Reducing Noise Power and Pushing it Around

Blog Post created by benz on Feb 13, 2017

  Jet engines, furry hoods, and a nod to four years of the Better Measurements blog

This blog gives me occasional freedom to explore technology and phenomena that have only a peripheral relationship to RF design and measurement. Sometimes it feels like a permission slip to cut class and wander off to interesting places with remarkable analogs to our world. I say this by way of warning that it may take me a while to get around to something central to RF engineering this time.

This little side trip begins with high-bypass turbofans and the artistic-looking scallops or chevrons on the outer nacelles and sometimes the turbine cores.

Turbofan jet engine with chevrons or scallops on trailing edge of engine nacelle and engine core, to reduce turbulence and noise.

The NASA-developed chevrons or scallops at the trailing edge of this turbofan engine reduce engine noise by causing a more gradual blending of air streams of different velocities. This reduces shear and the resulting noise-creating turbulence. They look cool, too. (Image from Wikimedia Commons)

In my mental model, shear is the key here. Earlier turbojets had a single outlet with very high velocity, creating extreme shear speeds as the exhaust drove into the ambient air. The large speed differences created lots of turbulence and corresponding noise.

Turbofans reduce noise dramatically by accelerating another cylinder of air surrounding the hot, high-speed turbine core. This cylinder is faster than ambient, but slower than the core output, creating an intermediate-speed air stream and two mixing zones. The shear speeds are now much lower, reducing turbulence and noise.

The chevrons further smooth the blending of air streams, so turbulence and noise are both improved. It’s a complicated technique to engineer, but effective, passive, and simple to implement.

Shear is useful in understanding many other technologies, modern and ancient. When I saw those nacelles I thought of the Inuit and the hoods of their parkas with big furry rims or “ruffs.” In a windy and bitter environment they reduce shear at the edges of the hood, creating a zone of calmer air around the face. A wind tunnel study confirmed Inuit knowledge that the best fur incorporates hairs of varying length and stiffness, anticipating—in microcosm—the engine nacelle chevrons.

The calm air zone reduces wind chill, and it also reduces noise. Years ago I had a parka hood with a simple non-furry nylon rim that would howl at certain air speeds and angles.

Another, more modern example is the large, furry microphone windscreen called (I am not making this up) a “dead cat.” At the cost of some size, weight, and delicacy (imagine the effect of rain) this is perhaps the most effective way to reduce wind noise.

The opposite approach to shear and noise is equally instructive. “Air knife” techniques have been used for years to remove fluids from surfaces, and you can now find them in hand dryers in public restrooms. They inevitably make a heck of a racket because the concentrated jet of air and resulting shear are also what makes them effective in knocking water from your hands. Personally, I don’t like the tradeoffs in this case.

In RF applications, we generally avoid the voltage and current equivalents of shear and undesirable signal power. When we can’t adequately reduce the power, we shape it to make it less undesirable, or we push it around to a place where it will cause less trouble. For example, PLLs in some frequency synthesizers can be set to optimize phase noise at narrow versus wide offsets.

Switch-mode power supplies are another example of undesirable power, typically because of the high dv/dt and di/dt of their pulsed operation. It isn’t usually the total power that causes them to fail EMC tests, but the power concentrated at specific frequencies. From a regulatory point of view, an effective solution can be to modulate or dither the switching frequency to spread the power out.

One final example is the tactic of pushing some noise out of band. Details are described in an article on delta-sigma modulation for data converters. Oversampling and noise shaping shift much of the noise to frequencies where it can be removed with filtering.

I’m sure that’s enough wandering for now, but before I finish this post I wanted to note that we’ve passed the four-year anniversary of the first post here. I’d like to thank all of you who tolerate my ramblings, and encourage you to use the comments to add information, suggest topics, or ask questions. Thanks for reading!