EN-03Environmental & Occupational Noise

Noise Barrier Insertion Loss

Path difference, Fresnel number and the Maekawa estimate for a thin screen.

Geometry

Source → barrier distance

Barrier → receiver distance

Source height

Receiver height

Barrier height

Frequency

Insertion loss @ 500 Hz

15.4dB

Path difference δ

0.541m

Fresnel number N

1.58

Line of sight

Blocked

Section viewvertical scale exaggerated

Insertion loss by octave bandMaekawa, thin screen, capped 20 dB

§ Theory of operation

Diffraction over a screen

A barrier does not stop sound completely, it forces it to diffract over the top edge. The longer the detour compared with the wavelength, the greater the attenuation. The detour is the path difference $\delta$ between the diffracted path (source → top edge → receiver) and the direct line:

\begin{aligned} \delta &= A + B - d \\ N &= \dfrac{2\delta}{\lambda} = \dfrac{2\delta\,f}{c} \qquad \text{(Fresnel number)} \end{aligned}

Maekawa's chart

Maekawa's 1968 laboratory measurements on thin screens are well fitted by:

IL \approx 10\,\log_{10}(\,3 + 20\,N\,)\ \ \mathrm{dB}, \qquad N > -0.1

At grazing incidence (N = 0, the top edge just touching the line of sight) the barrier still gives about 5 dB. When the line of sight is clear (negative N) the effect fades quickly. In practice, because of scattering by turbulence and flanking around the ends, a single thin barrier rarely gives more than 20 dB (about 25 dB for thick or double edges); this tool caps the result at 20 dB.

Design rules

Place barriers close to the source or the receiver, the path difference is largest there.
High frequencies (short wavelengths) are screened far better than low frequencies; traffic rumble passes over most walls.
The barrier itself must have a sound reduction at least 10 dB higher than the expected insertion loss (typically ≥ 10–15 kg/m²) and no gaps.
The barrier should be long enough that sound flanking around the ends is weaker than over the top.

For a more complete outdoor method including ground effects and meteorology, see ISO 9613-2.