For many years, I worked as an acoustical consultant in Southern California. We had seasons in SoCal, but it took several years before I easily recognized the subtle differences between summer and fall or between winter and spring. Summer brought daytime temperatures in the 80′s while wintertime temperatures tended to range in the upper 60′s. But for the most part, the temperate climate meant that days would be warm, dry, and sunny and nights would be cooler, dry, and cloudless. My point: long term monitoring of outdoor noise levels and sound propagation was easy.
After ten years on the West Coast, I returned to the Midwest to work for Acoustics By Design, Inc., and found myself back in the midst of weather – real weather – blizzards, sticky summers, and thunderstorms. Most people know that weather can influence sound propagation, but by how much?
First of all, wind alters sound propagation by the mechanism of refraction; that is, wind bends sound waves. Wind nearer to the ground moves more slowly than wind at higher altitudes, due to surface characteristics such as hills, trees, and man-made structures that interfere with the wind. This wind gradient, with faster wind at higher elevation and slower wind at lower elevation causes sound waves to bend downward when they are traveling to a location downwind of the source and to bend upward when traveling toward a location upwind of the source. Waves bending downward means that a listener standing downwind of the source will hear louder noise levels than the listener standing upwind of the source. This phenomenon can significantly impact sound propagation over long distances and when wind speeds are high.
Another factor that can impact sound propagation over long distances is temperature gradients in the atmosphere. On a typical sunny afternoon, air is warmest near the ground and temperature decreases at higher altitudes. This temperature gradient causes sound waves to refract upward, away from the ground and results in lower noise levels being heard at the listener’s position. In the evening, this temperature gradient will reverse, resulting in cooler temperatures near the ground. This condition, often referred to is a temperature inversion will cause sound to bend downward toward the ground and results in louder noise levels at the listener position. Like wind gradients, temperature gradients can influence sound propagation over long distances and further complicate measurements.
However, for the majority of outdoor noise assessment projects that we encounter involving sources such as freeways, factories, emergency electrical power generators, and commercial cooling towers, the residences or other noise sensitive receptors are relatively close to the noise source, usually within several hundred feet. Over these short distances, wind direction has a small impact on sound propagation as long as wind velocities are reasonably slow, 10 mph or lower.
So what about changes in humidity? Let’s look at some numbers. At a temperature of 60 degrees Fahrenheit, a decrease in relative humidity from 80% to 20% would decrease the sound level at a listener standing a ½ mile from the noise source by 3 dB (at 1,000 Hz).
What about changes in temperature? If we fix the relative humidity at 80%, an increase in temperature from 60 degrees to 95 degrees would decrease the sound level at a listener standing a ½ mile from the noise source by 3 dB (at 1,000 Hz).
So a relative humidity decrease lowers the noise level at the receiver and a temperature decrease raises the noise level at the receiver. But how much is a 3 dB change? Let’s take a moment to place this change into perspective. Generally, a 3 dB increase or decrease in sound is just barely detectable to the human ear (and that’s under controlled laboratory conditions). In the real world, a change of less than 3 dB is undetectable by those listening. A 5 dB change is considered to be easily discernible and significant, and a 10 dB change is perceived as a doubling (or halving) of the sound.
What should be obvious is that for wind, humidity, and temperature to have a significant, audible impact, the sound receptors must be located a long distance away from the noise source. In most situations that we encounter, the residents are much closer to the noise source, and the affects of wind, humidity, and temperature are insignificant.
That said, there are other ways that the weather can alter a sound reading, even if the measurement location is only a few hundred feet from the noise source. Wind rustling leaves can change the measurements, unless it is winter time and the trees are bare. The sound of rain falling near a measurement microphone can generate noise unrelated to the source under study. In the winter, the porous nature of snow can act as an acoustical absorber and soak up a portion of the sound before it reaches the microphone.
When taking outdoor noise measurements, there are numerous conditions that come into play, and many of these factors can be missed by the untrained eye (or ear, as the case may be). In less than usual conditions, where the measurement of sound propagation over long distances is desired, the weather itself can pose a significant factor. But with the vast majority of the outdoor noise measurements we take at ABD, it is the conditions caused by the weather that can affect the study.