PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Acoustic attenuators for aggressive environments
Ir. Christiaan Cornelis van Dijk1
1

Alara-Lukagro, the Netherlands

ABSTRACT
Alara-Lukagro has conducted research on the essence of acoustic attenuators and in recent years on thepossibility to create broadband attenuators without absorbing materials. This could be done using friction ofthe acoustic wave. An experimental setup using micro-perforates shows how the potential attenuation ofsystems without absorbing material relates to attenuators with absorbing materials, quantitatively, for both
broadband attenuation, flow generated noise and friction losses. The results will be presented, the principles
explained and the potential quantified.
Keywords: Attenuator, Insertion loss

1. INTRODUCTION
In noise control, one of the most interesting challenges lies in the combination of providing Sound
insulation whilst maintaining a significant opening for ventilation. In some industries the gasses are
so aggressive, hot, or sticky that splitter attenuators don't provide a sustainable solution. If the soundsource is broadband or variable or there is no absorption near the source, resonance attenuators can't
be made to fit the noise spectrum.
In theory the edge of a splitter attenuator still will hold its pressure drop properties. Therefore it
was decided to quantify the transmission loss of splitter attenuators without wool and micro -perforated
plates.
Because the outcome of this research is classified the geometries that were tested are not includedin this paper, but test results are. However the comparison with historical lab tests and simulations on
splitter silencers is. In this way the phenomena can still be quantified and a prove of concept can beprovided..

2. Test setup
The test was conducted at the test facility of Alara-Lukagro, which is at the same site as theproduction facility. As a result some test had to be carried out after working hours. The tests were all
conducted in accordance with ISO 7235; 2003. The measurements were all done in threefold, which
include a scan of the source surface, a measurement at a certain distance and an average sound pressurelevel in a reverberant room. The source was placed inside a thick steel casing with a big ventilator on
the roof. When the ventilator was not in operation, the opening was closed by an absorbing sandwich
panel. The channel leading from the casing to the reverberant receiving room was equipped with an
inline silencer half way.
For each setup reverberation times of the receiving room were re-evaluated. The reverberant soundpressure level, SPL difference between measurements with and without silencer was corrected for thechange in reverberation time T:ܶ଴‫ ܦ‬ൌ ܵܲ‫ܮ‬଴ െ ܵܲ‫ܮ‬ଵ ൅ ͳͲʉ  ൬ ൰ܶଵ(1)Note that the reverberation time of a reverberant room with an open channel is typically lower thanfor rooms with a partially open channel. Reverberation times can show strong inconsistencies in thelower frequencies. In this analytics we obtain a series of reverberation times in different open areas.
1Ch.van.dijk@alara-lukagro.com

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If linear functions are made to fit the reverberation time to the open area for each series of identicalelements, a fictive reverberation time can be obtained that suffers less deviations in the lower
frequencies. The fictive reverberation time:ܶ௙ ൌ ܶ௔௩ǡ௢௔ೌೡ ൅ ሺ‫ ܽ݋‬െ ‫ܽ݋‬௔௩ ሻʉσߜܶ௔௩σߜ‫ܽ݋‬௔௩(2)

Where oa is open area and av means average.
The reverberant measurements can now be evaluated in two ways. Where in one way thereverberation time suffers less inconsistencies, so only strong deviations in the average sound pressurelevel have a strong effect. In te other way the reverberation time and the average sound pressure level
have a strong effect on the deviations, but they are both actually measurement.
ISO 7235 (1) doesn't state which evaluation method should be preferred. Of the four attenuation
values two are derived from the average sound pressure level in the reverberant room. That averagesound pressure in the reverberation room doesn't suffer as much from manual inconsistencies, or notproperly evaluating standing waves due to the singularity of the position , as the other two attenuation
values. Therefore it can be allowed to weigh all four measurements equally and obtain a standard
deviation, which tells something about the accuracy of the measurement.

3. Results
3.1 Insertion loss
Two types of micro perforated attenuators were evaluated with different variations within eachtype. One variation is the relative open area. In figure 1 and 2 the resulting insertion loss of type 1
and 2 is plotted per 1/3 rd octave band for different open areas.

Figure 1 - Insertion loss micro-perforated attenuator type 18064

Figure 2 - Insertion loss micro-perforated attenuator type 2
The results are still inhomogeneous, therefore it is good to be aware of the standard deviation ofthese results. The standard deviation is given in figure 3 and 4.

Figure 3 - Standard deviation for type 1 results

Figure 4 - Standard deviation for type 2 results

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The insertion loss are reasonable from 400 Hz and above. The insertion loss is especially goodand reliable between 2 and 5 kHz. The insertion loss of attenuators with a wool filling issignificantly higher, see figure 5 and 6.

Figure 5 - Insertion loss for attenuators comparable to type 1 with wool infill

Figure 6 - Insertion loss for attenuators comparable to type 2 with wool infill
The insertion loss of attenuator with infill is around three times better than for attenuators without
infill. Nonetheless this shows that the pressure drop is a real useable mechanism. Note that both
attenuators use the same principles of transferring acoustic (kinetic) energy into a he at difference.
Another way to protect the wool is to use a protective foil layer. These foil layers have a muchstronger negative influence in the higher frequencies. In figure 7 the insertion loss is compared of two
attenuators with and without a protective layer. These tests were done without inline silencer, so there
is a strong phase-shift in the lower frequencies and the low frequency insertion loss varies strongly,and the low frequency insertion loss values are unreliable.

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Figure 7 - Insertion loss with and without foil
In figure 8 we see that the performance from around 2 kHz and higher is significantly better forthe micro-perforates compared to attenuators with infill than for attenuators with foil compared toattenuators without foil.

Figure 8 - Insertion loss with and without wool
The currently tested attenuators with micro-perforates show good performance above 2 kHz andreasonable performance above 400 Hz. For lower frequency performance some interesting trends wereobserved that require further investigation. There are more very important factors in play for highfrequency attenuation, such as the angle of incidence (2).
3.2 Real reverberation time correction compared to Fictive reverberation time correction
When the standard deviation of all data sets per frequency is compared between insertion loss thatwas obtained after the real reverberation time and insertion loss that was obtained using the fictive
insertion loss, the for fictive reverberation results are more in line with the other measurements. Thisis shown in figure 9.

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Figure 9 standard deviation for different reverberation time corrections

4. CONCLUSIONSThe currently tested attenuators with micro-perforates are market-ready for aggressive
environments were the performance above 2 kHz is essential. For lower frequency performance someinteresting trends were observed that require further investigation.

ACKNOWLEDGEMENTS
I'd like to acknowledge André Hameete and Alara-Lukagro for sponsoring this research and Jaspervan Hees for conducting the measurements.

REFERENCES
1. ISO 7235, Acoustics - Laboratory measurement procedures for ducted silencers and air-terminal units Insertion loss, flow noise and total pressure loss. INTERNATIONAL STANDARD; 2009(E).
2. van Dijk CC, Acoustic validation of calculation software for ducts, panels and room acoustics; ICA2019/788068