jun 02, 2023

Acoustic measurements: The effects of weather on sound propagation

  • Artículo
  • Environmental acoustics
  • Weather effects
  • noise

Weather conditions influence noise levels, especially over long distances, and need to be considered when taking acoustic measurements. In Québec, the Ministère de l’Environnement, de la Lutte contre les changements climatiques, de la Faune et des Parcs (MELCCFP) has established criteria for weather conditions that must be met in such situations. The criteria are set out in Note d’instructions [guidance note] 98-01.

  1. According to this note, a noise measurement is considered valid if the following are met:

    • The wind speed does not exceed 20 km/h
    • Humidity is not higher than 90%
    • The pavement is dry and there is no precipitation
    • The ambient temperature is within the tolerance limits specified by the manufacturer of the measuring equipment
  2. Wind

    Wind affects sound transmission over long distances by increasing or decreasing the speed of sound.

  3. Downwind conditions: wind in the same direction as the sound

    The speed of sound increases with altitude and sound waves are refracted toward the ground, increasing the expected noise level at a great distance. The stronger the wind, the more pronounced the effect.

  4. Upwind conditions: wind in the opposite direction of the sound

    In upwind conditions, the speed of sound decreases with height and sound waves are refracted away from the ground. A strong and persistent wind can also create a shadow zone (when the sound waves can’t propagate), as shown in the diagram below.

  5. Temperature

    Heating and cooling of the atmosphere follows the gas law: gas cools when it expands and heats when it is compressed into a smaller space, because the molecules are closer together and bump into each other more often. This process of heating and cooling occurs as air rises and falls in the atmosphere. Atmospheric pressure near the ground is higher than at higher altitudes, so the air near the ground is warmer. When an air mass is warmer than its surroundings, it rises. In addition, during the day, the ground heats up as it absorbs solar radiation.

  6. Temperature gradients

    A constant temperature with altitude produces no effect on sound transmission, but temperature gradients can produce bending in the same way as wind gradients.

    The air temperature above the ground is usually colder than at the ground and the denser air above the ground tends to bend sound waves upward, as shown in the figure below. In the case where the temperature decreases with the height, sound is refracted upward. For example, on a sunny day with no wind, the temperature decreases at higher altitudes, creating a zone that is not conducive to sound propagation.

  7. Inversion

    During a “temperature inversion,” warm air above the surface bends the sound waves toward the ground. On a clear night, for example, the temperature rises at higher altitudes, helping to push sound toward the ground. As such, air temperature increases with height, sound is refracted downward and is known as a favourable condition for sound propagation over long distances. Inversions are more pronounced in hilly or mountainous areas in the summer when the air mass is stable. In particular: its effects in mining pits are far from negligible.

  8. Atmospheric absorption

    As sound waves travel through the air, a small portion is absorbed by the air and the absorbed sound energy is converted into heat. The amount of absorption depends on temperature, humidity and sound frequency. Sound energy is lost due to collisions between air molecules. High frequencies are much more affected by atmospheric absorption than low frequencies. Atmospheric absorption increases linearly with distance; over short distances, absorption is negligible. The ISO 9613-2 standard for propagation calculations defines the absorption attenuation using the empirical formula A amt = ad/1000, where a is the coefficient of atmospheric absorption in each octave band. We can see below that the air absorption at various frequencies is dependent upon temperature and humidity.

    The graphs below show the dependence of atmospheric absorption on temperature and humidity, as obtained in experimental laboratory measurements. You can see that for the mid-range of voice frequencies (2 kHz), the absorption is generally 0.25 dB/100 m at a relative humidity of 30% and a temperature of 20°C (see the point in the figure to the left). It can reach 5 dB/100 m at 8 kHz (high frequency) when the temperature is 35°C and the humidity is 10% (see the point in the figure to the right).

  9. Effect of precipitation

    Sound propagates in two ways: it emanates from the source, and it refracts when it hits an object or the ground. Sound is refracted on the ground depending on the type and nature of the soil. Some is absorbed and some is reflected, interfering with the sound coming from the source; attenuation due to the ground is calculated according to the frequency and the type of soil.

    Precipitation modifies the soil and affects attenuation. Snow is conducive to quiet because there is very little effect of sound reflection. A flake of snow is dense. Thus, snow consists of many tiny air pockets, and the contact surface between ice crystals and air is uniform: sound waves are strongly absorbed. In addition, the snow can cause significant thermal gradients. On the other hand, rain generates noise, which contributes to the ambient noise level. For example, wet pavement increases the noise level of a road.

  10. In short, weather matters!

    Variations in wind, temperature and humidity are meteorological and atmospheric factors that change over time and that combine to influence sound propagation. This influence becomes significant when the acoustic environment simulation involves industrial activities that change over the course of the year (summer or winter) or when sources are located at a height (e.g., chimneys or wind turbines).

    With our expertise and experience in environmental acoustics, we can assess sound levels and perform simulations using appropriate weather conditions for our client’s operational context. This ensures that we obtain precise measurements based on best practices and avoids negative surprises for every client.

    Consider our acoustics expertise to address your sound and noise pollution issues.

  11. References

    Hannah, L. (2007). Wind and temperature effects on sound propagation, New Zealand Acoustics. 20;2.

    Brüel & Kjaer. Environmental Noise Handbook. [French only].

    United States Air Force (1995). Noise and vibration control, a technical Air Force manual. 88-3.

    Harris, C. Absorption of sound in air versus humidity and temperature. Journal of the Acoustical Society of America, 40:148.

    Wikipedia (2023). Refraction.

    Ministry of the environment, the fight against climate change, wildlife and parks (MELCCFP) (2006). Guidance note 98-01 [French only].

This content is for general information purposes only. All rights reserved ©BBA

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