean bass. And by eat it, we mean that we want huge, deep, musical bass, totally off the richter scale, but we don’t want to worry about our subwoofers self-destructing.

Our challenge has been to come up with a subwoofer design that avoids distortion and damage, but preserves as much of a sub’s dynamic capability and maximum output capacity as possible. Feed Forward Distortion Suppression Technology allows us to have our cake and blast it, too.

How to Kill a Subwoofer

We’ve found (through dedicated, and fun, research) that there are three main reasons why a subwoofer will self destruct.

  • Too much power from the amplifier causes overheating of the voice coil. This generates high levels of distortion, and ultimately causes the failure of the woofer driver.
  • Too little power coming from the amplifier can cause high distortion in the signal fed to the woofer driver, generating excess heat and ultimately burning out the amplifier.
  • Over-driving the low frequencies to the subwoofer will cause unnatural, excessive movement of the driver itself. This leads to distortion, then objectionable mechanical noises (that’s the technical term for something’s banging around in there!) and finally to actual physical damage as the driver literally tears itself apart.

Today’s subwoofer amplifiers are usually designed in conjunction with a particular sub’s mechanical system, and advances have been made in voice coil technology (making burn-out much less likely). So the first two dangers have become redundant.

But overdriving the low frequencies, and the mechanical damage that can result, can still be a problem. Distortion and damage can occur almost instantaneously regardless of how well matched the woofer driver and amplifier may be. It may be triggered by any number of complex signal combinations, which can be very difficult to predict. The cheap way to protect against these rogue signals is to just limit the output of the subwoofer far below its maximum output capacity. As an effort to guarantee a safe level of operation, this is an adequate solution. But if you love dynamic, musical bass (and lots of it), this solution is not the solution.

One Signal is Many Signals

Did you know that it takes different amounts of power from the amplifier to produce the same amount of sound at different frequencies? Because of this, combinations of signals at different frequencies can overload a woofer, while the same signal level at a single frequency would be no problem. It’s like asking a unitasker, someone who can only do one thing at a time, to become a multitasker, and to do many things at the same time. I resemble this remark.

As an example, let’s consider a hypothetical 10" powered subwoofer in a 2.5 cu ft ported box. Figure 3, shows the frequency response and impedance of the system. As you can see, the frequency response (the red line) is flat above 30Hz (below 30Hz it slopes down), but the impedance (the blue line) is far from flat. It rises from around 44 ohms [*] at 30Hz to 76 ohms at 20Hz, and 100 ohms at 50Hz. Ohms are the measure of the amount of power being drawn from the amplifier. In the dip, at 30Hz, the woofer is drawing only 1/9th as much power as it is at 20Hz, and 1/15th as much power as it draws at 50Hz. If the amplifier has enough power to drive the system to maximum output at 30Hz, it will have way too much power for the system to handle at 20Hz and 50Hz.

Unfortunately, all of these frequencies are important for the subwoofer. So you can see why it would be necessary to have some form of limiting protection, and why it’s so difficult to limit or protect the subwoofer without compromising its maximum performance capacity.

[*] What you absolutely do not need to know at this point is that this blue impedance curve has been offset upward by 40 ohms for clarity.

Signal Compression Sounds Muddy

Figure 4 shows the maximum acoustic output of our hypothetical subwoofer versus frequency, assuming a 500 W amplifier. As you can see, the maximum output at 30Hz is about 25dB greater than the maximum output at 20Hz, and at least 6dB greater than the output that occurs in the dip between 40Hz to 50Hz.

If we decided to use a variation on our cheap solution limiting overall output, and apply compression to the amplifier signal over the entire bandwidth of this subwoofer in order to protect it from being overdriven, we might roll off the low frequency response below 30Hz (even though we would lose significant low bass output). We might also limit the amplifier to a safe level well below the maximum output in the 40Hz to 50Hz range. It would be no surprise that this compression would tend to make a subwoofer sound muffled, with no extension and no punch at all.

Mechanical Limitations have Serious Limitations

Another way we could prevent a mechanical failure from overdriving would be to apply mechanical limiting to the actual movement of the woofer driver itself. Driven by a signal from your amplifier, the driver is vibrating back and forth on the pole piece. Using a position sensor in a servo feedback system, coupled to circuitry that compares the input signal to the cone movement, we can provide a correction signal. In theory, the correction signal can modify the amplifier’s original signal to correct for any over-drive, avoiding mechanical distortion before it begins.

In laboratory tests, servo feedback systems produce excellent results, keeping subwoofers safe from the dangers of over-driving. But laboratory tests are usually done with steady state (static) test tones. Real program material (i.e., music) is dynamic, and we’ve found that even the most complex servo feedback systems cannot correct for bad signals until the bad signal has already happened. The servo feedback systems are simply reactive. (And the more heavy-handed your servo correction algorithms are, the more sluggish the system sounds as it tries to measure, compare and correct in a dynamic signal environment.) We also had a lot of fun finding out that the correction signal has a tendency to go in the wrong direction, asking the woofer to correct itself by moving beyond its mechanical limits. Therefore, broadband limiting and compression are still required to protect the woofer driver against correction signals that might seriously overload the system themselves. So, what’s the point?

Be Proactive!

What we really needed was a system that limits only what absolutely needs to be limited, and that does it proactively rather than reactively.

To make this magic trick work, we had to think about the signal backwards. Instead of going from the amplifier to the subwoofer, we decided to go from the subwoofer to the amplifier. We programmed our digital subwoofer processors with an intricate mathematical model that knows in advance how the woofer’s own acoustic system will respond to the output signal from the amplifier. The digital processor monitors the amplifier for signal combinations that would cause distortion or damage, and alters only that part of the signal required to maintain safe operation. It does this instantly, before the signal ever gets to the woofer driver itself, so there’s no feedback correction time delay, and no need for limiting or compression.

This is how Polk’s proprietary Feed Forward Distortion Suppression Technology anticipates problems and fixes them before they happen.

The Magic of Distortion Free Bass Response

Another benefit of our proactive Feed Forward Technology is that performance limits can be set much higher, closer to the theoretical maximum output of the subwoofer, at each frequency, with no risk of excessive distortion or damage. In our hypothetical subwoofer, back in figure 4, the Polk Feed Forward System would know in advance that the sub’s output capability is lower at 50Hz. Even if limiting at 50Hz were required, Feed Forward would still preserve the full dynamics of the signals above and below that frequency (including the subwoofer’s low bass capabilities), while providing protection against overdriving any of the frequencies being reproduced.

Unlike the other intrusive distortion reduction or mechanical protection systems that we’ve discussed, Feed Forward Technology is undetectable in operation.

By knowing beforehand how the woofer driver will respond to complex signals, we can respond to those signals before they even reach the driver. With minimum intervention in the fast mechanics of bass response, we tune out damaging distortion even at big bass volumes. We never compromise low frequency dynamics or transient response, and we actually increase maximum output. And when it comes to bass, we wouldn’t have it any other way.

Feed Forward Distortion Suppression Technology is an exclusive Polk innovation. It guarantees that your subwoofer will deliver all the excitement and impact, or deep musical subtlety, you demand in your bass response. In other words, you can finally have your cake and eat it, too.

This article was last modified on Mar 26, 2013

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