Humboldt effect

In acoustics, the Humboldt effect is the daily change in sound intensity with which a sound source is perceived by the listener. The effect is named after Alexander von Humboldt , who was the first to formulate a useful qualitative explanation for this phenomenon.

The influence of temperature on the sound intensity

The sound waves emitted by a sound source are reflected in the atmosphere at temperature-related density inhomogeneities (sound scattering). As a result of the atmosphere, which is normally stabilized by cooling at night, the boundary layers that cause reflection are broken down, the atmosphere is homogenized and the sound scattering decreases. A temperature inversion that sometimes occurs when the air temperature increases with altitude increases this effect. The reduced disturbance of the sound waves is perceived by the listener as a greater volume . Therefore, one perceives the sound intensity of a certain sound source differently during the day.

History

Qualitatively, this phenomenon was already known in ancient times. Alexander von Humboldt noticed the effect during his trip to South America from 1799 to 1804 on the waterfalls of the Orinoco and on the volcanoes of the Andes. He also found that it diminishes at sea and at high altitude. Humboldt realized that this effect was not a result of reduced background noise at night. He saw the cause in the different warming of the earth's surface depending on the shape and vegetation of the ground and the resulting density inhomogeneity of the air, whereby the sound waves would be reflected more strongly. In it Humboldt found an application for the recently published theory of sound by Poisson .

literature

• Alexander von Humboldt: Sur l'acroissement de l'intensité du son. (Mémoire lu à l'Académie des sciences le 13 mars 1820.) In: Annales de chimie et de physique 1820, pp. 162-173.
• Alexander von Humboldt: About the increasing strength of sound at night. In: Annals of Physics . 65th Volume (1820), pp. 31-43.
• Karl Bruhns (Ed.): Alexander von Humboldt. A scientific biography. FA Brockhaus, Leipzig 1872, pp. 88-89 .
• Hans Ertel : A problem of meteorological acoustics. (The diurnal variation of the sound intensity). In: Meeting reports of the German Academy of Sciences in Berlin . Math, Physics, and Engineering Class No. 2, 1955.
• Astrid Ziemann: From the Humboldt effect to remote sounding methods - sound propagation in the atmosphere. In: Meeting reports of the Leibniz Society, Volume 71 (2004), pp. 79–88, here pp. 80–81.