Application of sound absorption measurements according to EN 1793-5 on gabion walls Wout Schwanen 1 M+P consulting engineers Vught, the Netherlands WillemJan van Vliet 2 Rijkswaterstaat Grote Projecten en Onderhoud Utrecht, the Netherlands

ABSTRACT The standard EN 1793-5 describes a method to determine the sound reflection index of road traffic noise reducing devices under direct sound field conditions in-situ. The method is commonly used to assess the intrinsic characteristics of conventional, absorptive noise barriers and yields results with a limited measurement uncertainty. Recently, the sound reflection index of a gabion wall was determined by means of the in-situ measurement method leading to unexpected results. An in-depth analysis was made of the results of the in-situ measurements on a conventional noise barrier and on a gabion wall. The main conclusion of this comparison is that application of the in-situ method on a gabion wall has a much higher measurement uncertainty than to be expected.

1. INTRODUCTION

The European standard EN 1793-5 describes a measurement method to determine the sound reflection of noise barriers in-situ. An array of nine microphones is used to measure incoming and reflected sound waves emitted by an external sound source. The amount of energy that is reflected from the barrier is a measure of the absorptive properties of a noise barrier. A filtering technique based on the expected path lengths of parasitic reflections (like ground reflections and reflections from other objects) in the impulse response domain is used to isolate the reflections from the barrier under test. For simple barrier geometries, the expected path lengths can be calculated easily. When measuring more complex geometries, like Gabion walls, these calculations become more complex. Gabion walls are typically composed of a solid concrete core, surrounded by a metal grid filled with stones of different sizes. In this case, the effective distance of the microphone array to the barrier can be largely frequency dependent. Low frequencies sound waves are expected to not be very much impacted by the stones, while higher frequencies waves are expected to scatter due to the stones. 2. CURRENT RESEARCH

M+P has been using the EN 1793-5 method for over five years now to measure the absorptive properties of noise barriers [2], [3]. We have participated in the inter laboratory test in 2018, organized

1 WoutSchwanen@mp.nl 2 WillemJan.Van.Vliet@rws.nl;

by the Austrian Institute of Technology (AIT). The current research compares the results of the various analysis steps of a simple absorptive barrier with a gabion wall and illustrate the difficulties that may arise when analyzing the results of measurements on a gabion wall. 3. DESCRIPTION OF THE NOISE BARRIERS

3.1. Gabion wall The gabion wall under testing is located along the railway track in the southwest of the Netherlands. It has a height of 3.5 meters and a thickness of approximately 1 meter. Inside the noise barrier a solid core of 20 cm thickness is present to improve the insulation of the gabion wall. The gabion is filled with rubble. Figure 1 shows several pictures of the gabion wall.

Figure 1: Gabion wall under testing

2.2. Reference noise barrier The reference barrier was tested in an inter-laboratory test (ILT) on sound reflection and airborne sound insulation properties of noise barriers. It consists of 8x0.5 m high stacked aluminum cassettes with perforated metal plates at the front and inner absorber. A picture of the noise barrier is given in figure 2. The measured reflection index is given in figure 3. For comparison this graph also depicts the accepted reference value of the interlaboratory testing. We see a good agreement between both results.

Figure 2: Reference noise barrier, that was tested in the ILT [4]

Figure 3:Reflection index for the reference noise barrier [4]

4. SOUND REFLECTION INDEX MEASUREMENTS

4.1. General principle The EN 1793-5 standard describes the method how to determine the sound reflection index in-situ. A simplified version of the process is schematically represented in the figure below.

ee os accepted reference value frequensy (12)

Figure 4: Schematic representation of the method for in-situ determination of the reflection index

according to EN 1793-5 A sound source emits a transient sound wave that travels past the measurement grid (microphone) position to the device under test and is then reflected. Each microphone that is placed between the sound source and the device under test receives both the direct sound pressure wave travelling from the sound source to the device under test and the sound pressure wave reflected (including scattering) by the device under test. The direct sound pressure wave can be better acquired with a separate free field measurement (reference measurement). The power spectra of the direct and the reflected components provides the basis for calculating the sound reflection index.

4.2. Calculation of impulse response The sound source emits an exponential sine sweep (e-sweep), which is later converted into an impulse response. Paragraph 5.4.2 of EN 1793-5 states that the sound source should have a smooth magnitude of the frequency response without sharp irregularities throughout the measurement frequency range, resulting in an impulse response under free-field conditions with a length not greater than 3 ms. Therefore, we calculated the amount of energy within 3 ms in comparison to the total amount of energy for microphone 5. For both the reference noise barrier and the gabion wall 99.9% of the total energy is within 3 ms.

Figure 5: Impulse response under free-field conditions. Left: Reference noise barrier. Right: Gabion

wall

measure e-sweep convert to IR determine window reference measure e-sweep convert to IR shift IR barrier to reference subtracted = IRparier ~ !Rreterence determine window subtracted apply windowing calculate RI:

4.3. Window placement and subtraction technique

Subtraction technique

Figure 6 shows the impulse response, the window placement and the subtracted signal, both for the reference noise barrier R2 (left side) and for the gabion wall (right side). Displayed is the result for microphone 5, the center microphone in the array.

Figure 6: Window application on the reference measurement (upper diagram) and on the subtracted

signal (lower diagram). Left: Reference noise barrier. Right: Gabion wall

The subtracted signal for the reference noise barrier shows one clear reflected peak in the subtracted signal. For the gabion wall, the sound is reflected over a much longer period, so there is not one clear peak in the subtracted signal. In fact, we observe reflections within the whole time window and even outside the window there seems to be noise coming back from the gabion wall. We did check the amount of energy outside the window, and it turned out that this energy is relatively low compared to the amount of energy within the window.

