Influence of temperature and humidity on the propagation of airborne sound in the atmosphere

​Airborne sound is attenuated as it propagates in the atmosphere depending on frequency, temperature and humidity; this is particularly relevant at medium and high frequencies and large distances between the sound source and the receiving location. With increasing temperature and medium to high humidity, both the speed of sound and the strength of absorption change significantly. Airborne sound absorption is the conversion of sound energy into heat through molecular friction and relaxation processes of the air components.The atmospheric absorption coefficient is usually given in dB per 100 m and increases sharply with frequency; low frequencies are much less attenuated than high frequencies.

Influence of temperature
The speed of sound in the air increases with temperature and is at 0 °C at about 331 m/s, at 20 °C at about 343 m/s and at 35°C at 352 m/s

. With increasing temperature, the relaxation processes of the air gases shift, which changes the frequency-dependent absorption; at high frequencies, the air attenuation can increase significantly with temperature.

Influence of humidity
Humidity changes the composition and relaxation properties of the air, which changes the atmospheric absorption coefficient as a function of frequency and relative humidity.
For many frequencies above about 1 kHz, the absorption initially increases with increasing relative humidity up to the range of 20% and then decreases again at even higher humidity. Very dry as well as very humid air therefore attenuate high-pitched sounds to varying degrees.

Practical significance
In normal indoor spaces, material and surface absorption dominate. Air absorption only becomes relevant in large rooms or outdoors at distances of tens to hundreds of meters. Particularly in the case of traffic noise, aircraft noise, noise from industrial plants or sound reinforcement over long distances, temperature and humidity profiles must be taken into account in measurements because they can noticeably reduce the levels of high frequencies at the receiving location.

Calculation / Standards
For technical calculations of air absorption, the ISO 96131 standard is often used, which indicates the atmospheric absorption coefficient as a function of temperature, relative humidity and frequency. The standard takes into account frequencies from 50 Hz - 10 kHz), temperatures from -20°C - +50°C, a relative humidity of 10% - 100%) and air pressure.
 

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