To calculate the wavelength of a wave, we need to know its frequency and speed. The speed of sound in the air at room temperature (20°C) is approximately 343 meters per second.

Wavelength (λ) = Speed of sound (v) / Frequency (f)

λ = 343 m/s / 400 Hz

λ ≈ 0.8575 meters

Therefore, the wavelength of a 400Hz sound wave in the air at room temperature is approximately 0.8575 meters.