Acoustics Engineering - Noise barriers

In Situ measurement of noise reducing devices

Roadside noise barriers come in all shapes and sizes, but there are few methods to measure their effectiveness. The CEN/TS 1793 series of standards describe in situ measurements of reflection, transmission and diffraction properties. Reflection is typically measured using the subtraction technique developed by Yuzawa[1], Mommertz [2] and others.

Since the first publication of the CEN/TS 1793-5 standard in 2003, Acoustics Engineering has supported this measurement method with the Zircon loudspeaker-microphone probe and the DIRAC software.

From 2010 to 2013 the CEN/TS 1793-5 European standard for in-situ measurement of sound reflection and airborne sound insulation characteristics of noise barriers has been significantly enhanced and validated in the frame of the EU funded QUIESST project. This resulted in the current CEN/TS 1793-5:2016 standard which is supported by the Zircon V2 system and Dirac software.

Reflection
In reflection measurements, a signal is played through a loudspeaker in the direction of the barrier. A microphone placed between the loudspeaker and the barrier picks up both the transmitted and the reflected signal. In a reference measurement without the barrier there is no reflection, and only the transmitted signal is recorded. Subtracting the reference measurement from the barrier measurement isolates the reflected signal. Comparing the transmitted and reflected signals allows a determination of the reflection coefficients of the barrier.

The image below shows a reflection measurement. The impulse response peak resulting from the direct sound from loudspeaker to microphone can be seen near the 4 ms mark. The reflected signal peak appears around 5.5 ms. Note that the delay between these two signals (1.5 ms) corresponds to the flight time from the microphone to the noise barrier and back again to the microphone. With a nominal distance of 0.25 m between microphone and barrier, this time is expected to be 2 x 0.25 / 343 = 1.46 ms.

The next image shows the reference measurement - without a barrier. This signal can be subtracted from the signal above to leave only the reflected signal.

The image below shows the processed signal with the isolated reflected component in the upper trace and the reference signal in the lower trace. Also visible are the Adrienne windows used to remove any parasitic reflections from the surroundings.

Transmission
The CEN/TS 1793-6 standard deals with sound transmission through noise barriers. The latest version of this standard appeared in 2016, and is supported by the Zircon V2 system and Dirac.

In transmission measurements there is no need to isolate any part of the measured signal, and the subtraction technique is not used. One simply compares the signal recorded with a noise barrier between the loudspeaker and the microphone, with the signal recorded without a noise barrier between loudspeaker and microphone.

Diffraction
When noise is propagated over the top of a noise barrier, it may experience diffraction and appear at the 'silent' side of the barrier. The diffraction behavior of sound barriers can be measured following the procedures described in the CEN/TS 1793-4 standard.

These measurements are mostly the same as those used for transmission measurements (with different microphone positions), and thus the Zircon V2 system together with the Dirac software can be used to find the diffraction properties.

Further reading
[1] M. Yuzawa, “A method of obtaining the oblique incident sound absorption coefficient through an on-the-spot measurement,” Appl. Acoust. 8, 27–41 (1975).

[2] E. Mommertz, “Angle-dependent in-situ measurements of reflection coefficients using a subtraction technique,” Appl. Acoust. 46, 251–263 (1995).

[3] QUIESST - QUIetening the Environment for a Sustainable Surface Transport. Final report summary.

Noise Barriers