Reduction factor R sub

The parameter is the reduction factor R sub , which is an indicator for the quality of the signal subtraction. Practical experience shows that a R sub value below 10 dB should be considered as a warning that the signal subtraction is not perfect. The figure below shows that for both noise barriers the reduction factor is above 10 dB for all microphones.

Figure 7: Reduction factor R sub for the measurement at the gabion wall and for reference noise

barrier

Signal to noise ratio

The signal to noise ratio is calculated by comparing the amount of energy in the windowed reflected signal with the amount of energy of the background noise. For the background noise we used the signal at the end of the impulse response. The length of the signal is equal to the length of the used time window. EN 1793-5 states that the effective signal to noise ratio (S/N ratio) shall be above 10 dB for the whole measurement range of interest. The S/N ratio is depicted in figure 8 for two microphone positions for the reference noise barrier and for the gabion wall. We can see that the S/N ratio is above 10 dB in all frequency bands.

Figure 8: Signal to noise ratio for two microphones positions. Left: reference noise barrier. Right:

gabion wall

smreference barter sui ‘eabion wall ssn postion

4.4. Calculation of reflection index The measurement was performed at three different positions along the gabion wall. The resulting reflection index per position is depicted in the figure below. The reflection index is the arithmetic average over the result of all nine microphones. The results for the induvial microphones show that for some microphone positions the reflection index in certain frequency bands has a value above 1, meaning for that microphone the reflected noise is higher than the direct noise from the sound source.

Figure 9: Reflection index for the gabion wall measured at three different positions along the noise

barrier

4.4. Single number rating We calculated the single number rating for sound reflection DL RI by weighting the sound reflection values according to the normalized traffic noise spectrum defined in EN1793-3 5. The values for each position are given below. For comparison reasons we have also depicted the value of DL RI for the reference noise barrier.

Table 1: Single-number rating DL RI

DL RI [dB]

position 1 4.1

position 2 4.1

position3 4.1

reference barrier 6.7

os oa oo position 1 Frequency (H2]

5. EVALUATION OF RESULTS

5.1. Variation within the results The reflection indices for the gabion wall shows a large variation over the microphone positions (see section 4.4). We have calculated the standard deviation for the reflection index over the nine microphones for the three positions for the gabion wall and for the reference noise barrier (see figure 10). For the frequencies up to 800 Hz, there is hardly any difference in standard deviation for the reference noise barrier and for the gabion wall. For frequencies from 800 Hz and above there is a large increase in standard deviation for the gabion wall. Secondly, we observe substantial differences in standard deviation between the different positions along the gabion wall.

Figure 10: Standard deviation over the nine for the reflection index for the gabion wall measured at

three different positions along the noise barrier

5.2 Influence of correction parameter C geo In EN 1793-5 the correction factor for geometrical divergence, C geo,k is specified as:

𝑑 𝑟,𝑘

𝑑 𝑖,𝑘 ) 2 (1)

𝐶 𝑔𝑒𝑜,𝑘 = (

Where d i,k is the distance from the front panel of the loudspeaker to the k -th measurement point, and d r,k the distance from the front panel of the loudspeaker to the source and microphone reference plane and back to the k -th measurement point following specular reflection. Note that C geo,k is not frequency dependent. In the case of a Gabion wall and for lower frequencies, the (effective distance) following the specular reflection is larger than for higher frequencies. This adds to the uncertainty of the measurement method for these kind of barrier surfaces.

6. CONCLUSIONS

The current study gained insight in the application of in-situ measurements for the reflection index in accordance with EN1793-5 on gabion walls. A first finding is that the gabion wall under testing has low values for the reflection index for lower frequencies. The value for the reflection index increases for higher frequencies. This is contrast with regular noise barriers. Secondly, the subtracted signal for the gabion wall is significantly different from the subtracted signal from a regular noise barrier. For the reference barrier there is one clear reflection peak. For the gabion wall there is no clear reflection peak, but noise seems to reflect from the gabion wall over a longer time.

|

We observe peaks in the value for reflection index clearly above 1. The standard does not prescribe how to deal with values above 1. We observe a large variance in the reflection indices between the various microphone positions in different frequency bands for the gabion wall. This variation is much larger than for a regular noise barrier and it’s unclear whether such a variance can render the measurement method to be invalid for irregular surfaces like Gabion walls. 6. REFERENCES

1. EN 1793-5: Road traffic noise reducing devices - Test method for determining the acoustic

performance - Part 5: Intrinsic characteristics - In situ values of sound reflection under direct sound field conditions, 2016 2. Wout Schwanen & Bert Peeters, In-situ testing of acoustical properties of noise barriers,

proceedings of INTER-NOISE 2016 , Hamburg, Germany, 2016 3. Wout Schwanen, Saskia Hardeman & Ysbrand Wijnant, Improving the in-situ determination of

the intrinsic characteristics of noise barriers, proceedings of ICSV24 , London 2017 4. Andreas Fuchs, Marco Conter, Reinhard Wehr, Inter-laboratory test on sound reflection and

airborne sound insulation properties of noise barriers, AIT report , 2019 5. EN 1793-3: Road traffic noise reducing devices - Test method for determining the acoustic

performance - Part 3: Normalized traffic noise spectrum, 1997