Theory

The level decreases with the spherical spreading of a point source, equation (1), and with the air absorption along the path, equation (2). The distance is measured in a straight line from the centre of the orifice; together with the outlet and microphone heights it gives the horizontal distance of equation (3), which is reported. A microphone farther above or below the outlet than $d$ is geometrically impossible and stops the calculation.

At 0.5 m the microphone lies in the near field of the outlet at low frequencies: below $f = c/(2\pi d)$ (about 110 Hz at 0.5 m in air at 25 °C) the point-source law is an approximation, and the report states this.

Equations

A_{div} = 20\lg\frac{d}{1\,\mathrm{m}} + 11\ \mathrm{dB}

(1)

A_{atm} = \alpha d

(2)

d_h = \sqrt{d^2 - (h_r - h_s)^2}

(3)

where	meaning	unit
$d$	Microphone distance (this setting)	m
$A_{div}$	Geometrical divergence	dB
$A_{atm}$	Air absorption	dB
$\alpha$	Atmospheric attenuation coefficient	dB/m
$d_h$	Horizontal distance	m
$h_s, h_r$	Outlet and microphone heights	m

Default and rule

Default: 0.5 m (ISO 5130)
Rule: 0.5 m: ISO 5130 (stationary road vehicles, survey method, 6.3.1) specifies a position 0.5 m from the outlet orifice, at 45° to the gas flow. The angle does not enter the calculation.
Source: The default follows the ISO 5130 measurement position; the propagation terms follow the point-source model of ISO 9613-2 as implemented by the program.

Valid range 0 < d ≤ 100 m; at least the height difference between outlet and microphone

Values

Above 0, at most 100 m. A distance other than 0.5 m is noted in the report.

Theory

Equations

Default and rule

Valid range 0 < d ≤ 100 m; at least the height difference between outlet and microphone

Related 4