Relative humidity

material.ambient.relative_humidity_pct Unit: % Step 04 · Material

Of the air OUTSIDE the muffler (Material step, Ambient air). It is used only for the air absorption of the radiated SPL. It is not the humidity of the gas inside the muffler (Relative humidity, Fluid: Humid air).

The relative humidity of the outside air at the measurement position. The water-vapour content sets the relaxation frequencies of oxygen and nitrogen, and through them the air absorption between outlet and microphone.

Last updated 2026-11-11

Theory

The attenuation coefficient (Ambient air temperature) requires the molar concentration of water vapour $h$ . The program derives it from the relative humidity, the saturation vapour pressure over liquid water at the air temperature, and the ambient pressure, equations (1) and (2) (ISO 9613-1 Annex B).

The absorption does not vary monotonically with humidity: the humidity shifts the relaxation frequencies, and with them the frequency range in which each relaxation process absorbs most.

Equations

h = h_{rel}\,\frac{p_{sat}/p_r}{p_a/p_r}

(1)

\frac{p_{sat}}{p_r} = 10^{\,C}, \qquad C = -6.8346\left(\frac{T_{01}}{T}\right)^{1.261} + 4.6151

(2)

where	meaning	unit
$h$	Molar concentration of water vapour	%
$h_{rel}$	Relative humidity (this setting)	%
$p_{sat}$	Saturation vapour pressure	kPa
$p_a$	Ambient pressure	kPa
$p_r$	Reference pressure, 101.325	kPa
$T$	Air temperature	K
$T_{01}$	Triple-point isotherm temperature, 273.16	K

Default and rule

Default: 70 %
Rule: 70 %. This is the program's default, chosen within the tabulated range of 10-100 %; it is not taken from a standard.
Source: ISO 9613-1 Annex B. The conversion is verified through the attenuation coefficients against tabulated values (see Ambient air temperature).

Valid range 10 to 100 %

Values

10 to 100 %.

Theory

Equations

Default and rule

Valid range 10 to 100 %

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