Air Speed Sound Attenuation Humidity Frequency

Overview

Engineering reference data for Air Speed Sound Attenuation Humidity Frequency in acoustics.

Key Formulas

Speed of Sound

$c = \sqrt{\gamma R T}$

Speed of sound in an ideal gas.

Sound Level

$L = 10 \log_{10}(I/I_0)$

Decibel level.

Wavelength

$\lambda = c / f$

Wavelength = speed / frequency.

Variables

Sound Attenuation in Air: Key Properties

Effect of Humidity on Sound Attenuation

At lower frequencies (below ~10 kHz), higher humidity generally reduces sound attenuation. This occurs because water vapor molecules facilitate the relaxation processes of nitrogen molecules, allowing more efficient energy transfer and less acoustic energy lost to heat.

At frequencies above ~100 kHz, the relationship reverses — higher humidity can increase attenuation due to viscous absorption in the air-water vapor mixture.

Frequency Dependence

Sound attenuation in air increases significantly with frequency. The relationship is approximately proportional to $f^2$ at frequencies below the relaxation frequency. This means:

• Low-frequency sounds (bass) travel farther with minimal loss
• High-frequency sounds are absorbed much more rapidly
• Speech intelligibility degrades over long distances due to selective high-frequency absorption

Typical Attenuation Values

Values at 20°C, 1 atm; actual attenuation depends on temperature, pressure, and exact humidity content.

Engineering Implications

1. Outdoor noise barriers are more effective at attenuating high-frequency noise
2. HVAC duct design must account for frequency-dependent attenuation over long runs
3. Acoustic modeling software requires humidity inputs for accurate predictions at distances >50 m
4. Measurement corrections are necessary when comparing sound levels measured on different days with varying humidity