June 2019 Newsletter: Gone with the Wind

As we all know, wind is moving air. Wind is caused by differences in the atmospheric pressure – when a difference in pressure exists, air moves from the higher to the lower pressure area, resulting in winds of various speeds. Sound is a vibration in the air hitting our eardrum, so both wind and sound are basically air movement. Is it possible for wind to blow away sound? The answer is yes! Read on to learn about how wind and other mother nature elements can influence sound.

Just like an arrow shot in the wind, sound propagation can be affected by enough force and completely “miss” its intended target. See the illustration below showing how a cross wind can blow sound away. Headwinds and tailwinds can widely affect how sound propagates as well.

What’s more interesting is the wind shear effect across a sound source, which refracts the sound. As a result, a sound shadow is created upwind.

Sometimes we are asked creative questions like “Can we stop the noise right at the property line with an invisible barrier?” Well, one possibility is to have a strong gust blowing across the noise source putting the property line under the sound shadow, but since mother nature is out of our control, we can’t rely on it as a noise control measure. Plus, wind itself is noisy.

Temperature gradients also refract sound as they bend sound propagation towards the cooler side, and a sound shadow may be created as the result of warmer air near the ground and cooler air above.

This phenomenon can help explain a common myth found in high school football field sound systems. A typical system has loudspeakers on top of the home bleachers aiming toward the visitor bleachers. Sound technicians often experience that the sound level at the visitor bleachers varies dramatically depending on the season, weather, and the time of a day, even if the system settings aren’t changed at all. If the visitors can’t hear the loudspeakers well, the system is probably just working fine, but the cooler air above the warmer ground may have put the visitor bleachers under a sound shadow.

While air movement and temperature gradients can manipulate sound propagation, what makes it even more complicated is that air itself also absorbs sound. The amount of absorption depends on the frequency, temperature, and humidity. These factors don’t have a linear relationship that can be calculated by a formula, but based on the measurement data shown below, we can deduce two facts: air that is extremely dry or very humid tends to be less absorptive (most sound is absorbed around 10-30% humidity); and air absorbs much more high frequency sound than low frequency sound.

We have worked on many outdoor amphitheaters to measure or predict the sound level at residential properties thousands of feet away. High frequency sound is rarely a problem thanks to air absorption; but we have observed distinct low frequency boom more than a mile away during an EDM concert.

It is impossible to take full control of mother nature, or any control at all! As acoustical consultants, we can only study its various impacts to sound propagation and use that knowledge to design solutions. You can never beat Mother Nature, but you can use it to your advantage sometimes.


July 2019 Newsletter: Beam me up Scotty
May 2019 Newsletter: Low End Theory