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Testing with pink noise

Noise test rig in Essen

© Udo Geisler

23 May 2017

Does your printer work quietly enough to merit the “Blauer Engel” eco-label? To what extent do wall hangings reduce noise levels in the office? And is the thumping music from next door still getting through the new windows? Such are the questions that occupy Marc Leisegang, Dirk Hausrad and Horst-Ulrich Pohl from TÜV NORD at the new noise test rig in Essen.

When you first enter the hall in in Essen you are confronted with a smooth white wall, nearly three metres high. Two steps lead up to a heavy white metal door. From behind it a noise is emanating which is reminiscent of an analogue radio that is not tuned to a channel. Some metres away, Dirk Hausrad, an expert in soundproofing, and metrologist Horst-Ulrich Pohl are watching eagerly as a jagged curve is described on the computer screen. After just a few seconds, the measurement is complete. Horst-Ulrich Pohl opens the door to the test rig, revealing a small, white-painted room. The noise becomes ear-splittingly loud before Pohl cuts it out by switching off the twelve-sided loudspeaker in the front corner of the room. This noise is known as pink noise, explains Dirk Hausrad: A frequency spectrum defined by the relevant standard as a test signal with which the average person experiences all the frequency ranges of the audible sound spectrum as equally loud. On this day in the wall test rig, the insulation material for a ship’s bulkhead is being tested. Several thick mats of mineral wool with or without aluminium coating are waiting their turn.

The wall test rig itself actually consists of two decoupled rooms; doors, sound insulation walls or, as is the case today, ship’s bulkheads are inset into the curtain wall. On the one side, what is referred to as the source room houses the speaker with the pink noise as well as a microphone to capture the noise level. On the other side of the wall, in the receiving room, there is also a microphone which slowly turns on its own axis during the measurement to record the sound that is arriving from all directions. In this way it is possible to determine how much sound the material in question is actually absorbing. To ensure that the sound does not pass by the test object, the rooms are designed as a room-in-a-room system and mounted on thick rubber cushions known as elastomers to completely eliminate any vibration.

Visibly proud, group leader for immission control Marc Leisegang shows off the new measurement and test equipment. Alongside the wall test rig there is also a window rig, an echo chamber and an anechoic room. Part of the equipment was previously housed at the old site in Langemarckstrasse. “Moving to the central Essen site here at Am Technologiepark enabled us on the one hand to massively expand our range of services for our customers and to deliver and install test pieces with ad ditional test rigs and a whole host of new options. On the other, we've been able to significantly improve the comfort and convenience of the testing conditions for our experts,” Marc Leisegang explains.

Noise from all sides

The echo chamber a few metres further on really lives up to its name: Anyone trying to hold a conversation here quickly ends up taking refuge in the corridor because all the noises generate a massive echo. From the ceiling hang white sails that are actually rather decorative: these are not test pieces but instead serve to properly distribute sound in the chamber, Mr Leisegang explains. The walls sealed with epoxy resin also reflect as much sound as possible. One of the tests carried out in the echo chamber records the effect of absorption elements. These include wall hangings, partition walls, specially padded chairs and pictures that are primarily used in offices to reduce the sound reverberation effect.

Sound-absorbing foam points

An absolute contrast – balm for the ears – is offered by the anechoic room in the next hall. Behind the heavy door there lies a room with hundreds of small and large foam points which are designed to swallow as much sound as possible. This space is known as a “free field” room because its walls and ceiling do not reflect sound, and the only sound that can be detected is the noise emitted by the test piece, as if it were standing on an open field.

Today, noise emissions from an air compressor are being measured in the anechoic room. Horst-Ulrich Pohl and Dirk Hausrad set up the microphones for the measurement. A total of nine microphones hang on rails under the ceiling and are pulled into the correct positions by ropes. They are arranged for the measurement on a preset enveloping surface around the test piece: The microphones hang around the object in an arrangement reminiscent of a cuboid placed over the compressor. When everything is set up, Mr Pohl starts the compressor which immediately begins to rattle loudly. Mr Pohl leaves the room, and the measurement can start. To start the printing process in compliance with the applicable standards, the experts have even constructed an “electric finger”: A metal construction with multiple joints that are moved by remote control is attached to a tripod. This allows the start button of the printer to be pressed and the prescribed procedure to be launched after all the staff have left the room and closed the heavy door behind them. Here, too, the complete measuring curve appears on Mr Hausrad’s computer after a few seconds. The experts will later generate the evaluation and the reports that go with it in the neighbouring office.

Neutral, recognised noise testers

Examples of other test pieces for the anechoic room include large electrical appliances, leaf blowers, impact wrenches, and, time and again, printers and copiers, says Marc Leisegang. “Normal appliances are generally tested directly by the manufacturer. But if a seal of quality such as the “Blauer Engel” is being sought or compliance with certain standards needs to be demonstrated, then we’re in demand as independent, accredited testers.”

Cooperation with DMT

With the advanced test rigs TÜV NORD can offer an array of component inspections in the field of acoustics and, together with DMT, pool them with fire protection audits. This is meeting with lots of interest in the industry. TÜV NORD working particularly closely on this with the DMT offices in Dortmund and Lathen. These services are a very good complement to our open-air immission measurements and pollution forecasts, for example, for new businesses.