Today’s subject is the result of one such posted topic. It was suggested by someone that SPL contests would be more challenging and meaningful if there was a distortion limit put on the competitors. In other words, they were suggesting that the measurement be done like an amp specification — put a limit on distortion, something like one percent, and don’t count anything over that. After all, we know by now that if distortion is ignored, virtually all amps can put out a lot more power than they can with a distortion limit. It sure seems reasonable on the surface, but what would it really entail?

For starters, making a distortion measurement of an acoustic device is never easy. This is especially true if the measurement has to be made quickly. Since most competitors in the dB lanes only do short bursts, it would be difficult even if there weren’t greater complications. Most distortion analyzers require some settling time, especially if the signal is unknown in level and frequency before the actual measurement.

Cost would certainly be an issue. It’s usually all that can be done to make a reasonably accurate SPL measurement. If the additional equipment to do a distortion measurement were added, things would complicate to a much greater extent.

So far, all we have talked about can be fixed with money. Money can fix a lot of things, but it can’t fix everything. One of the things it can’t fix is the fact that if we wanted to make distortion limits a part of SPL competition, it’s way too late. From the posts on the ‘Net, it’s obvious that sometimes a little knowledge can be dangerous. While the people posting to the Forum seemed to understand the basic concepts and normal procedures, we are way beyond where normal procedures can be used.

The problem has to do with air itself. Sound is nothing more than an alternating modulation of the atmospheric pressure that is around us all the time. We have alternating positive and negative peaks. In a low distortion sine wave, the positive and negative peaks are identical except for the fact that they have opposite pressure values. The ones we call plus are pressure increases and the ones we call minuses are pressure decreases.

Now, the pressure increases are not that impossible. All it takes to squeeze air is a lot of power (money) — lots of high-powered speakers driven with powerful amps will get the job done. To put this pressure thing in perspective, I like to convert dBs to a PSI (pounds per square inch) rating. Yes that “P” in SPL stands for pressure — air pressure that is. This is exactly the same kind of air pressure that’s in your car tires or air compressor tank. In fact, 200 dB is about 60 PSI. What’s it gonna take to hit 200 dB SPL? We’ll cover that in a future issue. For now, we’ll stay focused on distortion at high pressure.

Air behaves pretty well up to about 160 dB. After that, as we start to squeeze the air, it begins to act like a non-linear spring — the air gets a little stiff and the more you squeeze it the stiffer it gets. The situation can be compared to a set of variable rate springs such as the ones that are put on towing vehicles. When the springs are compressed a little, they move easily, but when they’re loaded, they get really stiff. The big dogs passed 160 dB several years ago. It just so happens that around 160 dB an interesting thing starts to occur. Let’s consider low distortion. By low distortion I mean perhaps one or two percent, such as we like to see in audio specs. Above this level the task gets exponentially more difficult, and when we approach 180 dB, things get really interesting.

At the very high pressures, we run into a complication that even money can’t fix. An SPL of 180 dB requires that the speakers on the forward pressure stroke displace 20 percent of the listening environment. (I hope it isn’t a listening environment, but a measuring environment.) And this 20 percent displacement is based on the condition that there be no air leaks or swelling of the measuring area. No doubt this is quite a task; but, someday, I believe somebody will be up to it. The real problem with doing high SPL with low distortion is on the return stroke.

This is a result of the fact that nature will not allow a pure vacuum. If our speaker were to displace 50 percent on the forward stroke, it would double the pressure (necessary for 190+ dB). But to come close to achieving an equal 50 percent pressure reduction on the return stroke would require 100 percent displacement. Even if the speaker were to move back several times the car’s entire volume, the pressure would never drop to zero. The resulting wave-form resembles a rectified sine wave. In fact, at 200 dB SPL, air that has been modulated by a perfect sinusoidal exciter looks just about like an AC electrical signal that has passed through a diode.

The distortion is horrific. Achieving 200 dB SPL at less than 30 percent distortion is simply impossible. If we ever achieve this level, the one thing that we can be sure of is that Mother Nature will not let us do it at a low distortion level. We are already way beyond that.