Lab for acoustic scale model technology
To reduce costs, traffic obstructions and lengthy examinations BASt uses this lab for investigations and measurements using models of the traffic structures and the residential structures surrounding them which are as little as a twentieth of the original size.
Lab for acoustic model technology (Photo: Christian Kruska)
Almost half the population in Germany feels very annoyed by road traffic noise. To protect people from noise, the legislator has passed the ”Act on the Prevention of Harmful Effects on the Environment caused by Air Pollution, Noise, Vibration and Similar Phenomena” (”Gesetz zum Schutz vor schädlichen Umwelteinwirkun-gen durch Luftverunreinigungen, Geräusche, Erschütterungen und ähnliche Vorgänge”), abbreviated to the Federal Immission Control Act (Bundes-Immissionsschutz-Gesetz). The details of this law are regulated by the 16th Federal Immission Control Ordinance (Bundes-Immissionsschutzverordnung), the so-called ”Traffic Noise Protection Ordinance” (”Verkehrslärmschutzverordnung”) and the ”Ordinance on Measures for Traffic Infrastructure Noise Protection” (”Verkehrswege-Schallschutzmaßnahmenverordnung”).
The Traffic Noise Protection Ordinance (Verkehrslärmschutzverordnung) requires the noise emissions made by newly built roads and railways, and roads and railways which have been subject to essential modification, to be lowered to such an extent that the noise perceived by the local residents stays below certain limit values. Emissions can usually be reduced sufficiently by implementing measures such as speed limits, noise barriers, noise protection walls, low noise pavements or a combination of these different measures. However, with further increases in traffic volumes and higher demands regarding noise protection, the usual measures to reduce noise annoyance are no longer sufficient and complex noise protection systems have to be erected:
- cladding of the noise barriers with highly absorbent material
- several noise barriers located one behind the other to attain additional diffraction edges for the noise
- wide earth walls with noise barriers located on top of them
- creating troughs through which to route roads, with or without partial covering
- installation of galleries: tunnel-like constructions open towards the side which is not noise-sensitive
- complete enclosure
On the one hand these measures entail considerable cost, and on the other hand the road construction authority must know what noise reduction can be attained with them. So far, however, it has only been possible to make approximate calculations of the noise-reducing effect of these protective devices. Their exact effectiveness would have to be measured in outdoor measurements of test systems. This requires constructions of up to 200 m in length. Installing these structures means high costs and often considerable hindrances to traffic. The characterisation of these systems is also time-consuming, dependent on the weather and requires a lot of personnel.
The effect of such special protective devices must therefore be determined using models. This is why the lab for acoustic model technology was established at the Federal Highway Research Institute.
Measurements on a scale of 1 : 20
The investigations and measurements in the lab are carried out using models of the traffic structures and the residential structures surrounding them which are as little as a twentieth of the original size. At a scale of, for example, 1:20, areas of 200 x 200 metres can be simulated on a 10 x 10 metres model table.
Testing of fin and grid covering
The wavelengths of the noise emitted by the vehicles on the roads are also shortened in accordance with the model scale. The model sound sources must emit the sound in wideband and evenly in all directions. Based on the real wavelengths occurring in traffic, this therefore results in frequencies in the region of 2,000 to 100,000 Hertz at a maximum scale of reduction of 1:20. To prevent sound frequencies, in particular the high frequencies, from being absorbed by the air, the lab air has to be dried to 5 % relative humidity at a room temperature of 20 °C during the course of the measurements.
The lab walls and the ceiling are double-wall clad with noise-absorbing material and equipped with a vapour barrier. This prevents the measurements from being influenced by noise or moisture from outside.
The model sound sources used in the lab work according to three different principles, depending on the required frequency range:
- modulation of a flame of ionised air (plasma)
- pressure release of compressed air via an aperture with a diameter of 1 mm
- ribbon tweeters which are made to oscillate by applying an electric signal
Compressed-air sound generator (lefth) and flame of ionised air (right)
Certain tasks require the two-dimensional measurement of acoustic fields. The lab is therefore equipped with an installation which automatically scans a surface of prescribed size and adjustable grid. This enables the readings to be easily recorded, stored and processed.
The investigations carried out in the lab for acoustic scale model technology have included the following:
- road trough as a means of noise abatement
- noise-reducing effect of galleries
- noise screens with several diffraction edges
- effectiveness of open noise barriers
- sound propagation at tunnel mouths and measures to reduce this, for example by cladding tunnel walls with absorbent material
The Federal Highway Research Institute uses the investigations in the lab for acoustic scale model technology to determine noise abatement alternatives which are both effective and at the same time cost-effective.
Special consultant: Dr. Wolfram Bartolomaeus
- Area: approx. 11 x 12 m
- Height: 4.30 m
- Sound sources: flame of ionised air, compressed-air sound generator or ribbon tweeter
- Fittings: noise-absorbing wall and ceiling cladding
- Air-conditioning: regulation of the relative air humidity of up to 5 percent at 20 °C